All living cells require oxygen for the oxidation of food molecules to release energy, and they produce carbon dioxide as a waste product. The process of gaseous exchange between the organism and its environment, along with the transport of gases to and from the cells, is essential for respiration. Breathing is the process by which organisms take in oxygen and give out carbon dioxide, while respiration is the cellular process that uses this oxygen to release energy. The respiratory organs vary among organisms: fish use gills, insects use tracheae, and mammals, including humans, use lungs.
In this chapter, the human respiratory system is studied in detail, including its structure, the mechanism of breathing, the transport of oxygen and carbon dioxide in the blood, and the regulation of respiration. The exchange of gases occurs across the respiratory membrane by diffusion, driven by the partial pressure gradients of oxygen and carbon dioxide between the alveoli and the blood. Understanding this system is fundamental to appreciating how the body maintains its internal environment and how respiratory diseases disrupt these processes.
Different organisms have evolved different respiratory organs: - Simple organisms like sponges, coelenterates, flatworms and earthworms exchange gases by simple diffusion through the body surface (skin). The earthworm uses its moist skin as the respiratory organ. - Insects have a tracheal system: an elaborate network of air tubes (tracheae) that branch and deliver oxygen directly to the tissues through spiracles. - Aquatic animals like fish use gills, which are richly supplied with blood vessels for gaseous exchange. - Amphibians respire through their moist skin, buccal cavity and lungs. - Reptiles, birds and mammals respire through lungs, which are the most efficient respiratory organs.
The human respiratory system consists of a pair of lungs, which are the main respiratory organs, along with a series of air passages. The air enters the body through the nostrils, passes into the nasal cavity, which is lined with hairs and mucus that filter, warm and moisten the air. From the nasal cavity, air passes through the pharynx, larynx (voice box), and into the trachea (windpipe). The trachea is supported by rings of cartilage, which prevent it from collapsing. The trachea divides into two primary bronchi, one entering each lung. Within the lungs, the bronchi divide into smaller bronchioles, which finally terminate in tiny air sacs called alveoli.
The lungs are paired, spongy, elastic organs situated in the thoracic cavity. The right lung has three lobes, and the left lung has two lobes. The lungs are enclosed in a double-walled membrane called the pleural membrane, with the pleural fluid between the two layers, which reduces friction during breathing. Each lung contains millions of alveoli, which provide a vast surface area (about 100 square metres) for gaseous exchange. The alveoli are richly supplied with blood capillaries. The wall of the alveolus and the wall of the capillary together form the respiratory membrane, across which gaseous exchange occurs by diffusion.
Breathing involves two processes: inspiration (inhalation), in which air is taken in, and expiration (exhalation), in which air is expelled. The mechanism of breathing is based on the pressure changes in the thoracic cavity, brought about by the movement of the diaphragm and the intercostal muscles.
During inspiration, the external intercostal muscles contract, and the diaphragm contracts and flattens. This increases the volume of the thoracic cavity, which decreases the intrapulmonary pressure below the atmospheric pressure. Air then rushes into the lungs through the nose. Inspiration is an active process.
During expiration, the internal intercostal muscles relax, and the diaphragm relaxes and becomes dome-shaped. The volume of the thoracic cavity decreases, increasing the intrapulmonary pressure above the atmospheric pressure, which forces air out of the lungs. Normal expiration is a passive process, but forceful expiration involves the contraction of abdominal muscles.
The exchange of oxygen and carbon dioxide between the air in the alveoli and the blood, and between the blood and the tissues, occurs by simple diffusion. The direction and rate of diffusion depend on the partial pressure of the gases. The partial pressure of oxygen (pO2) is higher in the alveolar air (about 104 mm Hg) than in the deoxygenated blood (about 40 mm Hg), so oxygen diffuses from the alveoli into the blood. The partial pressure of carbon dioxide (pCO2) is higher in the deoxygenated blood (about 45 mm Hg) than in the alveolar air (about 40 mm Hg), so carbon dioxide diffuses from the blood into the alveoli.
