UNIT 4 THE CIRCULATORY SYSTEM
UNIT 4: THE CIRCULATORY SYSTEM
Key Unit Competence
Relate the structures of the circulatory and lymphatic systems to their functions.
Learning objectives
– Explain the need for a transport system in animals.
– Explain the advantages and disadvantages of different types of circulatory
systems.
– Describe the external and internal structure of a mammalian heart.
– Explain how a heartbeat is initiated.
– Describe the main events of the cardiac cycle.
– Explain the relationship between the structure and function of blood vessels.
– Explain how blood circulation is controlled.
– Describe the effects of exercise on respiration and on circulation.
– Describe the process of blood clotting.
– Recall the structure of haemoglobin and explain how haemoglobin transports
oxygen.
– Explain how tissue fluid and lymph are formed.
– Describe the risk factors associated with cardiovascular diseases.
– Carry out an investigation on the effects of exercise on the pulse rate and blood
pressure.
– Distinguish between open and closed, single and double circulation with
reference to insects, earthworm, fish and mammals.
– Recognize blood vessels from their structures using a light microscope.
– Relate the structure of blood vessels to their functions.
– Differentiate between blood, tissue fluid, and lymph.
– Relate blood as a tissue to its functions.
– Interpret oxygen dissociation curves for haemoglobin and other respiratory
pigments.
– Appreciate the importance of the need for transport systems when animals
become larger, more complex and more active, to supply nutrients to, and
remove waste from, individual cells.
– Recognize possible risk factors as diet, stress, smoking, genetic predisposition,age and gender in relation to cardio vascular diseases.
Introductory activity
Mass sports in Rwanda has been encouraged, where people of all ages
participate in sports.Discuss the advantages of doing sports to a human health?
Physical activities can make people including students to be stronger and healthier.
They contribute also to lowering obesity rate. All individuals who practice physical
activities tend to; have lower body mass indexes, benefit from developing muscles
and burning calories. Physical activities help in lowering the rates of diabetes and
high blood pressure. Doing physical exercises regularly contribute to better heart
and lung function.4.1 Blood circulatory system in animals
Activity 4.1Observe the illustrations of animals below and answer the following questions
1. Do the above animals have the same circulatory system? Justify your
reasoning by distinguishing the type of circulatory system(s) found ineach animals?
All, except the smallest and tiniest animals need a system to transport substances
from cell to cell within themselves. The primary tasks of the system are to import,
distribute/deliver nutrients and oxygen to every cell and then to remove waste
products including carbon dioxide. The design of the transport system depends
upon the size of the organism and on how active it is.
In animals, there are two types of circulatory systems i.e.
i. Open circulatory systemii. Closed circulatory systems:
4.1.1 Open circulatory system and closed circulatory system
In animals, circulatory system is either open or closed. Table 4.1 below, showsdifferences between open and closed circulatory systems:
Table 4.1. A comparison between open and closed circulatory systems
a. Open circulatory system
The open circulatory system is common to molluscs and arthropods. In this system,
blood is pumped into a hemocoel where it comes into direct contact with body cells
and there after goes back to the tubular ‘heart’ via openings called ostia/pores.
Insects and other arthropods have a heart which is an elongated tube located
dorsally. The internal organs are suspended in a network of blood-filled sinuses
which collectively form the haemocoel. Blood from the heart mixes with the
interstitial fluid in the haemocoel to form haemolymph. The advantage this has, isthe direct exchange of materials between body cells and haemolymph.
Figure 4.1: Open circulation in insect Adapted from Campbell Biology 11th Edition
b. Closed circulatory system
Vertebrates, and a few invertebrates like earthworms, have a closed circulatory
system. Closed circulatory systems have the blood closed at all times within vessels
of different sizes and wall thickness. In this type of system, blood is pumped by theheart through vessels, and does not fill body cavities.
Figure 4.2: Closed circulation in annelids (adapted from Campbell Biology 11th edition
The earthworm possesses a closed circulation system whereby the blood is confined
to a series of blood vessels and not permitted to mix with the body tissues. Blood is
pumped around the system by muscular longitudinal and ventral vessels and five
pairs of lateral pseudo-hearts in segments 7 to 11. Backflow of blood is prevented
by valves. The blood itself contains haemoglobin dissolved in the plasma and some
phagocyte cells. It is advantageous for an organism to have closed circulatory system
because:
– It helps in control of distribution of blood to different parts of the body.
