Process of Respiration in Human Beings (With Diagram)

In this article we will discuss about the Process of Respiration in Human Beings.

Mechanism of Breathing:

It means the inflow (inspiration) and outflow (expiration) of air between atmosphere and the alveoli of the lungs. It is affected by the expansion and contraction of lungs. There are mainly two processes by which the lungs are expanded or contracted.

(i) The downward and upward movement of the diaphragm which increases and de­creases the diameter of the thoracic cavity (chest cavity).

(ii) The elevation and depression of the ribs, which lengthens and shortens the thoracic cavity.

1. Inspiration:

It is a process by which fresh air enters the lungs. The diaphragm, intercostal muscles and abdominal muscles play an important role.

(i) Diaphragm:

The diaphragm becomes flat and gets lowered by the contraction of its muscle fibres thereby increasing the volume of the thoracic cavity in length.

(ii) External intercostal muscles:

They occur between the ribs. These muscles contract and pull the ribs and sternum upward and outward thus increasing the volume of the thoracic cavity.

(iii) Abdominal Muscles:

These muscles relax and allow compression of abdominal organs by the diaphragm. The abdominal muscles play a passive role in inspiration. The muscles of the diaphragm and external intercostal muscles are principal muscles of inspiration.

Movement of Fresh Air into the Lungs:

Thus overall volume of the thoracic cavity increases and as a result there is a decrease of the air pressure in the lungs.

The greater pressure outside the body now causes air to flow rapidly into external nares (nostrils) and through nasal cavities into internal nares.

Thereafter the sequence of air flow is like this:

External nares → Nasal cavities → Internal nares → Pharynx → Glottis → Larynx → trachea → Bronchi → bronchioles → alveolar ducts → alveoli.

From the alveoli oxygen passes into the blood of the capillaries and carbon dioxide diffuses out from the blood to the lumen of the alveoli.

2. Expiration:

It is a process by which the foul air (carbon dioxide) is expelled out from the lungs. Expiration is a passive process which occurs as follows.

(i) Diaphragm:

The muscle fibres of the diaphragm relax making it convex, decreas­ing volume of the thoracic cavity.

(ii) Internal Intercostal Muscles:

These muscles contract so that they pull the ribs downward and inward decreasing the size of me thoracic cavity.

(iii) Abdominal Muscles:

Contraction of the abdominal muscles such as external and internal oblique muscles compresses the abdomen and pushes its contents (viscera) to­wards the diaphragm. The internal intercostal and abdominal muscles are muscles of expiration.

Movement of Foul Air out of the lungs:

Thus overall volume of the thoracic cavity decreases and foul air goes outside from the cavities of the alveoli in the following manner:

Alveoli → alveolar ducts → bronchioles → bronchi → trachea → larynx → glottis → pharynx → internal nares → nasal cavities → external nares → outside. The process of expiration is simpler than that of inspiration.

Thoracic Vs. Abdominal Breathing:

In human males, lateral movement of thorax constitutes 25% of breathing while abdomi­nal movement accounts for 75% of breathing. In pregnant women, almost the entire breath­ing is through lateral movement of thorax. Therefore, breathing of women is often regarded as thoracic while that of males as abdominal.

Advantages of Nasal Breathing:

Breathing through nose is healthier because it is a natural process. The air which is inhaled contains dust, bacteria, etc., get filtered in the nose. Thus the air which goes into lungs is cleaner. The conchae of the nose also filter and warm up the air.

Respiratory or Pulmonary Volumes and Capacities:

The quantities of air the lungs can receive, hold or expel under different conditions are called pulmonary (= lung) volumes. Combinations of two or more pulmonary volumes are called pulmonary (= lung) capacities.

Respiratory or Pulmonary Volumes (Lung Volumes):

1. Tidal Volume (TV):

It is the volume of air inspired or expired during normal breath. This is about 500 mL, i.e., a healthy man can inspire or expire about 6000 to 8000 mL of air per minute. The lowest value is of tidal volume.

