Biology SME copy 1

6. The following diagram shows the nephron.

Key unit competence: Explain general principles of homeostatic mechanisms, excretion and osmoregulation.

Introductory activity 9

Analyze the following water treatment plant and answer the questions that follow:

The photo above shows a water treatment plant located in Kigali in Kimisagara. The water in the river that supplies the water treatment plant may become polluted with sediments, animal waste, urine of people, but the water treatment plant removes these wastes. In an analogous way, the cells of all body systems produce waste products, and these wastes end up in the blood.

a. Which system in our body could be compared to a water treatment plant? 

b. Which organs make the system that you have named above? 

c. Which fluid produced by the body that contains the metabolic waste products? 

d. Compare the process of removing the waste products from the water to the process by which our body removes the metabolic waste products from the blood. Are there any similarities?

9.1 Significance of constant internal environment and factors kept constant in the body

Activity 9.1

Use your biological knowledge to the answer the questions that follow:

1. Define the following biological terms

a. Homeostasis

b. Internal environment

2. State ant three factors that need to be maintained constant in the human body?

3. Explain why our body temperature is kept constant independently of the external environment.

All living organisms have an ability to maintain stable internal conditions. It requires continuous adjustments to the changes occurring in both internal and external environment. This self-regulating property of living beings to maintain a constant internal environment is termed as ‘homeostasis’ (‘homeo’, “similar,” and ‘stasis’, “stable”). Homeostasis is a key concept in the understanding of biological mechanisms that play an important role in survival of individual cells, to an entire body.

Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties such as temperature or pH. It was defined by Claude Bernard and later by Walter Bradford Cannon in 1926, 1929 and 1932. Typically used to refer to a living organism, the concept came from that of milieu interieur that was created by Claude Bernard and published in 1865. Multiple dynamic equilibrium adjustment and regulation mechanisms make homeostasis possible.

9.1.1 Meaning of internal environment

 Internal environment or interstitial fluid (or tissue fluid) is a solution that bathes and surrounds the cells of multicellular animals. It is the main component of the extracellular fluid, which also includes plasma and transcellular fluid. The interstitial fluid is found in the interstitial spaces, also known as the tissue spaces. On average, a person has about 11 liters of interstitial fluid, providing the cells of the body with nutrients and a means of waste removal.

9.1.2 Factors of homeostasis to be kept constant in the body

 To function efficiently, organisms have control systems to keep internal conditions near constant, a feature known as homeostasis. This requires information about conditions inside the body and the surroundings, which are detected by sensory cells. Some of the physiological factors controlled in homeostasis in mammals are:

• Core body temperature: The maintenance of a steady body temperature involves mechanisms such as sweating or shivering. These mechanisms occur whenever the internal body temperature becomes high or low.

 • Blood glucose concentration: When glucose levels are high, a hormone called insulin is released by beta cells of the pancreas. Insulin stimulates the conversion of glucose as insoluble glycogen by the body cells. This lowers the glucose concentration in the blood. A condition called as diabetes occurs due to the deficiency of insulin in the body, due to which glucose level of blood increases. When the blood glucose levels are low, another hormone known as glucagon is released by the alpha cells of pancreas. Glucagon breaks down stored glycogen in the form of glucose. The addition of glucose in blood returns the body glucose levels to normal.

 • Metabolic wastes, particularly carbon dioxide and urea

 • Blood pH: The pH of the blood is regulated at 7.365 (a measure of alkalinity and acidity). The tolerable lower and upper limit for a human body is about pH 7.0 and pH 7.8, respectively. To prevent a change in the pH, all body fluids, including cell cytoplasm are buffered (buffer is a chemical or a combination of chemicals) absorbing either hydrogen ions (H+ ) or hydroxide ions.

• Water potential of the blood: Whenever the water content of the blood and lymph fluid gets low, it is restored initially by extracting water from the cells. Also, the throat and mouth become dry. These symptoms of thirst motivate humans to drink water.

 • The concentrations in the blood of the respiratory gases, oxygen and carbon dioxide: A change in breathing and heart rate occurs in humans due to various activities like exercise. As a result, the amount of carbon dioxide produced and oxygen demand in the body increases. The heart rate increases so that the blood flows rapidly to the tissues to fulfil the oxygen requirement and remove the carbon dioxide from the cells. Also, the speed and depth of breathing increases. The body works to normalize breathing and heart rate when activity stops.

Application activity 9.1

1. In your own words, explain the significance of a constant internal environment by giving suitable examples.

2. State four factors that must be kept constant in the internal environment of the body.

3. What are the main internal and external causes of change in the internal environment?

9.2 Feedback mechanisms 

Activity 9.2 

The temperature in the house can be maintained constant using a thermostat. The thermostat sends a message to the furnace to produce heat. Heat returns to the thermostat. The heat will cause the thermostat to stop stimulating the furnace. If temperature drops below the set point of the thermostat then the furnace will be stimulated again. The furnace will turn on and off several times a day to keep the temperature constant. How can this heating system be compared to our body temperature regulation?