– Muscular walls of vessels can constrict and dilate to vary the amount of flow
through specific vessels
– Blood pressures are fairly high and the circulation can be vigorous
– It is more efficient hence the blood can reach further distances
– Allows for more control over oxygen delivery
All vertebrates including; fish, amphibians, reptiles, birds and mammals possess
a prominent muscular heart which pumps blood around the body. The closedcirculatory system can be single, partial and double.
1. Single circulation in fish
Fish have a two-chambered heart made of one atrium and one ventricle.
Deoxygenated blood from the body is pumped by the heart to the gills. Here blood
is oxygenated before passing around the body and ultimately returning to the
heart. Blood has to pass through two capillary systems, the capillaries of the gills
and then those of the body before returning to the heart. The two system capillaries
offer considerable resistance to the flow of blood. This means that in fish there is a
marked drop in blood pressure before the blood completes a circuit. In this type of
circulation, it is an advantage that the blood circulating in the body cells has alreadybeen oxygenated in the gills.
2. Partial double circulation in amphibians
All amphibians and most of the reptiles possess a heart with two atria and one
ventricle. Blood from the body enters the right atrium and is pumped to the lungs by
the common ventricle. It returns to the heart and enters the left atrium before being
pumped around the body. It is called partial because the only one ventricle receivedoxygenated and non-oxygenated blood which can be mixed as illustrated below:
Figure 4.3: Illustration of partial double circulation in amphibians
A spiral valve called conus arteriosus helps to keep deoxygenated and oxygenated
blood separate to some extent. The figures below distinguish how closed circulationoccurs in fishes and in amphibians.
Figure 4.4: Closed circulation in fish and amphibian
3. Complete double circulation in mammals
This circulation is called double circulation because blood must pass twice in the
heart for one complete circuit. The right side of the heart delivers oxygen poor
blood to the capillary beds of the gas exchange tissue in lungs, where there is a
net movement of O2 into the blood and of CO2 out of the blood. This part of the
circulation is called a pulmonary circuit or pulmonary circulation.
After the oxygen enriched blood leaves the gas exchange tissues (the lungs), it
enters the left side of the heart. Contraction of the left part of the heart propels this
blood to the capillary beds in organs and tissues throughout of the body. Following
the exchange of O2 and CO2 as well as nutrient and waste products, then the oxygen
poor blood returns to the right part of the heart, completing the systemic circuit orthe systemic circulation.
Mammals and birds have a four-chambered heart and a complete double circulation.
The following are some of the advantages of double circulation:
– The heart can increase the pressure of the blood after it has passed through the
lungs, so the blood flows more quickly to the body tissues.
– There is no mixing of oxygenated blood with deoxygenated blood.
– Blood is pumped exactly where it is needed
– The blood pressure must not be too high in the pulmonary circulation, otherwiseit may damage the delicate capillaries in the lungs
Figure 4.5: Closed double circulation in mammals and birds
The following table 4.2 indicates the comparison between single and doublecirculation
Table 4.2: Comparison between single and double circulation.
Application 4.1
1. Briefly explain why animals need a transport system.
2. Explain how open and closed circulatory systems differ.
3. Describe the differences between single and double circulation.4. Describe how circulation take place in humans.
4.2 Structure of the human heart
Activity 4.2
– Obtain an intact heart of sheep or goat from a butcher’s shop or slaughter
house
– Rinse it under a tap to remove excess blood
– Observe the surface of the heart and identify the main visible features
– The blood vessels may have been cut off, but it is possible to identify where
these would have been attached later in the dissection
– Gently squeeze the ventricles. They can be distinguished because the wall
of the right ventricle is much thinner than that of the left ventricle
– Using a pair of sharp scissors or a scalpel, make an incision from the base of
the left ventricle, up to the left atrium and then from the base of the right
ventricle up to the right atrium
– Using a pair of forceps, remove any blood clots lying in the exposed chambers
– Identify the main components of internal structure of the heart– Compare the thickness of the left ventricle wall to that of the right ventricle
The human heart is made up of a cardiac muscle which contracts in order to propel
blood throughout the body. It is located between the two lungs, behind the
sternum in the thorax. The heart is surrounded by a tough sac called pericardium. A
pericardial fluid is secreted between the membranes allowing them to move easily
over each other. The pericardium protects the heart from overexpansion caused by
elastic recoil when it is beating very fast. The heart (Figure 4.6) is divided into a leftand a right side separated by the septum.
