Blood & Circulation

Human Circulatory System
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Small organisms don’t have a bloodstream, but instead rely on the simple diffusion of materials for transport around their cells. This is OK for single cells, but it would take days for molecules to diffuse through a large animal, so most animals have a circulatory system with a pump to transport materials quickly around their bodies. This is an example of a mass flow system, which means the transport of substances in the flow of a fluid (as opposed to diffusion, which is the random motion of molecules in a stationary fluid). The transport of materials in the xylem and phloem of plants is another example of mass flow. Mass flow systems work together with the specialised exchange systems (such as lungs).

Humans have a double circulatory system with a 4-chambered heart. In humans the right side of the heart pumps blood to the lungs only and is called the pulmonary circulation, while the left side of the heart pumps blood to the rest of the body – the systemic circulation. The circulation of blood round the body was discovered by William Harvey in 1628. Until then people assumed that blood ebbed and flowed through the same tubes, because they hadn't seen capillaries.

Blood Vessels  [back to top]

Arteries carry blood from the heart to every tissue in the body. They have thick, elastic walls to withstand the high pressure of blood from the heart. The arteries close to the heart are particularly elastic and expand during systole (heart muscles contracting) and recoil again during diastole (heart muscles relaxing), helping to even out the pulsating blood flow. The smaller arteries and arterioles are more muscular and can contract (vasoconstriction) to close off the capillary beds to which they lead; or relax (vasodilation) to open up the capillary bed. These changes are happening constantly under the involuntary control of the medulla in the brain, and are most obvious in the capillary beds of the skin, causing the skin to change colour from pink (skin arterioles dilated) to blue (skin arterioles constricted). There is not enough blood to fill all the body’s capillaries, and at any given time up to 20% of the capillary beds are closed off.

Veins carry blood from every tissue in the body to the heart. The blood has lost almost all its pressure in the capillaries, so it is at low pressure inside veins and moving slowly. Veins therefore don’t need thick walls and they have a larger lumen that arteries, to reduce the resistance to flow. They also have semi-lunar valves to stop the blood flowing backwards. It is particularly difficult for blood to flow upwards through the legs to heart, and the flow is helped by contractions of the leg and abdominal muscles:

The body relies on constant contraction of these muscles to get the blood back to the heart, and this explains why soldiers standing still on parade for long periods can faint, and why sitting still on a long flight can cause swelling of the ankles and Deep Vein Thrombosis (DVT or “economy class syndrome”), where small blood clots collect in the legs.

Capillaries are where the transported substances actually enter and leave the blood. No exchange of materials takes place in the arteries and veins, whose walls are too thick and impermeable. Capillaries are very narrow and thin-walled, but there are a vast number of them (10⁸ m in one adult!), so they have a huge surface area : volume ratio, helping rapid diffusion of substances between blood and cells. Capillaries are arranged in networks called capillary beds feeding a group of cells, and no cell in the body is more than 2 cells away from a capillary.

Blood and Blood Cells  [back to top]

Blood is a specialised tissue containing a number of different living cell types floating in a non-living watery liquid called plasma. 55% of blood is made up of plasma and the balance is various blood cells that are suspended in this watery matrix. This pale yellow liquid is made up of 90% water and 10% dissolved substances.

The four main components in blood are shown in the diagram below:

There are dozens of different substances in blood, all being transported from one part of the body to another. Some of the main ones are listed in this table:

Substance	Where	Reason
Oxygen	Red blood cells	Transported from lungs to all cells for respiration
Carbon dioxide	Plasma	Transported from all cells to lungs for excretion
Nutrients (e.g. glucose, amino acids, vitamins, lipids, nucleotides)	Plasma	Transported from small intestine to liver and from liver to all cells
Waste products (e.g. urea, lactic acid)	Plasma	Transported from cells to liver and from liver to kidneys for excretion
3   4   Ions (e.g. Na+, K+, Ca2+, Mg2+, Cl-, HCO3, HPO2-, SO2-)	Plasma	Transported from small intestine to cells, and help buffer the blood pH.
Hormones	Plasma	Transported from glands to target organs
Proteins (eg albumins)	Plasma	Amino acid reserve
Blood clotting factors	Plasma	At least 13 different substances (mainly proteins) required to make blood clot.
Antigens and antibodies	Plasma	Part of immune system
Water	Plasma	Transported from large intestine and cells to kidneys for excretion.
Bacteria and viruses	plasma
Heat	Plasma	Transported from muscles to skin for heat exchange.

