NEET Biology Breathing and Exchange of Gases Notes

Breathing and exchange of gases is a key NEET Biology chapter in Class 11 that explains external and internal respiration, the human respiratory system, the mechanism of breathing, the regulation of breathing rate, the air volumes measured by a spirometer, and the transport of oxygen and carbon dioxide in blood. This NEET Biology notes page on Breathing and Exchange of Gases covers the respiratory structures of animals and humans, the oxyhaemoglobin dissociation curve, Bohr's effect, Haldane's effect, the chloride shift, respiratory disorders and important terms.

Chapter 17: Breathing and Exchange of Gases

While going through the chapter pay special attention to the following:

Terms and Definitions: 1. Tidal volume 2. Inspiratory capacity 3. Vital capacity 4. Functional residual capacity 5. Hypercapnia 6. Hering-Breuer reflex 7. Asphyxia 8. Emphysema 9. Byssinosis 10. p50-value

Laws and effects: 1. Bohr's law 2. Haldane's law 3. Chloride shift

Differences: 1. Larynx and Syrinx 2. Carboxyhaemoglobin and Carbaminohaemoglobin 3. Bronchi and bronchioles 4. Oxygenation and Oxidation of haemoglobin

Relationship: 1. pH of blood with the shifting of oxyhaemoglobin dissociation curve 2. p50 value and affinity of haemoglobin with oxygen.

At a Glance: External Respiration; Internal Respiration; Survey of respiratory structures in animals; Respiratory System in Human (Nasal passage, Pharynx, Larynx, Trachea, Bronchi and Bronchioles, Alveoli, Lungs, Diaphragm, Ribs, Breathing rate, Factors affecting breathing rate, Control of breathing, Composition of air, Air volumes, Transportation of gases); Important terms and disorders of respiratory system.

Topics Covered

1. Respiration and its two parts: external and internal respiration
2. Survey of respiratory structures in animals
3. Respiratory system in humans
4. Mechanism of breathing: inspiration and expiration
5. Breathing rate and factors affecting it
6. Control of breathing
7. Composition of air and air volumes
8. Transportation of gases
9. Important terms and disorders of the respiratory system

RESPIRATION AND ITS PARTS

Respiration is a physicochemical process involving the exchange of gases and the oxidation of food material to release energy. It is a catabolic process and includes two parts.

1. External Respiration
External respiration includes the intake of oxygen and the removal of carbon dioxide from the body. It is also known as ventilation or breathing.
In aquatic animals, oxygen from the water enters through gills, and carbon dioxide leaves through the gills. In terrestrial animals, oxygen from the atmosphere enters through lungs, and carbon dioxide leaves through the lungs. Gills are found in fishes, and lungs are found in mammals.

2. Internal Respiration
Internal respiration includes the gaseous exchange between the cells and the fluid medium, the utilisation of oxygen by the cells, and the production of carbon dioxide by the cells.

SURVEY OF RESPIRATORY STRUCTURES IN ANIMALS

1. Protozoans, porifers, coelenterates and helminthes have no specialised respiratory structures. Gases are exchanged through the general body surface.
2. In annelids, respiration is either cutaneous (for example, earthworm and Nereis) or branchial.
3. In arthropods, there are various types of respiratory structures: gills (in crustaceans), book lungs (in arachnids) and tracheal tubes (in insects).
4. In prawn, the gills (8 pairs) are of 3 types: (i) podobranchs, 1 pair; (ii) arthrobranchs, 2 pairs; (iii) pleurobranchs, 5 pairs. All the gills are of the phyllobranch type.
5. In cockroach, there are three pairs of longitudinal tracheal tubes. Each lateral tube contains 10 spiracles, so there are 10 pairs of spiracles in total.
6. In molluscs, the respiratory structures are mainly gills.
7. In echinoderms, the respiratory structures are the gills, body wall, respiratory tree and tube feet (which are primarily locomotory structures).
8. In protochordates and cyclostomes, the respiratory structures are gills.
9. In fishes, the gills are the main respiratory structures. In lungfishes (Dipnoi), both gills and lungs are well developed.
10. In amphibians, the respiratory structures are either gills (branchial respiration) or lungs (pulmonary respiration). In frog, respiration is cutaneous, bucco-pharyngeal as well as pulmonary.
11. In reptiles, birds and mammals, respiration is pulmonary.
12. The lungs in birds are non-elastic and contain air sacs. Residual air is absent in such cases.