Figure 4.6: Structure of human heart (From Campbell 11th Edition)
The heart of mammals and birds is composed of 4 chambers including 2 upper atria
and 2 lower ventricles. The right side deals with deoxygenated blood and the left
side with oxygenated blood. The muscular wall of the left ventricle is thicker than
that of the right ventricle because the left ventricle has to pump blood to the whole
body with much higher pressure.
The left atrium is separated from the left ventricle by a bicuspid or mitral valve, whilst
a tricuspid valve separates the right atrium from the right ventricle. Jointly, these
two valves are known as atrioventricular valves. Atrioventricular valves are pushed
open when atria contract but, when ventricles contract they close and produce the
first sound of the cardiac cycle, the second being that of the closing semilunar valves(aortic and pulmonary valves).
Application 4.2
1. Suggest a reason for each of the following:
a. The right atrium is larger than the left atrium.
b. The left ventricle has a thicker muscular wall than the right ventricle.
2. Discuss the functions of pericardium and pericardial fluid that surroundthe heart.
4.3 Heart beat and mammalian cardiac cycle
Activity 4.3
Work to find out the number of pulses of each other using a thumb above the
vein ahead of your wrest or a sphygmomanometer.
a) Record in a table the number of pulses for the class
b) Who has the highest number? The lowest number?c) Explain significance of such a technique.\
4.3.1. Initiation of a heartbeat
Heart beat is a rhythmic sequence of contractions of the heart. It is coordinated by
two small groups of cardiac muscle cells called the sinoatrial (SA) and atrioventricular
(AV) nodes. The sinoatrial node (SAN), often known as the cardiac pacemaker,
is found in the upper wall of the right atrium and is responsible for the wave of
electrical stimulation that starts atrial contraction by creating an action potential.
The action potential causes the cardiac cells to contract. This wave of contraction
spreads across the cells of the atria, reaching the atrioventricular node (AV node/
AVN) which is found in the lower right atrium.The atrioventricular node/AVN conducts the electrical impulses that come from the
SA node/SAN through the atria to the ventricles. The impulse is delayed there before
being conducted through special bundles of heart muscle cells called the bundle of
His. This delay allows for the ventricles to be filled with blood before they contract
There is a collection of heart muscle cells (fibres) specialized for electrical conduction
that transmits the electrical impulses from the AVN, through the Purkinje fibres,which leads to a contraction of the ventricles.
Figure 4.7: The initiation (origin) of heart beat.
4.3.2. Mammalian cardiac cycle and cardiac sounds
The cardiac cycle refers to the sequence of events which take place during the
completion of one heartbeat. It involves repeated contraction (systole) and relaxation
(diastole) of the heart muscle. The three steps in cardiac cycle are the followings:
1. Atrial systole and ventricular diastole
In this brief period of 0.1 seconds that follows atrioventricular diastole, blood from
the vena cava and pulmonary vein enter the both atria and they get filled with blood.
The walls of the atria undergo contraction (systole) forcing blood into the ventricles
via bicuspid and tricuspid valves. During this time, the ventricles are relaxed and
semilunar valves remain closed.
Ventricular systole and atrial disatole
During this stage, the ventriles undergo contraction (systole) hence forcing blood
out of the heart via the semilunar valves into the aorta and pulmonary artery. At
this time, the atria relax and expand waiting to be filled with blood. The contraction
of ventricles causes the atrioventricular valves to close simultaneously in order to
prevent back flow of blood. The closure of the valves produces the first heart soundtermed as ‘lub’.
2. Atrioventricular diastole
Upon expelling of blood, ventricles relax and their pressure lowers compared to
aorta and pulmonary artery pressures. This would cause back flow of blood to the
heart but it is prevented by sudden closure of the semilunar valves. The closure of
the semilunar valves causes a second heart sound called ‘dub’.
Note: The two sounds ‘lub’ and ‘dub’ are so close and often describes as ‘lub –dub’
and they form a single heartbeat.
The atrioventricular diastole ends the cardiac cycles and is followed by the atrial
systole. Hence the cycle restarts. When the heart rate is 75/min, which means 75heartbeats per minute, the period of one cardiac cycle is 0.8 sec.
Figure 4.8: The cardiac cycle
Figure 4.9: The relationship between heart sounds and key events in cardiac cycle
The electrical activity of the heart can be monitored using an Electrocardiogram
(ECG) as shown in figure 4.10. This involves attaching of sensors to the skin. Some
of the electrical activity generated by the heart spreads through the tissue next to
the heart and onwards to the skin. The sensors on the skin pick up the electrical
excitation created by the heart and convert this into a trace. The trace of a health
person has particular shape. it consists of a series of waves that are labelled P, Q, R, S
and T. Wave P shows the excitation of the atria, while QRS indicates the excitation of
the ventricles and T shows diastole.