2. Inspiratory Reserve Volume (IRV):

It is the extra amount of air that can be inspired forcibly after a normal inspiration. Thus it is forced inspiration. It is about 2500 to 3000 ml. of air.

3. Expiratory Reserve Volume (ERV):

It is the extra amount of air that can be expired forcibly after a normal expiration. Thus it is forced expiration. It is about 1000 to 1100 ml.

4. Residual Volume (RV):

It is the volume of air which remains still in the lung after the most forceful expiration. It is about 1100 mL to 1200 ml.

Respiratory or Pulmonary Capacities (Lung Capacities):

1. Inspiratory Capacity (IC):

It is the total volume of air a person can inspire after a normal expiration. It includes tidal volume and inspiratory reserve volume (TV + IRV).

2. Expiratory Capacity (EC):

It is the total volume of air a person can expire after a normal inspiration. This includes tidal volume and expiratory reserve volume (TV + ERV).

3. Functional Residual Capacity (FRC):

Volume of air that will remain in the lungs after a normal expiration is called functional residual capacity. This includes residual volume and the expiratory reserve volume (RV + ERV).

4. Vital Capacity (VC):

The maximum volume of air a person can breathe in after a forced expiration or the maximum volume of air a person can breathe out after a forced inspiration is called vital capacity. This includes tidal volume, inspiratory reserve volume and expiratory reserve volume (TV + IRV + ERV).

In fact total lung capacity minus residual volume is called vital capacity. VC varies from 3400 mL to 4800 ml. depending upon age, sex and height of the individual. The vital capacity is higher in athletes, mountain dwellers than in plain dwellers, in men than women and in the young ones than in the old persons.

5. Total Lung Capacity (TLC):

It is the total volume of air present in the lungs and the respiratory passage after a maximum inspiration. It includes vital capacity and the residual volume (VC + RV). All pulmonary volumes and capacities are about 20 to 25 per cent less in women than in men and they are greater in tall persons and athletes than in small and asthenic (slight build) people.

Respiratory Quotient (RQ):

Respiratory quotient is the ratio of the volume of carbon dioxide produced to the volume or oxygen consumed over a period of time in respiration.

RQ = Volume of CO₂ evolved/Volume of O₂ absorbed

Respiratory quotient varies with different foods utilized in respiration. For glucose, RQ (RQ 6CO₂/6O₂ – 1), for fats it is about 0.7, for proteins it is about 0.9 and for organic acids it is about 1.3 or 1.4.

In anaerobic respiration, there is no consumption of oxygen. Carbon dioxide is produced in most of the cases. Therefore R.Q. is infinity. The respiratory quotient indicates the type of food oxidized in the body of the animal during respiration.

Exchange of Gases:

(A) Exchange of gases between alveoli and blood (Fig. 17.9 & 17.11):

The exchange of gases (i.e., oxygen and carbon dioxide) between lung alveoli and pulmonary capillaries is called external respiration.

It occurs as follows:

The wall of the alveoli is very thin and has rich network of blood capillaries. Due to this, the alveolar wall seems to be a sheet of flowing blood and is called respiratory membrane (= alveolar-capillary membrane).

The respiratory membrane (Fig. 17.10) consists mainly of:

(a) Alveolar epithelium,

(b) Epithelial basement membrane,

(c) A thin interstitial space

(d) Capillary basement membrane and

(e) Capillary endothelium.

All these layers form a mem­brane of 0.2 mm thickness. The respiratory membrane has a limit of gaseous exchange between alveoli and pulmonary blood. It is called diffusing capacity. The diffusing capacity is defined as the volume of gas that diffuses through the membrane per minute for a pressure difference of 1 mm Hg.

It is further dependent on the solubility of the diffusing gases. In other words at the particular pressure difference, the diffusion of carbon dioxide is 20 times faster than oxygen and that of oxygen is two times faster than nitrogen.

