Respiratory System – SS2 Biology Lesson Note (First Term)

Respiration in animals involves taking in oxygen and giving out carbon dioxide and water vapour. Unicellular organisms exchange gases through the cell membrane, insects through tracheae, fish through gills, toads through the skin, mouth and lungs, and mammals through lungs. This note describes each mechanism and the breathing process in humans.

Behavioural objectives

By the end of the lesson, students should be able to:

• describe how unicellular organisms, insects and fish respire;
• describe how tadpoles and adult toads respire;
• name the parts of the mammalian respiratory system and trace the path of air through them;
• explain the mechanisms of inhalation and exhalation in man;
• compare the composition of inhaled and exhaled air.

How do lower animals respire?

Unicellular organisms

Unicellular organisms require no elaborate respiratory system. They respire through their entire body surface, and the cell membrane acts as the respiratory surface. Gases move by simple diffusion. Oxygen, which is highly concentrated in the environment, diffuses into the organism, where the concentration is lower.

Insects

Insects use a tracheal system for respiration. Air passes through a spiracle and enters the tracheal trunk. The trunk leads into a complex, branching network of tracheal tubes that subdivide into smaller and smaller diameters and reach every part of the body.

At the end of each tracheal branch, a special cell (the tracheole) provides a thin, moist interface for the exchange of gases between atmospheric air and a living cell. Oxygen in the tracheal tube first dissolves in the liquid of the trachea and then diffuses into the cytoplasm of an adjacent cell. At the same time, carbon dioxide, produced as a waste product of cellular respiration, diffuses out of the cell and, eventually, out of the body through the tracheal system.

Fish

The gill is the respiratory organ of fish. The gills are enclosed in the gill chamber and are about three to four in number. Each gill is made up of the gill filament (where gaseous exchange takes place), the gill raker (which prevents food particles from entering the gill chamber) and the gill arch (in which the filaments are built). The gills are concealed by the operculum.

The fish starts breathing by closing the operculum, opening the mouth and lowering the floor of the mouth. Water containing dissolved oxygen rushes into the mouth. The fish then closes its mouth, and water rushes into the gill chamber and moves across the gill filaments. Oxygen in the water diffuses into the gill filaments, while CO₂ diffuses out into the water. The water is expelled when the fish opens its operculum.

Toads

Tadpole

A tadpole is the larval stage of a toad or frog. It hatches from an egg and can only survive in water. Tadpoles have tiny external gill flaps that extract oxygen from water as it passes over them. They open their mouths as they swim and take in water. As the mouth closes, muscles transfer the water to the gills.

The gills consist of thin membranes called lamellae, which take oxygen from the water into the bloodstream by diffusion. Tadpoles can also rise to the surface and gulp air. As tadpoles mature, the gills are absorbed by the body as other respiratory systems develop.

Adult toad

The adult toad respires in three ways: through the mouth, the skin and the lungs.

Buccal (mouth) gaseous exchange

The toad can use its mouth as a respiratory organ because of the following:

• It is very large, so it has a large surface area.
• It has a thin mucus membrane for easy diffusion.
• It is well supplied with blood capillaries.

To start breathing, the toad closes its mouth, the nostrils are opened and the floor of the buccal cavity is lowered, so air is drawn through the nostrils into the buccal cavity. After this, the capillaries and the glottis close and gaseous exchange takes place between the blood and the inhaled air. To get rid of the air, the floor of the buccal cavity is raised, increasing the air pressure and thereby forcing the nostrils to open. The air in the buccal cavity, which contains carbon dioxide, then flows out.

Skin (cutaneous) respiration in toads

This is possible because of the large surface area of the skin. The skin is moist because the mucus gland secretes continuously, there is an adequate supply of blood capillaries, and the skin is thin. A simple diffusion of gases takes place through it, both on land and in water.

Lung (pulmonary) respiration in toads

This is similar to buccal respiration, with a slight difference. To draw air into its mouth, the toad lowers the floor of its mouth, which causes the throat to expand. The nostrils then open, allowing air to enter the enlarged mouth. The nostrils close, and the air in the mouth is forced into the lungs by contraction of the floor of the mouth.

To eliminate the carbon dioxide in the lungs, the floor of the mouth moves down, drawing the air out of the lungs and into the mouth. Finally, the nostrils are opened and the floor of the mouth is moved up, pushing the air out through the nostrils.

How do mammals respire?

The respiratory system in mammals

The respiratory system of mammals is the most complex of all respiratory systems. It comprises a pair of lungs enclosed in the thorax and connected to the outside by a series of branched air tubes.

In human beings, air can be drawn in through the mouth or nose. Both lead into the pharynx, a short passageway which branches at the end into two directions. One leads to the digestive tract, while the other leads to the larynx (voice box) and the lower air pathway.

The entrance of the larynx is called the glottis. It is covered by a cartilaginous flap (the epiglottis), which prevents food from entering the windpipe. For air to enter the larynx, the glottis must remain open.

The trachea (windpipe) branches into two bronchi. The cartilaginous rings in the trachea and bronchi prevent them from collapsing when the air pressure in them is low. Each bronchus leads to a lung, where it branches into small tubes called bronchioles. The alveoli are richly supplied with blood capillaries, and gaseous exchange takes place at their surface.

As oxygen follows this pathway from the outside to the lungs, CO₂ is released from the lungs to the outside by the same pathway.

Mechanism of respiration in mammals

Respiration in mammals has two phases: external and internal respiration.

Breathing is the taking in of oxygen (inspiration or inhalation) and the giving out of CO₂ and water vapour (expiration or exhalation).

Mechanism of inhalation in man

1. The intercostal muscles contract.
2. The ribs are moved upward and outward.
3. The diaphragm becomes flattened.
4. The volume of the thoracic cavity increases.
5. Consequently, air is drawn from the nostrils to the trachea, bronchi, bronchioles and finally to the alveoli.

Mechanism of expiration (exhalation) in man

1. The intercostal muscles relax.
2. The ribs are moved downward and inward.
3. The diaphragm becomes dome-shaped.
4. The volume of the thoracic cavity decreases.
5. Consequently, air containing waste products such as CO₂ and water vapour is forced out from the alveoli of the lungs through the bronchioles, bronchi and trachea, and finally to the exterior through the nose.

What is the composition of inhaled and exhaled air?

Evaluation

1. Name the respiratory organ of a fish and state the function of the gill raker.
2. State three reasons why the toad can use its mouth as a respiratory organ.
3. Name the three ways in which an adult toad respires.
4. Describe what happens to the intercostal muscles, ribs and diaphragm during inhalation.
5. State the percentage of oxygen and carbon dioxide in inhaled air and in exhaled air.

Assignment

1. Describe how an insect takes in oxygen and gives out carbon dioxide.
2. Trace the path of air from the nose to the alveoli in man.