Explain physiology of Respiration


Respiration is the sign of life and the index of all biological activities, taking place in the body. Living orainsms require energy to perform their activities which they get from the oxidation of food materials by oxygen. So respiration includes breathing and oxidation. Thus it can be defined as “a catabolic process in which the respired oxygen is used in the oxidation of food, resulting in release of energy.”

Respiratory organs in vertebrates:

Respiration is carried in vertebrates by the following means:


(a) Direct respiration through skin and pharyngeal regions e.g., Rana and salamander and some fishes.

(b) Gills as the most important respiratory structures in aquatic bodies which may be external and internal. E.g., Larval amphibians and fishes.

(c) Air bladder is modified in some fishes to ac as respiratory surfaces.

e.g., Dipnoi.


(d) The lugs are well developed respiratory organs in land vertebrates, which become more complicated and efficient in higher vertebrates.

e.g., all amphibians.

Types of Respiration:

1. Direct respiration is seen in invertebrates.


2. Indirect respiration is seen in higher invertebrates and vertebrates.

Respiratory System:

Respiratory system can be divided two phases:

(a) External respiration, which includes the exchange of gases between environment and blood.


(b) Internal respiration, which involves the exchange of gases between blood and body cells.

Respiratory Tract:

The respiratory passage include nasal chamber, reparatory chamber, nasopharynx, larynx,trachea, bronchi and two lungs.

Inside the lungs the bronchus capillaries ending in alveoli.


Thoracic cavity:

The thoracic cavity is separated from the abdominal cavity by a dorne shaped, muscular partition wall, and diaphragm. The thoracic cavity contains a pleural cavity which is ventrally covered by sternum, dorsally by vertebral column and laterally by ribs and anteriorly by the neck.

The pleural cavity is made up of outer parietal pleura and inner visceral pleura. Pleural fluid is present from lubrication. It is an air tight chamber.

Mechanism of Breathing:

Ventilation or breathing is acyclic event which occurs 16-18times per min. The expansion and contraction of the chest occurs by the action of some voluntary and involuntary muscles. The expansion and contraction of thoracic cavity bring about the expansion and contraction of wings. The inter coastal muscle help in breathing.

It is of two types.

(1) External intercostals muscle: These are attached with the lower border of upper rib is extended downwards to be attached to the upper border of the lower rib.

(2) Internal intercostals muscle: These are extended in downward and backward directions from the lower borders of the rib below to the upper borders of the rib above. They cross external intercostals muscles at right angles.

Diaphragm: It is a done shaped, musculo-fibrous structure attached with radiating muscle fibres.

Process of Breathing:

It occurs in two phases.

(1) Inspiration: During the process, lungs are enlarged by the enlargement of thoracic cavity to allow entry of fresh air.

The external intercostals muscles contract to raise the ribs upwards and outwards to increase the capacity laterally. The diaphragm flattens, so that the volume of the pleural cavities increase so also the lungs expand. The air pressure falls and is filled up by air, passing through respiratory tract. It is an active process.

(2) Expiration: It is known as passive process. When the external intercostals muscle relaxes the ribs are drawn inwards and downward. The diaphragm becomes dome-shaped. These changes reduce the volume of the pleural cavity so also the lungs. The air is expelled out of equalize the pressure. Internal intercostals muscle acts only during vigorous expiration

(3) Pause: There is a pause which marks the completion of one breathing.

Exchange of gases in the lungs:

(1) The alveoli gets filled with fresh air during breathing.

(2) The partial pressure of oxygen in the blood of alveolar capillary is considerably low and the concentration of O2 is much lower in comparison to atmospheric air.

(3) The diffusion of O2 occurs from alveolar air into the blood and CO2 (40mlHg) out of blood into the alveoli.

(4) Diffusion occurs only I dissolved condition. There is marked diffusion gradient which determinesthe direction of their flow.

Transportation of gases by blood:

Blood carries both O2 and CO2.

1. As physical solution: Oxygen is transported from the lungs as physical solution in plasma at a concentration of 0.3 ml per 100ml of blood.

2. As oxyhaemoglobin: The solubility of O2 in water is very low. This short-coming is ove-come by haemoglobin. The haemoglobin present in RBC contains haems coupled with globin which acts as the carries of O2. Haemoglobin forms a loose vhemical combination with O2 at high O2 tension of form oxyhaemoglobin.

(i) When the oxyhaemoglobin reaches the tissue, it immediately dissociates releasing oxygen content. It occurs in areas where there is low concentration of oxygen and high concentration of CO2.

Oxygen dissociation curve: At low oxygen pressure oxygen dissociates from haemoglobin. At any given oxygen concentration, there is a definite proportion between the amount of haemoglobin and oxyhaemoglobin. The actual relationship between the partial pressure of oxygen and the degree of saturation of haemoglobin with O2 is shown by the remarkable, O2 – hemoglobin dissociation curve.