The partial pressure of oxygen (PO₂) in the alveoli is higher (104 mm Hg) than that in the deoxygenated blood in the capillaries of the pulmonary arteries (95 mm Hg.). As the gases diffuse from a higher to a lower concentration, the movement of oxygen is from the alveoli to the blood. The reverse is the case in relation to carbon dioxide.

The partial pressure of carbon dioxide (PCO₂) is higher in deoxygenated blood (45 mm Hg) than in alveoli (40 mm Hg), therefore, carbon dioxide passes from the blood to the alveoli. The partial pressure of nitrogen (PN₂) is the same (537 mm Hg) in the alveoli as it is in the blood. This condition is maintained because nitrogen as a gas is not used up by the body.

(B) Exchange of gases between blood and tissue cells (Fig. 17.11):

The exchange of gases (i.e., oxygen and carbon dioxide) between tissue blood capillaries and tissue cells is called internal respiration. The partial pressure of oxygen is higher (95mm Hg) than that of the body cells (40 mm Hg) and the partial pressure of carbon dioxide is lesser (40 mm Hg) than that of the body cells (45 mm Hg).

Therefore, oxygen diffuses from the capillary blood to the body cells through tissue fluid and carbon dioxide diffuses from the body cells of the capillary blood via tissue fluid. Now the blood becomes deoxygenated. The latter is carried to the heart and hence to the lungs.

Transport of Gases (Fig. 17.11):

Blood transports oxygen and carbon dioxide.

(A) Transport of Oxygen in the Blood:

Blood carries oxygen from the lungs to the heart and from the heart to various body parts.

Oxygen is transported in the following manners:

(i) As dissolved gas. About 3 per cent of oxygen in the blood is dissolved in the plasma which carries oxygen to the body cells.

(ii) As oxyhaemoglobin. About 97% of oxygen is carried in combination with hae­moglobin of the erythrocytes.

Haemoglobin (Hb) consists of a protein portion called globin and a pigment portion called heme. The heme portion contains four atoms of iron, each capable of combining with a molecule of oxygen. Four molecules of oxygen bind one molecule of haemoglobin. Oxygen and haemoglobin combine in an easily reversible reaction to form oxyhaemoglobin.

Under the high partial pressure, oxygen easily binds with haemoglobin in the pulmonary (lung) blood capillaries. When this oxygenated blood reaches the different tissues, the partial pressure of oxygen declines and the bonds holding oxygen to haemoglobin become unstable. As a result, oxygen is released from the blood capillaries.

A normal person has about 15 grams of haemoglobin per 100 ml of blood. 1 gram of haemoglobin binds about 1.34 ml of O₂. Thus on an average 100 ml of blood carries about 20 ml (19.4 ml exactly) of O₂ Hence under normal conditions, about 5 ml of oxygen is transported to tissues by 100 ml. of blood.

During exercise or under strenuous conditions, the muscle cells consume more oxygen. The partial pressure of oxygen in the tissue falls, as a result of which, the blood at the tissue level has only 4.4 ml of oxygen/100 ml of blood. Thus about 15 ml. of oxygen is transported by haemoglobin during exercise.

Oxygen-haemoglobin Dissociation curve (=Oxygen Dissociation Curve):

The amount of oxygen that can bind with haemoglobin is determined by oxygen tension. This is expressed as a partial pressure of oxygen (PO₂). The percentage of haemoglobin that is bound with O₂ is called percentage saturation of haemoglobin.

The rela­tionship between the partial pressure of oxygen (PO₂) and percentage satu­ration of the haemoglobin with oxy­gen (O₂) is graphically illustrated by a curve called oxygen haemoglobin dissociation curve (also called oxy­gen dissociation curve).