All homeostatic control mechanisms have at least three interdependent components for the variable being regulated: The receptor is the sensing component that monitors and responds to changes in the environment. When the receptor senses a stimulus, it sends information to a “control center”, the component that sets the range at which a variable is maintained. The control center determines an appropriate response to the stimulus. In most homeostatic mechanisms, the control center is the brain. The control center then sends signals to an effector, which can be muscles, organs or other structures that receive signals from the control center. After receiving the signal, a change occurs to correct the deviation by either enhancing it with positive feedback or depressing it with negative feedback. (https://biologydictionary.net/positiveand-negative-feedback-homeostasis/

The homeostatic mechanisms in mammals require information to be transferred between different parts of the body. There are two coordination systems in mammals that do this: the nervous system and the endocrine system. 

• In the nervous system, information in the form of electrical impulses is transmitted along nerve cells (neurons).

 • The endocrine system uses chemical messengers called hormones that travel in the blood, in a form of long-distance cell signaling.

Positive feedback mechanisms are designed to accelerate or enhance the output created by a stimulus that has already been activated. Unlike negative feedback mechanisms that initiate to maintain or regulate physiological functions within a set and narrow range, the positive feedback mechanisms are designed to push levels out of normal ranges. To achieve this purpose, a series of events initiates a cascading process that builds to increase the effect of the stimulus. This process can be beneficial but is rarely used by the body due to risks of the acceleration’s becoming uncontrollable. One positive feedback example event in the body is blood platelet accumulation, which, in turn, causes blood clotting in response to a break or tear in the lining of blood vessels. Another example is the release of oxytocin to intensify the contractions that take place during childbirth. Another example of a positive feedback mechanism is milk production by a mother for her baby. As the baby suckles, nerve messages from the mammary glands cause the hormone prolactin, to be secreted by the mother’s pituitary gland. The more the baby suckles, the more prolactin is released, which stimulates further milk production by the mother’s mammary glands. In this case, a negative feedback loop would be unhelpful because the more the baby nursed, the less milk would be produced.

Table 9.1: Negative and positive feedback compared

Application activity 9.2

a. State any to examples of a negative feedback and two examples of positive feedback in our body.

b. What are the main differences between positive feedback and negative feedback?

c. Explain why a positive feedback cannot be effective in homeostatic regulation?

9.3 Negative feedback mechanisms related to the endocrine and nervous systems in homeostatic activities 

Activity 9.3: Research activity

Have you ever thought about how your body maintains the same blood calcium level independently of whether you have eaten or not? Do you know about homeostasis and why it is required? Use library and Internet sources to collect information about the regulation of blood calcium level.

In the human body, all the organs and organ systems are controlled by nervous and endocrine systems. The nervous system controls the activities of body parts by reacting quickly to external and internal stimuli. The endocrine system regulates those activities slowly but its effects are long lasting. The hypothalamus is a part of the brain (nervous control center) located just above the brain stem and consists of a group of neurons that forms the primary link between the nervous system and the endocrine system. This small part of the brain is responsible for regulating many key body processes, including internal body temperature, hunger, thirst, blood pressure, and daily body rhythms.

Nervous system consists of receptive nerve cells which transmit the signal to the brain, which in turn, command the effector nerve cells, muscles and glands to respond. For instance, humans maintain a constant body temperature, usually about 37.4°C. It increases during the day by about 0.8°C and decreases slightly during sleeping. The core body temperature is usually about 0.7-1.0°C higher than skin or axillary temperature. A change in temperature is sensed by receptors found in the skin, veins, abdominal organs and hypothalamus. The receptors in the skin provide the sensation of cold and transmit this information to brain. The brain process and commands for the vasoconstriction of blood vessels in the skin and limb. This drops the surface temperature, providing an insulating 335 layer (fat cell) between the core temperature and the external environment. The major adjustment in cold is shivering to increase the metabolic heat production. On the contrary, if the body temperature rises, blood flow to the skin increases, maximizing the potential for heat loss by radiation and evaporation.

The endocrine system consists of glands which secrete hormones into the bloodstream. Each hormone has an effect on one or more target tissues. 

 In this way, it regulates the metabolism and development of most body cells and its systems through feedback mechanisms, mostly negative. For example, when blood calcium becomes too low, calcium-sensing receptors in the parathyroid gland become activated. This results in the release of Parathyroid Hormone (PTH), which acts to increase blood calcium by release from the bones. This hormone also causes calcium to be re-absorbed from urine and the gastrointestinal tract. Calcitonin, released from the thyroid gland functions in reverse manner, i.e., decreasing calcium levels in the blood by causing more calcium to be fixed in bones.

Application activity 9.3

a. Which part of the body is involved in temperature regulation?

b. Which hormones are involved in blood calcium regulation?

c. What are the functions of the nervous system and the nervous system in homeostatic regulation?

9.4 Causes of changes in the internal environment 

Activity 9.4: 

Research activity Nowadays there are many people suffering from different disorders such as diabetes mellitus and diabetes insipidus. Make a search on the internet and in the library in order to know the cause of these disorders.

Homeostasis is maintained through a series of control mechanisms. When homeostatic process is interrupted, the body can correct or worsen the problem, based on certain influences. There are internal and external causes influencing the body’s ability to maintain homeostatic balance.

9.4.1 Internal causes: heredity 

 Genetic/Reproductive: A variety of diseases and disorders occur due to the change in the structure and function of genes. For example, cancer can be genetically inherited or can be induced due to a gene mutation from an external source such as UV radiation or harmful drugs. Another disorder, Type 1 diabetes, occurs due to the lack or inadequate production of insulin by the pancreas to respond to changes in a person’s blood glucose level.

