Summary
Highlights
Key blood vessels include arteries, arterioles, capillaries, venules, and veins. Arteries have thick muscular and elastic layers to modulate blood flow and maintain high pressure. Arterioles have thicker muscular layers to restrict blood flow into capillaries. Capillaries are one cell thick for efficient gas exchange and tissue fluid formation, with a narrow diameter to slow blood flow. Veins and venules have thinner walls and contain valves to prevent backflow due to lower pressure. Identifying these vessels in micrographs is a crucial skill, distinguishing arteries by their thick, circular walls and small lumen from veins with thinner, often distorted walls.
Identifying blood components like red blood cells (biconcave), monocytes (kidney-bean nucleus), neutrophils (lobed nucleus, granular cytoplasm), and lymphocytes (large, round nucleus) is important. Tissue fluid forms at capillary beds due to hydrostatic pressure, forcing water and small molecules out of capillaries to surround cells. This fluid delivers nutrients and removes waste. Large plasma proteins remain in capillaries, lowering water potential and increasing oncotic pressure towards the venule end, causing water to re-enter capillaries by osmosis. Unreabsorbed tissue fluid enters the lymphatic system, becoming lymph, which eventually drains back into the bloodstream.
Oxygen transport involves hemoglobin, a globular protein with a quaternary structure found in red blood cells. Myoglobin, a similar protein in muscle tissue, has a higher affinity for oxygen. Oxyhemoglobin dissociation curves illustrate hemoglobin's affinity for oxygen at different partial pressures. Oxygen is loaded in high partial pressure regions (lungs) and unloaded in low partial pressure regions (respiring tissues). The sigmoid shape of the curve reflects cooperative binding, where initial oxygen binding changes hemoglobin's shape, increasing its affinity for subsequent oxygen molecules. The Bohr effect describes how high carbon dioxide concentrations (indicating active respiration) shift the curve to the right, lowering hemoglobin's oxygen affinity and promoting oxygen unloading where it's needed most.
Hemoglobin affinity varies among organisms and conditions. Fetal hemoglobin has a higher oxygen affinity than adult hemoglobin, allowing the fetus to extract oxygen from the mother's blood. Animals at high altitudes, like llamas, also have higher affinity hemoglobin to compensate for lower ambient oxygen. Organisms with high metabolic rates, such as doves, have lower affinity hemoglobin to readily release oxygen to their active tissues. Earthworms and deep-diving animals have higher affinity hemoglobin to cope with low oxygen environments. Carbon dioxide is transported in four ways: dissolved in plasma, as carbaminohemoglobin, as hydrogen carbonate ions, and as hemoglobin acid. Approximately 85% is transported as hydrogen carbonate ions, formed by the enzyme carbonic anhydrase within red blood cells. The chloride shift maintains electrical balance by exchanging chloride and hydrogen carbonate ions across the red blood cell membrane. Carbaminohemoglobin forms when CO2 binds to globin proteins, facilitating CO2 removal from tissues to the lungs.
The heart, made of myogenic cardiac muscle, automatically contracts and relaxes. It never fatigues, provided a constant supply of oxygen and glucose by the coronary arteries. The heart is encased in an inelastic pericardial membrane. It has four chambers: two atria and two ventricles. The left ventricle has a much thicker muscular wall to pump oxygenated blood at high pressure to the entire body via the aorta. The right ventricle has thinner walls, pumping blood to the lungs at lower pressure to prevent capillary damage. Atria have thin walls, only needing to pump blood a short distance into the ventricles.
The cardiac cycle involves three stages: diastole (relaxation), atrial systole (atrial contraction), and ventricular systole (ventricular contraction). During diastole, both atria and ventricles relax, blood enters the atria, increasing pressure and opening atrioventricular valves. Atrial systole contracts the atria, pushing blood into the ventricles. Ventricular systole contracts the ventricles, closing atrioventricular valves and opening semilunar valves, forcing blood into the arteries. Valves ensure unidirectional blood flow, controlled by pressure changes. Cardiac output (volume of blood pumped per minute) is calculated by heart rate multiplied by stroke volume (blood per beat). Heart rate is controlled by electrical activity initiated by the sinoatrial node (SAN, pacemaker) in the right atrium. This wave of depolarization spreads across the atria, causing them to contract. A delay at the atrioventricular node (AVN) ensures ventricles fill completely before contracting. The AVN then sends the impulse down the bundle of His and Purkinje fibers, causing ventricular contraction from the apex upwards. Repolarization leads to diastole again.
This video covers Topic 8: Transport in Mammals for Cambridge International A-level Biology, focusing on the circulatory system, oxygen and carbon dioxide transport, and the heart. Large animals require circulatory systems due to high metabolic rates and a lower surface area to volume ratio, ensuring resource delivery and waste removal. Circulatory systems transport gases and nutrients in liquid (blood) via vessels and a pump. Mammals typically have a double closed circulatory system with two loops, where blood always remains within vessels.