RESPIRATORY SYSTEM IN HUMANS

In humans, the respiratory structures are the lungs, which are endodermal in origin. The respiratory system contains the following important components.

Nasal passage
It contains hair for filtering out dust particles. Bowman's glands secrete mucus. The turbinate bones warm up the air. The sensory lining of the nasal passage is called the Schneiderian membrane.

Pharynx
It is a common passage for the gut and the respiratory tract. The openings of the internal nostrils in the pharynx are called choanae. The opening between the pharynx and the larynx is called the glottis, which is closed by the epiglottis during swallowing of food. The opening between the pharynx and the oesophagus (gut) is called the gullet. The lateral sides of the pharynx contain the openings of the Eustachian tubes.

[Figure: Respiratory system of human showing nasal cavity, nose hairs, paranasal sinuses, pharynx, epiglottis, oesophagus, larynx, trachea, bronchi, right and left lungs, pulmonary vessels, heart, ribs, diaphragm, intercostal muscles, pleural membrane, thyroid, vocal cords and arytenoid cartilage]

Larynx
The larynx is commonly called the voice box or sound box. In birds, the sound box is present at the base of the trachea and is called the syrinx.
In humans, it contains 9 cartilages, three paired and three unpaired. The unpaired cartilages are thyroid, cricoid and epiglottis. The paired cartilages are arytenoid, corniculate and cuneiform.
The thyroid cartilage is collar-shaped, and in males it forms the Adam's apple. (In frog, the thyroid cartilage is absent.)
The cricoid cartilage is ring-shaped (complete).
The arytenoid cartilage is pyramidal. The vocal cords (1 pair) are attached to this cartilage.
The thyroid, cricoid and arytenoid cartilages are hyaline cartilages, while the epiglottis is an elastic cartilage.

[Figure: Inner view (diagrammatic) of the pharynx showing the opening of the Eustachian tube, epiglottis, glottis and gullet]

Trachea
The trachea is commonly called the wind pipe. It contains C-shaped rings of hyaline cartilage, which prevent it from collapsing. These cartilaginous rings are dorsally incomplete. The tracheal lining is pseudostratified epithelium. (In frog, the trachea is reduced due to the absence of a neck. In giraffe, it is very long. In penguins, it is paired.)

Bronchi and bronchioles
The trachea divides into two principal (main) bronchi. These further divide into secondary bronchi and tertiary bronchi. Cartilaginous rings are present in all bronchi.
The bronchi divide into terminal bronchioles, which in turn divide into respiratory bronchioles. The bronchioles do not have cartilaginous rings.
The respiratory bronchioles divide into alveolar ducts, which form alveolar sacs. Each alveolar sac is surrounded by alveoli.

Alveoli
The inner lining of the alveoli is simple squamous epithelium. The number of alveoli in both lungs is 300 million (3 × 10⁸). The exchange of gases occurs mainly through the alveoli.

[Figure: Bronchial tree showing the principal bronchus, cartilaginous rings, secondary and tertiary bronchi, terminal bronchiole, respiratory bronchiole, alveolar duct, alveolar sac and alveoli, with an enlarged view of the respiratory membrane made of alveolar epithelium, basement membrane and endothelium of the blood vessel]

Lungs
The lungs in humans are solid and spongy (in frog, they are hollow). They are present in the thoracic cavity. The left lung has 2 lobes, while the right lung has 3 lobes.
Each lung is surrounded by two membranes called the pleura. The outer pleural membrane is parietal or somatic, while the inner pleural membrane is visceral or splanchnic. The cavity between the two pleura (pleural cavity) is filled with a fluid (perilymph), which reduces friction during breathing.

Diaphragm
The diaphragm is a muscular partition between the thorax and the abdomen of all mammals. It is dome-shaped at rest. It is responsible for about 75% of the breathing volume during quiet respiration.

Ribs
The number of ribs in humans is 12 pairs. Between the ribs are the intercostal muscles, which have two sets: external intercostal muscles and internal intercostal muscles. These muscles move the ribs during breathing.

[Figure: Intercostal muscles showing the ribs, costal cartilage, sternum, internal intercostal muscles and external intercostal muscles]

MECHANISM OF BREATHING

The breathing process includes two phases: 1. inspiration and 2. expiration.