Normal Oxygen Haemoglobin Dissociation Curve:

Under normal conditions, the oxygen haemoglobin dissociation curve is sigmoid shaped or ‘S’ shaped (Fig. 17.12). The lower part of the curve indicates dissocia­tion of oxygen from haemoglobin. The upper part of the curve indicates the acceptance of oxygen by haemoglo­bin.

When the partial pressure of oxy­gen is 25 mm Hg the haemoglobin gets saturated to about 50%. It means the blood contains 50% oxygen. The partial pressure at which the haemoglobin saturation is 50% is called P₅₀. At 40 mm Hg of partial pressure of oxygen, the saturation is 75%. It becomes 95% when the partial pressure of oxygen is 100 mm Hg.

Factors Affecting Oxygen Haemoglobin Dissociation Curve:

The oxygen haemoglo­bin dissociation curve is shifted either to right or left by various factors.

Shift to Right:

Shift to right indicates dissociation of oxygen from haemoglobin.

The oxygen-haemoglobin curve is shifted to right in the following conditions:

(1) Decrease in partial pressure of oxygen.

(2) Increase in partial pressure of carbon dioxide (Bohr effect).

(3) Increase in hydrogen ion concentration and decrease in pH (acidity).

(4) Increased body temperature.

(5) Excess of 2, 3 diphosphoglycerate (DPG). DPG is a by-product in glyco­lysis. It is present in RBCs.

Shift to Left:

Shift to left indicates acceptance (association) of oxygen by haemoglobin.

The oxygen haemoglobin dissociation curve is shifted to left in the following conditions:

(1) In the foetal blood, because, foetal haemoglobin has more affinity for oxygen than the adult haemoglobin.

(2) In the low temperature and high pH.

Bohr Effect:

An increase in carbon dioxide in the blood causes oxygen to be displaced from the haemoglobin. This is Bohr effect. This is an important factor increasing oxygen transport. It is named after the Danish physiologist Christian Bohr (1855-1911).

The pres­ence of carbon dioxide decreases the affinity of haemoglobin for oxygen and increases release of oxygen to the tissues. The pH of the blood falls as its CO₂ content increases so that when the PCO₂ rises the curve shifts to the right and the P₅₀ rises. As stated in the oxygen haemoglobin dissociation curve, the partial pressure at which the haemoglobin satu­ration is 50% is called P₅₀.

Factors Influencing Bohr Effect:

All the factors, which shift the oxygen haemoglobin dissociation curve to the right (mentioned above) increase the Bohr effect.

(B) Transport of Carbon dioxide:

In the oxidation of food, carbon dioxide, water and energy are produced. Carbon dioxide in gaseous form diffuses out of the cells into the capillaries, where it is transported in three ways.

(i) Transport of CO₂ in Dissolved Form:

Because of its high solubility, about 7 percent carbon dioxide gets dissolved in the blood plasma and is carried in solution to the lungs. Thus as compared to O₂, a much larger volume of CO₂ is transported in dissolved form. This is about 7% of all the CO₂ transported by blood from tissues to the lungs.

(ii) Transport of CO₂ as Bio-carbonate:

The largest fraction of carbon dioxide (about 70%) is converted to bicarbonate ions (HCO₃^_) and transported in plasma. When carbon dioxide diffuses into the RBCs, it combines with water, forming carbonic acid (H₂CO₃). H₂CO₃ is unstable and quickly dissociates into bicarbonate ions and hydrogen ions:

Although this reaction also occurs in plasma, it is thousands of times faster in erythro­cytes because they (and not plasma) contain carbonic anhydrase (karbon’ ik an-hi’ dras), an enzyme that reversibly catalyzes the conversion of carbon dioxide and water to carbonic acid.

Hydrogen ions released during the reaction bind to hemoglobin, triggering the Bohr effect; thus, no oxygen release is enhanced by carbon dioxide loading (as HCO₃~). Because of the buffering effect of hemoglobin, the liberated hydrogen ions cause little change in pH under resting conditions. Hence, blood becomes only slightly more acidic (the pH declines from 7.4 to 7.34) as it passes through the tissues.