Erythrocytes (red blood cells) make up approximately 45% of blood. They are non-nucleated biconcave discs that carry oxygen around the body. A pigment called haemoglobin combines with the oxygen in the lungs to form oxyhaemoglobin. New red blood cells (RBC) are produced in the red bone marrow of the ribs, sternum, vertebrae, skull and long bones.

The main function of RBC is the transport of respiratory gasses.  Oxygen is carried in RBC bound to the protein haemoglobin.  A haemoglobin molecule consists of four polypeptide chains, with a haem prosthetic group at the centre of each chain.  Each haem group contain one iron atom, and one oxygen molecule binds to each iron atom.  Therefore haemoglobin can bind up to four oxygen molecules.  RBC have specific features (listed below) that make it efficient in absorbing and transporting respiratory gasses:

• They have a small size - They are much smaller than most other cells in the body.  This means that all the haemoglobin molecules are close to the surface, allowing oxygen to be picked up and release rapidly.

• Shape - RBC are biconcave shapes discs.  It allows the cell to contain a lot of haemoglobin while still allowing efficient diffusion through the plasma membrane

• Organelles – RBC do not contain  either nuclei or mitochondria.  This allows more space inside the cell for haemoglobin.

Leucocytes (white blood cells) are nucleated amoeboid cells, which are much larger than red blood cells and protect us against disease. They have the ability of leaving the blood vessels to invade diseased tissues. White blood cells (WBC) are arranged according to whether they are granular or agranular. Granular WBC are large cells containing a nucleus and granular cytoplasm. There are three kinds - neutrophils, eosinophils and basophils. Agranular WBC have round or kidney-shaped nuclei and cytoplasm that lacks any granules. There are two types - lymphocytes and monocytes.

Thrombocytes are bits of broken up blood cells that help clot the blood when we cut ourselves and bleed. When we bleed, platelets, chemicals and substances called clotting proteins (prothrombin) help to form an insoluble 'plug' to seal off the bleeding point.  

Tissue Fluid  [back to top]

These substances are all exchanged between the blood and the cells in capillary beds. Substances do not actually move directly between the blood and the cell: they first diffuse into the tissue fluid that surrounds all cells, and then diffuse from there to the cells.

1.   At the arterial end of the capillary bed the blood is still at high hydrostatic pressure, so blood plasma is squeezed out through the permeable walls of the capillary. Cells and proteins are too big to leave the capillary, so they remain in the blood.

2.   This fluid now forms tissue fluid surrounding the cells. Materials are exchanged between the tissue fluid and the cells by all four methods of transport across a cell membrane. Gases and lipid-soluble substances (such as steroids) cross by lipid diffusion; water crosses by osmosis, ions cross by facilitated diffusion; and glucose and amino acids cross by active transport.

3.   At the venous end of the capillary bed the blood is at low pressure, since it has lost so much plasma. Water returns to the blood by osmosis since the blood has a low water potential. Solutes (such as carbon dioxide, urea, salts, etc) enter the blood by diffusion, down their concentration gradients.

4.   Not all the plasma that left the blood returns to it, so there is excess tissue fluid. This excess drains into lymph vessels, which are found in all capillary beds. Lymph vessels have very thin walls, like capillaries, and tissue fluid can easily diffuse inside, forming lymph.

The Lymphatic System  [back to top]

The lymphatic system consists of a network of lymph vessels flowing alongside the veins. The vessels lead towards the heart, where the lymph drains back into the blood system at the superior vena cava. There is no pump, but there are numerous semi-lunar valves, and lymph is helped along by contraction of muscles, just as in veins. Lymph vessels also absorb fats from the small intestine, where they form lacteals inside each villus. There are networks of lymph vessels at various places in the body (such as tonsils and armpits) called lymph nodes where white blood cells develop. These become swollen if more white blood cells are required to fight an infection.

Remember the difference between these four solutions:

Plasma            The liquid part of blood. It contains dissolved glucose, amino acids, salts and vitamins; and suspended proteins and fats.

Serum              Purified blood plasma used in hospitals for blood transfusions.

Tissue Fluid     The solution surrounding cells. Its composition is similar to plasma, but without proteins (which stay in the blood capillaries).

Lymph             The solution inside lymph vessels. Its composition is similar to tissue fluid, but with more fats (from the digestive system).

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Last updated 20/06/2004