 Arteries  Capillaries  Veins

Muscular Walls

Arteries  These vessels receive blood at high pressure from the heart  Thick Muscular walls help them to withstand that pressure  The walls (outer structure) of arteries contain smooth muscle fibre that contract and relax under the instructions of the sympathetic nervous system.  Transport blood away from the heart;  Transport oxygenated blood only (except in the case of the pulmonary artery).  They maintain the blood pressure to all parts of the body

Arterioles  Arterioles are tiny branches of arteries that lead to capillaries. These are also under the control of the sympathetic nervous system, and constrict and dilate, to regulate blood flow.  Transport blood from arteries to capillaries;  Arterioles are the main regulators of blood flow and pressure.

Capillaries  Walls are only one cell thick which permits exchanges of material between the contents of the capillary and the surrounding tissue.  Permeable to small particles eg. Molecules of gas, nutrients and waste.  Capillaries branch repeatedly to form a network within tissues.  Capillaries are tiny (extremely narrow) blood vessels, of approximately 5-20 micro-metres (one micro-metre = metre) diameter.  There are networks of capillaries in most of the organs and tissues of the body. These capillaries are supplied with blood by arterioles and drained by venules.  Function is to supply tissues with substances in the blood & substances carried by the blood, and also to remove waste from the surrounding cells... as opposed to simply moving the blood around the body (in the case of other blood vessels);  Exchange of oxygen, carbon dioxide, water, salts, etc., between the blood and the surrounding body tissues.

Veins  Return blood at low pressure to the heart from the tissues  Their wide lumens helps to reduce the resistance of blood flow  Reduced pressure means that they have only thin walls  The walls (outer structure) of veins consist of three layers of tissues that are thinner and less elastic than the corresponding layers of arteries.  Veins contain valves that aid the return of blood to the heart by preventing blood from flowing in the reverse direction. One way flow is maintained.  Transport blood towards the heart;  Transport deoxygenated blood only (except in the case of the pulmonary vein).

Venules  Venules are minute vessels that drain blood from capillaries and into veins. Many venules unite to form a vein.  Drains blood from capillaries into veins, for return to the heart

Comparison between Arteries and Veins Arteries  Transport blood away from the heart;  Carry Oxygenated Blood (except in the case of the Pulmonary Artery);  Have relatively narrow lumens (see diagram above);  Have relatively more muscle/elastic tissue;  Transports blood under higher pressure (than veins);  Do not have valves (except for the semi-lunar valves of the pulmonary artery and the aorta). Veins  Transport blood towards the heart;  Carry De-oxygenated Blood (except in the case of the Pulmonary Vein);  Have relatively wide lumens (see diagram above);  Have relatively less muscle/elastic tissue;  Transports blood under lower pressure (than arteries);  Have valves throughout the main veins of the body. These are to prevent blood flowing in the wrong direction, as this could (in theory) return waste materials to the tissues.

Major Organs and Associated Blood Vessels

Formation of Tissue fluid Tissue Fluid

Formation of Tissue Fluid cont... Tissue Fluid (Interstiti al Fluid)

Tissue Fluid  Blood flow slows down in these vessels  High Blood Pressure at the arterial end forces the plasma out through the capillary walls and the dissolved substances in it  The plasma that baths the cells is know as tissue fluid  No cell is more than 100 micrometers from a capillary  Low Blood Pressure at the venous end allows some of the tissue fluid to re-enter the veins

Tissue Fluid cont...  The blood is more concentrated at the venous end  The rest of the tissue fluid enters the lymphatic capillaries which make up part of the lymphatic system  The lymphatic system eventually rejoins the circulatory system and its contents are returned

Table summarizing blood Vessels ArteriesCapillariesVeins Blood carried Pressure Blood Flow Gases Valves Position of Vessels