Gas Exchange in Mammals

Gas Exchange in Mammals

The mammalian respiratory system consists of the nasal cavity, pharynx, glottis, larynx, trachea, bronchi, bronchioles and air sacs (alveoli).

The respiratory passages are lined by the presence of cilia and mucus-secreting cells, which keep them moist and trap dust, dirt and bacteria. The lungs are encased a rib cage. The rib cage is composed of the breastbone or sternum and ribs. At the bottom of the rib cage is a muscular diaphragm.
Breathing in mammals. During inhalation, the rib cage is pulled outwards and upwards by the contraction of the intercostal muscles, while the diaphragm is lowered or flattened. The volume of the thoracic cavity is increased, the air enters the lungs. During exhalation, the intercostal relax, the rib cage is lowered and pulled inwards while the diaphragm contracts and is raised. This reduces the volume of the thoracic cavity, and the air is forced out of the lungs.
During inhalation, the intercostal muscles of the ribs contract, pulling the rib cage upwards and outwards. This stretches the diaphragm outwards, creating a large empty space in the thoracic cavity. The air is then drawn into the lungs. During inhalation the intercostal muscles relax, the rib cage falls back to its original position and the diaphragm arches upwards. The result is that the volume of the thoracic cavity is reduced and the air is expelled from the lungs.
The amount of air inhaled at each breath is called the tidal volume and is about 500 ml for an adult human being. Only about 350 ml of the tidal volume reaches the lung. The remaining 150-ml of air, known as the dead space volume remains within the respiratory passages.
Under a strong physical exercise, the lungs can hold up to between 4500 ml and 5000 ml of air. This is called the vital capacity of the lungs. It is impossible to empty the lungs completely. Some air always remains in the lungs, which is called the residual volume.
Control of Breathing
Breathing is under the control of a centre in the medulla oblongata. The breathing centre is very sensitive to changes in the concentration of CO2 and the pH of the blood. An increase in CO2 concentrations increases the rate and the depth of breathing. Sometimes excessive vigorous exercise can reduce the level of CO2 concentration in the body so much so that the breathing centre stops sending signals to the lungs. When this happens, breathing stops, causing loss of consciousness or fainting.
Effects of Smoking on Health
Smoking is of great risk to health. Smoking reduces a persons live expectancy. It has been estimated that a twenty – five year old who smokes two packs of cigarettes a day has his life expectancy shortened by 8.3 years. The greater the number of cigarettes one smokes daily the shorter his life expectancy.
Cigarette smoking is the major cause of cancer in both men and women. Nicotine can be absorbed into the blood stream to exert its additive effects. Lung cancer is the most widely known and most harmful effect of smoking. Tobacco smoking is also associated with chronic bronchitis, emphysema, coronary artery disease, peripheral vascular disease and stroke. The harmful components of tobacco include tar, carbon monoxide, nitrogen dioxide and nitric oxide.
Cancer of the lungs, due to smoking, starts first by thickening the mucus producing cells that line the bronchi. This is followed by a loss of cilia so that it is impossible to prevent dust and dirt from settling in the lungs. Later cancer cells appear in the thickened lining. Some cancer cells break loose and penetrate the other lung tissues spreading the cancer. The incidence of cancer of the pharynx, mouth, oesophagus, bladder and pancreas appears to be higher in smokers than in non-smokers.
Cigarette smokers are 4 to 25 times likely to suffer from emphysema than non-smokers. Smoking thickens the lining of the bronchioles obstructing the free movement of air so that breathing becomes difficult and the air is trapped within the alveoli. The trapped air may cause the walls of the alveoli to break and the thickening of the surrounding capillary vessels, thereby reducing the amount of oxygen reaching the brain. This causes the heart to beat faster in order to get enough oxygen to the brain. In the end, this damages the heart.
Smoking is responsible for deaths due to coronary heart disease.
Pregnant mothers who smoke run the risk of having stillborn babies and babies who are underweight. Such children are usually underdeveloped and are socially unadjusted.