Cycles in Biology

1055 Words5 Pages
Cycles in Biology A cycle is an event of series which is repeated in the same order. Cycles are one of the main aspects in biology, as they occur in the body, plants and ecosystems. One of the most essential cycles for aerobic organisms is the ventilatory cycle, or better known as the gas-exchange system. In human beings, the lung acts as the interface for exchange of gases and to maintain diffusion across the alveolar epithelium of the lungs, air must constantly be moved in and out of the lungs. This is called ventilation and consists of inspiration and expiration. During inspiration, external intercostal muscles contract, which cause the ribcage to move upwards and forwards. At the same time, internal intercostal muscles relax and the the diaphragm muscles contract and flatten. All these steps cause the volume of the thorax to increase and this on the other hand will decrease the pressure in the lungs to below atmospheric pressure. Therefore air can move into the lungs via the trachea and bronchi and provide the lung with oxygen. Before expiration, gas exchange takes place on the gas exchange surface, the alveoli. As alveoli are tiny air sacs, they provide a large surface area which speeds up the rate of diffusion. After air moved into the lung, oxygen diffuses down the concentration gradient through the epithelial cells of the alveoli and through the endothelial cells of the capillaries. Furthermore they diffuse through plasma membranes of red blood cells and bind to the haemoglobin. By the cardiac cycle, oxygen gets transported to cells all over the body, where it’s used for aerobic respiration. One of the products during respiration is carbon dioxide. It is diffused in blood plasma and also transported by the cardiac cycle, back to the lungs. There the carbon dioxide will diffuse down the concentration gradient through endothelial cells in capillaries
Open Document