Test: Breathing and Exchange of Gases - Assertion and Reasoning - NEET MCQ

Although atmospheric air contains far more oxygen content than the water (air contains 21% oxygen and water contains 0.5-0.9% oxygen by volume depending on the temperature), still most fish when out of water die of suffocation due to lack of oxygen. When fish is taken out of water the gills stick together thereby reducing the surface area. Reduced surface area lowers gas exchange and so death occurs.

Gills are the main respiratory organs of aquatic animals. Each gill bears rows of comb-like, soft, thin gill-filament, each gillfilament bears many flat, parallel membranelike gill-lamellae. Each gill lamella carries many blood capillaries. Water taken through the mouth, is made to flow from the pharynx in a single direction between the gill lamella. This greatly helps in the gaseous exchange across the lamellar membrane between the capillary blood and the flowing water.

Mammalian skin is impermeable so that water loss through it, is minimised. But mammals need far more oxygen to maintain their high metabolic rates than lower animals; so they need a more extensive respiratory surface. Thus a complex respiratory system has evolved in mammals to meet this need. The mammalian respiratory system consists of the nasal cavity, nasopharynx, larynx, trachea, bronchi, bronchiole and lungs.

As the integument of insect is thick and impermeable to minimise lose of body water, they can not carry out gas exchange through their body surface. To overcome this difficulty they have developed a complex system of air tubes called trachea to reach the air directly near the tissue cells. Each trachea communicates with the exterior through openings in the body wall.

Coughing and sneezing should to take place because these reactions serve to keep the air passages free from foreign matter. Coughing is a reflex action under nervous control. The minute receptors found in the wall of trachea, bronchi, bronchioles and alveoli are highly sensitive to foreign matter (smoke, dust etc.) Like coughing, sneezing is also a reflex action triggered because of irritation to nasal passages. In this, sensory impulses travel from nasal passages to the medulla through trigeminal nerves. The reaction involves the same series of events as in cough reflex, but the air explodes out both through nose and mouth, expelling the foreign matter from nasal passages.

In most animals, tissue oxidation are carried out by aerobic respiration. But sometimes in aerobically respiring animals, anaerobic metabolism take place in certain tissues like skeletal muscles which do not immediately get as much oxygen as in necessary for metabolize glucose during vigorous movements.

There is no air in trachea, it does not collapse due to the presence of C-shaped narrow cartilagenous ring or discs.

Inspiration is the result of muscular contraction. The diaphragm and external intercoastal muscle contracts simultaneously. The lateral thoracic wall moves outward and upward.

Aerobic respiration involves the exchange of respiratory gases at two places in muticellular animals-one between the body surface and surrounding medium, the other between the individual cells and the extracellular fluid. Lungs are involved in the first step of exchange, but not the heart.

The solubility of the gases as well as the thickness of the membranes involved in diffusion are important factors that affect the rate of diffusion. A gradient of partial pressure is present for oxygen from alveoli to blood and blood to tissue. Similarly, a gradient of CO₂ is present in the opposite direction i.e., from tissues to blood and blood to alveoli. The pO₂ in alveoli is higher (104 mm Hg) than that in deoxygenated blood in the capillaries arising from pulmonary arteries (95 mm Hg), therefore, oxygen diffuses from the alveoli to the blood. Similarly, the partial pressure of carbon dioxide (pCO₂) is higher in deoxygenated blood (45 mm Hg) than that in alveoli (40 mm Hg), therefore carbon dioxide passes from the blood to the alveoli. It is further dependent on the solubility of the diffusing gases. As the solubility of CO₂ is 20-25 times higher than that of O₂, the amount of CO₂ that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher compared to that of O₂. Therefore, all the factors in our body are favourable for diffusion of O₂ from alveoli to tissues and that of CO₂ from tissues to alveoli. Alveolus is the primary site of exchange as it has an extensive network of blood capillaries and consists of squamous epithelium. Due to very intimate conact of blood capillaries with the alveoli, the exchange of gases takes place easily.

The cartilage rings are present in the trachea to prevent it from collapsing. This enables the lumen of trachea to stay open during breathing.

Pneumotaxic centre in the pons region of the brain can moderate functions of the respiratory rhythm centre. Neural signals from this centre can reduce the duration of inspiration and thereby alter the respiratory rate. A chemosensitive area is situated adjacent to the rhythm centre to make necessary adjustments in the respiratory process. Receptors associated with aortic arch and carotid artery also can recognize changes in CO₂ and H⁺ concentration and send necessary signals to the rhythm centre for remedial actions. The role of oxygen in the regulation of respiratory rhythm is quite insignificant.

In the walls of bronchi and bronchioles stretch receptors are located and are stimulated by overstretching of the lungs. Nerve impulses are sent along the vagus nerve to inhibit the inspiratory area. The result is that expiration begins. Therefore, it is mainly a protective mechanism for preventing excessive inflation of the lungs.

The lungs are situated in the thoracic chamber which is anatomically an air-tight chamber. The thoracic chamber is formed dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs and on the lower side by the dome-shaped diaphragm. The anatomical setup of lungs in thorax is such that any change in the volume of the thoracic cavity will be reflected in the lung (pulmonary) cavity. Such an arrangement is essential for breathing, as we cannot directly alter the pulmonary volume.

Inside the larynx are present two pairs of vocal cords. One pair is of false vocal cords which has little to do with sound production and the second inner pair is true vocal cords. When air is forced through the larynx, it causes vibration of the true vocal cords and sound is produced. The pitch of sound is determined by the tension on the vocal cords-the greater the tension, the higher the pitch.

During inspiration (breathing in) the pressure of air falls in the thorax because of increase in its volume which is brought about by the contraction of diaphragm and inspiratory muscles. Since lungs are situated in the thorax, therefore a fall of pressure in thorax also lowers the pressure inside lungs, due to which air from outside rushes into the lungs through nostrils, trachea and bronchi.

If a person first inspires with his maximum effort and then expires also with maximum effort the volume of air breathed out is called the vital capacity. An athlete requires more oxygen during exercise on a regular basis. This gradual effort of an athlete to meet his oxygen demand ultimately increases the vital capacity.

In forceful expiration requiring effort, a different group of intercostal and some abdominal muscles contract to reduce the volume of thorax more than that in ordinary respiration. So, a larger volume of air is breathed out, such muscles are called expiratory muscles.

Histamine is a derivative of the amino acid histidine produced by damaged cells of vertebrates. When released, it has the effect of dilating capillaries and lowering blood pressure. Histamine is involved in allergic and inflammatory reactions

Blood is colourless in insects. Insects have tracheal respiration. It is carried on by an extensive system of inter-communicating tubes called tratrachea.