9.4.2 External causes: lifestyle 

Nutrition: A diet lacking specific vitamin or mineral leads to the cellular malfunction. A menstruating woman with iron deficiency will become anaemic. As iron is required for haemoglobin, an oxygen transport protein present in red blood cells, the blood of an anaemic woman will have reduced oxygen-carrying capacity. 

Physical Activity: Physical activity is essential for proper functioning of our cells and bodies. Adequate rest and exercise are examples of activities that influence homeostasis. Lack of sleep causes ailments such as irregular cardiac rhythms, fatigue, anxiety that and headaches. Overweight and obesity are related to poor nutrition and lack of physical activity that greatly affects many organ systems and their homeostatic mechanisms. It increases a person’s risk of developing heart disease, Type 2 diabetes, and certain forms of cancer.

Mental Health: Both the physical and mental health is inseparable. Negative stress (also called distress) leads to thoughts and emotions harmful for homeostatic mechanisms in the body.

9.4.3 Environmental exposure

Many substances act as toxins, including pollutants, pesticides, natural and synthetic drugs, plants and animal products interfering at cellular levels. Modern medicines practice can also be potentially harmful in case of wrong or over dosage. For instance, drug overdose affects the central nervous system, disrupts breathing and heartbeat in human body. It can further result in coma, brain damage, and even death. Therefore, alterations or interruption of beneficial pathways, whether caused by an internal or external factor will result in harmful change in homeostasis. Therefore, adequate positive health influences are to be taken into consideration in order to maintain homeostasis.

Application activity 9.4 

What are the causes of the changes in the internal environment? State any two genetic causes and two environmental causes.

9.5 Formation, composition and movement of tissue fluid and its relationship to the blood and lymph 

Activity 9.5

You may have observed a clear fluid after having an injury. This fluid does not have a red color. This fluid is not blood because it does not have a red color. What is the name of this fluid? What is the composition of this fluid? What are some similarities and differences between this fluid and blood?

The blood supplies nutrients and essential metabolites to the cells of a tissue and collects back the waste products. This exchange of respective constituents between the blood and tissue cells occurs through interstitial fluid or tissue fluid formed by the blood. The fluid occupies the spaces between the cells known as tissue spaces. It is the main component of the extracellular fluid, which also includes plasma and transcellular fluid. On an average, a person has about 10 liters of interstitial fluid making 16% of the total body weight.

9.5.1 Formation 

The formation of the tissue fluid is based on the difference in pressure of flowing of blood through capillaries. A hydrostatic pressure is produced at the arterial end of blood capillaries which is generated by the heart. This results in expulsion of water and other solutes (known as plasma) from capillaries except blood proteins (like serum albumin). This retention of solutes in capillaries creates water potential. The osmotic pressure (water moves from a region of high to low concentration) tends to drives water back into the capillaries in an attempt to reach equilibrium. At the arterial end, the hydrostatic pressure is greater than the osmotic pressure, so the net movement favours water along with solutes being passed into the tissue fluid. At the venous end, the osmotic pressure is greater, so the net movement favours tissue fluid being passed back into the capillary. The equilibrium is never attained because of the difference in the direction of the flow of blood and the solutes imbalance created by the net movement of water (Figure 11.6).

9.5.2 Composition 

As the blood and the surrounding cells continually add and remove substances from the interstitial fluid, its composition continually changes. 

 Water and solutes can pass between the interstitial fluid and blood via diffusion across gaps in capillary walls called intercellular clefts; thus, the blood and interstitial fluid are in dynamic equilibrium with each other. Generally, tissue fluid consists of a water solvent containing sugars, salts, fatty acids, amino acids, coenzymes, hormones, neurotransmitters, as well as metabolic waste products from the cells.

Not all of the contents of the blood pass into the tissue, which means that tissue fluid and blood are not the same. Red blood cells, platelets, and plasma proteins cannot pass through the walls of the capillaries. The resulting mixture that does pass through is, in essence, blood plasma without the plasma proteins. Tissue fluid also contains some types of white blood cells, which help to combat infection.

9.5.3 Movement 

To prevent a buildup of tissue fluid surrounding the cells in the tissue, the lymphatic system plays an important role in its transport. Tissue fluid can pass into the surrounding lymph vessels where it is then considered as lymph. The lymphatic system returns protein and excess interstitial fluid to the blood circulation. Thus, it is transported through the lymph vessels to lymph nodes and ultimately with blood in the venous system, and tends to accumulate more cells (particularly, lymphocytes) and proteins.

Application activity 9.5

a. What are the main components of lymph?

b. What are the main differences between blood and lymph?

c. How does lymph differ from tissue fluid?

9.6 Structure and functions of excretory organs in mammals

Activity 9.6

a. What are the main excretory organs of humans?

b. What are the main excretory waste products of mammals?

c. Identify the organs of the human excretory organs and their functions. 

Excretion the removal of toxic waste products of metabolism from the body. The term is generally taken to mean nitrogenous wastes such as; urea, ammonia and uric acid but other materials like carbon dioxide and the bile pigments are also waste products of metabolism, and their removal is as much a part of excretion as the elimination of urea.

