Biology and Health Sciences

Key unit competency 

To be able to explain gaseous exchange in plants and human beings.

Learning objectives 

After studying this unit, I should be able to: 

• Explain the characteristics and adaptive features of gaseous exchange surface. • Define gaseous exchange and why it is an important process. 

• Describe gaseous exchange in the lungs and alveolus. 

• Demonstrate the process of inspiration and expiration. 

• Explain the process of gaseous exchange in plants. 

• Identify common respiratory diseases and suggest their prevention and treatment. 

• Dissect mammalian lung and relate its features to gaseous exchange surfaces. 

• Construct bell-jar model from locally available materials.      

• Appreciate learning through using locally available materials. 

Introduction 

In senior one, you learnt about gaseous exchange in human beings. How did you define gaseous exchange? What did you learn as the importance of gaseous exchange? 

Look at the cycle below. 

What is the importance of the cycle shown in the diagram? Can you identify gases X and Y? What is their importance to each of the organisms represented in the cycle? Based on the diagram, predict what you will learn in this unit. 

9.1   The concept of respiration and respiratory surfaces

Respiration is the process where food taken in by organisms is burnt to produce energy required for their body functions. During respiration, oxygen is used and carbon dioxide is removed. 

Cellular respiration takes place constantly in all living cells. It creates a constant demand for oxygen and a need to eliminate carbon dioxide gas. Gaseous exchange is the biological process through which these gases are transported through the body of an organism across a specialised respiratory surface. Organisms such as plants need to take in or release oxygen and carbon dioxide at one time or another during respiration and photosynthesis. Animals, on the other hand, always take in oxygen and release carbon dioxide during respiration. Gaseous exchange therefore is: 

Therefore an efficient system for the exchange of gases is extremely important in living organisms. Gaseous exchange is necessary because organisms are able to obtain useful gases from their environment and get rid of waste gases into the environment.

The environments that organisms exchange gases with include: air for some organisms and water for others. Air is the main source of oxygen and carbon dioxide. Organisms that live on land exchange these gases directly with air. Oxygen and other gases from the air diffuse into lakes, rivers and oceans. The air dissolved in water is used for gaseous exchange by organisms that live in water. 

Discussion corner 

1. Find out the following with a friend. 

2.  Write down answers in your note books. 

3.  Discuss your findings with fellow classmates. 

In large multicellular animals, the surface area to volume ratio is small. Many cells are deep inside the body of the animal, away from the surface. Diffusion of gases alone is not efficient enough in moving gases to and from all the cells. Therefore, large multicellular animals have specialised structures or organs with special surfaces over which gaseous exchange takes place. These special surfaces are called respiratory surfaces. 

Examples of specialised structures for gaseous exchange in animals include the following: 

Gaseous exchange takes place over the respiratory surfaces. A respiratory surface has a number of characteristics that make it efficient for gaseous exchange. Some of these characteristics include: 

Not all respiratory surfaces are in direct contact with the medium through which gaseous exchange occurs, such as water or air around the organism. Therefore, there is need for a process that can ensure a continuous supply of fresh water or air to and from the respiratory surface. This is achieved by the process of ventilation which continuously brings water or air containing more oxygen to the respiratory surface. It also removes water or air containing a lot of carbon dioxide from it. Ventilation, therefore, is important because it maintains a high diffusion gradient at the respiratory surface. It also ensures a high rate of gaseous exchange. Breathing is an example of ventilation. 

9.2 The mechanism of breathing in humans 

In human beings, the process of breathing (ventilation) is the first part of the gaseous exchange processes. The second part is the exchange of these gases between the lungs and blood. Breathing provides a continuous supply of fresh air to the gas exchange surface. It also helps to maintain a large diffusion gradient across the gas exchange surface. The volume of gases exchanged during breathing, changes according to physiological demands placed on the body, for example, during an exercise. The breathing rate is controlled by the respiratory center of the brain.

Activity 9.1: To observe the movement of the chest during breathing 

Working in pairs, carry out the following exercise. 

1. Ask your partner to breathe in deeply then hold their breath. 

2.  Describe what you see happening to the chest as your partner breathes in. 

      • Does it remain in the same position? 

3. Let your partner now breathe out. 

      • What do you see happening to the chest? 

4. Record the form of movements noticed. 

The process that brings air into the lungs and removes it again is known as breathing. Breathing involves two phases called inhalation and exhalation. Since the lungs contain no muscle tissue, they are not capable of independent movement. However, they are elastic, and during breathing they are forced to expand or contract as a result of pressure changes around them. These pressure changes are caused by the movement of the muscular diaphragm, ribs and intercostal muscles (rib muscle), and by the force of atmospheric pressure.

