- For the biochemical process, see respiration
The respiratory system, also called the gas exchange system, is the body getting rid of carbon dioxide and taking in oxygen. Carbon dioxide, a waste product, goes out of the body. Oxygen, which the body needs, comes in.
The first step in this process is breathing in air, or inhaling. The taking in of air rich in oxygen into the body is called inhalation and giving out of air rich in carbon dioxide from the body is called exhalation. The second step is gas exchange in the lungs where oxygen is diffused into the blood and the carbon dioxide diffuses out of the blood. The third process is cellular respiration, which produces the chemical energy that the cells in the body need, and carbon dioxide. Finally, the carbon dioxide from cellular respiration is breathed out of body from the lungs.
For respiration to happen, the body needs a constant supply of oxygen, which is done by breathing. Inhalation is the breathing in of air. To inhale, the lungs expand, decreasing the air pressure in the lungs. This is caused by the diaphragm (a sheet of muscular tissue that separates the lungs from the abdomen) and the muscles between the ribs contracting to expand the chest, which also expands the lungs. As the air pressure inside the lungs are lower when it has expanded, air from outside at higher pressure comes rushing into the area of low pressure in the lungs. Air first passes through the nose and mouth, then through the larynx (voice box), then down the trachea (windpipe), and into the lungs and comes out
The lungs are made of many tubes or branches. As air enters the lungs, it first goes through branches called the bronchi, then through smaller branches called bronchioles, and finally into the air sacs. Gas exchange occurs in the air sacs where oxygen is exchanged with carbon dioxide. The carbon dioxide in the air sacs now need to be exhaled, or breathed out. In the reverse process to inhaling, the diaphragm and the rib muscles relax, causing the lungs to be smaller. As the air pressure in the lungs is greater when the lungs are smaller, air is forced out. The exhaled air has a high concentration of carbon dioxide and a low concentration of oxygen. The maximum volume of air that can be inhaled and exhaled is called the vital capacity of the lungs and is up to five liters.
The inhaled air goes down to the air sacs at the end of each bronchiole. The air sacs are called alveoli — they have a large surface area, and are moist, thin, and close to a blood supply. The inhaled air has a much greater concentration of oxygen than carbon dioxide whilst the blood flowing to the lungs has a more carbon dioxide than oxygen. This creates a concentration gradient between the air in the air sacs and the blood, meaning there is more oxygen in the air than the blood. As the membrane, oxygen can easily diffuse in and out. Oxygen at high concentration in the air sacs diffuses into the blood where oxygen concentration is low, and carbon dioxide at high concentration in the blood diffuses into the air sacs where carbon dioxide concentration is low. The oxygen in the blood enters the circulatory system and is used by the cells in the body. The carbon dioxide in the air sacs are exhaled out of the body.
The effect of the contraction of the accessory muscles of inhalation, pulling the front of the rib cage upwards. This increases the antero-posterior diameter of the thorax, contributing to the expansion in the volume of the chest. A similar effect causes the transverse diameter of the chest to increase, because not only do the ribs slant downwards from the back to the front, but, in the case of the lower ribs, also from the midline downwards to the sides of the chest.
A cartoon illustrating the mechanisms for forceful inhalation (left), and forceful exhalation (right). During forceful inhalation the diaphragm (the domed, almost horizontal structure in red, between the thoracic cavity (upper compartment) and abdominal cavity (lower compartment), contracts forcing the abdominal contents downwards, causing the abdomen to bulge outwards. At the same time the accessory muscles of inhalation cause the transverse diameter of the thorax to increase as described in the illustration of the movement of the ribs on the left. During forced exhalation the powerful muscles of the abdominal wall pull the lower edges of the rib cage downwards decreasing the antero-posterior and transverse diameters of the chest cavity, while at the same time forcing the abdominal organs up against the diaphragm causing it to bulge deeply into the chest.The force with which air can be expelled from the lungs is considerable greater than the force with which air can be inhaled. This is the result of the power of the muscles of the abdominal wall exceeding that of all the accessory muscles of inhalation. The most vigorous exhalatory efforts occur during coughing, sneezing and the blowing out of, for instance, candles.
The changes in the composition of the alveolar air during a normal breathing cycle at rest. The scale on the left, and the blue line, indicate the partial pressures of carbon dioxide in kPa, while that on the right and the red line, indicate the partial pressures of oxygen, also in kPa (to convert kPa into mm Hg, multiply by 7.5).
A highly diagrammatic illustration of the process of gas exchange in the mammalian lungs, emphasizing the differences between the gas compositions of the ambient air, the alveolar air (light blue) with which the pulmonary capillary blood equilibrates, and the blood gas tensions in the pulmonary arterial (blue blood entering the lung on the left) and venous blood (red blood leaving the lung on the right). All the gas tensions are in kPa. To convert to mm Hg, multiply by 7.5.
A diagrammatic histological cross-section through a portion of lung tissue showing a normally inflated alveolus (at the end of a normal exhalation), and its walls containing the pulmonary capillaries (shown in cross-section). This illustrates how the pulmonary capillary blood is completely surrounded by alveolar air. In a normal human lung all the alveoli together contain about 3 liters of alveolar air. All the pulmonary capillaries contain about 100 ml blood.
A histological cross-section through an alveolar wall showing the layers through which the gases have to move between the blood plasma and the alveolar air. The dark blue objects are the nuclei of the capillary endothelial and alveolar type I epithelial cells (or type 1 pneumocytes). The two red objects labeled "RBC" are red blood cells in the pulmonary capillary blood.
Aerial photo of Mount Everest from the south, behind Nuptse and Lhotse.
Respiratory system for Kids. Kiddle Encyclopedia.