The shape of the ECG trace can sometimes indicates the parts of the heart muscles
which are not healthy. It can show if the heart is being beating irregularly, fibrillation
(the heart beat is not coordinated), or if it is suffering the heart attack (myocardial
infarction). It can also show if the heart has enlarged or if the Purkinje fibre is notconducting electrical activity properly.
Figure 4.10: Electrocardiogram normal wave and electrocardiogram machine.
Application 4.3
1. Briefly describe the main events of cardiac cycle.
2. During the mass sports the medical doctor made a check-up and found
the following data from three participants A, B and C.
a. Among the three participants, who shows more signs of
cardiovascular problem? Why?b. Differentiate between systolic and diastolic ventricular pressures.
3. Observe the illustration below and answer to the following questions:
a. Describe the shape of the electrocardiogram trace above.
b. Explain why the QRS complex has a larger peak than the P wave.
4.4 Control of the heart rate.
Activity 4.4
a. Place your middle finger on the artery found near the opening of the ear
then count the number of pulses and write it down. Repeat this 3 times,
then calculate the average of the heart beat per minute.
b. Do some warm up exercises within 2 minutes, again place your thumb
finger on the artery found at the back of the wrist then count the number
of pulses after the exercise. Repeat this 3 times then calculate the average
of the heartbeat per minute. Use the stop clock or a watch to count the
number of pulse (beatings) within one minute.
i. How does your heart rate immediately after a warm up exercises differ
from that while at rest?ii. How would you explain the differences?
4.4.1. Nervous and hormonal control of heart rate
In the nervous control of the heartbeat, there is a cardiovascular center located in
the medulla oblongata of the hindbrain which controls the activities of the SAN. Thecenter has two nerves from the autonomic nervous system i.e. sympathetic nerve
whose stimuli accelerates activity of the SAN (increases heartbeat) and vagus nerve
whose stimuli slows down the activity of SAN (decreases heartbeat).
With regard to the hormonal control, the adrenal glands under influence of
hypothalamus secrete the hormone adrenaline into blood. Upon reaching the heart,
adrenaline will speed up the activity of the SAN thus increasing heartbeat. The
reduction comes about when the levels of adrenaline reduce through a negativefeedback mechanism.
4.4.2. Other factors controlling heart rate
Other factors affecting heart rate include; the levels of carbon dioxide, temperature,pH and mineral ions.
a. Carbon dioxide
Chemically, high CO2 levels stimulate the vasomotor Centre (VMC) to vasoconstrict
arterioles. The resulting high blood pressure transports CO2 more rapidly to the
lungs for expulsion and exchange with O2. Where tissues suddenly become active,
they produce more CO2. This causes vasodilation of local blood vessels, thus
increasing their blood supply and allowing more oxygen and glucose to reach them
for respiratory purposes.
b. Body temperature
When the body temperature changes, so does the heart rate. This is one of the
thermoregulatory changes that occur to prevent the body’s core temperature of
370C from increasing or decreasing. Heart rate increases when heat is gained by the
body such as in hot climates and during physical exercise in order to transfer more
heat away from the body. When the body loses heat such as in cold weather or a cold
shower, heart rate decreases to preserve core temperature.
c. pH and mineral ions
The importance of plasma electrolytes and pH levels in determining heart rate is
not yet well grounded. A significant heart rate increase was obtained after a decrease
of potassium and calcium and an increase in pH levels and with no significant
variations in indices of autonomic activity. The analysis revealed that changes
in physiological range of; potassium, calcium, and pH could cause large heart rate
variations from 60 to 90 bpm. It was concluded that electrolyte and pH changes in
physiological range have an important complex impact on the pacemaking rhythm
independently of autonomic outflow.
Effect of drugs, and physical activity on cardiac frequency
a. Physical exercise
The heart rate and blood pressure both rise during physical exercise. Over time,
regular physical exercise can help lower the resting blood pressure and heart rate.
This is because physical exercise training improves the health of the heart and blood
vessels, allowing the cardiovascular system to function more efficiently. This enables
increased blood flow to muscles without putting excess pressure on blood vessel
walls. While blood pressure rises during exercise, it is too much smaller degree than
the increase in heart rate. Like the heart rate, blood pressure returns to resting level
a few minutes after the end of physical exercise.
b. Caffeine and Other Drugs
Caffeine found in coffee, tea and soda is a stimulant drug that influences the nervous
system to increase heart rate. It mimics the effect of adrenaline, a natural hormone
in the body responsible for elevating heart rate. Other stimulants such as cocaine
and ephedrine work in a similar manner.