Activity 9.2: Construction and use of a model to demonstrate breathing mechanism 

Requirements 

Procedure 

1.  Assemble the materials as shown below.

2.  Study the model and state what the following parts represent in the human respiratory system  

      • Bell jar 

      • Balloon  

      • Rubber sheet 

3.  Pull down the rubber sheet at the bottom of the bell jar. 

       • What happens to the balloons? 

4.  Now push up the rubber sheet.  

      • What happens to the   balloons? 

Study questions 

(a) Draw sketches to show the appearance of the balloons in (2) and (3) above. 

(b) Explain your observation. 

(c) Explain how the model is similar to the working of the thorax in humans beings during breathing.

a) Inhalation (breathing in) 

Inhalation is also known as inspiration. This is the active phase of breathing which draws air into the lungs. During inhalation, the diaphragm muscles contract causing it to flatten, look at Fig. 9.4. In the ribs region, the external intercostal muscles contract while the internal intercostal muscles relax. This causes the rib cage to move upwards and outwards.

The contraction of the diaphragm and external intercostal muscles increases the volume in the chest cavity. However, it causes a decrease in the pressure of air inside compared to atmospheric air. Air rushes through the air passages into the lungs, forcing them to expand. 

 b. Exhalation (breathing out) 

Exhalation is also known as expiration. This is the phase of breathing, which expels air out of the lungs. During exhalation, the diaphragm muscle relaxes making it to move upward and regain its dome shape. The external intercostal muscles relax and the internal intercostal muscles contract. This causes the rib cage to move downward and inwards. The volume of the chest cavity decreases and the pressure increases compared to the atmospheric air. Increased pressure forces air out of the lungs, which become deflated. 

Table 9.1: Differences between inhalation and exhalation

Activity 9.3: Dissection of a small mammal to display the structures of the respiratory system 

Requirements 

Procedure 

1.  Pin the animal onto the dissection board with the ventral side up. 

2.  Open the thorax by cutting with scissors along the mid section from the neck to the hind legs.

3.  Cut the skin further to expose the abdomen as shown in Figure 9.6 above. 

Note the following:  

4.  Remove the lungs out and place it on the board.  

5.  Cut through the lungs. 

6. Identify the main airways. 

7. Note your observations. 

8.  Share your findings withother  class.

Gas exchange in the alveoli 

Air passes the nose or mouth as it moves down the trachea. The trachea is divided into the left and right bronchi. Each bronchus is divided into smaller bronchioles and each bronchiole is in turn attached to numerous alveolar sacs. The groupings of alveoli take the same shape like a bunch of grapes. 

Gaseous exchange at the alveolus takes place between the phases of inhalation and exhalation. The alveolus is a suitable point for gaseous exchange because:

•  It is supplied with blood which carries the gases being exchanged. 

•  It has a very thin wall across which gases diffuse between it and the blood. 

•  It is lined with a thin film of moisture to dissolve the diffusing gases. 

•  A ventilation process brings in and takes away air containing the gases being exchanged. 

•  It has a very large number of alveoli to increase their surface area for gaseous exchange. 

Gas exchange between the air within the alveoli and the pulmonary capillaries occurs by diffusion. Oxygen in air, in the alveolar space is at a higher concentration than that in the blood capillaries. It therefore first dissolves in the water layer in the alveolar lining then diffuses across the alveolus and then the capillary walls into the red blood cells. This becomes oxygenated blood which is carried to the heart by the pulmonary vein. 

Carbon dioxide in the blood diffuses across the capillary and alveolus walls into the alveolar space and is eventually expelled during exhalation.

A diffusion gradient is essential for rapid gaseous exchange in the alveolus.  The following factors contribute towards maintaining this diffusion gradient. 

(a) Lung ventilation: Breathing movements transport respiratory gases to and from the alveolus. 

(b)  Blood flow: This constantly replaces oxygenated blood with deoxygenated blood. The pulmonary artery brings blood low in oxygen concentration and high in carbon dioxide concentration. The pulmonary vein takes away blood high in oxygen concentration and low in carbon dioxide concentration. 

 (c)  Haemoglobin: It quickly combines with oxygen and prevents its accumulation in the alveolus.

Activity 9.4: To analyse the  gas present in exhaled air 

Requirements 

Procedure 

1. Set up the apparatus as shown in the diagram below.

2.  Breath in and out through the mouth-piece. 

3.  What do you notice in the lime water in: 

         • Flask A?           • Flask B? 

