Cardiovascular Physiology: Microcirculation

Microcirculation Structure

• The microcirculation of each organ is organized to serve the organ's specific needs
• Each artery entering an organ branches 6-8 times before becoming small enough to be called arterioles
• Arterioles branch 2-5 times until they become terminal arterioles that supply blood to capillaries
• Terminal arterioles are called metarterioles
• Metarterioles do not have a continuous muscular coat, but smooth muscle fibers encircle the vessel at intermittent points

Precapillary Sphincter

• The point where each true capillary originates from a metarteriole has a smooth muscle fiber that usually encircles the capillary, called a precapillary sphincter
• The precapillary sphincter can open or close the entrance of the capillary

Capillaries

• Not all capillaries are always perfused with blood, it depends on metabolic needs
• The capillary wall is composed of a single layer of endothelial cells and a thin basement membrane
• Special types of pores occur in the capillaries of certain organs, such as the brain, liver, and kidney

Venous Structure

• Venules are larger than arterioles and have a much weaker muscular coat
• The pressure is lower and allows constriction despite the weak muscle

Capillary Fluid Exchange

• Capillary arterial pressure = 35 mmHg, capillary venous pressure = 17 mmHg
• Interstitial fluid hydrostatic pressure (Pif) drives fluid out of the interstitial space into the capillaries
• Capillary oncotic pressure (πp) is also called colloid osmotic pressure, and is exerted by plasma proteins (80% by albumin), favoring reabsorption
• Interstitial fluid oncotic pressure (πif) is determined by the interstitial fluid protein concentration and favors filtration
• Net filtration pressure = hydrostatic pressure - oncotic pressure

Forces Involved in Fluid Exchange

• Hydrostatic pressure pushes fluid out of the capillaries
• Osmotic pressure pulls fluid inside of the capillaries
• If the result of these forces is positive, there will be a net fluid filtration across the capillaries
• If the result is negative, there will be a net fluid reabsorption from the interstitial fluid into the capillaries

Permeability of the Capillary Wall

• Permeability is increased in capillary injury (e.g. toxins, burns, inflammation), which increases permeability to water
• Hydraulic conductance (Kf) is the water permeability of the capillary wall and varies among different types of tissues

Lymphatic System

• The lymphatic system is an accessory route through which fluid can flow from the interstitial spaces into the blood
• Lymphatics carry proteins and large particle matter away from the tissue spaces
• Lymphatic capillaries possess one-way flap valves that permit interstitial fluid and protein to enter but not to leave
• Lymphatic capillaries merge into larger lymphatic vessels/thoracic duct, and lymph flow depends on contraction of the smooth muscle in the lymph vessels and compression of the lymph vessels by activity of the surrounding skeletal muscle
• The lymphatic system plays a key role in controlling interstitial fluid protein concentration, volume, and pressure

Clinical Application: Edema and Effusion

• Changes in Starling forces can influence the direction and magnitude of fluid movement
• Edema is an increase in interstitial fluid volume (swelling)
• Effusion is an abnormal collection of fluid in a hollow space or between tissues (e.g. pleural effusion)
• Congestive heart failure can lead to edema or effusion due to an increase in capillary hydrostatic pressure