There are two main types of blood vessels in the circulatory system artery and veins with characteristic differences among them. Artery have relatively thick walls that consist of heavy strong layer of Elastic fibres and smooth muscles. As the artery is branch the diameter becomes smaller and the relative amount of muscle tissue increases in proportion to the elastic tissue. The smallest units of the artery are the capillaries which are single cell thick and play a major role in all exchange between tissue and blood which takes place through the walls of the capillaries. The veins also contain smooth muscles and fibres.
The walls of the arteries and veins are largely composed of living cells and need to be supplied with nutrition. the walls of the artery and veins also produce waste. for this reason these blood vessels have their own blood supply in the form of capillary network known as “Vasa vasorum” or “vessels within vessels”.Minute nerves are also present within the walls of the capillaries of the Vasa vasorum which control and coordinates the construction and relaxation of these capillaries.
ARTERIES
The arteries except for the Pulmonary artery carry deoxygenated blood from the heart to the various cells and tissue.The arteries have four main functions
- To act as a conduct for oxygenated blood (except for Pulmonary artery which carries deoxygenated blood) between heart and capillaries
- To act as a pressure is aware for forcing blood into small diameter arterioles.
- To produce a more or less even flow of blood by means of the capillaries
- To control distribution of blood to different capillaries network via selective construction of the terminal branches.
There is a precise control on arterial blood pressure. the nature of the arterial wall and the volume of blood pumped into the arteries determines the pressure will stop if any of these are change the pressure will also changed.Normally arterial blood pressure varies a little as a cardiac output and the capillary flow are evenly matched.
Elastic properties of arterial wall vary. close to the heart the arteries are elastic and dampen (reduced )oscillation and pressure and blood flow generated by the contraction of the heart.
VEINS
The veins bring back the oxygenated blood (except the Pulmonary vein which carries oxygenated blood) to the heart from the capillary via the venules.The form a large volume low pressure system. the vessels have a larger internal diameter and Pocket valves.In mammals 50% of the total blood volume is present in the veins and the pressure in the veins is approximately 10 25 mm hg. if there is any blood loss the venous volume is decreased and not the arterial volume.2So that arterial blood pressure and capillary blood flow is maintained.
The flow of blood in veins is affected by several factors. pressure exerted by the diaphragm on the gut and the activity of the limbus both help to squeeze the Veins of those regions.The squeezing and also the action of the pocket valves that (prevent back flow) help the flow of blood towards the heart. breathing in mammals also help in drawing the blood from the veins in the head and abdominal cavity.
Smooth muscles in veins also help in regulating blood supply in the venous system. when a person changes his position from sitting to standing the change in the relative position of heart in brain with respect to gravity activates the nerve fibres that are present in the veins of the limb.This cause a contraction of smooth muscles of the veins and result in pooled blood to be redistributed.
CAPILARIES
Most Tissue have such an extensive network of capillaries that any single cell is hardly 2–3 cells away from any capillary.Small terminal artery subdivide to form arteries which divides to form metarterioles and then capillaries. The capillaries join the venous which contain the venous blood of the capillaries.The smooth muscles of the arterioles become discontinuous in the metarterioles and end in a muscle ring the precapillary sphincter that controls the blood supply to each capillary bed.
Through the precapillary Spencer pressure supply to the capillary bed can be bypassed altogether and blood can be diverted to areas of Greater demand.All capillaries of an animal have the potential to hold 14% of total blood volume. however only 30% to 50% off all capillaries are open at a time and has only 5–7 % of the total volume is contained in them. several venules join together to form the veins.
Capillaries are made up of a single layer of endothelial cells surrounded by a basal membrane.The walls are thin and projectile but because of their small diameter can resist stretching In response to capillary blood pressure.
Water and dissolved substance of small molecular weight (gases, salt ‚sugar, amino acid etc) can diffuse easily in and out of the capillaries. In addition flute is forced out to the capillary walls. substance of molecular weight more than 70000 Dalton mostly proteins do not pass in or out of the capillary walls. these proteins of molecular weight more than 70010 Dalton when present in the capillary exit and osmotic pressure called the colloidal osmotic pressure,Which tends to draw water back into capillary from the surrounding tissue fluid (reabsorption).Another force known as hydrostatic pressure of blood tends to push the water across the endothelial cell layer and so out of capillaries (filtration).When the hydrostatic pressure within the capillary exceed colloidal osmotic pressure fluid is passed out through the capillary wall into the tissues.However when the hydrostatic pressure in the capillary Falls below the colloidal osmotic pressure then fluid is drawn in from the tissue.At the arterial end of capillary the hydrostatic pressure is higher than the colloidal osmotic pressure while at the venous and it is often lower.Therefore fluid is filtered out at the arterial and and taken out at the venous end.
The amount of fluid which is forced out and out and the amount of fluid re-entering the capillary varies greatly. usually outflow exceeds inflow and excess fluid remains in the interstitial spaces. this is forms the Lymph.