On the other hand, there are specific drugs used in lowering heart rate such as betaand
calcium channel blockers. Beta-blockers work by interfering with the receptors
that adrenaline binds to, subsequently decreasing hormonal influence on heart
rate. Calcium channel blockers reduce the amount of calcium that enters the heart
muscle. Because calcium is needed for muscle to contract, the heart beats at a slowerrate when this drug is taken.
Application 4.4
1. Discuss how both nervous and hormonal systems are involved in
regulation of heart beat rate.2. Discuss how some drugs like caffeine affect the heart beat rate.
4.5 Blood vessels
Activity 4.5
1. Use a microscope to observe prepared slides of blood vessels.
2. Draw and label the observed blood vessels.
3. Compare those blood vessels.4. Explain the relationship between each blood vessel and its function.
Blood vessels include; arteries, capillaries and veins. Illustrations, structure of
walls, lumen, valves, branching, and functions of arteries, capillaries and veins aresummarized in the figure 4.11
Figure 4.11: Illustration of blood vessels.
Table 4.3. A comparison between arteries, capillaries and veins.
Application 4.5
1. Associate the following vessels with their functions
2. Explain how each blood vessel is adapted to its function.
4.6 Body fluids, composition and functions
Activity 4.6
1. List the main body fluids.2. Look at the figure below and answer the questions that follow.
a. Identify the blood components represented by the letters A, B, C, D, E, F, G,
H, I.
b. Suggest the functions of each of those blood components.c. State the origin of each blood component.
4.6.1. Main types of body fluids and their compositions
Body fluids are liquids originating from inside the body of living humans. The main
body fluids are; blood, plasma, serum, tissue fluid and lymph which are describedbelow in the table 4.4.
Table 4.4. Body fluids and their composition
4.6.2. Composition and functions of blood
The main blood components are formed elements and plasma. Formed elements
are erythrocytes (red blood cells), leukocytes (white blood cells) and thrombocytes(platelets).
Figure 4.12: Blood sample in a test tube.
a. Erythrocytes
Erythrocytes also called red blood cells, their core function is to carry oxygen from
the respiratory organs to tissues and their structure are well modified accordingly to
perform the purpose. There are five million per cubic millimetre each having about
8 μm in diameter and 3 μm thick in widest part. The cell has red pigment calledHaemoglobin a complex protein c
ontaining four iron haem groups.
b. Leukocytes
Leukocytes (white blood cells) are involved in immune system that fights against
infections. . white blood cells are responsible for destroying infectious agents and
infected cells, and secrete protective substances such as antibodies, which fight
infections. Leukocytes are divided into:
– Granulocytes or polymorph nuclear cells. They are neutrophils, basophils
eosinophils. They take the name from the possession of numerous granules in
their cytoplasm.
– Agranulocytes or monomorphonuclear cells: They are lymphocytes andmonocytes. They lack granules in the cytoplasm.
Thrombocytes
Thrombocytes are also called platelets, are small cell fragments with 2-3 mm in
diameter. They are formed from cytoplasm of large cells (mega karyotypes. Normal
quantitative value is between 250,000 and 450,000 platelets per mm³. They help inblood clotting. A comparison between formed elements is summarized in the table
4.5 below.
Table 4.5: Blood composition
Application 4.6
1. Discuss the functions of:
a. Macrophage.
b. T-lymphocytes.
c. Erythrocytes2. Explain the relationship between blood and tissue fluid.
4.7 Transport of respiratory gases
Activity 4.7
Refer to unit 8 in S4 to answer the following questions:
1. What protein is responsible for the transport of oxygen in human blood?
Describe it.
2. Explain how that protein behaves when blood reaches the alveoli in thelungs and when blood reaches active muscle cells.
a. Structure of haemoglobin of red blood cells.
Haemoglobin is a red protein responsible for transporting oxygen in the blood of
vertebrates. It is also involved in the transport of carbon dioxide. Haemoglobin is
composed of haem and globin (polypeptide chains). Haem is an iron porphyrin
compound. Iron occupies the centre of the porphyrin ring and establishes linkages
with all the four nitrogen of all the pyrrole rings.