Chloride Shift (= Hamburger’s Phenomenon):

Since the rise in the HCO₃^_ content of red cells is much greater than that in plasma as the blood passes through the capillaries, about 70% of the HCO₃ formed in the red cells enters the plasma.

The excess HCO₃ leaves the red cells in exchange for Cl^– (Fig. 17.13). This exchange is called the chloride shift. Because of it, the Cl^– content of the red cells in venous blood is, therefore, significantly greater than in arterial blood. The chloride shift occurs rapidly and is essentially complete in 1 second. Consequently, the red cells take up water and increase in size.

(iii) Transport of CO₂ as Carbaminohaemoglobin. About 23 per cent CO₂ is carried by haemoglobin as carbaminohaemoglobin. In addition to reacting with water, carbon dioxide also reacts directly with amine radicals (NH₂) of haemoglobin to form an unstable compound carbaminohaemoglobin (Hb CO₂). This is reversible reaction.

CO₂ + Hb (Haemoglobin) ⇋ HbCO₂ (Carbominohaemoglobin).

Every 100 mL of deoxygenated blood delivers approximately 4 mL of CO₂ to the alveoli.

Release of Carbon Dioxide in the Alveoli of Lung:

The pulmonary arteries carry deoxygenated blood to the lungs. This blood contains carbon dioxide as dissolved in blood plasma, as bicarbonate ions and as carbaminohaemoglobin.

(i) CO₂ is less soluble in arterial blood than in venous blood. Therefore, some CO₂ diffuses from the blood plasma of the lung capillaries into the lung alveoli.

(ii) For the release of CO₂ from the bio-carbonate, a series of reverse reactions takes place. When the haemoglobin of the lung blood capillaries takes up O₂, the H⁺ is released from it.

Then, the Сl^– and HCO₃^– ions are released from KC1 in blood, and NaHCO₃ in the RBC respectively. After this HCO₃^– reacts with H⁺ to form H₂CO₃^_ As a result H₂CO₃ splits into carbon dioxide and water in the presence of carbonic anhydrase enzyme and CO₂ is released into the alveoli of the lungs.

(iii) High PO₂ in the lung blood capillaries due to oxygenation of haemoglobin favours separation of CO₂ from carbaminohaemoglobin.

Haldane Effect:

It was proposed by J.S. Haldane, a Scotish physiologist, 1860-1936. Binding of oxygen with haemoglobin tends to displace carbon dioxide from the blood. This is called Haldane effect. It is far more important in promoting carbon dioxide than is the Bohr effect which promotes oxygen transport. The Haldane effect encourages CO₂ exchange in both the tissues and lungs.

It is quantitatively far more important in promoting CO₂ transport than the Bohr effect in promoting O₂ transport. Thus, Haldane effect and Bohr effect complement each other. In the tissues addition of CO₂ to the blood facilitates unloading of O₂ by Bohr effect. In turn, O₂ unloading favours uptake of CO₂ by Haldane effect.

Cellular Respiration:

As the name indicates it occurs inside the cells. It takes place in all types of living cells. Respiratory substrates are those organic substances which can be catabolized to liberate energy inside the living cells. The most common respiratory substrate is glucose. Fats are used as respiratory substrates by a number of organisms because they contain more energy as compared to carbohydrates.

However, fats are not directly used in respiration. Instead they are first broken to intermediates common to glucose oxidation, viz., acetyl CoA, glyceraldehyde phosphate. Proteins are used rarely in respiration.

Proteins are hydrolysed to form amino acids from which organic acids are produced through deamination. Organic acids enter Krebs cycle, e.g., aspartic acid, glutamic acid. At other times, proteins are employed as reparatory substrates under starvation conditions only when carbohydrates and fats be­come unavailable.

