9.3) Blood and lymphatic vessels
Arterioles, shunt vessels and venules:
- The small arteries and arterioles have proportionately less elastic tissue and more muscle fibres than the great arteries.
- When the muscle fibres of the arterioles contract, they make the vessels narrower and restrict the blood flow (process called vasoconstriction). In this way, the distribution of blood to different parts of the body can be regulated.
- Shunt vessels, linking the arterioles with venules, dilate to allow the blood to bypass the capillaries. This helps to reduce further heat loss.
Main blood vessels:
- Pulmonary Veins: This transports oxygenated blood from the lungs to the left atrium
- Aorta: This transports oxygenated blood from the heart to the rest of the body and the brain.
- Vena Cava: This transports deoxygenated blood from the rest of the body to the heart.
- Pulmonary Artery: Transports deoxygenated blood from body to the heart to the lungs.
- Hepatic Artery: This supplies oxygenated blood to the liver (also pylorus (part of stomach), duodenum and pancreas).
- Hepatic Portal Vein: This transports blood from the gastrointestinal tract and spleen to the liver, rich in nutrients to be processed by the liver.
- Renal Artery: This supplies kidney with oxygenated blood.
- Renal Vein: This transports blood away from the kidney, into the inferior Vena Cava.
The lymphatic system:
- Not all the tissue fluid returns to the capillaries. Some of it enters blind-ended, thin-walled vessels called lymphatics.
- The lymphatics from all parts of the body join up to make two large vessels, which empty their contents into the blood system.
- The fluid in the lymphatic vessels is called lymph.
- Some of the larger lymphatics can contract, but most of the lymph flow results from the vessels being compressed from time to time when the body muscles contract in movements such as walking or breathing.
- There are valves in the lymphatics, so that when the lymphatics are squashed, the fluid in them are forced in one direction only: towards the heart.
- At certain points in the lymphatic vessels there are swellings called lymph nodes.
- Lymphocytes are stored in the lymph nodes and released into the lymph to eventually reach the blood system.
- There are also phagocytes in the lymph nodes. If bacteria enter a wound and are not ingested by the white cells of the blood or lymph, they will be carried in the lymph to a lymph node and white cells there will ingest them.
- The lymph nodes thus form part of the body’s defence system against infection.