transport in humans

• humans have a double circulation:

  • pulmonary circulation: deoxygenated blood to be pumped to the lungs to be reoxygenated and transported back to the heart at a lower pressure
  • systemic circulation: oxygenated blood to be pumped from the heart to all parts of the body at a higher pressure
• advantages of a double circulation:

  • Blood entering lungs is at a lower pressure, allowing sufficient time for blood to be oxygenated.
  • Blood pumped in the systemic circulation is at a high pressure, allowing oxygenated blood to quickly reach all tissues of the body.
• Plasma transports the dissolved substances and blood cells around the body.
• red blood cells (erythrocytes)

  • Transport oxygen from the lungs to all parts of the body
  • Produced in the bone marrow and destroyed in the spleen and liver
• Haemoglobin binds reversibly to oxygen to form oxyhaemoglobin, which helps to transport oxygen around the body
• Each molecule of haemoglobin can bind up to four molecules of oxygen
• Red blood cells have no nucleus. This allows more space in the cell for packing more haemoglobin and transporting more oxygen
• Red blood cells are biconcave in shape. This shape increases the surface area-to-volume ratio of the cell, allowing quicker diffusion of oxygen in and out of the cell
• Red blood cells are elastic and flexible. This allows the cell to change its shape to squeeze through small and narrow capillaries
• Carbon monoxide binds tightly with haemoglobin to form carboxyhaemoglobin. Less haemoglobin is available to bind with oxygen, resulting in reduced oxygen transport around the body.
• Sickle cell anaemia is a genetic disorder that causes red blood cells to become misshapen and stick together, reduces the surface area-to-volume ratio for absorption of oxygen.
• Phagocytes carry out phagocytosis to engulf and digest bacteria
• Lymphocytes produce antibodies that can

  • neutralise toxins produced by bacteria
  • bind to the surface of bacteria, causing them to rupture
  • bind to the surface of bacteria, tagging them for phagocytosis by phagocytes
  • cause bacteria to clump together, so that they can be easily ingested by phagocytes
• Transplantation is the replacement of diseased tissue or organs by healthy donor ones.
• Tissue rejection occurs when the recipient’s immune system recognises the transplant organ as foreign tissue.
• Prevention of tissue rejection: Tissue matching or donor matching, Immunosuppression by using drugs to inhibit the recipient’s immune system
• Agglutination is the clumping of red blood cells.

  When antibody A recognises and binds to A-antigen on the surface of red blood cells, agglutination occurs.
• Platelets (thrombocytes)

  • Upon injury, damaged tissues and platelets release an enzyme, thrombokinase.
  • In the presence of calcium ions, thrombokinase converts inactive prothrombin (in plasma) to the active thrombin.
  • Thrombin then converts soluble fibrinogen to insoluble fibrin threads.
  • The fibrin threads forms a mesh that traps blood cells. This is a blood clot.
• Blood clots defend the body by: preventing excessive loss of blood

  from a wound, preventing entry of bacteria through the wound
• There are 3 main types of blood vessels: arteries, veins, capillaries
• walls of arteries and veins consists of three layers.

  • The innermost layer is called the endothelium.

  • The middle layer consists of smooth muscle tissue and elastic fibres.

  • The outer layer consists of connective tissue like collagen fibres.
• Artery: Blood vessels that carry blood away from the heart.
• Arteries branch out to form smaller blood vessels called arterioles.
• Most arteries contain oxygenated blood. But there are exceptions:

  • Pulmonary artery transports deoxygenated blood from the heart to the lungs for gaseous exchange.

  • Umbilical artery transports deoxygenated blood from a fetus to the placenta of its mother.
• structure of artery:

  • thick muscular walls: allows the diameter of the vessel to be adjusted (muscles in the walls contract and relax to bring about vasodilation or vasoconstriction), control the blood pressure inside the vessel and regulate blood flow
  • numerous elastic fibres: elasticity for blood vessel to dilate, stretch and recoil to enable it to withstand increases in blood pressure in the vessel, prevents blood vessel from bursting, stretch and recoil also help to maintain the high pressure of blood flowing through the artery
• Vein: Blood vessels that carry blood towards the heart.
• Most veins contain deoxygenated blood. But there are exceptions:

  • Pulmonary vein transports oxygenated blood from the lungs to the heart before the blood is pumped around the body.

  • Umbilical vein transports oxygenated blood from the mother’s placenta to the fetus.
• structure of vein:

  • thinner, less muscular and less elastic walls: blood pressure in the veins is lower, less muscular tissue is needed to support the vessel
  • semilunar valves present: valves prevent the backflow of blood, allows blood to flow in one direction only
  • veins located between the large muscles of the body: contraction of the body muscles will help to push against the walls of the veins and push blood forward to the heart, against the effects of gravity
• Blood flows this way: arteries → arterioles → capillaries → venules → veins
• capillaries enable exchange of substances between the blood and body cells via diffusion.
• structure of capillaries:

  • capillary walls are one-cell thick: for quick diffusion of substances between blood and body cells
  • small gaps between the endothelial cells: causes some components of the blood to leak out from the plasma, forming tissue fluid, gaps allow white blood cells to squeeze through, gaps are too small for large molecules to pass through
  • large network of capillaries: large total surface area available for diffusion of substances, blood pressure in capillaries is low, speed of blood flow is low, allows more time for diffusion of substances to take place
• Tissue fluid (also known as interstitial fluid) is a fluid that bathes the cells of most tissues.
• Tissue fluid functions to provide a medium for diffusion to allow for the exchange of materials between the capillaries and the body cells.
• The main force that pushes plasma out of the blood is hydrostatic pressure, which is the blood pressure against the capillary wall.
• Why do red blood cells have no nucleus?

  more space to pack more haemoglobin to transport more oxygen
• The chambers of the heart work in this manner:

  • They fill up with blood, then their muscular walls will contract to push blood out of the chamber.

  • Atria will force blood into the ventricles.

  • The ventricles will force blood out of the heart.
• The bicuspid and tricuspid valves are also called the atrioventricular valves.
• The bicuspid valve is also called the mitral valve.
• walls of the atria are less muscular compared to the walls of the ventricles as the muscles in the walls of the atria contract and force blood into the ventricles which is a short distance away
• the wall of the left ventricle is more muscular than the wall of the right ventricle

  • the muscular wall of the left ventricle functions to force blood to all parts of the body (except lungs)
  • this is a longer distance away from the heart thus a higher blood pressure needs to be generated