 Excretion is an essential process in all forms of life. When cells metabolize or break down nutrients, waste products are produced. For example, when cells metabolize amino acids, nitrogen wastes such as ammonia are produced. Ammonia is a toxic substance and must be removed from the blood and excreted from the body.

Although the kidneys are the main organs of excretion of wastes from the blood, several other organs are also involved in the excretion, including the; liver, skin, and lungs.

 - The large intestine eliminates waste products from the bile synthesis. 

- The liver breaks down excess amino acids in the blood to form ammonia, and then converts the ammonia to urea, a less toxic substance. The liver also breaks down other toxic substances in the blood, including alcohol and drugs. 

- The skin eliminates water and salts in sweat. - The lungs exhale water vapor and carbon dioxide.

Importance of excreting wastes

 i. To maintain life processes, the body must eliminate waste products, many of these which can be harmful. The lungs eliminate carbon dioxide, one of the products of cellular respiration. The large intestine removes toxic wastes from the digestive system.

 ii. The liver transforms ingested toxins, such as alcohol and heavy metals, into soluble compounds that can be eliminated by the kidneys.

9.6.1 Kidneys and excretion 

The kidneys are part of the urinary system (Figure 9.4). The kidneys work together with other urinary system organs in the function of excretion.

a. Urinary system 

In addition to the kidneys, the urinary system includes the; ureters, bladder, and urethra. The main functions of the urinary system are to; filter waste products and excess water from the blood and remove them from the body. 

From the kidneys, urine enters the ureters. Each ureter is a muscular tube about 25 centimetres long. Peristaltic movements of the muscles of the ureter send urine to the bladder in small amount. Ureters carry urine to the bladder. The bladder is a hollow organ that stores urine. It can stretch to hold up to 500 millilitres. When the bladder is about half full, the stretching of the bladder sends a nerve impulse to the sphincter that controls the opening to the urethra. In response to the impulse, the sphincter relaxes and lets urine flow into the urethra. 

The urethra is a muscular tube that carries urine out of the body. Urine leaves the body through another sphincter in the process of urination. This sphincter and the process of urination are normally under conscious control/voluntary system.

b. Kidneys 

The kidneys are a pair of bean-shaped, reddish brown organs about the size of a fist. They are located just above the waist at the back of the abdominal cavity, on 342342 either side of the spine. The kidneys are protected by the ribcage. They are also protected by a covering of tough connective tissues and two layers of fat, which help cushion them. Located on top of each kidney is an adrenal gland. The two adrenal glands secrete several hormones. Hormones are chemical messengers in the body that regulate many body functions. The adrenal hormone aldosterone helps regulate kidney functions. The functional unit of a kidney is a nephron.

9.6.2 Structure and the functions of the nephron

 Nephrons are the structural and functional units of the kidneys. A single kidney may have more than a million nephrons. An individual nephron (Figure 9.6) includes a glomerulus, Bowman’s capsule, and renal tubule.

a. Parts of the nephron and their functions 

- The glomerulus is a cluster of arteries that filters substances out of the blood.

- Bowman’s capsule is a cup-shaped structure around the glomerulus that collects the filtered substances.

- The renal tubule is a long, narrow tube surrounded by capillaries that reabsorbs many of the filtered substances and secretes other substances.

b. Ultra-filtration, selective reabsorption and tubular secretion

The renal arteries, which carry blood into the kidneys, branch into the capillaries of the glomerulus of each nephron. The pressure of blood moving through these capillaries forces some of the water and dissolved substances in the blood through the capillary walls and into Bowman’s capsule. Bowman’s capsule is composed of layers. The space between the layers, called Bowman’s space, fills with the filtered substances.

The process of filtering substances from blood under pressure in the glomerulus is called ultra-filtration, while the fluid that collects in Bowman’s space is called glomerular filtrate. The filtrate is mainly composed of; water, salts, glucose, amino acids, hormones and urea. Larger structures in the blood including; the protein molecules, blood cells, and platelets do not pass into Bowman’s space. Instead, they remain in the main circulation. 

From Bowman’s space, the filtrate passes into the renal tubule whose main function is reabsorption. Reabsorption is the return of needed substances in the glomerular filtrate back to the bloodstream. It is necessary because some of the substances removed from the blood by filtration including; water, salts, glucose, and amino acids which are useful and needed by the body. About 75 % of these substances are reabsorbed in the renal tubule.

Under conditions in which the kidney conserves as much water as possible, urine can reach an osmolality of about 1200 milliosmoles (mOsm/L), considerably hypertonic to blood (about 300 mosm/L). Osmolarity is the solute concentration expressed as molarity. This ability to excrete nitrogenous wastes with a minimal loss of water is a key terrestrial adaptation of mammals. The loop of Henle is known as a countercurrent multiplier. The term countercurrent refers to the fact that the fluid flows in opposite directions in the two sides of the loop, down one side and up in the other. The multiplier effect is seen by comparing the osmolality of the fluid in the cortex and that in the renal medulla at the hairpin end of the loop.

The remaining fluid enters the distal tubule. The distal tubule carries the fluid, now called tubular fluid, from the loop of Henle to a collecting duct. As it transports the fluid, the distal tubule also reabsorbs or secretes substances such as calcium and sodium following the influence of hormones (e.g. aldosterone). The process of secreting substances into the tubular fluid is called secretion.