As stated earlier respiration is of two main types: anaerobic and aerobic. In anaerobic respiration food is oxidised without using molecular oxygen. Less energy is produced in anaerobic respiration. In aerobic respiration organic food is completely oxidised with the help of oxygen into carbon dioxide and water. 686 Kcal of energy is also liberated per mole of glucose.

Aerobic respiration consists of four steps:

(i) Glycolysis:

It is a first step which is common to both anaerobic and aerobic modes of respiration. It occurs in cytoplasm and does not require oxygen. Glycolysis consumes ATP molecules. No carbon dioxide is released in glycolysis. Water and ATP molecules are released,

(ii) Krebs Cycle:

It is the second step in respiration. It operates inside mitochondria and uses oxygen and, therefore, occurs only in aerobic respiration. It does not consume ATP but liberates ATP molecules. Water and carbon dioxide are produced during Krebs cycle,

(iii) Electron Transport Chain (ETC):

It is a series of coenzymes and cytochromes that take part in the passage of electrons from a chemical to its ultimate acceptor. The enzymes involved in electron transport chain are components of the inner mitochondrial membrane. Thus ETC occurs in mitochondria. Oxygen is the ultimate acceptor of electrons.

It becomes reactive and combines with protons to form metabolic water 2H⁺ + O^2- → 2H₂O]. (iv) Oxidative Phosphorylation. It is the synthesis of energy rich ATP molecules with the help of energy liberated during oxidation of reduced co-enzymes (NADH, FADH₂) produced in respiration.

The enzyme required for this synthe­sis is called ATP synthase. ATP synthase is located in F₁ or head piece of F₁ or elementary particles. The particles are present in the inner mitochondrial membrane.

The net gain from complete oxidation of a molecule of glucose in muscle and nerve cells is 36 ATP molecules. However, in aerobic prokaryotes, heart, liver and kidneys, 38 ATP molecules are produced per glucose molecule oxidised.

Artificial Respiration:

Conditions when artificial respiration is required. It is required when persons have stopped breathing because of (i) drowning, (ii) electric shock, (iii) accidents, (iv) gas poisoning, or (v) anesthesia.

Methods of Artificial Respiration:

Two methods of artificial respiration are (i) manual methods and (ii) mechanical methods.

1. Manual Method:

Manual method of artificial respiration can be applied quickly without waiting for the availability of any mechanical aids. The mouth to mouth breathing is very common.

2. Mechanical Methods:

During the respiratory failure due to paralysis of respiratory muscles or some other cause, the manual method of respira­tion is not useful because in these conditions, the resuscitation should be given for a longer period.

This can be done only by means of mechanical methods which are of two types:

(i) Drinker’s method:

The machine used in this method is called iron lung or Drinker’s respiration or tank respiration invented by Philips Drinker in 1929. By using the tank respirator, the patient can survive for a longer time, even up to the period of one year till the natural respiratory functions are restored

(ii) Ventilation Method:

A rubber tube is introduced into the trachea of the patient through the mouth. When air is pumped, inflation of lungs occurs, when it is stopped expiration occurs, and the cycle is repeated. The apparatus used for this is called ventilator.

Exercise and Respiration:

On the basis of severity, the exercise is classified into three types:

1. Severe Exercise:

It includes strenuous muscular activity but the severity can be maintained only for short duration. Fast running for a distance of 100 or 4(X) metres is the best example of this type of exercise. Complete exhaustion occurs at the end of severe exercise.

2. Moderate Exercise:

This type of exercise can be performed for a longer period. The examples of this type of exercise are fast walking and slow running. Exhaustion does not occur at the end of moderate exercise.

3. Mild Exercise:

This is very simple form of exercise like slow walking. So exhaustion does not occur at the end of mild exercise.

After a period of severe muscular exercise the amount of oxygen consumed is enor­mously more. The oxygen required is more than quantity available to muscles.

This much of oxygen is utilized for reversal of the following metabolic processes:

(i) Reformation of glucose from lactic acid accumulated during exercise,

(ii) Re-synthesis of ATP and creatine phosphate, and

(iii) Restoration of amount of oxygen dissociated from haemoglobin and myoglobin.