Globin part is made of four polypeptide chains, two identical α-chains and two
identical β-chains in normal adult haemoglobin. Each chain contains a “haem” in the
so called ‘haem pocket’ and one haemoglobin molecule possess four haem units.
Haem pockets of α-subunits are of just adequate size to give entry to an O2 molecule.Entry of O2 into haem pockets of β-subunits is blocked by a valine residue.
Figure 4.13: Structure of haemoglobin.
b. Transport of carbon dioxide (CO2)
At systemic capillaries in the body cells, CO2 enters red blood cells. Some CO2combines with Hb to form HbCO2 (Carbaminohaemoglobin):
I.e. Hb + CO2 →HbCO2 (Carbaminohaemoglobin)
Most CO2 is converted to HCO3- (bicarbonate ion), which is carried in the plasma.
Haemoglobin is in relation with chloride shift. It is a process which occurs in
a cardiovascular system and refers to the exchange of bicarbonate (HCO3−)
and chloride (Cl−) across the membrane of red blood cells (RBCs). The chloride shiftoccurs in this way:
Figure 4.14: Chloride shift and transport of carbon dioxide by haemoglobin erythrocyte.
: H + Hb is reduced haemoglobin which is haemoglobin combined with hydrogenion (H+).
c. Transport of oxygen
Haemoglobin gets oxygen in lungs from external environment to form a compound
called oxyhaemoglobin (HbO8). , In this form, oxygen is transported to the body cellsto sites where it is needed for aerobic respiration.
Figure 4.15: Oxygen dissociation curve
The curve above in figure 4.15 shows the oxygen dissociation curve by haemoglobin.
Oxygen dissociation curves determined by plotting the partial pressure of
oxygen in blood against the percentage of haemoglobin combined with
oxygen in the form of ox haemoglobin. The S-shape of the oxygen dissociation
curve can be explained by the behaviour of a haemoglobin molecule as it combines
with or loses oxygen molecules. When an oxygen molecule combines with one haem
group, the whole haemoglobin molecule is slightly distorted. The distortion makes
it easier for a second and third oxygen molecules to combine the haem groups. It is
then still easier for the fourth and final oxygen molecule to combine.
If all the oxygen binding sites contain oxygen, then the oxygen saturation is
100%. Oxygen saturation is defined as the ratio of oxyhaemoglobin to the total
concentration of haemoglobin present in the blood The Bohr Effect is a physiological
phenomenon in which a raise of carbon dioxide in the blood and a decrease in pH
results in a reduction of the affinity of haemoglobin for oxygen. This causes the
oxygen dissociation curve for haemoglobin to shift to the right. The Bohr Effectoccurs in this way:
Figure 4.16: Bohr effect curve (Adapted from brainscape.com)
Application 4.7
1. Explain the importance of hemoglobin to a human being.
2. In a healthy adult human, the amount of haemoglobin in 1 dm3 of
blood is about 150 g. Given that 1 g of pure haemoglobin can combine
with 1.3 cm3 of oxygen at body temperature, how much oxygen can becarried in 1 dm3 of blood?
4.8 Blood clotting and common cardiovascular diseases
Activity 4.8
Warning to medical staff!
Doctor NINA called upon all medical staff and warned them about three major
causes of death in the theater. She said into the: “Please, pay attention to
hemophilic people though they are rare in Rwanda. Embolus and thrombosis
are now reported from time to time. Beware!”
1. Differentiate between:
a. a hemophilic and non-hemophilic person
b. embolus and thrombosis
2. why Dr Nina warns medical practitioners about above cases:3. Explain the mechanism of blood clotting.
a. Blood clotting
Blood clotting also known as blood coagulation is the process by which blood becomes
thick and stops flowing, forming a solid cover over any place where the skin has
been cut or broken. Blood that has been converted from a liquid to a solid state
is called blood clot. A blood clot called thrombus is stationary within a vessel or
the heart. If a blood clot moves from that location through the bloodstream, it isreferred to as an embolus.
Figure 4.17: Illustration of blood clotting process
Blood clotting is a series of different processes:
Step 1: The blood coagulation process begins when the endothelium of a vessel is
damaged, exposing the connective in the vessel wall to blood. Platelets adhere to
collagen fibres in the connective tissue and release a substance that makes nearby
platelets sticky.
Step 2: The thrombocytes form a plug that provides emergency protection against
blood loss.
Step 3: This seal is reinforced by a clot of fibrin when vessel damage is severe.