4.  Explain your observation in  

         • Flask A              • Flask B

Task: From the table above, explain the difference in composition of gases in inhaled and exhaled air.

Self-evaluation Test 9.1 

1.  What role does the nasal passages and the diaphragm play in gaseous exchange? 

2.  Describe the characteristics of a respiratory surface. 

3.  Describe the mechanism by which carbon dioxide in the blood capillaries around the alveoli passes into the alveoli and finally is exhaled.

9.3 Respiratory diseases and smoking 

Research Activity 

1.  Using handouts, textbooks and internet, research on the following: 

2. Share your findings with the class.

The respiratory system is affected by many diseases and disorders. Some of these are caused by micro-organisms while others are genetic. The most common respiratory diseases are: 

1.  Asthma This is a disease that comes about when the air passages in the lungs suddenly narrow as a result of contraction of their smooth muscles. It is also characterised by an inflow of mucus which clogs the narrow passages even more. Some substances can trigger an asthma attack. They are called allergens. They include pollen grains, some type of proteins in milk, pet hairs, dust and even flavours in food. It can also caused by stress and anxiety. In some families the disease is inherited.

Symptoms of asthma 

Prevention and treatment 

Asthma is treated by two types of medication: long-term control and quick-relief medicines that help reduce airway inflammation and prevent asthma symptoms. 

2. Bronchitis This is an infection of the inner walls of the bronchi.  It is caused by bacteria or air pollutants such as smoke in inhaled air. The infection causes the mucous membrane in the respiratory tract to produce excess mucus. This causes the cells lining the bronchi and bronchioles not to function properly. As a result, the air passage to the alveoli becomes blocked with mucus. Severe coughing occurs in an attempt to get rid of the excess mucus. Breathing also becomes difficult. Bronchitis may be acute or chronic. 

Acute bronchitis starts quickly and stops after a few days. The symptoms of acute bronchitis are like those of a cold. 

Chronic bronchitis starts slowly and lasts for a long time. It is a more serious kind of infection. It is commonly caused by smoking and air pollution. 

Symptoms of bronchitis 

Prevention and treatment 

3. Emphysema 

This results from long untreated bronchitis where the bronchioles in the lungs become blocked. This causes damage to delicate walls of the alveoli due to high pressure when coughing. This leads to reduced surface area for gaseous exchange. The lungs become distended and inelastic that gases cannot be exchanged efficiently. The patient becomes weak due to insufficient oxygen supply to tissues.  Running and walking can prove to be hard when one has this condition. 

Prevention and treatment 

Emphysema is treated according to the severity of symptoms. Bronchodilators are normally given to help relieve coughing, shortness of breath and breathing problems. 

Early treatment of bronchitis with antibiotics to prevent secondary infection can help to prevent emphysema. 

4. Pneumonia 

Pneumonia is an infection of the lungs. It is caused by bacteria called Pneumococcus that spreads through the air. It can also be caused by a virus or a fungi. Infection proceeds from the mouth down into the lungs. As a result of the infection, a fluid is produced which collects in the alveoli. The lungs become solid and have no air. This prevents exchange of gases in the lungs. 

Signs and symptoms 

Prevention and treatment  

5. Tuberculosis 

Tuberculosis (TB) is caused by bacteria called Mycobacterium tuberculosis. The source of infection may be droplets containing bacteria sprayed from the air passages during breathing or sneezing. It can also be caused by infected dry sputum in particles of dust. Tuberculosis bacteria may attack any part of the body, but they usually invade the lungs, causing pulmonary tuberculosis. Another source of infection is by drinking raw milk from a cow suffering from bovine tuberculosis. 

Signs and symptoms 

In addition to tuberculosis of the lungs, there are other forms of the disease in which bacteria attack the lymphatic glands, bones and other parts of the body. 

Prevention and treatment 

• The patient should consult a doctor for adequate treatment. Treatment for TB will usually involve a long course of antibiotics lasting 6-9 months. 

• Overcrowding increases the risk of spread of tuberculosis. 

• Avoid taking raw milk. Boil all milk or drink pasteurised milk. 

• Immunisation with B.C.G. vaccine in children. 

• Isolating patients 

6. Whooping cough 

Whooping cough is caused by bacteria called Bordetella pertussis. The mode of infection is from one person to another through inhalation of infected droplets.

Signs and Symptoms 

• Whooping cough starts like a cold with fever, running nose and cough. 