Application activity 9.6

1. What are the main parts of a nephron?

2. In which part of the nephron does each of the following processes takes place?

a. Ultrafiltration

b. Reabsorption

c. Secretion

3. What is the function of the loop of Henle?

9.7 Formation of urea and urine

Activity 9.7

a. Where is urea produced?

b. Where is urea excreted?

c. What the main steps involved in the formation of urine?

d. What is the importance of the ornithine cycle?

Urine formation depends on three processes including ultrafiltration, selective reabsorption and secretion/tubular secretion.

a. Ultra-filtration 

Each nephron of the kidney has an independent blood supply, which moves through the afferent arteriole into the glomerulus, a high-pressure filter. Normally, pressure in a capillary bed is about 25 mm Hg. The pressure in the glomerulus is about 65 mm Hg. Dissolved solutes pass through the walls of the glomerulus into the Bowman’s capsule. Although materials move from areas of high pressure to areas of low pressure, not all materials enter the capsule.

b. Selective reabsorption 

 The importance of reabsorption is emphasized by examining changes in the concentrations of fluids as they move through the kidneys. On average, about 600 mL of fluid flows through the kidneys every minute. Approximately 20% of the fluid, or about 120 mL, is filtered into the nephrons. This means that if none of the filtrate were reabsorbed the quantity of around 120 mL of urine each minute would be formed and the amount of at least 1 L of fluids would be consumed every 10 minutes to maintain homeostasis.

c. Secretion 

Secretion is the movement of wastes from the blood back into the nephron. Nitrogen containing wastes, excess H+ ions, and minerals such as K+ ions are examples of substances secreted. Even drugs such as penicillin can be secreted. Cells loaded with mitochondria line the distal tubule. Like reabsorption, tubular secretion occurs by active transport, but, unlike reabsorption, molecules are shuttled from the blood into the nephron.

Formation of urea 

The body is unable to store proteins or amino acids, and any surplus is destroyed in the liver. Excess amino acids which are brought to the liver by the hepatic portal vein, are deaminated by the liver cells.

In this process the amino (NH2 ) group is removed from the amino acid, with the formation of ammonia. The amino acid residue is then fed into carbohydrate metabolism and oxidized with the release of energy. Meanwhile the ammonia must not be allowed to accumulate because it is highly toxic even in small quantities. Under the influence of specific enzymes in the liver cells, the ammonia enters a cyclical series of reactions called the ornithine cycle, in which it reacts with carbon dioxide to form the less toxic nitrogenous compound urea. The urea is then shed from the liver into the bloodstream, and taken to the kidney which eliminates it from the body.

Application activity 9.7

1. What are the main components of urine?

2. The table below shows the percentage of various components in the blood plasma in the part labelled A, the fluid in the part labelled B and in the urine of a human.

9.8 Kidney transplants and dialysis machines

Activity 9.8

a. Explain why some people need to have their kidney replaced?

b. What is the use of a dialysis machine?

Dialysis is a medical procedure in which blood is filtered with the help of a machine. Blood from the patient’s vein enters the dialysis machine through a tube. Inside the machine, excess water, wastes, and other unneeded substances are filtered from the blood. The filtered blood is then returned to the patient’s vein through another tube. A dialysis treatment usually lasts three to four hours and must be repeated three times a week. Dialysis is generally performed on patients who have kidney failure. Dialysis helps them stay alive, but does not cure their failing kidneys.

Kidney transplantation or renal transplantation is the organ transplant of a kidney into a patient with end-stage renal disease. Organ transplantation is a medical procedure in which an organ is removed from one body and placed in the body of a recipient, to replace a damaged or missing organ. Kidney transplants are sometimes performed on people who suffer from severe renal failure. Usually, the donor has suffered an accidental death and had granted permission to have his or her kidneys used for transplantation. An attempt is made to match the immune characteristics of the donor and recipient to reduce the tendency for the recipient’s immune system to reject the transplanted kidney. Even with careful matching, however, recipients have to take medication for the rest of their lives to suppress their immune systems so that rejection is less likely. The major cause of kidney transplant failure is rejection by the recipient’s immune system.

Application activity 9.8

What is the difference between dialysis and kidney transplantation?

9.9 Role of the hypothalamus, pituitary gland, adrenal gland and nephron in varying the osmotic pressure of blood

Activity 9.10

a. Which hormone are involved in the regulation of the osmotic pressure of the blood?

b. Where are those hormones produced?

c. What are the function of these hormones?

The body adjusts for increased water intake by increasing urine output. Conversely, it adjusts for increased exercise or decreased water intake by reducing urine output. These adjustments involve nervous system and the endocrine system.

9.9.1 Regulation by antidiuretic hormone (ADH) 

 A hormone called antidiuretic hormone (ADH) helps to regulate the osmotic pressure of body fluids by causing the kidneys to increase water reabsorption. When ADH is released, more concentrated urine is produced, thereby conserving body water. ADH is produced by specialized nerve cells in the hypothalamus, and it moves along specialized fibres from the hypothalamus to the pituitary gland, which stores and releases ADH into the blood. Specialized nerve receptors, called osmoreceptors, located in the hypothalamus detect changes in osmotic pressure when there is a decrease in water intake or increase in water loss by sweating, causing blood solutes to become more concentrated. This increases the blood’s osmotic pressure. Consequently, water moves into the bloodstream, causing the cells of the hypothalamus to shrink. When this happens, a nerve message is sent to the pituitary, signaling the release of ADH, which is carried by the bloodstream to the kidneys. By reabsorbing more water, the kidneys produce more concentrated urine, preventing the osmotic pressure of the body fluids from increasing any further.

