What is Blood vessels ? Types, Functions

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.

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Blood Ves­sel

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.

Table of Contents

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

  1. To act as a con­duct for oxy­genat­ed blood (except for Pul­monary artery which car­ries deoxy­genat­ed blood) between heart and capillaries
  2. To act as a pres­sure is aware for forc­ing blood into small diam­e­ter arterioles.
  3. To pro­duce a more or less even flow of blood by means of the capillaries
  4. 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.

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Artery

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.

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Veins

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.

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Capi­lar­ies

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.

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