A Counter Current System
Bony fish have gills that use a counter current principle. The blood flows through the gill filaments and secondary lamellae in the opposite direction from the water passing the gills. This is important for obtaining all of the available oxygen out of the water and into the blood. However, if the blood flowed in the same direction as the water passing it, then the blood would only be able to achieve half of the obtainable oxygen from the water. The blood and water would reach equilibrium in oxygen content and diffusion would no longer take place. Due to the blood flowing in the opposite direction and the water also passing over the gill plate in the opposite direction a diffusion gradient is maintained between the blood and water across the gill plate. The counter current system allows the gills of a bony fish to achieve an 80% extraction of oxygen from water. If fish are taken out of water they suffocate this is due to their gill arches collapsing and also there is not enough surface area in order for diffusion to take place. The walking catfish however, can survive out of water this is because they have modified lamellae allowing them to breathe air. Fish may also smother in the water; this could happen if the oxygen in the water has been used up by another biotic source such as bacteria decomposing a red tide.
How humans breathe
Breathing consists of two phases; inspiration and expiration. During inspiration (breathing in) the external intercostals muscles contract and the internal intercostals muscles relax, raising the ribs upwards and outwards. At the same time, the muscular diaphragm contracts and flattens. Both these actions increase the volume inside the thorax causing the pressure inside the thorax to decrease. Since atmospheric pressure is greater, air rushes into the lungs and they inflate. During expiration the internal intercostals muscles contract and the external intercostal muscles relax. This lowers the ribs downwards and inwards. The diaphragm relaxes and bulges upwards due to pressure from the organs below (for example the liver). Both these actions decrease the volume inside the thorax causing the pressure inside the thorax to increase, Air is forced out of the lungs as the elastic tissue of the alveoli recoils.
Around each lung and lining the thorax are the pleural membranes, between these two membranes is a space called the pleural cavity, which contains pleural fluid. During breathing the pleural fluid acts as a lubricant. This permits friction-free movement of the lungs against the inner wall of the thorax. The alveoli do not collapse when we breathe out due to an anti-sticking chemical called a surfactant covering the surfaces. This chemical acts by reducing the surface tension and so keeps the alveoli open.
Internal respiration is the exchange of gases between the blood in the capillaries and the body’s cells. The body tissues need the oxygen and have to get rid of the carbon dioxide, this is so the blood carried throughout the body exchanges oxygen and carbon dioxide with the body’s tissues.
External respiration is the exchange of oxygen and carbon dioxide between the air and the blood in the lungs. Blood enters the lungs via the pulmonary arteries; it then proceeds through arterioles and into the alveolar capillaries. Oxygen and carbon dioxide are exchanged between the blood and the air. This blood then flows out of alveolar capillaries, through venuoles and back to the heart via the pulmonary veins.
Gas exchange at the alveolus
Gas exchange takes place in the lungs at the tiny air sacs or alveoli. Ventilation ensures that air is moved into and out of the air passages of the lungs regularly. This helps to maintain the necessary concentration gradients of oxygen and carbon dioxide.
The capillaries around the alveoli provide efficient blood transport of gases, this prevents a build-up of oxygen and carbon dioxide and maintains gas concentration diffusion gradients. The red blood cells contain the respiratory pigment haemoglobin. This has a high affinity for oxygen, making the removal of oxygen from the alveoli even more efficient.
Deoxygenated blood enters the capillaries around the alveolus. This blood has less oxygen and more carbon dioxide than the air inside the alveolus. Oxygen diffuses out of the capillary into the air in the alveolus.
The lining of the alveolus is moist and gases diffuse in solution. The walls of the alveolus and blood capillary are each only one cell thick, making it easy for diffusion to take place.
