Structure+and+composition

=The Structure and Composition of the blood=

=__Topic B : Blood Vessels__= = = = =
 * Tissues**: Tissues are made up of only 1 type of cell. capillaries are tissues as they are made of only endothelial tissue, in which each cell is the same.

An artery is a organ, and its function is to carry blood to the tissues in all parts of the body such as the capillaries. Arteries are made up of a number of different tissues such as endothelium, which ensures smooth blood flow and elastic tissue to enable the artery to expand and deal with high blood pressures.
 * Organs:** organs have a specific function and are made up of a number of different tissues.

The Blood Vessels
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 * Arteries**: Arteries carry blood **A**way from the heart to the lungs and organs and therefore need to withstand higher pressures than the vein. The artery has thick walls made up of smooth muscle and elastic tissues to withstand these higher pressures.


 * Arterioles:** Small arterioles occur when arteries divide mear an organ. Arteriole walls contain a relatively large proportion of smooth muscle tissue to enable the opening and closing of artery lumens when diverting blood from one organ to another (for example anerobic respiration). During excercise we need more blood to flow to our muscle tissue rather than our digestive system.


 * Veins:** Ve**IN**s carry blood **INto** the heart. The blood pressure in a vein is very low and the blood flows very slowly and may even stop. The lumen (width) of the vein is much larger than an artery because the pressure is much lower. Veins have Valves to maintain a one way system and prevent the back flow of blood as slow speeds. Valves are controlled by pressure and contraction of muscle within the vein.


 * Capillaries**: Capillaries are the smallest blood vessel, they are only one cell thick. The walls of the cappilary are made up if one layer of endothelical cells. It is vital for the cell to have a wall this thin to provide a small diffusion pathway for a high diffusion rate of oxygen from the capillary through the cell to the tissue which the capillary serves.

**The Return of Tissue Fluid to the Blood Stream**

 * At the arterial end of the capillary bed**: The hydrostatic pressure of the blood is higher than the water potential of the blood at the arterial end of the capillary because water and small solutes are forced via plasma out of the capillary at +1 KPa due to high pressures, this increases the concentration of protein. Overall water is forced **OUT** of the capillary and collects as tissue fluid between the cells.


 * At the venous end of the capillary bed:** As blood passes along the capillry bed, its hydrostatic pressure decreses untill it becomes numerically lower than the water potential. It is at this point water moves back into the capillaries via osmosis at a net pressure of 2 KPa. The small solute molecules also move back into the capillary via diffusion.


 * Tissue Fluid is Forced OUT of the Capillary at the ARTERIAL END and reabsorbed IN at the VENOUS END.

Gaseous Exchange
The alveoli are the sites of pulmonary gaseous exchange. the walls of the alveoli (like the capillary) are extrememly thin, which is vital to provide a short diffusion pathway for rapid diffusion. The blood entering the alveoli has a high concentration of carbon dioxide compared to the atmosphere which causes it to diffuse into the air space of the alveolous.