Capillaries, Fluid Exchange and the Lymphatic System

Hi, in this post we'll be talking about the different types of capillaries and how substances can diffuse through capillary walls. I'll also talk about the role of the lymphatic in returning fluid to the circulatory system as well as venous pressure. 

Types of Capillaries

Capillaries are blood vessels that allow the rapid exchange of substances, there are 3 types of capillaries:

1. Continuous Capillaries: the endothelial cell wraps around the lumen and is sealed by tight junctions. This type of capillary can be found in places such as the muscles and lungs.
2. Fenestrated capillaries: the endothelial cells have windows for increased permeability. They can be found in areas such as the endocrine glands and intestinal tract. 
3. Discontinuous Capillaries: these have large gaps between endothelial cells to allow the passage of proteins. They can be found in places such as the bone marrow and spleen. 

This website has some nice diagrams which show what each of these look like. 


Control of Flow Through Capillary Beds

Not all capillaries carry blood at all times. Arterioles (which precede capillaries) alternate between constriction and dilation to periodically reduce or stop blood flow to capillary beds. This is under the control of local factors and the autonomic nervous system. Metarterioles have isolated rings of smooth muscle and act as shunts which directly connect arterioles to venules. When the metarterioles are open blood bypasses the capillary beds and when closed blood flows through the capillary beds. There are also precapillary sphincters which reduce flow to individual capillaries. They are under the control of only local factors such as carbon dioxide.   

Factors Affecting Diffusion Across Capillaries 

Water soluble substances are able to pass through the water-filled pores located between the endothelial cells. Lipid-soluble substances diffuse across the endothelial cells while some proteins cross via a process called transcytosis. There are several factors which affect the rate at which substances diffuse into capillaries:

1. concentration difference
2. area available for diffusion
3. diffusion distance: the distance from the tissue cell to the nearest capillary
4. diffusion coefficient: which increases with an increase in temperature.

These are all based on Fick's Law and are physiologically adjustable.

Bulk Flow  

Bulk flow refers to the movement of water and solutes along pressure gradients. Across the capillary, it is the movement of protein free plasma where filtration (movement out of capillary) and absorption (movement into the capillary) are possible. The purpose of bulk flow is to distribute the extracellular fluid. Bulk flow is driven by hydrostatic and osmotic pressures (known as Starling forces). Hydrostatic pressures are forces due to the fluid pressing on the vessel walls and the pressure in the interstitial tissue. Water moves from high hydrostatic pressure to low hydrostatic pressure. Osmotic pressure is the pressure exerted by the movement of water down its concentration gradient. Water moves from areas of low osmotic pressure to areas of high osmotic pressure. There are four Starling forces which act on capillaries:

1. Capillary hydrostatic pressure: the mean functional capillary pressure (17.3 mmHg)(filtration)
2. Interstitial hydrostatic pressure: the interstitial free fluid hydrostatic pressure. (-3 mmHg)(filtration)
3. Plasma colloid osmotic pressure: the capillary pressure associated with proteins in blood.(28 mmHg)(absorption)
4. Interstitial colloid osmotic pressure:the interstitial tissue pressure associated with proteins. (8 mmHg)(filtration)

The total amount of force tending to move fluid out the capillary is 28.3mmHg while the amount of force tending to move fluid into the capillary is 28.0 mmHg. Thus there is a net movement of blood into the interstitial fluid. The lymphatic system picks up this fluid and returns it to the circulatory system.

 The Lymphatic System

As mentioned earlier, the capillaries experience a net filtration of 0.3 mmHg. This may not seem like much, however, in an adult human this amounts to 2mL/min being lost from the blood stream. This adds up to 3 L/day of fluid lost from the circulatory system. This fluid is returned to the circulation via the lymphatics. 

Central Venous Pressure

Central venous pressure is the pressure in the large veins of the thoracic cavity that lead into the right atrium. Several factors affect central venous pressure including:

• The activity of the muscle pump
• the activity of the respiratory pump
• The activity in the sympathetic nerves: an increase in activity leads to an increase in venomotor tone (the degree of tension in the muscle coat of a vein which determines its shape) which leads to a decrease in compliance of the veins.
• Blood volume 

An increase in all of these will lead to an increase in venous pressure. An in crease in venous pressure results in increased driving force for venous return which leads to an increase in preload. This results in an increase in end diastolic volume which increases the stroke volume and cardiac output. This causes an increase in the amount of blood flow into the systemic circuit. 




And that's all for this post, see you next time :)