• Two weeks later, the whooping begins. The patient coughs rapidly many times without taking a breath, until one coughs up a mass of sticky mucus, and the air rushes back into the lungs with a loud whoop sound. After the “whoop”, the patient may vomit. Between coughing bouts the patient seems fairly healthy. 

Prevention and treatment 

1.  Patients should consult a doctor for adequate treatment. Treatment for whooping cough involves taking antibiotics early before coughing fits begin. 

2.   Patients should be isolated from contact with other people. 

3.   Immunisation with vaccines against whooping cough. In infants, the vaccine against whooping cough is usually combined with those against diphtheria, tetanus and poliomyelitis. 

7. Common cold 

A common cold is an illness caused by a virus infection located in the nose. Colds also involve the sinuses, ears and bronchial tubes. Colds last on average for one week. Mild colds may last only 2 or 3 days while severe colds may last for up to 2 weeks. A cold is a milder illness than influenza. 

Nasal secretions containing cold viruses contaminate the hands of people with colds as a result of nose blowing, covering sneezes and touching the nose. Cold viruses may contaminate objects and surfaces in the environment of a patient. 

Note: Young children are prone to colds. 

Cold virus, which is expelled into the air in coughs and sneezes, may land in the nose or eye of another person and cause infection. Hence transmission. 

Signs and symptoms 

Prevention and treatment 

Cold is caused by a virus, therefore it will clear after several days. Treatment is mainly to lessen the symptoms. If a cold persists seek medical advice.

To prevent catching a cold, limit contact with known cold patients, especially during the first three days of their illness. Practice preventive measures which keep cold virus from entering the nose: 

8. Influenza 

Influenza or 'the flu' is a highly contagious disease caused by infection from influenza type A or B (or rarely C) virus. These viruses infect the upper airways and lungs. The flu is highly contagious. 

Flu is not the same as a common cold, and can be a serious illness. For some people, such as the elderly and those with underlying medical conditions, the flu can cause serious complications which require hospitalisation. It can sometimes lead to death. 

Flu is usually spread through infected people coughing and sneezing, which temporarily contaminates the surrounding air and surfaces with infected droplets 

Signs and Symptoms 

Symptoms usually appear 1–3 days after being infected. A person can spread flu to others 1–2 days before they become unwell and up to 5 days after symptoms develop. The symptoms of influenza can include: 

Prevention and treatment 

1. Generally, uncomplicated flu is managed by simply resting in bed, drinking plenty of fluids (particularly water) and taking over-the-counter medication to help relieve the symptoms. 

2. Antiviral medications reduce the length of time symptoms last and help people infected return to their daily routines earlier. 

3. Good hygiene is essential to protect yourself and others 

4. You can reduce the risk of infection by getting vaccinated. 

Effect of smoking on the respiratory system 

The respiratory system is not designed to cope with tobacco smoke. Tobacco smoke paralyses the cilia in the respiratory tract and stops their movement. 

In addition, tobacco smoke increases the production of mucus in the air passages. A cough by a smoker is an attempt to remove the excess mucus from the respiratory system. Besides irritating the trachea and bronchi, smoke particles interfere with the uptake of oxygen in the air sacs. 

When cigarette smoke is inhaled, about one-third of the particles remain in the alveoli. Phagocytic cells called macrophages can slowly remove many of the particles. However, an excess of particles from smoking or from other sources of air pollution breaks down the walls of the air sacs and causes the formation of inelastic tissue. This reduces the functional area of the respiratory surface and in severe cases may lead to a disease called emphysema. In some cases, lung cancer also develops.

Self-evaluation Test 9.2 

1.  Suppose you are nursing a patient suffering from a respiratory disease. How would you prevent contracting the disease? 

2.  Is it proper to avoid friends who smoke? 

3.  Why is it not advisable to take antibiotics without prescription from a doctor?

9.4 Gaseous exchange in plants 

The principal gaseous exchange surfaces for plants are the leaves. Plant leaves have stomatal pores on their surface where gaseous exchange occurs. Plants do not have a specialised respiratory system like animals. This is because they are metabolically less active than animals. 

Activity 9. 5: To demonstrate the presence of stomata in leaves 

Requirements 

Procedure 

1.  Paint a 1 cm 2 (or larger) square of thick nail polish on the underside surface of the leaf being studied. Allow the nail polish to dry fully. 

2.  Once the patch of nail polish is dry, tape a piece of clear cellophane tape to the patch on the leaf. 

3.  Carefully pull on a corner of the tape and gently peel the nail polish off the leaf.  The layer of cells that sticks on the nail polish is what you will examine under the microscope. 

4. Tape the peeled nail polish and its layer of cells to a clean microscope slide, using scissors to trim any excess tape.

