Brent Cornell

    
Structure and Function

The function of capillaries is to exchange materials between the cells in tissues and blood travelling at low pressure (<10mmHg)

• Arteries split into arterioles which in turn split into capillaries, decreasing arterial pressure as total vessel volume is increased
• The branching of arteries into capillaries therefore ensures blood is moving slowly and all cells are located near a blood supply
• After material exchange has occurred, capillaries will pool into venules which will in turn collate into larger veins

Capillaries have specialised structures in order to accomplish their task of material exchange:

• They have a very small diameter (~ 5 µm wide) which allows passage of only a single red blood cell at a time (optimal exchange)
• The capillary wall is made of a single layer of cells to minimise the diffusion distance for permeable materials
• They are surrounded by a basement membrane which is permeable to necessary materials
• They may contain pores to further aid in the transport of materials between tissue fluid and blood

Capillaries structure may vary depending on its location in the body and specific role:

• The capillary wall may be continuous with endothelial cells held together by tight junctions to limit permeability of large molecules
• In tissues specialised for absorption (e.g. intestines, kidneys), the capillary wall may be fenestrated (contains pores)
• Some capillaries are sinusoidal and have open spaces between cells and be permeable to large molcules and cells (e.g. in liver)

Capillary Structure

    
Flow of Blood

Blood flows through the capillaries very slowly and at a very low pressure in order to allow for maximal material exchange

• The high blood pressure in arteries is dissapated by extensive branching of the vessels and the narrowing of the lumen

The higher hydrostatic pressure at the arteriole end of the capillary forces material from the bloodstream into the tissue fluid

• Material that exits the capillaries at body tissues include oxygen and nutrients (needed by the cells for respiration)

The lower hydrostatic pressure at the venule end of the capillary allows materials from the tissues to enter the bloodstream

• Materials that enters the capillaries at body tissues include carbon dioxide and urea (wastes produced by the cells)

Material Exchange in the Capillaries