The Mammalian Transport System
- Large active organisms such as mammals need a transport system, in which fluid driven by a pump carries oxygen and nutrients to all tissues, and removes waste products from them. Mammals have a double circulatory system.
- Blood is carried away from the heart in arteries, passes through tissues in capillaries, and is returned to the heart in veins. Blood pressure drops gradually as it passes along this system.
- Arteries have thick, elastic walls to allow them to withstand high blood pressures and to smooth out the pulsed blood flow. Capillaries are only just wide enough to allow the passage of red blood cells, and have very thin walls to allow efficient and rapid transfer of materials between blood and cells. Veins have thinner walls than arteries and possess valves, to help blood at low pressure flow back to the heart.
- Plasma leak from capillaries to form tissue fluid. This is collected into lymphatics and returned to the blood in the subclavian veins.
- Red blood cells carry oxygen in combination with hemoglobin. Hemoglobin picks up oxygen at high partial pressures of oxygen in the lungs, and releases it at low partial pressures of oxygen in respiring tissues. It releases oxygen more easily when the carbon dioxide concentration is high. Myoglobin and fetal hemoglobin have a higher affinity for oxygen than adult hemoglobin. Myoglobin acts as an oxygen store in muscle.
- Carbon dioxide is mostly carried as hydrogen carbonate ions in blood plasma, but also in combination with hemoglobin in red blood cells and dissolved as carbon dioxide molecules in blood plasma.
- White blood cells aid in defense against disease.
- At high altitudes, the partial pressure of oxygen is so low that altitude sickness can be caused which can be fatal. The body can adapt to gradual changes, however, by producing more red blood cells and hemoglobin.