1. Inspiration
Inspiration is an active process.
The external intercostal muscles contract, and the ribs move outward and upward to increase the volume of the thoracic cavity.
The diaphragm muscles contract, and the shape of the diaphragm changes from dome-shaped to flat.
With this contraction, the lower surface of the lungs is also pulled downward.
The lungs expand, and air is drawn into them.
The lungs in humans, like other mammals, are negative pressure type. Negative pressure means that the pressure in the lungs is reduced and becomes lower than the atmospheric pressure, due to which air is drawn into the lungs. In frog, the lungs are positive pressure type. Positive pressure means that the pressure outside the lungs (for example, in the bucco-pharynx of the frog) increases, and air is forced into the lungs.

2. Expiration
Quiet expiration is a passive process.
The diaphragm becomes dome-shaped again, and the external intercostal muscles relax.
In forced expiration, the internal intercostal muscles contract. This is an active process. The abdominal muscles also contract to decrease the thoracic volume.
Forced expiration is a voluntary process and is, therefore, controlled by the cerebrum.
(For breathing in frog, refer to 'Frog' in Chapter 7.)

BREATHING RATE

The normal breathing rate in an adult is 12 to 15 per minute. In a newborn child, the breathing rate is about 40 per minute.

Factors affecting breathing rate
1. Size: Breathing rate increases with decrease in size (B.R. ∝ 1/size).
2. Sex: Breathing rate is higher in males.
3. Altitude: Breathing rate is directly proportional to altitude.
4. Exercise: Breathing rate is directly proportional to exercise.
5. Temperature: Breathing rate is directly proportional to temperature.
6. Pressure: Breathing rate is inversely proportional to pressure (B.R. ∝ 1/pressure).
7. Carbon dioxide concentration: Breathing rate is directly proportional to the CO₂ concentration.

Check Point
1. Fill in the blanks with suitable words or statements.
(i) ............... muscles contract to move the ribs ............... in inspiration.
(ii) The basic respiratory rhythm is maintained by the neurons of the ............... group.
(iii) Breathing rate ............... with the increase in body size.
(iv) The left and right lungs have ............... and ............... lobes respectively in humans.
(v) The number of alveoli in human lungs is ............... .
(vi) ............... connects the middle ear to the pharynx.
(vii) The trachea divides into right and left bronchi at the ............... thoracic vertebra.
(viii) Breathing of air volume is estimated by ............... .
(ix) The function of the respiratory rhythm is moderated by ............... .
(x) ............... glands secrete mucus in the nasal passage.

CONTROL OF BREATHING

Breathing is controlled by two mechanisms: I. nervous control and II. chemical control.

I. Nervous control
Nervous control can be voluntary (through the cerebral cortex) or involuntary (through the medulla oblongata or pons).
Involuntary nervous control is carried out by three respiratory groups of neurons:
(a) Dorsal respiratory group (DRG): It is present in the dorsal part of the medulla oblongata and generates the basic respiratory rhythm. Nervous signals from this group are transmitted to the diaphragm, which is the primary inspiratory muscle.
(b) Ventral respiratory group: It is located on the ventro-lateral side of the DRG in the medulla oblongata. It does not play any role in the basic respiratory rhythm and remains inactive during quiet breathing. When the requirement of oxygen increases, signals from this part are transmitted to the inspiratory and expiratory muscles for faster breathing.
(c) Pneumotaxic centre: It is located dorsally in the pons varolii. It is the switch-off point for breathing.

[Figure: Medulla oblongata showing the dorsal respiratory group, ventral respiratory group and the fourth ventricle]

Hering-Breuer reflex
The Hering-Breuer inflation reflex is an increase in the duration of expiration produced by steady inflation of the lungs, due to stimulation of stretch receptors present in the walls of the bronchi and bronchioles. When the lungs become over-inflated (over-stretched), signals from the stretch receptors are transmitted to the DRG, and by a feedback mechanism it switches off inspiration. The same occurs in the pneumotaxic centre. In humans, this reflex is activated when the tidal volume increases to more than 1500 ml. It is a protective device that prevents excess inflation of the lungs.
The Hering-Breuer deflation reflex is a decrease in the duration of expiration produced by marked deflation of the lungs. The stimuli in both cases are mediated by the vagus nerve.

