Blood Vessels and their Functions

Questions: State two factors that make it more likely that an organism will have a circulatory pump such as the heart. What is the main advantage of the double circulation system found in mammals?

Learning Objectives: What are the structures of arteries, arterioles and veins? How is the structure of each of the blood vessels related to its function? What is the structure of capillaries and how is it related to their function?

Blood Vessels Arteries: Carry blood away from the heart Arterioles: Control blood flow from arteries to capillaries Capillaries: Link arterioles to veins Veins: Carry blood towards the heart

Structure of Blood Vessels Tough outer layer – resists pressure Muscle layer – can contract and control the flow of blood Elastic layer – can stretch and recoil to maintain blood pressure Endothelium – smooth layer to prevent friction Lumen – not a layer – a cavity

Structure of Arteries Thick muscle layer – control the flow of blood Thick elastic layer – smooth surges from the heart No valves

Structure of Arterioles Thicker muscle layer than arteries Thinner elastic layer than arteries No valves

Structure of Veins Thin muscle layer Thin elastic layer Valves

Structure of Capillaries No muscle No elastic No valves Thin layer of cells only

Capillary Structure to Function Thin layer of cells – short diffusion distance. Numerous and highly branched – large SA for diffusion. Narrow diameter – keep all cells close by. Narrow lumen – bring RBC close to the cells = short diffusion distance. Spaces between cells – allow WBC to escape.

Capillary – endothelium – large number – large surface area for exchange Wall - one cell thick – short diffusion distance Endothelium is continuous throughout circulatory system Capillary Artery Vein Narrow lumen; High pressure Highly elastic – expand and recoil Thick muscular wall – to withstand force; more elastic fibres (recoil) No valves (except aortic and pulmonary semilunar at the start) Oxygenated blood from heart – except pulmonary artery to lungs Pulsatile blood flow (expansion + recoil) Pulse can be felt – e.g. wrist Wide lumen; Low pressure Thin wall - less elastic and less muscular Valves (semilunar) – prevent backflow Deoxygenated blood to heart from tissues - except pulmonary vein from lungs Non pulsatile – smooth flow of blood

Tissue Fluid

Tissue Fluid What is the role of tissue fluid? It is the fluid which allows the exchange of substances between the blood and cells What substances are found in tissue fluid? glucose, amino acids, fatty acids, salts and oxygen = all delivered to the cells. carbon dioxide and other waste substances = removed from the cells.

Hydrostatic Pressure As the capillaries are narrower than the arterioles, a pressure builds up which forces tissue fluid out of the blood plasma = hydrostatic pressure. This pressure is resisted by: Pressure of the tissue fluid on the capillaries (from the outside) The lower water potential of the blood (caused by plasma proteins – too large to leave the blood) Overall, pressure pushes tissue fluid and small molecules out of the capillary, leaving cells and large proteins behind = ultrafiltration.

Return of tissue fluid Most tissue fluid is returned to the blood plasma via the capillaries. Hydrostatic pressure at the venule end of the capillary is higher outside the capillary and tissue fluid is forced back in. Osmotic forces (resulting from the proteins in the plasma) pull water back into capillaries. Remaining tissue fluid enters the lymph vessels – drain back into the veins close to the heart.

Hydrostatic pressure

Lymph System

Lymph Lymph is moved by: Hydrostatic pressure Contraction of body muscles (aided by valves in the lymph vessels)