Alveoli are air sacs in the lungs where gas exchange takes place. Oxygen and carbon dioxide are small, uncharged molecules. They diffuse (i.e. the net movement of molecules from an area of high concentration to an area of low) freely across the phospholipid bilayer of a cell’s plasma membrane. Oxygen diffuses from the alveolus into the blood and is transported to respiring cells. Carbon dioxide diffuses from the blood into the alveolus and is breathed out. These average about 100 micrometers in diameter. There are approximately 750 million alveoli in adult human lungs, with a total surface area for gas exchange of about 80m2. Oxygen entering the alveolus dissolves in the film of water on its wall. Within the walls of the alveoli are numerous capillaries. These are very small blood vessel where water, solutes and respiratory gases are exchanged with body tissues. Capillaries carry blood from arterioles to venules. Their walls are only one cell thick, so that oxygen and nutrients can pass easily into the surrounding tissues. Similarly, waste materials, such as urea and carbon dioxide, can enter capillaries from the tissues and be transported away for excretion.
Considering Fick’s law of diffusion, the body has adapted to maximise diffusion by increasing the surface area for gas exchange (many alveoli, and capillaries), by having a small diffusion pathway (blood supply is very close to gas exchange surface, alveoli are only one cell thick), and by maintaining a large concentration difference (blood always moves oxygenated blood away from lungs, and partial pressure of oxygen in alveoli is always greater than in capillaries due to breathing).
At the capillaries oxygen is diffused into the blood. Blood is a suspension of cells in solution that acts as a transport medium within an animal. In humans, blood consists of about 55% by volume of plasma and 45% by volume of cells. The cells consist of red blood cells (erythrocytes), white blood cells (leucocytes) and small cell-fragments called platelets (thrombocytes). Blood has a number of important biological functions; the one in question is transport. Nutrients, such as glucose and amino acids; respiratory gases (oxygen and carbon dioxide); excretory products, such as urea; hormones, such as insulin; and heat from vasodilation/constriction are all transported in the blood. In the blood the oxygen is transported by red blood cells (erythrocytes) by an iron-containing protein called haemoglobin. Haemoglobin has a quaternary structure consisting of four polypeptide chains, each linked to a haem group. Its function is to transport oxygen around the body of many animals; each haem group has one iron molecule within it, and so can therefore only combine with one molecule of oxygen. It can combine with haemoglobin reversibly to form oxyhaemoglobin. In addition to its oxygen-carrying function, haemoglobin also transports some carbon dioxide (as carbomino compounds, not carboxyhaemoglobin) and is an important buffer in the blood. The relationship between the concentration of oxygen in the blood and the percentage of saturation of haemoglobin is shown by the oxygen dissociation curve.
The heart is a muscular organ that pumps blood around the body of an animal. The pumping action of the heart is described by the cardiac cycle. The heartbeat is initiated by a small area of muscle tissue in the wall of right atrium known as the sino-atrial node (SAN) or pacemaker. This causes the following events: atrial systole (contraction), where the atria contract, forcing blood into the ventricles through the atrioventricular (AV) valves. This is shortly followed by ventricular systole where the ventricles contract, forcing blood out of the heart to the body and lungs via the aorta and pulmonary artery respectively. After systole both atria and ventricles go into a stage of diastole (relaxing) where the hearts fills with deoxygenated and oxygenated blood into the right and left sides from the vena cavae and pulmonary vein respectively. The medulla oblongata coordinates breathing and heart rate. The blood system in fish is known as a single circulation as blood passes through the heart only once in its passage around the body. Mammals, on the other hand, have a double circulation as blood passes through the heart twice as it travels around the body.
When the blood reaches the respiring muscles oxygen is given from oxyhaemoglobin to myoglobin. Myoglobin is an iron-containing protein found in muscle. Myoglobin consists of a polypeptide chain linked to a haem group. Its function is to store oxygen in muscle tissue for use during strenuous exercise. Myoglobin has to obtain its oxygen from haemoglobin in the blood. It therefore has a high affinity for oxygen and its oxygen dissociation curve lies to the left of that of haemoglobin. The myoglobin gives off its oxygen to the respiring cells, more importantly the organelles such as mitochondria that require it for respiration. These are found in the cytoplasm. This is the content of a cell surrounding the nucleus and enclosed by the cell surface membrane. Cytoplasm consists of organelles suspended in jelly-like matrix (cytosol), which contains enzymes and numerous small molecules, such as amino acids and nucleotides. The cytoplasm is metabolically active, being the site of glycolysis and fatty acid synthesis. Glycolysis is the initial stage of respiration where glucose is broken down into pyruvate, and the products are used in the Kreb’s cycle and in the electron transport chain to produce more ATP for metabolic reactions.