II. Chemical control
A rise in pCO₂ or H⁺ concentration of arterial blood, or a drop in pO₂, increases the rate of respiration by stimulating the activity of neurons in the medulla.
The chemoreceptors are located mainly in the aortic and carotid bodies. There are usually two aortic bodies, present near the arch of the systemic aorta. Each carotid body (two in total) is situated near the bifurcation of the carotid artery. These receptors are stimulated when the arterial pO₂ is low. (These receptors are not stimulated in anaemia and CO poisoning, since in such conditions the amount of dissolved oxygen reaching the receptors remains the same.)
Chemoreceptors are also present in the medulla. These receptors are stimulated by CO₂ that diffuses into the cerebrospinal fluid (CSF) through the blood-brain barrier. This CO₂ is hydrated to form H₂CO₃, which dissociates to increase the concentration of H⁺. The medullary receptors are sensitive to this H⁺. (More CO₂ produces more H⁺ ions.)
Remember that the breathing or ventilation rate is doubled when the concentration of oxygen decreases by 75%. However, an increase of even 0.25% in CO₂ can double the breathing rate.

COMPOSITION OF AIR

Dry air has the following composition.

Nitrogen (N₂): 78%, partial pressure about 600 mm of Hg
Oxygen (O₂): 21%, partial pressure about 160 mm of Hg
Carbon dioxide (CO₂): 0.03 to 0.04%, partial pressure about 0.3 mm of Hg
Inert gases: 0.9%

AIR VOLUMES

Respiratory air volumes are measured with the help of a spirometer.

1. Tidal volume (TV) = 500 ml: The amount of air exchanged during each quiet (normal) breath.
2. Inspiratory reserve volume (IRV) or complementary air = 2.5 to 3 litres: The maximum amount of air that can be breathed in after the tidal volume.
3. Inspiratory capacity (IC) = 3 to 3.5 litres: The maximum amount of air that can be breathed in or inspired.
Inspiratory capacity = TV + IRV
4. Expiratory reserve volume (ERV) or supplementary air = 1 to 1.2 litres: The maximum amount of air that can be breathed out after the tidal volume, when inspiration is normal.
5. Expiratory capacity (EC) = 1.5 litres: The total amount of air that can be breathed out after normal inspiration.
Expiratory capacity = TV + ERV
6. Vital capacity (VC) = 3.5 to 5 litres: The maximum amount of air that can be breathed out after the deepest inspiration.
Vital capacity = TV + IRV + ERV
7. Residual volume (RV) = 1.5 litres: The amount of air left in the lungs after breathing out the vital capacity.
8. Functional residual capacity (FRC) = 2.5 litres: The amount of air left in the lungs after a normal expiration.
Functional residual capacity = ERV + RV
9. Total lung capacity (TLC) = 5 to 6 litres: The maximum amount of air that can be filled in the lungs.
10. Dead space air = 150 ml: The air present in the trachea, bronchi and bronchioles, which is not used in respiration.
11. Diffusing capacity: The volume of gas that diffuses through the membrane per minute for a pressure difference of 1 mm of Hg. It depends on the partial pressure and the solubility of the gases. The order of diffusing capacity is CO₂ greater than O₂, greater than N₂. The diffusing capacity of CO₂ is 20 times greater than that of O₂.

[Figure: Graph of lung volume (in litres, 0 to 6) against time showing the tidal volume, inspiratory reserve volume, expiratory reserve volume, residual volume, vital capacity and total lung capacity]

Check Point
1. Differentiate between:
(i) The lungs of birds and the lungs of humans.
(ii) Larynx and syrinx.
(iii) Respiratory structures of insects and crustaceans.
(iv) External respiration and internal respiration.
(v) Positive pressure and negative pressure lungs.
(vi) Residual capacity and functional residual capacity.
(vii) Complemental air and supplementary air.
(viii) Diffusing capacity of CO₂ and O₂.