Fibrin is formed via a multistep process where clotting factors released from the
clumped platelets or damaged cells mix with clotting factors in the plasma, forming
an activation that converts a plasma protein called prothrombin to its active form,
called thrombin. This is facilitated by calcium ions and vitamin K. Thrombin itself is
an enzyme that catalyses the final step of the clotting process. This final step is the
conversion of fibrinogen to fibrin. The threads of fibrin become interwoven into a
patch. And the blood clot is formed. These threads trap red blood cells and otherblood components, preventing the continuous bleeding.
b. Common cardiovascular diseases
1. Stroke
Stroke is a cardiovascular disease due to the lack of oxygen to the brain which may
lead to reversible or irreversible paralysis. The damage to a group of nerve cells in
the brain is often due to interrupted blood flow, caused by a blood clot or blood
vessel bursting. Since atherosclerosis is a body wide process, similar events can also
occur in the arteries to other parts of the body, including the brain. A stroke is a loss
of brain function due to a stoppage of the blood supply to the brain. It can be caused
by a stationary blood clot known as thrombus, a free-floating clot moving blood
clot or embolus that gets caught in a blood vessel, or by bleeding (haemorrhage).
Hypertension or high blood pressure promotes atherosclerosis and increases the
risk of heart attack and stroke.
2. Atherosclerosis
Atherosclerosis is a cardiovascular disease characterized by the progressive narrowing
and hardening of the arteries over time. Atherosclerosis normally begins in later
childhood, and is usually found in most major arteries. It does not usually have any early
symptoms. Causes of atherosclerosis include a high-fat diet, high cholesterol, smoking,
obesity, and diabetes. Atherosclerosis becomes a threat to health when the plaque
build-up interferes with the blood circulation in the heart known as coronary circulation
or the brain known as cerebral circulation. A blockage in the coronary circulation, canlead to a heart attack, and blockage of the cerebral circulation can lead to a stroke.
Figure 4.18: Plaque formation in blood vessels
3. Coronary heart disease
Coronary heart disease (CHD) is a disease in which a waxy substance called plaque
builds up inside the coronary arteries. Cardiac muscle cells are fed by the coronary
arteries. Blocked flow in a coronary artery can result in oxygen starvation and death
of heart muscle. Most individuals with coronary heart disease have no symptoms for
many years until the first sign, often a heart attack, happens.
c. Risk factors associated with cardiovascular diseases
There are several risk factors for heart disease. Some of those factors are controllable
and others are uncontrolled. Uncontrollable factors include the gender (males are
at greater risk), age (old people have higher risk), and family history in relation to
heart diseases as well post-menopausal stages for females. Making some changes in
lifestyle can reduce chance of having heart disease. Controllable risk factors includesmoking, high blood pressure, physical inactivity, obesity, diabetes, stress and anger
Application 4.8
1. State the role of fibrinogen, calcium and thrombin in blood clotting.
2. Explain the cause and effects of stroke.
3. Describe the impact of smoking on the cardiovascular system.
4. Discuss the effects of high consumption of lipids such as fats and oilson the body.
4.9 Lymphatic system
Activity 4.9
1. Define the following terms:
a. Lymph
b. Lymph nodes
c. Lymphatic vessels
2. Describe the function of lymphatic system.
3. Explain how the tissue fluid and lymph are formed.
4. Suggest any 2 similarities and 2 differences between a circulatorysystem and a lymphatic system.
4.9.1 Structure of a lymphatic system
A lymphatic system is a system composed of tissues and organs, including; bone
marrow, spleen, thymus, and lymph nodes that produce and store cells that fight
infection and disease. The channels that carry lymph are also part of this system.So, the lymphatic system is part of the circulatory system and an important part of
the immune system.
Figure 4.19: Structure of human lymphatic system.
4.9.2 Functions of a lymphatic system
– Drainage of fluid from blood stream into the tissues: The circulating blood
through narrow vessels leads to leakage of fluid or plasma into the tissues
carrying oxygen and nutrients to the tissues and taking waste materials from
the tissues into the lymph channels. The leaked fluid drains into the lymph
vessels.
– Filtration of the lymph at the lymph nodes: The nodes contain white blood
cells that can attack any bacteria or viruses they find in the lymph as it flows
through the lymph nodes.
– Filtering blood: This is done by the spleen which filters out bacteria, viruses
and other foreign particles.
– Raise an immune reaction and fight infections: The lymphatic system
especially the lymph nodes are over active in case of an infection the lymph
nodes or glands often swell up in case of a local infection in so doing, thelymphocytes fight the foreign bodies trapped in the lymph nodes.