9.9.2 Kidneys and blood pressure 

The kidneys play a role in the regulation of blood pressure by adjusting for blood volumes. A hormone called aldosterone acts on the nephrons to increase Na+ reabsorption. The hormone is produced in the cortex of the adrenal glands which lies above the kidneys. Not surprisingly, as NaCl reabsorption increases, the osmotic gradient increases and more water move out of the nephron by osmosis.

 Aldosterone is secreted by the adrenal cortex in response to a high blood potassium levels, to a low blood sodium levels, or to a decreased blood pressure. When aldosterone stimulates the reabsorption of Na+ ions, water follows from the filtrate back to the blood. This helps maintain normal blood volume and blood pressure. In the kidneys, aldosterone increases reabsorption of Na+ and water so that less is lost in the urine. Aldosterone also stimulates the kidneys to increase secretion of K+ and H+ into the urine. With increased water reabsorption by the kidneys, blood volume increases.

Application activity 9.9

What is the effect of drinking a lot of water on the production of the following hormones?

a. ADH

b. Aldosterone

9.10 Excretion and osmoregulation in protists, insects, fish, amphibians and birds 

Activity 9.11

1. What are the excretory organs in the following animals?

a. Amoeba

b. Housefly

c. Tilapia

2. Explain the osmoregulation in fresh water fishes

a. Osmoregulation in protists such as Amoeba 

Amoeba makes use of contractile vacuoles to collect excretory wastes, such as ammonia, from the intracellular fluid by diffusion and active transport. As osmotic action pushes water from the environment into the cytoplasm, the vacuole moves to the surface and disposes the contents into the environment.

b. Excretion in insects

 Insects and other terrestrial arthropods have organs called Malpighian tubules that remove nitrogenous wastes and also function in water balance. The Malpighian tubules extend from dead-end tips immersed in haemolymph (circulatory fluid) to openings into the digestive tract. The filtration steps which are common to other excretory systems are absent. Instead, the transport epithelium that lines the tubules secretes certain solutes, including nitrogenous wastes, from the haemolymph into the lumen of the tubule.

Water follows the solutes into the tubule by osmosis, and the fluid then passes into the rectum. There, most solutes are pumped back into the haemolymph and water reabsorption by osmosis follows. The nitrogenous wastes mainly insoluble uric acid, are eliminated as nearly dry matter along with the faeces. Capable of conserving water very effectively, the insect excretory system is a key adaptation contributing to their success on land.

c. Excretion in birds and reptiles 

Most birds live in environments that are dehydrated. Like mammals, birds have kidneys with juxtamedullary nephrons that specialize in conserving water. However, the nephrons of birds have loops of Henle that extend less far into the medulla than those of mammals. Thus, bird kidneys cannot concentrate urine to the high osmolarities achieved by mammalian kidneys. Although birds can produce hyperosmotic urine, their main water conservation adaptation is 352352 having uric acid as the nitrogen waste molecule. Since uric acid can be excreted as a paste, it reduces urine volume.

The kidneys of reptiles having only cortical nephrons, produce urine that is osmotic or hypo-osmotic to body fluids. However, the epithelium of the chamber called the cloaca helps conserve fluid by reabsorbing some of the water present in urine and feces. Also like birds, most reptiles excrete their nitrogenous wastes as uric acid.

Freshwater fishes and amphibians 

Freshwater fishes are hyperosmotic to their surroundings, so they must excrete excess water continuously. In contrast to mammals and birds, freshwater fishes produce large volumes of very dilute urine. Their kidneys, which contain many nephrons, produce filtrate at a high rate. Freshwater fishes conserve salts by reabsorbing ions from the filtrate in their distal tubules, leaving water behind.

 Amphibian kidneys function much like those of freshwater fishes. When in fresh water, the kidneys of frogs excrete dilute urine while the skin accumulates certain salts from the water by active transport. On land, where dehydration is the most pressing problem of osmoregulation, frogs conserve body fluid by reabsorbing water across the epithelium of the urinary bladder.

Marine bony fishes

 The tissues of marine bony fishes gain excess salts from their surroundings and lose water. These environmental challenges are opposite to those faced by their freshwater relatives. Compared with freshwater fishes, marine fishes have fewer and smaller nephrons, and their nephrons lack a distal tubule. In addition, their kidneys have small glomeruli, and some lack glomeruli entirely. In keeping with these features, filtration rates are low and very little urine is excreted.

Application activity 9.10

 Compare the osmoregulation in unicellular organisms such as amoeba and insects.

9.11 Principles of osmoregulation in marine, freshwater and terrestrial organisms.

Activity 9.12 

Obtain a live fish from an aquarium or a lake in a bucket. Increase the concentration of salts in the water to see what happens to the fish and record your observation. Why does the fish die? You can use another animal that lives in fresh water such as tadpoles.

