A&P II- Capillaries

Vessel Function

 Function:

 Carry and control the flow of blood.

 Capillaries – site of nutrient, waste, gas exchange

Vessel Structure and Function

 1 basic layer or “tunic” which comprises the vessel wall:

• Tunica interna (intima): epithelial tissue and basement membrane

Capillaries :

• only sites in the entire vasculature where gases, water and other nutrients are exchanged.

 Metarteriole :

• Branch from arteriole to venule

• Precapillary sphincter controls flow to capillary bed

Three types of capillaries:

 Continuous capillaries:

• most common

• continuous tube with

• small intercellular clefts.

 Fenestrated capillaries (fenestra = windows):

• kidneys, villi of small intestines, and endocrine glands

• Pores through endothelial cells

 Sinusoids: liver, spleen

• Large pores through which blood can percolate

Fluid Exchange- Starling Forces

 Starling Forces: hydrostatic and osmotic forces at the capillaries

 determine

• Filtration: movement of fluid through the walls of the capillary into the interstitial fluid.

• Reabsorption: movement of fluid from the interstitial fluid back into the capillary

Hydrostatic pressure physically pushes fluid out, generated by mechanical forces like the heart pumping.

Osmotic pressure pulls fluid in, created by solute concentration gradients

Filtration:

• Blood hydrostatic pressure (BHP) : decreases from 35 to 16 from the arterial to the venous end of the capillary

• Interstitial fluid osmotic pressure (IFOP): constant at about 1 mmHg

Reabsorption:

• Blood colloid osmotic pressure (BCOP): due to the presence of plasma proteins too large to exit the capillary

 averages 26 mmHg on both ends.

• Interstitial fluid hydrostatic pressure (IFHP) : close to zero except in states of edema

 Most (about 85%) of fluid filtered is reabsorbed