TRANSPORTATION OF GASES

(a) Transportation of oxygen
1. Through plasma (in dissolved form): 2 to 3%
2. Through RBCs (in combination with haemoglobin): 97 to 98%
1 gram of haemoglobin combines with 1.34 ml of oxygen.
100 ml of blood, having about 15 g of haemoglobin, contains 20.1 ml of oxygen bound to Hb at 100% saturation. Since there is slight mixing of venous blood in the pulmonary capillaries, the haemoglobin in arterial blood is only 97% saturated. It contains 19.8 ml of O₂, of which 19.5 ml is bound to Hb and 0.3 ml is in soluble form.
Haemoglobin is customarily written as Hb₄, since one molecule of haemoglobin has 4 sub-units (each sub-unit has 1 haem and 1 polypeptide chain).
Haemoglobin combines with oxygen to form oxyhaemoglobin. This process is called oxygenation, not oxidation, because the iron in the haem remains in the ferrous form.
Hb₄ + 4O₂ → Hb₄O₈
One molecule of haemoglobin, when fully saturated (about 95% saturation), carries 4 molecules of oxygen.
One molecule of myoglobin, which is structurally similar to one of the four sub-units of haemoglobin, can carry only 1 molecule of oxygen.
During one round through the body, about 25% of the oxygen is dissociated from oxyhaemoglobin, and about 70% of the oxygen is still associated with haemoglobin in venous blood. (Remember that 100 ml of arterial blood delivers about 25% of O₂, that is, about 5 ml, to the tissues.)
Oxidation of haemoglobin (in the presence of an oxidising agent) forms methaemoglobin, in which the ferrous form of iron changes into the ferric form. Methaemoglobin is an abnormal form of haemoglobin and is incapable of carrying oxygen.

Oxyhaemoglobin dissociation curve
When the saturation of haemoglobin is plotted against the partial pressure of oxygen, a sigmoid (S-shaped) curve is obtained.
The partial pressure of oxygen at which 50% of haemoglobin is saturated is called the p50 value.
p50 value ∝ 1 / affinity of haemoglobin for O₂
In normal adults, the p50 value is about 27 mm of Hg. In the foetus, the affinity of haemoglobin is higher than in the adult, so the p50 value is lower and the curve lies on the left-hand side.

[Figure: Oxyhaemoglobin dissociation curve. The x-axis shows partial pressure of oxygen (pO₂) and the y-axis shows percentage saturation of haemoglobin. The curve reaches 97% saturation at about 100 mm of Hg, and the p50 value of 27 mm of Hg is marked.]

Partial pressure of oxygen in blood
Arterial blood: pO₂ about 100 mm of Hg, at 97% saturation.
Venous blood: pO₂ about 40 mm of Hg, at 70 to 75% saturation.

Shifting of the oxyhaemoglobin dissociation curve
In all the following conditions, the dissociation curve (sigmoid) shifts to the right, and the affinity of haemoglobin for oxygen decreases, so that more oxygen is released to the tissues.
1. When the concentration of carbon dioxide increases.
2. When the temperature increases.
3. When acidity increases, or pH decreases.
4. When altitude increases.
5. When the amount of 2,3-DPG (diphosphoglycerate, or biphosphoglyceraldehyde, BPG) increases in RBCs.
During exercise, the following changes occur:
(a) A large amount of heat is produced, so the temperature rises.
(b) Due to rapid metabolism, there is a lack of oxygen and an accumulation of CO₂.
(c) The pH decreases due to the accumulation of lactic acid in muscles.
All these changes shift the curve to the right, which increases the unsaturation of haemoglobin. When the conditions are opposite, the curve shifts to the left.

[Figure: (A) Comparison of oxyhaemoglobin dissociation curves in foetal and maternal blood, with the foetal curve on the left. (B) Effect of temperature (20°C, 37°C and 42°C) on the oxyhaemoglobin dissociation curve, with the curve shifting right as temperature rises]

In the foetus, haemoglobin (HbF) has 2 α chains and 2 γ chains, and the γ chain is present in place of the β chain of adults. The γ chain does not bind easily with 2,3-DPG, which facilitates the movement of O₂ from maternal to foetal blood.
Myoglobin has a higher affinity for oxygen than haemoglobin, and therefore it unloads oxygen only when the blood is almost fully deoxygenated. The p50 value for myoglobin is very low, about 5 mm of Hg. The oxygen dissociation curve for myoglobin is a rectangular hyperbola.

Bohr's effect
When the pH of the blood decreases (acidity increases), the p50 value increases and the affinity of haemoglobin for oxygen decreases. This effect is called Bohr's effect.
The total amount of O₂ in arterial blood is 19.8 ml per 100 ml of blood (19.5 ml in combined form and 0.3 ml in dissolved form). In venous blood, the amount of O₂ is 15.2 ml per 100 ml (15.1 ml in combined form and 0.12 ml in dissolved form).