Organisms in aquatic and terrestrial environments must maintain the right concentration of solutes and amount of water in their body fluids. This involves excretion through the skin and the kidneys. 

a. Marine animals 

Marine bony fishes, such as the salmon, constantly lose water by osmosis. Such fishes balance the water loss by drinking large amounts of seawater. They then make use of both their gills and kidneys to rid themselves of salts. In the gills, specialized chloride cells actively transport chloride ions (Cl-) out, and sodium ions (Na+) follow passively. In the kidneys, excess calcium, magnesium, and sulphate ions are excreted with the loss of only small amounts of water.

b. Freshwater animals 

The body fluids of fresh water animals must be hypertonic because animal cells cannot tolerate salt concentrations as low as those of lake or river water. Having internal fluids with an osmolality higher than that of their surroundings, freshwater animals face the problem of gaining water by osmosis and losing salts by diffusion through their gills. Many freshwater animals, including fishes, solve the problem of water balance by drinking almost no water and excreting large amounts of very dilute urine. At the same time, salts lost by diffusion and in the urine are replaced by those found in the food they eat.

 c. Land animals 

The threat of dehydration is a major regulatory problem for terrestrial plants and animals. Humans, for example, die if they lose as little as 12% of their body water. Adaptations that reduce water loss are key to survival on land. Much as a waxy cuticle contributes to the success of land plants, the body coverings of most terrestrial animals help prevent dehydration.

 Examples are the waxy layers of insect exoskeletons, the shells of land snails, and the layers of dead, keratinized skin cells covering most terrestrial vertebrates,including humans. Despite these and other adaptations, most terrestrial animals lose water through many routes: in urine and feces, across their skin, and from moist surfaces in gas exchange organs. Land animals maintain water balance by drinking and eating moist foods and by producing water metabolically through cellular respiration. A number of desert animals, including many insect-eating birds and other reptiles, are well enough adapted for minimizing water loss that they can survive without drinking water. A noteworthy example is the kangaroo rat loses so little water that 90% replaced by water generated metabolically; the remaining 10% comes from the small amount of water in its diet of seeds.

Application activity 9.11 

Compare the osmoregulation in fresh water fishes with the excretion in salt water fishes.

9.12 Excretion in plants 

Activity 9.13

a. Name three excretory waste products of plants.

b. Plants do not have complex excretory organs. State three reasons.

c. What are hydathodes? What is their importance in the excretion in plants?

Compared to animals, plants do not have a well-developed excretory system to throw out nitrogenous waste materials. This is because of the differences in their physiology. Therefore, plants use different strategies for excretion. 

The gaseous waste materials produced during respiration (carbon dioxide) and photosynthesis (oxygen) diffuse out through stomata in the leaves and through lenticels in other parts of the plant. Excess water evaporates mostly from stomata and also from the outer surface of the stem, fruits, etc., throughout the day. This process of getting rid of excess water is called transpiration. The waste products, like oxygen, carbon dioxide and water, are the raw materials for other cellular reactions such as photosynthesis and cellular respiration. The excess of carbon dioxide and water are used up in this way. The only major gaseous excretory product of plants is oxygen. 

Many plants store organic waste products in their permanent tissues that have dead cells, for example in heartwood. Plants also store wastes within their leaves or barks, and these wastes are periodically removed as the leaves and barks fall off. Some of the waste products are stored in special cells or cellular 355 vacuoles. Organic acids, which might prove harmful to plants, often combine with excess cations and precipitate out as insoluble crystals that can be safely stored in plant cells. Calcium oxalate crystals accumulate in some tubers like yam.

Aquatic plants lose most of their metabolic wastes by direct diffusion into the water surrounding them. Terrestrial plants excrete some wastes into the soil around them. Plants do not have complex excretory systems. This is because of the following reasons: 

- There is very little accumulation of toxic wastes. Often the plant wastes are utilized by the plant. For example, carbon dioxide is used for photosynthesis and oxygen for respiration. 

- The extra gaseous waste is removed from the plant by simple diffusion through the stomata and the lenticels. - Most of the waste substances formed in plants are not harmful and can be stored in the plant tissues. 

- Some plants store other waste such as resins in their tissues in a nontoxic form. These tissues or organs later fall off the plant. 

- Excess water and dissolved gases are removed by the process of transpiration through the stomata. 

- Some plants remove waste products by exudation, for example gums, resins, latex and rubber. 

- In some plants water with dissolved salts oozes out through hydathodes. This is called guttation.

Note that hydathodes are specialized structures and they are mainly responsible for secreting water in liquid form. They are generally restricted to the apex or the serrated edges of the margins of leaves.

Application activity 9.12 

Most of the waste products produced by plants are useful to humans. Name any three waste products produced by plants that humans may benefit.

9.13 Adaptations of organisms to different environmental conditions

Activity 9.14

a. Name any three animals adapted to live in very cold areas.

b. List the adaptations that these animals have in common.

c. The camels are animals adapted to live in deserts. What are the adaptations of these animals that help them to survive in the deserts?

Every organism has certain features or characteristics which enables it to live successfully in its particular habitat. These features are called adaptations, and the organism is said to be adapted to its habitat. Organisms living in various habitats need different adaptations in order to maintain homeostasis. The animals adapt to such changes in their environment which threatens their chances of survival. The main threats are temperature, lack of water and food. Besides the environmental threats, many animals also need to be able to defend themselves from predators and pathogens.

Different organisms have adapted to the great diversity of habitats and distinct conditions in the environment. Although, the adaptations are many and varied, they can be categorized into mainly three types: Structural, physiological and behavioural.

