CIE Categories Archives: 8. Respiration

8. Respiration

8.1 Respiration.

1 . Respiration is the process of breaking down food to release energy in the presence of oxygen.

2 . There are two types of respiration:

1) Aerobic respiration.

2) Anaerobic respiration.

3 . Aerobic respiration is the process of breaking down food in living cells to release a large amount of energy in the presence of oxygen.

4 . Anaerobic respiration is the process of breaking down food in living cells to release a small amount of energy in the absence of oxygen.

5 . Aerobic respiration

6 . Anaerobic respiration:

8.2 Human Respiratory System.
1 . Breathing is a mechanical process that involves taking in air into the lungs & takes out air from the lungs.

2 . Inspired and expired air:

3 . Temperature of:

1) Inspired air is lower than body temperature.

2) Expired air is about the same as body temperature.


8.3 Human Gaseous Exchange.

1 . The respiratory system consists of:

  • Lungs.
  • Nose.
  • Mouth.
  • Trachea.
  • Bronchi (bronchus sg.)
  • Bronchioles.
  • Alveoli (alveolus sg.)

2 . The gas exchange system in man:

Fig.8.1 The gas exchange system in man (left lung cut open to show alveoli)

3 . Inspiration & Expiration:

Fig.8.2 (a) Inspiration and (b) expiration.
A) Inspiration is an active process involving the contraction of:

  • The external intercostal muscles; and
  • The diaphragm muscles.

B) Contraction of the external intercostal muscles pulls the rib cage upwards and outwards, while contraction of the diaphragm muscles result in the flattening of the diaphragm.

C) The net result is an increase in the volume of the thorax (the part of the body between the neck and the abdomen where the heart and lungs lie).

D) Pressure in the thorax is thus reduced.

E) Air is then sucked into the lungs, inflating the alveoli until the internal pressure equals that of the atmosphere.

F) Expiration is a passive process.

G) During expiration, only the internal intercostal muscles contract. The rib cage drops, mainly due to its own weight.

H) The diaphragm relaxes and is forced into a dome shape by the falling rib cage.

I) The volume of the thorax is decreased.

J) Pressure is then exerted on the air in the lungs, forcing it out.

K) During forced breathing, such as during exercise or sneezing, expiration becomes a much more active and forceful process as the ribs are moved more vigorously downwards and the diaphragm is moved upwards.

4 . Structure of the alveolus:

A) The alveoli are where gaseous exchange takes place between the air in the sacs and the blood in the capillaries surrounding each alveolus.

B) The distance over which the gases must diffuse is about 0.01 mm -two cell layers thick.

C) A thin film of moisture covers the surface of the alveolar wall. This has been found to contain a chemical which lowers surface tension. This makes it easier for the lungs to be inflated during breathing in. Without this chemical or surfactant, the alveoli would collapse and become stuck together each time air passes out of the lungs.


5 . Diffusion across the alveoli is enhanced by the following features:

  • Large surface area of the alveoli;
  • Short distance between the air and the blood;
  • Steep concentration gradient maintained by constant movement of blood through the tissues and ventilation of the lungs; and
  • Ability of blood to carry oxygen and carbon dioxide.

6 . Exchange of oxygen:

  • Each haemoglobin molecule present in red blood cells can combine reversibly with up to four molecules of oxygen, forming the compound oxyhaemoglobin.
  • As the red blood cells move through the blood capillaries surrounding the alveoli, the narrowness of the capillaries forces them to slow down and become distroted.
  • This increases the time available for gaseous exchange to take place, as well as exposes a larger surface area of the cell.

7 . Exchange of carbon dioxide:

  • Carbon dioxide is transported in a variety of ways by blood.
  • A very small percentage is dissolved in plasma as carbonic acid.
  • Some carbon dioxide becomes attached to haemoglobin to form the compound carbamino haemoglobin.
  • Most of the carbon dioxide is carried by the plasma as hydrogen carbonate ions.


Continue Reading 0