(b) Transportation of carbon dioxide
1. In dissolved form (mainly through plasma): 5 to 10%. The solubility of CO₂ in plasma is 20 times that of oxygen.
2. In the form of carbamino compounds (through RBCs): about 20%. CO₂ combines with reduced haemoglobin to form carbamino-haemoglobin. This is a non-enzymatic and reversible reaction.
3. In the form of bicarbonates (through RBCs and plasma): 70 to 80%. CO₂ enters the RBCs and combines with H₂O in the presence of the enzyme carbonic anhydrase to form carbonic acid, which soon dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃^-). The H⁺ combines with HbO₂ (oxyhaemoglobin) to reduce it to haemoglobinic acid (HHb), and it releases O₂ to the tissues.
The concentration of bicarbonate ions then increases. Some of these bicarbonate ions combine with K⁺ of the RBCs to form KHCO₃, but most of them are released from the RBCs into the plasma. In plasma, HCO₃^- combines with Na⁺ to form NaHCO₃. Maximum CO₂ is carried in plasma in the form of sodium bicarbonate (NaHCO₃).
To compensate for the loss of HCO₃^- from the RBCs, or to maintain electroneutrality, chloride ions diffuse from the plasma into the RBCs. This is called the chloride shift, or Hamburger's reaction.

[Figure: Schematic representation of the chloride shift, and of the formation of carbamino compounds and bicarbonates in RBCs. CO₂ and H₂O form H₂CO₃ with carbonic anhydrase (C.A.); H₂CO₃ gives H⁺ and HCO₃^-; HCO₃^- leaves the RBC as NaHCO₃ in plasma while Cl^- enters the RBC; HbO₂ releases O₂ as H⁺ binds to HHb.]

Connecting concepts
Carbonic anhydrase is the fastest working enzyme in the human body. It can hydrate 36 million molecules of CO₂ per minute.

In the lungs, oxyhaemoglobin is formed first, which releases H⁺ from HHb. These H⁺ combine with HCO₃^- to form carbonic acid. In the presence of carbonic anhydrase, CO₂ is released in the lungs. Thus, the whole process of bicarbonate formation is reversed. (100 ml of venous blood delivers about 4 ml of CO₂ into the lungs.)

Partial pressure of carbon dioxide in blood
Arterial blood: pCO₂ about 400 mm of Hg.
Venous blood: pCO₂ about 46 mm of Hg.

Check Point
1. Write approximate numerical value of the following:
(i) Oxygen in tidal volume.
(ii) Partial pressure of CO₂ in arterial and venous blood.
(iii) Amount of oxygen carried by 100 ml of oxygenated blood.
(iv) Normal breathing rate in human.
(v) Amount of O₂ dissociated in each round of blood through the body.
(vi) Expiratory capacity.
(vii) Partial pressure of oxygen in deoxygenated blood.
(viii) Percentage of carbon dioxide carried in dissolved state through plasma.

The amount of CO₂ in arterial blood is 49 ml per 100 ml of blood (46.4 ml in combined form and 2.6 ml in dissolved form). In venous blood, it is 52.7 ml per 100 ml (49.7 ml in combined form and 3.0 ml in dissolved form).

Hypercapnia
Hypercapnia is the retention of CO₂ in the body. Initially it stimulates respiration, then it causes depression of the central nervous system, and ultimately it leads to coma and death.

Haldane's effect
Deoxyhaemoglobin (Hb) binds with more H⁺ than oxyhaemoglobin (HbO₂) does. Deoxyhaemoglobin, present in venous blood, therefore carries more CO₂ than oxyhaemoglobin. This is called Haldane's effect. (Oxyhaemoglobin, on the other hand, is acidic and releases more H⁺.)

Connecting concepts
Carbon monoxide combines with haemoglobin to form carboxyhaemoglobin. The affinity of carbon monoxide for haemoglobin is 200 to 250 times greater than that of oxygen. Carboxyhaemoglobin is a stable compound and does not release oxygen to the tissues. If this compound is formed in the body due to inhalation of carbon monoxide, oxygen starvation will occur, and this will cause coma and finally death.