4.9.3 Formation of tissue (interstitial) fluid
Fluids and some soluble proteins leak from the blood capillaries into the interstitial
fluid that bathes the cells of tissues. This occurs due to the arterial end of capillary,
where the blood pressure is greater than osmotic pressure so that fluid flows out
of capillary into the interstitial fluid. This process is called pressure filtration or
ultrafiltration
4.9.4 Formation of lymph
The lymph is the tissue fluid that moves within the lymphatic vessels. The lymphatic
vessels recover some leaked fluid and proteins, and carry them to large veins at thebase of the neck (figure 4.20).
Figure 4.20: The close association of lymphatic vessels and blood capillaries.
4.9.5 Comparison between lymphatic and circulatory systems
Both the cardiovascular and lymphatic systems are vascular networks carrying bodyfluids. Differences and similarities are summarized in the table 4.6.
Table 4.6. Differences between lymphatic and circulatory system
Application 4.9
Observe the figure below and respond to the following questions.
a. Identify the organs W, X, Y, Z shown on this figure
b. Describe the functions of the organs W, X, Y, Z.
End of unit assessment 4
1. Blood returning to the mammalian heart in a pulmonary vein drains first into
the:
a. Vena cava
b. Left ventricle
c. Right ventricle
d. Left atrium
2. Pulse is a direct measure of:
a. Blood pressure.
b. Breathing rate.
c. Cardiac output
d. Heart rate.
e. Stroke volume
3. Complete the following paragraph by filling in the blank spaces.
Blood is ………………in the lungs. The red pigment ………………has a high
affinity for oxygen. The pumping action of the……………creates pressure
which pushes the blood around the body. In the tissues the partial pressure
of…………….is low. This causes the ………………of the oxyhaemoglobin. In
the tissues, the oxygen is used in the process of……………………. Most of the
carbon dioxide produced in this process enters the……………. cells. Here it is
converted to carbonic acid by the action of the enzyme carbonic anhydrase.
The carbon dioxide is transported as ………………. back to the lungs
4. How many oxygen molecules can each haemoglobin molecule transport?
5. Explain the function of fibrinogen.
6. Distinguish between plasma and serum.
7. a) Explain why haemoglobin is called conjugated protein.
b) Describe the effect of high carbon dioxide concentrations on the
oxygen dissociation curve of haemoglobin.
8. a) By which process does fluid leave the blood and enter the tissue fluid?b) Which component of the blood does not enter the tissue fluid?
9. The figure below shows a cross section through the human heart
a. Label the structure A-E
b. What are the functions of the structures A and B
10. Why is it important that the AV node delay the electrical impulse moving from
the SA node and the atria to the ventricles?
11. Draw a pair of simple diagrams comparing the essential features of single and
double circulation.
12. The figure below shows pressure changes to the left side of the heart and theaorta during the cardiac cycle.
a. State what is happening at point A-D on the graph. Explain your answer.
b. If the time taken for one complete cardiac cycle is 0.8 seconds, how
many cardiac cycles are there in one minute?
13. Explain any two advantages of closed double circulatory system and two
disadvantages of open circulatory system
14. a) Where is the radial pulse taken?
b) Suggest what will happen to the heart rate if the vagus nerve is cut off.
15. The diagram shows a vertical section through a human heart. The arrows
represent the direction of movement of the electrical activity which starts
muscle contraction Carefully, observe the following and answer the questionsthat follow.
a. Name the structure denoted by the letter A
b. Explain why each of the following is important in then pumping of blood
through the heart.
i. There is a slight delay in the passage of electrical activity that takes place
at the point A
ii. The contraction of the ventricles starts at the base
c. Describe how stimulation of the cardiovascular centre in the medulla mayresult in an increase in heart rate
16. Read the following passage and answer the questions that follow
The human heart is a double pump adapted to forcing blood, at the same rate but
at different pressures, along the two systems of double circulation. High pressure
in the systemic circulation has evolved with lower pressure in the pulmonary
circulation and low pressure lymphatic circulation. Each heart beat is controlled
by a wave of electrical excitation. In turn, the cardiac output of the heart adapts
to meet the body needs and is influenced by nervous and hormonal control.
a. Based on the statement: “The human heart is a double pump adapted to
forcing blood, at the same rate but at different pressures, along the two
systems of double circulation”. Explain how the mechanism that controls
each heartbeat, and the structure of the heart, enable it to do this.
b. Describe the role played by hormones and the nervous system in
controlling heart rate.c. Describe the formation of lymph fluid.