Fluid Physiology

4.1 Microcirculation

What is the 'microcirculation'?

The microcirculation refers to the smallest blood vessels in the body:

The lymph vessels are not included. The arterioles contain vascular smooth muscle and are the major site of systemic vascular resistance. In skeletal muscle and other tissues, a large number of capillaries remain closed for long periods due to contraction of the precapillary sphincter. These capillaries provide a reserve flow capacity and can open quickly in response to local conditions such as a fall in pO2 when additional flow is required.

The microcirculation of some tissues (eg skin) contain direct AV connections which act as shunts. The flow in these shunts does not participate in transfer of gases, nutrients or wastes. These AV shunts are under the control of the nervous system. In the skin, opening or closing of these shunts is important in heat regulation.

The smooth muscle of the metarterioles and the precapillary sphincters contracts and relaxes regularly causing intermittent flow in the capillaries: this is known as vasomotion. A local drop in pO2 is the most important factor causing relaxation of the precapillary sphincters. The intermittent flow is not due to the cyclical rise and fall of the blood pressure as these fluctuations are almost completely damped out by the arterioles.

The principal function of the microcirculation is to permit the transfer of substances between the tissues and the circulation. This transfer occurs predominantly across the walls of the capillaries but some exchange occurs in the small venules also. Substances involved include water, electrolytes, gases (O2, CO2), nitrogenous wastes, glucose, lipids and drugs.

Electrolytes and other small molecules cross the membrane through pores. Lipid soluble substances (including oxygen and carbon dioxide) can also easily cross the thin (1 mm) capillary walls. Proteins are large and do not cross easily via pores but some transfer does occur via pinocytosis (endocytosis/exocytosis).

Water molecules are smaller than the size of the pores in the capillary and can cross the capillary wall very easily. The capillary endothelial cells in some tissues (eg glomerulus, intestinal mucosa) have gaps (called fenestrations) in their cytoplasm which are quite large. The water conductivity across these capillaries is much higher then in non-fenestrated capillaries in other tissues of the body.

The transfer of water across the capillary membrane occurs by two processes: diffusion and filtration.

Diffusion

The total daily diffusional turnover of water across all the capillaries in the body is huge (eg 80,000 liters per day) and is much larger than the total capillary blood flow (cardiac output) of about 8,000 liters per day.

Diffusion occurs in both directions and does not result in net water movement across the capillary wall. This is because net diffusion is dependent on the presence of a concentration gradient for the substance (Fick’s Law of Diffusion) and there is ordinarily no water concentration difference across the capillary membrane. Net diffusional flux is zero.

Filtration

This is actually ultrafiltration as the plasma proteins do not cross the capillary membrane in most tissues. This filtration is considered to occur because of the imbalance of hydrostatic pressures and oncotic pressures across & along the capillary membrane (Starling’s hypothesis - see following section).

For the whole body, there is an ultrafiltration outward of 20 liters per day and inwards of 18 liters per day. The difference (about 2 liters/day) is returned to the circulation as lymph.

Filtration results in net movement of water because there is an imbalance between the forces promoting outward flow and the forces promoting inward flow. These forces are variable so net movement could be inwards or outwards in a particular tissue at a certain time. The forces also change in value along the length of the capillary and the typical situation is to have net movement outward at the arterial end and to have net movement inward at the venous end of the capillary.

Comparison of Diffusion and Filtration in Capillaries

Diffusion:

  • HUGE volumes of water are involved
  • bidirectional along the whole length of the capillary
  • net movement is governed by concentration gradient
  • hydrostatic & oncotic pressures (Starling forces) are not involved in diffusion
  • this is the process responsible for net movement of gases, nutrients and wastes (as these substances move down their concentration gradients)
  • · there is no net movement of water across the capillary wall due to diffusion -this is remarkable considering the huge volume of water involved.

Filtration

  • really ultrafiltration as proteins cannot easily cross most capillary membranes
  • volumes involved are much smaller then the diffusional flux
  • fluid movement can be either inwards (absorption) or outwards, but not both at any particular position along the capillary
  • net movement is governed by the balance of the hydrostatic and oncotic pressure gradients (the Starling forces)
  • this process is not important for net movement of gases, nutrients & wastes the net movement of water is important.
Fick’s Law of Diffusion

This states that the amount of diffusion (or flux) of a substance across any membrane is proportional to the concentration difference (C2-C1) across the membrane and to the surface area (A) of the membrane and is inversely proportional to the thickness (t) of the membrane. The constant of proportionality (k) is a measure of the permeability of the membrane to the substance:

Flux = k x A(C2 - C1) / t