5.  Place the slide on the microscope stage. Using the low power objective lens, focus the slide until the stomata are visible. 

Each stoma is bordered by two sausage-shaped cells, called guard cells, which are normally smaller than epidermal cells. Unlike other cells in the epidermis, guard cells contain chloroplasts. 

6.  Sketch what you observe under the microscope, labelling the stoma, guard cells, epidermal cells, and chloroplasts. 

7.  Count the number of stomata in your field of view, then estimate the number of stomata on the sample being examined. 

Stomata are pores between guard cells. They are found on the upper or lower epidermis or both. Stomata  allow: 

Stomata are the main structures for gaseous exchange in leaves of plants. They are tiny openings found in the epidermis of leaves. With the exception of submerged plants, stomata are present in all the leaves of plants. Most land plants have stomata on the lower surface of the leaf.

Stomata allow gaseous exchange to take place in leaves. Let us examine the structure of stomata to understand how gaseous exchange takes place through them.

The guard cells control the opening and closing of each stoma. 

Discussion corner 

1. Discuss the following questions with some of your classmates: 

2. Share your findings with the rest of the class.

Gas exchange in plants 

When the stomata open, gases come into the leaf while others come out. Oxygen which is a product of photosynthesis diffuse out of palisade cells to the air spaces. It eventually comes out through the stomata to the atmosphere. On the other hand, carbon dioxide from the atmosphere enters into the leaf through the stomata into the airspaces. It then diffuses from the air spaces into palisade cells where it is used during photosynthesis. 

When the stomata are open, air from the surrounding enters the leaf and occupies the air spaces. Oxygen and carbon dioxide diffuse into or out of the leaf cells along their concentration gradient. 

During photosynthesis, carbon dioxide is used up by the palisade cells, and oxygen is produced.

This means that the concentration of carbon dioxide in the palisade cells becomes lower than in the air spaces outside the cells. Therefore during photosynthesis carbon dioxide diffuses into the palisade cell from the airspaces. On the other hand, photosynthesis produces oxygen whose concentration becomes higher inside the palisade cells than in the air spaces surrounding the cells. Oxygen therefore diffuses out of the palisade cells into the air spaces. This is the process of gaseous exchange in leaves. 

Gases first dissolve in the film of moisture surrounding the cell, before they diffuse into or out of the cell. Note that when air is entering or leaving the plant through stomata, the process is not gaseous exchange, but diffusion. Gaseous exchange refers to the movement of gases between the cells and their surroundings.

Stems of woody terrestrial trees and shrubs have areas of loosely arranged cells with large air spaces between them. These cells together form a structure called a lenticel. Lenticels are formed when the epidermis is replaced by the bark. Lenticels appear scattered on the surface of the stem as small raised openings. They allow gaseous exchange of oxygen and carbon dioxide between the atmosphere and the internal tissues of the stem. 

Self-evaluation Test 9.3 

1.  What are the structural differences between guard cells and other epidermal cells? 

2.  Although the leaves are the main organs of gaseous exchange in plants, the roots also absorb oxygen in the soil. Why is this necessary?

Unit summary 

• Gaseous exchange is the process by which animals exchange respiratory gases while plants exchange both respiratory and photosynthetic gases. 

• Gaseous exchange between a living cell and its surrounding always takes place by diffusion across a moist cell membrane. 

• The surface over which gaseous exchange takes place is called a respiratory surface. 

• All respiratory surfaces have a large moist surface, basically a mechanism of transporting gases to and from the respiratory surface. 

• The direction that a gas takes during diffusion depends on the concentration of the gas on either side of the respiratory surface. 

• In large multicellular organisms such as human beings, most cells are deep inside the body.  Diffusion alone is not enough to move gases across the large number of cells that lies inside. 

• Large multicellular organisms have special respiratory structures for gaseous exchange. These include the tracheal system in insects; the skin, buccal cavity and lungs in frogs; the gills in fish and the lungs in humans. 

• In humans beings, breathing is controlled by medulla oblongata in the brain which is sensitive to the concentration of carbon dioxide. When this exceeds a certain level the medulla oblongata stimulates the ribs and diaphragm to contract more rapidly. 

• Plants also require carbon dioxide for photosynthesis and produce oxygen as a waste product. 

• In plants, gaseous exchange occurs through stomata in leaves, lenticels in the stem and roots and through the epidermis of the root. 

• Certain diseases of the respiratory system such as asthma, bronchitis, pulmonary tuberculosis, pneumonia and whooping cough interfere with the functioning of the respiratory structures.