The capillaries are tiny vessels (measuring 5–10 μm in diameter) arranged in an interweaving network called the capillary bed. Capillaries have two main roles.
The capillaries are tiny vessels (measuring 5–10 μm in diameter) arranged in an interweaving network called the capillary bed. Capillaries have two main roles:
Delivery of nutrients to and removal of metabolites from the tissues;
Distribution of body water between intravascular and interstitial fluid compartments.
In contrast to arteries and veins, the capillary wall is almost entirely composed of endothelium and is only one cell thick, supported by a basement membrane.
There are three main capillary types:
Continuous capillaries, the most common class of capillary, found in muscle, brain and connective tissue. Features include:
– A continuous basement membrane.
– Blood–brain barrier. In the brain, tight junctions hold the endothelial cells especially close and are surrounded by astrocyte foot processes (see Chapter 47). Only the smallest molecules such as water, O2 and CO2 can then freely diffuse from one side of the cell to the other. The diffusion of larger molecules (nutrients, metabolites and drugs) across the capillary is dependent on carrier-mediated transport mechanisms.
Fenestrated capillaries, found in the renal glomeruli, intestinal mucosa and choroid plexus. Fenestrated capillaries have large pores within the endothelial cell called fenestrations, which makes them much more permeable than continuous capillaries. These fenestrations are large enough (60–80 nm) to allow passage of all but the largest plasma proteins (i.e. albumin).
Sinusoidal capillaries, a particular type of fenestrated capillary found in the bone marrow and lymph nodes. The fenestrations are large enough to allow white blood cells and red blood cells (RBCs) to pass through (up to 10 μm). In the liver and spleen, even greater movement of cells is required – in addition to large fenestrations, their sinusoidal capillaries also lack tight junctions. These vessels are called discontinuous sinusoidal capillaries.
Capillary exchange involves the matrix properties of the capillary basement membrane as well as the features of the endothelial layer itself. It takes place through three overall mechanisms:
– Gases (e.g. O2 and CO2) and small lipophilic molecules (e.g. anaesthetic agents) are able to diffuse across the phospholipid bilayer of the endothelial cell.
– Small water-soluble molecules traverse the capillary either through pores in the cell membrane or through gaps between endothelial cells.
The rate of diffusion is affected by a number of factors; most importantly, the concentration (or partial pressure) gradient of the substance across the capillary wall (Fick’s law – see Chapter 10).
Bulk flow. Water is filtered through the fluid-filled pores within (fenestrations) or between (tight junctions) endothelial cells. Any dissolved solutes (e.g. electrolytes) can be dragged along with the water. This mechanism is particularly important in the fenestrated renal glomerular capillaries. Filtration and reabsorption of fluid across the capillary is governed by the balance of Starling filtration forces.
Pinocytosis. This is an energy-consuming type of endocytosis where substances in the capillary lumen are enveloped by the endothelial cell membrane to form a vesicle. The vesicle is then transported across the endothelial cell and its contents released into the interstitium (see Chapter 4). Pinocytosis makes only a minor contribution to capillary exchange.
Capillary exchange is facilitated by a blood flow pattern known as bolus flow, which is another example of the non-Newtonian nature of blood. Capillaries have approximately the same diameter as the RBC at around 7 μm. For this reason, the RBC only just fits through the capillary, often having to deform its biconvex shape. Flow therefore takes place as intermittent RBC and plasma boluses. As previously discussed, turbulence increases resistance and therefore decreases flow; this is therefore usually avoided in the larger vessels. However, turbulence can be used advantageously in the capillary as a method of mixing the plasma and potentially facilitating exchange at the endothelium. Effective viscosity is only increased by approximately 30% in bolus flow, which is much less than would be expected from turbulent flow. Capillary bolus flow therefore allows controlled pockets of turbulence to occur for mixing whilst maintaining a relatively low resistance.
How do the Starling filtration forces determine transmembrane fluid flow?
The net fluid filtration across the capillary wall results from the balance of the four opposing Starling filtration forces (Figure 36.1):
Forces tending to move fluid out of the capillary:
– Capillary hydrostatic pressure Pc;
– Interstitial fluid oncotic pressure πi.
Forces tending to move fluid into the capillary:
– Interstitial fluid hydrostatic pressure Pi;
– Plasma oncotic pressure πc.
Net fluid filtration pressure across capillary wall = Kf [(Pc – Pi) – σ(πc – πi)], where Kf is the filtration coefficient, a constant related to the permeability of the capillary wall (high Kf indicates high water permeability, whilst low Kf indicates low water permeability); σ is the reflection coefficient, a constant that represents the permeability of the capillary to proteins (σ = 1 implies that the capillary wall is 100% impermeable).
As capillary walls are normally relatively impermeable to proteins, the Starling filtration equation can be simplified as:
Net fluid filtration pressure across capillary wall ∝ [(Pc – Pi) – (πc – πi)]
Note: Starling pressures are usually measured in mmHg.