When the partial pressure of O2 is 100 mm Hg (completely saturated), no mote O2 is taken out by haemoglobin, but at lower oxygen pressure O2 is given up by haemoglobin saturation and it continues further. Thus the degree of haemoglobin saturation is lowered with the fall of partial pressure of O2. The O2 capapcity in mammalian blood is about 20 vols. Per 100 vols. of blood.

Factors affection O2 dissociation:

1. Partial pressure of O2 and partial pressure of CO2 affects oxygen absorption and dissociation.

2. A rise in temperature causes decrease I O2 carrying capacity of the blood.

3. The amount of haemoglobin determines the quantity of oxygen to be carried.

4. The pH also affects the degree of saturation.

Transport of CO2

CO2 is evolved as a result of oxidation of glucose, which diffuses into tissue fluid. It reaches the respiratory surface through mostly venous blood circulation. Normally the venous blood contains 60ml of CO2/100 ml of blood

Transport of CO2 and physical collation of blood

CO2 diffuses into the water content of plasma to form carbonic acid.

Only 5% of CO2 is carried by blood plasma.

Transport of CO2 as carbamino compounds

CO2 combines with amino group of haemoglobin to form carbamino haemoglobin. Only 10% of CO2 is carried in this form.

Transport of CO2 as bicarbonate:

Nearly 85% of CO2 is carried back as bicarbonate. The carbonic acid formed by water and CO2 dissociates into hydrogen ion NSS HCO3. The latter combines with sodium and potassium ions to form bicarbonates.

Function of carbonic anhydrase:

It is present in erythrocytes, which increases the speed of reaction during the formation of carbonic acid. This it is largely essential.

It also acts as catalyst in splitting carbonic acid into water and CO2.

OXyhaemoglobin is strongly acidic which helps the release of CO2 from bicarbonates, carbonic acids and carbamino haemoglobin.

Internal or cellular or Tissue Respiratory:

The ultimate function of respiratory cactivity is cellular oxidation of digested food materials to release energy.

C6H12O6 + 6O2 à 6CO2 + 6H2O + 673K.CAL.

The breakdown of glucose occurs under two major steps.

1. Glycolysis.

2. Kreb cycle.

Glcolysis: The glycolysis the 6-carbon glucose is broke down in 3 carbon pyruvic acid. The end products of glycolysisi are alcohol and lactic acid which are still rich in energy.

Kreb cycle:

It occurs in mitochondria which is packed with the enzymes. The acetuic acid is converted into acetyl co-A wich combines with oxaloacetic acid to form citric acid. It passes through different stages releasinf energy. IT is suggested that nearly 60 ATP Molecules are yielded from one molecule of glucose storing 38 ATP molecules for further use.

Oxidation of other food materials:

The fatty acids are converted into acetic acid where as the amino acids are converted into pyruvic or ketoglutaric acid. These are converted into CO2 and H2O releasing energy.

Control of Breathing:

This process is involuntary and automatic. The pneumotaxic centre of pons varolu and inspiratory and expiratory centre of medulla oblongata are responsible for controlling respiration.

Factors affecting breathing centre:

The breathing centre is influenced by some physical and chemicall factors.

1. Partial pressure of CO2.

2. Increased acidity.

3. Increased concentration of CO2 and lacitic acid during muscular exercise.

4. Blood pressure and body temperature.

Respiratory Pigment:

The haemoglobin is best oxygen carrying pigment of vertebrates.

Constituents :

It is present in the RBC. It is a conjugated protein of chromoprotein containing an iron containing protoporphyrin (or haem), and a protein called globin. The globin varies considerably in size, amino acid composition, solubility and other physical properties from animal to animal. IT has a molecular wt of about 68-72 *103. The mammalian haemoglobin contains 0.336% of iron.

Respiratory rate :

It is the rate of inspiration and expiration per min i.e., 15.20 times in healthy man. The total air inspired per min is about 10 liters normally.

Tidal volume:

It is the volume of air inspired and expired during a normal breathing (500 ml in man.)

Residual air: It is the amount of air left in the lungs after expiration.

Vital capacity: It is the amount of air breathed in and out of lungs with maximum effort.

Respiratory quotient:

It is the ratio of volume of CO2 released to the volume of oxygen consumed during same time.

e.g. R.Q. for glucose is 6CO2/ 6O2 =1

Anaerobic respiration:

When a respiration goes I absence of oxygen is called anaerobic respiration e.g.

(i) Fermaentation in the yeast

(ii) Respiration by microbes

(iii) Glycolysis in tissues

C6H12O6 à 2CH3-CH2-OH+2CO2+ Energy (50 Kcal)

In the glycolysis of tissues it differs.

C6H12O6 à 2CH3CH(OH) COOH + ENERGY (36Kcal)

It occurs due to absence of oxygen and liberates comparatively less energy.


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