The circulatory system

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Blood vessels

These are tubes through which blood flows. There are 3 main types:

(1) Arteries carry oxygenated blood away from the heart. (exception = pulmonary artery, which carries deoxygenated blood from the heart to the lungs). Blood is transported under high pressure to tissues. Arteries have thick walls which are elastic. Inner walls are made of involuntary muscle which expands (vasodilation) or contracts (vasoconstriction) to control flow of blood. The pulse can be felt through arteries. Arteries branch into smaller vessels called arterioles which further subdivide into capillaries.

(2) Capillaries are the smallest of all blood vessels, and lie between arteries and veins, connecting them both. Capillaries exist in very large numbers in and around muscles and organs. Walls of capillaries are extremely thin (one cell thick) allowing substances to pass into and out of muscles.

(3) Veins – capillaries lead into venules and then into veins. Veins carry deoxygenated blood and waste products to the heart (exception = pulmonary vein, which carries oxygenated blood from the lungs to the heart). Blood is carried through veins under very little pressure, and has to rely on one-way valves to stop back-flow of blood. Veins have thin walls. The pulse cannot be felt through veins. When blood is flowing towards the heart through veins, the valves automatically open to allow blood to flow freely through. If blood tries to flow backwards away from the heart, the valves are automatically forced shut, stopping all blood flow.
vein valves
blood vessels

Blood pressure

The heart pumps blood under pressure. The pumping action of the heart creates this pressure, and the pressure varies in different parts of the body. The highest pressure exists in the arteries, and the pressure reduces, until it reaches virtually zero in the veins. Measuring blood pressure gives two readings:

  1. Systolic pressure is the pressure as the heart contracts
  2. Diastolic pressure is the pressure when the heart relaxes.

Blood pressure is always stated in the form of systolic over diastolic, for example, 120/80, said “120 over 80”. Blood pressure is often used as an indicator of general health. Many factors can affect blood pressure:

  • BP increases during exercise
  • BP is lower in children than in adults
  • There are variations between men and women
  • Stress or emotional tension may increase BP
  • If the circulatory system is in poor condition, BP can increase
  • Obesity can lead to high blood pressure (as well as other circulatory disorders such as CHD and metabolic disorders such as diabetes)

120/80 is considered healthy; 140/90 is considered as mild hypertension; and 160/100 is considered hypertension. High blood pressure can lead to strokes.

Red blood cells

Red blood cells are bi-concave discs (doughnut shaped). They contain haemoglobin – a compound of iron and protein which is reddish in colour – and has a strong affinity to oxygen and carbon dioxide. When it combines with oxygen in the lungs, oxyhaemoglobin is formed. Red blood cells are made in bone marrow. They pick up oxygen where it is plentiful (lungs) and give it up where there is little or where it is needed (in muscles, for example).

White blood cells

White blood cells are larger than red blood cells and are irregular in shape. They defend the body against infection and disease. White blood cells are produced in the bone marrow, as well as the lymph glands.

Functions of blood

(1) Transportation – many materials are transported in the blood:

  • Oxygen combines with haemoglobin in the blood which gives up oxygen to body tissue when needed
  • Carbon dioxide is transported in blood from all tissues to the lungs;
  • Glucose and nutrients are transported from the small intestine to all parts of the body
  • Heat is transported through the blood (originating in muscles) going to all parts of the body. As blood moves around the body, it transports heat to the cooler parts to maintain an even temperature
  • Waste products are transported from all body tissues to the kidneys
  • Hormones are transported from the glands to where they are needed

(2) Protection – the blood helps to protect the body in many ways:

  • Antitoxins are produced to fight toxins that might enter the body
  • Antibodies are produced by the lymphocytes to fight disease and are kept in the blood stream. They offer immunity to certain illnesses
  • Destruction is carried out by cells known as phagocytes. When bacteria cause a threat to the body, the phagocytes attach themselves to the harmful organisms and eat them!
  • Repair – nutrients and other materials are needed to repair damaged tissue
  • Clotting of blood is activated by platelets. When a blood vessel is cut, platelets join together forming a temporary plug to stop immediate blood loss

Short-term effects of aerobic exercise (during exercise)

  • Increase in blood pressure
  • Increase in heart rate
  • Increase in stroke volume
  • Increase in breathing rate
  • Build-up of carbon dioxide in the blood
  • Capillaries dilate
  • More blood is circulated around the body
  • Blood is diverted from soft organs to working muscles
  • Blood transports heat from muscles to surface of the body where it can escape

Long-term adaptations of aerobic training

  • Increase in size of heart
  • Lower resting heart rate
  • Increase in stroke volume
  • Cardiac output stays approximately the same
  • An increase in the size and number of mitochondria
  • Faster recovery (heart returns to resting heart rate faster after exercise has finished)
  • Reduced risk of coronary heart disease (CHD)
  • More red blood cells are found in the blood, improving the blood’s ability to carry oxygen
  • Blood supply to muscle fibres is improved
  • Return of deoxygenated blood to the heart is improved (venous return)
  • Blood pressure may be lowered
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