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Microcirculation

It describes the circulation of blood in the smallest type of blood vessels “the capillary” which

has a wall made up of a single layer of endothelial cells. This has two ends “arteriolar end and venous

end”. Between the arteriolar end and the beginning of   the capillary there is precapillary sphincter 

“metarteriole”, this serves as a switch determining the blood flow to the capillary bed. 

  All the substances cross the capillary walls by the simple diffusion.

  Lipid soluble substances “O2 and CO2” cross the capillary wall by diffusing through the

endothelial cells.

  Water soluble substances “amino acids, glucose and ions” cross the capillary wall by diffusing

through the aqueous pores between the endothelial cells.

  Proteins have large molecular size so they cannot cross the cross the capillaries, except the

glomerular capillaries where the gaps are large enough for the proteins to diffuse between

them.

  Fluids movement mainly depends on the osmotic pressure “Starling Force”.   The fluid exchange process is basically determined by two types of pressures:

1-  Hydrostatic pressure.

2-  Effective osmotic pressure AKA “oncotic pressure” or “colloid osmotic pressure”, which

depends on the protein concentration.

  Starling Equation: describes the movement of fluids across a capillary wall.

Jv = Kf [ (Pc – Pi) – ( c – i) ]

  Jv “Fluid movement”. 

  Kf “Hydraulic conductance”:  it indicates the water permeability across the capillary wall. The

glomerular capillaries have the highest Kf , whereas the cerebral capillaries have the lowest K f .

  Pc “Capillary hydrostatic pressure”: is a force favoring filtration out of the capillary. The value of

Pc is closer to the arterial pressure. Pc is more affected by changes in the venous pressure except

in glomerular capillaries. Pc declines along the length of the capillaries, therefore Pc is higher at

the arteriolar end of the capillary.

  Pi “Interstitial hydrostatic pressure”: is a force opposing filtration. Pi is normally equal to zero,

or it may be slightly negative.

  c “Capillary oncotic pressure”:  is the force opposing filtration. And it depends on the protein

concentration.Protein concentration c Filtration

  i “Interstitial oncotic pressure”: is a force favoring filtration. It is determined by the interstitial

fluid protein concentration. 

  Filtration force is indicated by the “+” sign. 

  Absorption force is indicated by the “-“ sign. 

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  Pc and c are the major forces, Pc favors filtration out of the capillary but c favors the

absorption.

Lymph

The lymphatic system takes the lymph from the interstitial spaces and returns it into the large veins

“unidirectional movement”. 

Lymph “in interstitial spaces” Thoracic duct Large veins

The lymph flow is promoted by:

1-  Contraction of smooth muscle cells in the lymph vessels.

2-  Compression of lymph vessels by the activity of the surrounding skeletal muscles.

Edema: is the increase in the interstitial fluid volume due to increased filtration out of the capillaries.

This is due to:

1-  Increased Kf : due to destruction of the capillary wall or release of cytokines and histamines.

2-  Increased Pc: due to arteriolar dilation, venous constriction, heart failure or increased ECF

volume.

3-  Decreased c: due to less plasma proteins “malnutrition or nephrotic syndrome”.  

Other causes:

  Long standing.

  Removal of lymph nodes.

  Parasitic infection of lymph nodes.