The rate of diffusion is directly proportional to the surface area available and the difference in partial pressures, and inversely proportional to the thickness of the membrane. The lungs provide a vast surface area of about 100 square metres and a thin respiratory membrane (about 0.2 micrometres thick), making diffusion rapid and efficient.
Oxygen is transported in the blood in two ways: about 3 percent is dissolved in the plasma, and about 97 percent is carried by haemoglobin in the red blood cells as oxyhaemoglobin. The binding of oxygen to haemoglobin is a reversible reaction: Hb + O2 gives HbO2. Each haemoglobin molecule can bind four oxygen molecules. The oxygen-haemoglobin binding is affected by the partial pressure of oxygen, the partial pressure of carbon dioxide, temperature and pH. The oxygen dissociation curve shows that at high pO2, haemoglobin is almost fully saturated, and at low pO2, it releases oxygen. In the tissues, where pO2 is low, oxyhaemoglobin releases oxygen for the cells to use.
Carbon dioxide is transported in three ways: - About 7 percent is dissolved in the plasma. - About 20-25 percent is carried by haemoglobin as carbamino-haemoglobin (CO2 binds to the amino groups of the haemoglobin, which is not oxygen-dependent in binding). - About 70 percent is transported as bicarbonate ions (HCO3-). In the red blood cells, carbon dioxide reacts with water, catalysed by the enzyme carbonic anhydrase, to form carbonic acid, which dissociates into hydrogen ions and bicarbonate ions. The bicarbonate ions diffuse into the plasma, while chloride ions enter the red blood cells to maintain electrical neutrality, a process called the chloride shift. In the lungs, this process is reversed: bicarbonate is converted back to carbon dioxide, which is exhaled.
The respiratory rhythm is controlled by the respiratory centre in the medulla oblongata of the brain. The pneumotaxic centre in the pons can moderate the function of the respiratory centre by reducing the duration of inspiration. A chemosensitive area is adjacent to the respiratory centre, which is sensitive to the changes in carbon dioxide, oxygen and hydrogen ion concentration in the blood. An increase in the carbon dioxide concentration and hydrogen ion concentration (decrease in pH) in the blood stimulates the respiratory centre, increasing the rate and depth of respiration. Oxygen level has a lesser direct effect on the respiratory centre. The overall regulation ensures that the blood gas levels are maintained within normal limits.
| Volume | Definition | Value (approx.) |
|---|---|---|
| Tidal volume (TV) | Air in/out in normal breathing | 500 ml |
| Inspiratory reserve volume (IRV) | Extra air after normal inspiration | 2500-3000 ml |
| Expiratory reserve volume (ERV) | Extra air after normal expiration | 1000-1100 ml |
| Residual volume (RV) | Air left after forceful expiration | 1100-1200 ml |
| Vital capacity (VC) | Max exhale after max inhale | TV + IRV + ERV |
| Gas | Mechanism | Percentage |
|---|---|---|
| O2 in plasma | Dissolved | 3% |
| O2 with haemoglobin | Oxyhaemoglobin | 97% |
| CO2 in plasma | Dissolved | 7% |
| CO2 with haemoglobin | Carbamino-haemoglobin | 20-25% |
| CO2 as bicarbonate | HCO3- in plasma | About 70% |
| Location | pO2 | pCO2 |
|---|---|---|
| Alveolar air | 104 mm Hg | 40 mm Hg |
| Deoxygenated blood | 40 mm Hg | 45 mm Hg |
| Tissues | Low | High |
Breathing and the exchange of gases are the first steps in the continuous supply of oxygen that every cell needs for energy release. The human respiratory system, from the filtering nasal cavity to the vast alveolar surface, is exquisitely designed for efficient gaseous exchange by diffusion. Breathing is achieved through the elegant interplay of the diaphragm and intercostal muscles, which create the pressure gradients that move air in and out of the lungs. The transport of oxygen by haemoglobin and of carbon dioxide as bicarbonate ions ensures that the tissues receive oxygen and the waste carbon dioxide is eliminated. The medullary respiratory centre, modulated by chemical signals, keeps blood gas levels in balance, while diseases like asthma and emphysema remind us how sensitive this system is. This understanding forms the basis for the study of blood and circulation in the next chapter.