Microcirculation

# Overview
- The microcirculation is the blood flow through blood vessels smaller than 100 m (i.e. arterioles, capillaries, and venules), this contrasts with macrocirculation, which transports blood to and from the organs.
- The main functions of the microcirculation are transporting blood cells and substances to/from the tissues, and acting as body coolant in thermoregulation processes.
- It also contributes to tissues color and stiffness.
- Diffusion is the capillary exchange of substances across the endothelial cells membranes.

# The Components of the Microcirculation
- The vessels on the arterial side of the microcirculation are called the arterioles, which are well innervated, are surrounded by smooth muscle cells, and are 10-100 m in diameter.
- Arterioles carry the blood to the capillaries, which are not innervated, have no smooth muscle, and are about 5-8 m in diameter.
- Blood flows out of the capillaries into the venules, which have little smooth muscle and are 10-200 m. The blood flows from venules into the veins.
- In addition to these blood vessels, the microcirculation also includes lymphatic capillaries and collecting ducts.

# Determinants of Capillary Exchange
- 1- Surface area of the capillary bed
It is dependent on the metabolic activity of the tissue.
- It is dependent on the metabolic activity of the tissue.
- 2- Permeability of the capillary bed
It is dependent on the characteristic of each organ, for example the brain is relatively impermeable to most substances whereas the kidneys have very high permeability.
- It is dependent on the characteristic of each organ, for example the brain is relatively impermeable to most substances whereas the kidneys have very high permeability.
- 3- Free diffusion coefficient of the solutes to be exchanged
It is dependent on the solutes size, charge and lipid solubility.
- It is dependent on the solutes size, charge and lipid solubility.
- 4- Concentration of the solutes to be exchanged
The difference in concentration of the solutes in the blood and in the interstitium drives the flow of substances across the capillary bed.
- The difference in concentration of the solutes in the blood and in the interstitium drives the flow of substances across the capillary bed.

# Starling's Law
- The Starling equation is an equation that illustrates the role of hydrostatic and oncotic forces (the so-called Starling forces) in the movement of fluid across capillary membranes.
- According to Starling's equation, the movement of fluid depends on six variables:
Capillary hydrostatic pressure ( Pc )
Interstitial hydrostatic pressure ( Pi )
Capillary oncotic pressure ( c )
Interstitial oncotic pressure ( i )
Filtration coefficient ( Kf )
Reflection coefficient (  )
- Capillary hydrostatic pressure ( Pc )
- Interstitial hydrostatic pressure ( Pi )
- Capillary oncotic pressure ( c )
- Interstitial oncotic pressure ( i )
- Filtration coefficient ( Kf )
- Reflection coefficient (  )
- Starling's equation reads as follows:

# Transvascular Exchange of Water
- Water moves across the capillary walls based on the same principles of Starling's law and thus it depends on the difference of hydrostatic pressures and oncotic pressures in the blood vessels and in the interstitium.
- Filtration is the movement of water out of the vessels
- Reabsorption is the movement of water in the vessels.