9.13.1 Structural Adaptations 

Structural (or morphological) adaptations are the physical features of the organism. It includes shapes or body covering as well as its internal organisation. Microscopic organisms which includes protozoans and bacteria employ encystment (a state of suspended form, separated by the outside world by a solid cell wall) to surpass hostile conditions for long periods of time, even millions of years. Larger animals like polar bears are well adapted for survival in the cold climate of Arctic region. They have a white appearance to camouflage from prey on the snow and ice. Also, polar bear have thick layers of fat and fur, for insulation against the cold and a greasy coat which sheds water after swimming.

Dolphins are fish-like mammals which have streamlined shape and fins instead of legs. They also have blowholes on the tops of their heads for breathing, rather than their mouth and nose. Desert animals like camels have many adaptations that allow them to live successfully in hot and dry conditions. They have long eyelashes and nostrils that can close and open to prevent entry of sand. Thick eyebrows shield the eyes from the desert sun. Camels store fat in the hump which can be metabolised for energy. A camel can go a week or more without water, and they can last for several months without food. Their huge feet help them walk on sand without sinking into it.

9.13.2 Physiological Adaptations 

Physiological adaptations are related to the working of an organism’s metabolism. 

These adaptations enable the organism to regulate their bodily functions, such as breathing and temperature, and perform special functions like excreting chemicals as a defence mechanism (Sea stars). Chameleon (a reptile) changes colour or body markings in order to blend into its surroundings. Marine mammals such as whales are endothermic/warm blooded (able to maintain a constant body temperature). They cope with the temperature changes during migration over large distances and can spend time in arctic, tropical and temperate waters. 

9.13.3 Behavioural Adaptations 

Behavioural adaptations are learned adaptations that help organisms to survive. The whales produce sounds that allow them to communicate, navigate and hunt prey. Bears hibernate or ‘sleep’ through the coldest part of the year. Bryozoans are water dwelling small individual animals found in colonies in high numbers on the continental shelf in New Zealand. These animals band together for collecting food and survive predation. Penguins are the flightless birds found in the oceans around Antarctica. During extreme winter, Emperor penguins show social behaviour by huddling together in groups comprising several thousand penguins to stay warm.

Application activity 9.13

Describe two structural adaptations, two physiological adaptations and two behavioral adaptations.

Skills lab 9

Dissection of the rabbit to study the urinary system

Materials required: A mature rabbit, dissecting tray, and dissecting kit, chloroform.

- Place the rabbit in the dissecting tray, ventral side up.

 - Tie the legs securely to the corners of the tray by passing a string or rubber bands (2 bands together) under the tray from front leg to front leg and hind leg to hind leg. 

 - Be sure that the specimen is held firmly before you begin dissecting. 

 - Find the lower edge of the sternum (breastbone) and make an incision through the skin from that point to the pelvis. This will expose the layers of the abdominal muscles.

 - Strip the skin well back to the sides and examine the muscle layer.

 - Using the scissors or the scalpel, make another incision through the muscle layer. This will expose a thin membrane, the peritoneum, which lines the abdominal cavity. 

 - Cut through the peritoneum to expose the abdominal organs. 

 - Open the abdominal cavity wide by making several lateral cuts and pulling the skin and muscle layer well to the side. 

 - Use pins to pin back the cut sections of skin and muscle.

 - Discard the digestive organs and examine the kidneys. 

 - Cut under each kidney and remove it along with the ureter tube. 

 - Cut a kidney laterally and examine its internal structure.

 - You should find a spongy cortex on the other curved side and a hollow pelvis on the inner concave side. See if you can find the renal blood vessels which lead to and from the kidneys. Discard the kidneys.

Identify the functions of each part of the urinary system.

End unit assessment 9

1. The most important function of the kidney is:

a. Removal of water from the body. 

b. Regulating blood composition.

c. Storage of salts in the body. 

d. Elimination of urea from the blood.

2. Glucose is small enough to be filtered from the blood in glomeruli in the kidney, but is not normally found in the urine. This is because glucose is:

a. Reabsorbed in distal convoluted tubules

b. Reabsorbed in proximal convoluted tubules

c. Reabsorbed along the whole length of the nephrons

d. Respired by cells in the kidney

3. Which of these does not contribute to the process of filtration in the kidney?

a. High hydrostatic blood pressure in glomerular capillaries.

b. Large surface area for filtration.

c. Permeability of glomerular capillaries.

d. Active transport by epithelial cells lining renal tubules.

4. a. Name the nitrogenous waste substances excreted by mammals.

b. Explain why it is important that carbon dioxide and nitrogenous  wastes are excreted and not allowed to accumulate in the body.

5. Observe the diagram below and identify the following structures:

a. The structure that filters blood

b. The structure that carries urine from the kidney

c. The structure that carries blood containing urea into the kidney

d. The structure that stores urine

6. The following diagram shows the nephron.

a From the diagram above write the number that represents the: 

i. Collecting duct 

ii. Bowman’s Capsule 

b On the diagram above label the loop of Henle. 

c Name structure X. 

d Compare the blood pressure in the afferent and efferent arterioles and explain the cause of this difference. 

e Proteins are not present in the glomerular filtrate but amino acids are absent. Explain.

f Compare the urea concentration in the renal artery with that in the renal vein.

Name TWO organs that excrete urea.