Circulation In Humans
Übersicht
Put a finger on the inside of your wrist and wait. That small, regular push is a muscular pump the size of your fist forcing blood out of your chest and down your arm, and it has done so without a single day off since before you were born. Every cell you own is fed and cleaned by that pump. Starve any part of the network for a few minutes and the tissue it serves begins to die.
This lesson takes the whole circulatory system apart and puts it back together: the four chambers of the heart and the order in which they squeeze, the pacemaker cells that set the beat, the three kinds of vessel and why each one is built the way it is, and the four components of blood and the jobs they do. You will also meet the medicine that grows out of the biology, including stents, replacement valves, artificial hearts, transplants and the ABO blood groups, and you will learn to weigh a mechanical fix against a biological one the way an examiner expects.
Ziele
- Substances are transported from where they enter the body to the cells, or from the cells to where they are removed from the body, by the circulatory system (the heart, the blood vessels and the blood).
- The heart is an organ that pumps blood around the body in a double circulatory system. Much of the wall of the heart is made from muscle tissue.
- The heart has four main chambers (right and left atria and right and left ventricles).
- The natural resting heart rate is controlled by a group of cells that act as a pacemaker, located in the right atrium. Artificial pacemakers are electrical devices used to correct irregularities in the heart rate.
- In coronary heart disease, layers of fatty material build up inside the coronary arteries and narrow them. This reduces the flow of blood through the coronary arteries, resulting in a lack of oxygen for the heart muscle. Stents are used to keep the coronary arteries open.
- In some people heart valves may become faulty. There are two main faults - the heart valve tissue might stiffen, preventing the valve from opening fully, or the heart valve might develop a leak. Faulty heart valves can be replaced using: biological valves - valves from humans or other mammals; mechanical valves.
- Artificial hearts are occasionally used to keep patients alive whilst waiting for a heart transplant, or to allow the heart to rest as an aid to recovery.
- Blood enters the atria of the heart. The atria contract and force blood into the ventricles. The ventricles contract and force blood out of the heart. Valves in the heart ensure that blood flows in the correct direction. Knowledge of the names of the heart valves is not required.
- Blood flows from the heart to the organs through arteries and returns through veins. There are two separate circulation systems, one for the lungs and one for all other organs of the body. Knowledge of the blood vessels associated with the heart is limited to aorta, vena cava, pulmonary artery, pulmonary vein and coronary arteries.
- Arteries have thick walls containing muscle and elastic fibres. Veins have thinner walls and often have valves to prevent back-flow of blood.
- In the organs, blood flows through very narrow, thin-walled blood vessels called capillaries. Substances needed by the cells in body tissues pass out of the blood and substances produced by the cells pass into the blood, through the walls of the capillaries.
- Blood is a tissue consisting of a fluid called plasma, in which the white blood cells, platelets and red blood cells are suspended.
- Blood plasma transports: carbon dioxide from the organs to the lungs; soluble products of digestion from the small intestine to other organs; urea from the liver to the kidneys.
- Red blood cells have no nucleus. They are packed with a red pigment called haemoglobin. Red blood cells transport oxygen from the lungs to the organs. In the lungs haemoglobin combines with oxygen to form oxyhaemoglobin. In other organs oxyhaemoglobin splits up into haemoglobin and oxygen.
- White blood cells have a nucleus. They form part of the body’s defence system against microorganisms.
- Platelets are small fragments of cells. They have no nucleus. Platelets help blood to clot at the site of a wound.
- Blood clotting is a series of enzyme-controlled reactions, resulting in the change of fibrinogen to fibrin, which forms a network of fibers trapping blood cells and forming a clot.
- Antigens are proteins on the surface of cells.
- In organ transplants a diseased organ is replaced with a healthy one from a donor. The recipient’s antibodies may attack the antigens on the donor organ as they do not recognise them as part of the recipient’s body. To prevent rejection of the transplanted organ: a donor organ with a ‘tissue-type’ similar to that of the recipient is used; the recipient is treated with drugs that suppress their immune system. Students should be able to evaluate the advantages and disadvantages of treating organ failure by mechanical devices or transplant.
- There are four main types of human blood: O, A, B and AB. Blood group O is the universal donor. Students should understand: the need for blood typing; the ABO compatibility table.
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- Which chamber of the heart pumps blood into the aorta? A. The right atrium B. The left atrium C. The right ventricle D. The left ventricle Answer: D
- Which blood vessel carries deoxygenated blood away from the heart? A. The aorta B. The pulmonary artery C. The pulmonary vein D. The vena cava Answer: B
- Where are the cells that act as the heart's natural pacemaker found? A. In the right atrium B. In the left atrium C. In the right ventricle D. In the wall of the aorta Answer: A
- Which component of the blood has no nucleus and is packed with haemoglobin? A. The plasma B. A white blood cell C. A red blood cell D. A platelet Answer: C
- Why is blood group O described as the universal donor? A. Its red blood cells carry both the A and the B antigen B. Its red blood cells carry neither the A nor the B antigen C. Its plasma contains no antibodies at all D. Its red blood cells have no nucleus Answer: B
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