Blood vessels

• Blood circulates from the heart through the arteries to the capillaries to the veins and back to the heart.
• There is a decrease in blood pressure as blood moves away from the heart.
• The endothelium lining the central lumen of blood vessels is surrounded by layers of tissue.
• Arteries have an outer layer of connective tissue containing elastic fibres and a middle layer containing smooth muscle with more elastic fibres.
• The elastic walls of the arteries stretch and recoil to accommodate the surge of blood after each contraction of the heart.
• The smooth muscle can contract or relax causing vasoconstriction (contract) or vasodilation (relax) to control blood flow.
• Veins have an outer layer of connective tissue containing elastic fibres but a much thinner muscular wall than arteries.
• Valves prevent the backflow of blood through the veins.
• Capillaries allow exchange of substances with tissues.

Exchange of substances between plasma and tissue fluid

• When blood arrives at a capillary bed, the pressure forces some of the plasma out through the thin walls into the space outside by pressure filtration.
• Tissue fluid and blood plasma are similar in composition, with the exception of plasma proteins, which are too large to be filtered through the capillary walls.
• Tissue fluid allows exchange of materials between capillaries and tissue cells.
• Tissue fluid supplies cells with glucose, oxygen and other substances.
• Carbon dioxide and other metabolic wastes diffuse out of the cells and into the tissue fluid to be excreted.
• Much of the tissue fluid returns to the blood.
• Lymphatic vessels absorb excess tissue fluid and return the lymph fluid to the circulatory system.