Breathing and Exchange of Gases notes

This chapter explains how humans obtain oxygen and remove carbon dioxide through breathing, diffusion, transport and regulation. The respiratory system includes nostrils, nasal chamber, pharynx, larynx, trachea, bronchi, bronchioles and lungs, ending in alveoli where gas exchange occurs. Breathing is caused by pressure changes produced by diaphragm and intercostal muscles. Oxygen and carbon dioxide diffuse according to partial pressure gradients at alveoli and tissues. Oxygen is mainly carried as oxyhaemoglobin, while carbon dioxide is mostly transported as bicarbonate ions. Respiratory centres in the medulla and pons regulate rhythm, especially by sensing carbon dioxide and hydrogen ion levels. NEET commonly tests respiratory volumes, partial pressure tables, chloride shift, oxyhaemoglobin dissociation curve and disorders like asthma, emphysema and occupational lung diseases.

The human respiratory system is designed to conduct, condition and exchange respiratory gases. Air enters through nostrils, passes into the nasal chamber, pharynx, larynx, trachea, bronchi and bronchioles, finally reaching alveoli. The conducting part transports air and removes dust using mucus and cilia, while the respiratory part includes alveoli where diffusion occurs. Lungs are paired, spongy organs present in the thoracic chamber, protected by ribs and covered by pleura. Pleural fluid reduces friction and helps lung movement during breathing. Alveoli are thin-walled, moist and surrounded by capillaries, giving a large surface for exchange. Respiratory volumes such as tidal volume, inspiratory reserve volume, expiratory reserve volume and residual volume combine to form capacities like vital capacity and total lung capacity.

Breathing or pulmonary ventilation is the movement of air into and out of lungs due to pressure differences between atmosphere and alveoli. Inspiration is an active process in normal breathing. The diaphragm contracts and becomes flat, external intercostal muscles lift ribs and sternum, thoracic volume increases and intrapulmonary pressure falls below atmospheric pressure, so air enters. Expiration during quiet breathing is mostly passive. Diaphragm and external intercostals relax, ribs and sternum return, thoracic volume decreases, intrapulmonary pressure rises above atmospheric pressure and air moves out. Forced expiration uses internal intercostal and abdominal muscles. The key NEET logic is inverse relation between volume and pressure: increase in thoracic volume decreases pulmonary pressure; decrease in volume increases pulmonary pressure.

Gas exchange occurs by simple diffusion across thin respiratory membranes and depends mainly on partial pressure gradients. In alveoli, pO2 is higher than in deoxygenated blood, so oxygen diffuses from alveolar air into blood. pCO2 is higher in deoxygenated blood than in alveolar air, so carbon dioxide diffuses from blood into alveoli. In tissues, the reverse happens: tissue pO2 is lower than blood pO2, so oxygen enters tissues; tissue pCO2 is higher, so carbon dioxide enters blood. Exchange is efficient because alveoli have a huge surface area, thin squamous epithelium, moist surface, rich capillary supply and a short diffusion distance. NEET often asks the partial pressures in alveoli, blood and tissues and the factors affecting diffusion.

Blood transports respiratory gases between lungs and tissues. About 97 percent of oxygen is carried by haemoglobin as oxyhaemoglobin, while about 3 percent is dissolved in plasma. Each haemoglobin molecule can bind four oxygen molecules. Oxygen loading occurs in lungs where pO2 is high, and unloading occurs in tissues where pO2 is low, pCO2 is high, H+ concentration is high and temperature is higher. This is shown by the sigmoid oxyhaemoglobin dissociation curve. Carbon dioxide is transported in three forms: dissolved in plasma, bound to haemoglobin as carbaminohaemoglobin and mainly as bicarbonate ions. In RBCs, carbonic anhydrase catalyses CO2 hydration. Bicarbonate exits RBCs into plasma and chloride ions enter RBCs to maintain electrical balance; this is called chloride shift.

Respiration is regulated mainly by neural centres in the brain, with chemical feedback modifying the rhythm. The respiratory rhythm centre in the medulla generates the basic breathing rhythm. The pneumotaxic centre in the pons can reduce the duration of inspiration and thereby alter respiratory rate. Chemosensitive areas near the respiratory centre are highly sensitive to carbon dioxide and hydrogen ions. Peripheral chemoreceptors in the carotid and aortic bodies also sense changes in CO2, H+ and oxygen, though oxygen usually becomes a major stimulus only when it falls significantly. During exercise, increased CO2, H+, temperature and neural signals from muscles increase ventilation. The goal of regulation is to maintain proper blood pO2, pCO2 and pH.

Respiratory disorders reduce ventilation, diffusion or oxygen delivery. Asthma is an allergic or inflammatory condition causing narrowing of bronchi and bronchioles, leading to wheezing and difficulty in breathing. Emphysema is a chronic disorder, commonly linked to cigarette smoking, in which alveolar walls are damaged and respiratory surface area decreases. Occupational respiratory disorders occur due to long exposure to dust in industries such as mining, stone grinding or asbestos work; inflammation and fibrosis reduce lung capacity. Pneumonia is infection and inflammation of alveoli, often causing fluid accumulation and poor gas exchange. Tuberculosis is a bacterial disease mainly affecting lungs, with chronic cough and systemic symptoms. NEET focuses on cause, affected part and physiological consequence.