Composition of inspiratory and expiratory air

IMPORTANT TERMS AND DISORDERS OF THE RESPIRATORY SYSTEM

1. Eupnea: Normal breathing, at a rate of 12 to 15 per minute.
2. Hypopnea: Decrease in breathing rate.
3. Hyperpnea: Increase in breathing rate.
4. Dyspnea: Distressed breathing, or breathing with difficulty.
5. Hypoxia: Lack of oxygen at the tissue level. This term is preferred over anoxia.
6. Anoxia: No oxygen at the tissue level.
7. Asphyxia: Suffocation due to lack of oxygen and retention of CO₂. It is produced by occlusion of the airway, or by acute hypercapnia and hypoxia developing together.
Asphyxia = Hypoxia + Hypercapnia
8. Cyanosis: Bluish or violet colouration of the skin and mucous membranes. It develops due to the presence of an excessive amount of reduced haemoglobin, or partially deoxygenated blood, or due to an inadequate amount of oxygen in the blood. It develops at low temperature, when the enzyme carbonic anhydrase becomes inactive.
9. Emphysema: Irreversible distension of the respiratory bronchioles, alveolar sacs and alveoli. The walls separating adjacent alveoli break down, and air remains filled even after expiration. It develops mainly due to tobacco (cigarette) smoking.
10. Asthma: An allergic disorder due to inflammation of the bronchi and bronchioles. It causes wheezing. The bronchioles constrict due to spasm of smooth muscles, and expiration becomes difficult.
11. Hay fever: An allergic disorder of the nasal lining. It develops due to hypersensitivity of the lining to pollen grains or other foreign particles. It causes an episode of sneezing that begins following an allergy.
12. Atelectasis or respiratory distress syndrome (hyaline membrane disease): Inability of the lungs to expand at birth. This is mainly due to a deficiency of surfactant. Surfactant, a phospholipid (lecithin), reduces the surface tension at the alveolar surface, which reduces the effort needed to expand the lungs at birth.
13. Blue baby syndrome: The skin of the child becomes bluish (violet). It can develop due to a congenital disorder of the blood vascular system, such as persistent truncus arteriosus or a defect of the ventricular septum. It can also develop due to the formation of methaemoglobin, the oxidised form of haemoglobin, which is incapable of transporting oxygen. Methaemoglobin is formed due to high nitrate contamination (from fertilisers) in groundwater.
14. Occupational lung diseases (diseases of industrial workers): Diseases due to the inhalation of organic and inorganic pollutants are called pneumoconiosis. It includes:
(a) Anthracosis (pneumoconiosis of coal workers): Develops due to inhalation of coal dust in coal-mine workers. Fibrous tissue is deposited in the lungs, causing bronchitis and emphysema.
(b) Silicosis: Due to long exposure to dust containing silicon compounds. Workers in the glass industry, pottery, and gold and copper mines develop progressive fibrosis in the liver.
(c) Asbestosis: Due to inhalation of asbestos fibres, which may result in cancer of the pleura.
(d) Byssinosis: Due to inhalation of fibres of cotton, flax and hemp.
(For pneumonia, tuberculosis (TB), bronchitis, common cold, etc., refer to 'Human Health and Diseases'.)
15. Drowning: In about 10% of drowning cases, the person suffocates in water. When the subject loses the struggle and stops breathing, there is laryngospasm, and death results from asphyxia without any water in the lungs. In other cases, the glottis muscles relax and water enters the lungs. Fresh water is rapidly absorbed, diluting the plasma and causing haemolysis.
16. Decompression sickness: When a diver ascends gradually from a dive, the nitrogen dissolved at high pressure under water diffuses from the tissues into the lungs, and no harmful effects are noticed. But if the ascent is rapid, N₂ escapes from the tissues and blood in the form of bubbles, causing decompression sickness. These bubbles cause severe pain in the joints. In the blood, the bubbles obstruct the arteries to the brain, spinal cord or other parts, causing paralysis, choking (bubbles in the pulmonary artery), myocardial disorder (bubbles in the coronary artery), respiratory failure or damage to the spinal cord. Decompression sickness may also develop in atmospheric air by a sudden change of altitude, for example, a rise from sea level to 8000 m.

CHECK POINT

1. Name the following:
(i) The maximum volume of air that can be breathed in.
(ii) The instrument used for measuring respiratory air volumes.
(iii) The shape of the oxyhaemoglobin dissociation curve for myoglobin.
(iv) The form in which the maximum CO₂ is carried through plasma.
(v) The shift of chloride ions from plasma to RBCs.
(vi) The common term for hypoxia and hypercapnia.
(vii) Inflammation of bronchi and bronchioles due to allergy.
(viii) The centre that moderates the functions of the respiratory rhythm centre.
(ix) The disease due to inhalation of coal dust.
(x) The combination of CO and haemoglobin.