Regulation of Respiration (= Regulation of Breathing):

Respiration is under both nervous and chemical regulation.

1. Neural Regulation:

Normal quiet breathing occurs involuntarily. Adult human beings breathe 12 to 14 times per minute, but human infants breathe about 44 times per minute. In each breathe in human beings, inspiration accounts for about two and expiration for about three seconds.

The respiratory centre is composed of groups of neurons located in the medulla oblongata and pons varolii. Hence respiratory centre is divided into the medullary res­piratory centres and pons respiratory centres.

Medullary Respiratory Centres:

(i) Dorsal Respiratory Group (DRG):

It is located in dor­sal portion of the medulla oblongata. The dorsal respiratory group mainly causes inspiration.

(ii) Ventral Respiratory Group (VRG):

It is located in the ventrolateral part of the me­dulla oblongata. The ventral respiratory group can cause ei­ther inspiration or expiration, depending upon which neurons in the group are stimulated.

Pons Respiratory Centres:

(i) Pneumotaxic centre:

It is located in the dorsal part of pons varolii. The function of the pneu­motaxic centre is primarily to limit inspiration.

(ii) Apneustic Centre:

There is another strange centre called the apneustic centre, located in the lower part of the pons varolii. The function of this centre is not well understood but it is thought that it operates in association with the pneumotaxic centre to control the depth of inspiration. Apneustic centre is considered hypothetical.

2. Chemical Regulation:

The largest number of chemoreceptors is located in the carotid bodies. However, a sizeable number of chemoreceptors are in the aortic bodies.

The carotid bodies are lo­cated bilaterally in the bifurcation of the common carotid arteries and their afferent nerve fibres pass through glossopharyngeal cranial nerves and hence to the dorsal respiratory area of the medulla oblongata. The aortic bodies are located along the arch of the aorta and their afferent nerve fibres pass through the vagi (sing, vagus), cranial nerves and hence to the dorsal respiratory area.

Excess carbon dioxide or hydro­gen ions mainly stimulate the respiratory centre of the brain and increase the inspiratory and expiratory signals to the respiratory muscles.

Increased CO₂ lowers the pH resulting acido­sis. However, oxygen does not have a significant direct effect on the respiratory centre of the brain in controlling respiration. Thus carotid and aortic bodies send chemical signals to the respiratory centre in the medulla oblongata.

Functions of Respiration:

1. Energy Production:

The energy required for daily metabolic activities is derived from the oxidation of food going on continuously in the body.

2. Excretion:

Respiration excretes carbon dioxide, water, etc.

3. Maintenance of Acid-base Balance:

Elimination of CO₂ maintains the acid-base balance in the body.

4. Maintenance of Temperature:

A large amount of heat is expelled out during expi­ration which maintains the body temperature.

5. Return of Blood and lymph:

During inspiration the intra-abdominal pressure in­creases and the intrathoracic pressure decreases. This results the return of blood and lymph from the abdomen to the thorax.

Mountain Sickness:

Definition:

Mountain sickness is the condition characterized by the ill effect of hypoxia (shortage of oxygen) in the tissues at high altitude. This is commonly developed in persons going to high altitude for the first time.

Symptoms:

In mountain sickness, the symptoms occur mostly in digestive system, respiratory system and nervous system.

1. Digestive System:

Loss of appetite, nausea and vomiting occur because of expan­sion of gases in the gastrointestinal tract.

2. Respiratory System:

Breathlessness occurs because of pulmonary oedema. Pulmo­nary oedema develops because of the response of the pulmonary blood vessels to hypoxia.

3. Neural System:

The symptoms are headache, depression, disorientation, irritability, lack of sleep, weakness and fatigue. These symptoms are developed because of cerebral oedema.

Treatment:

The symptoms of mountain sickness disappear by breathing oxygen.