What are capillaries?
Alongside veins and arteries, capillaries are the third type of blood vessel in the circulatory system. They make up only around five percent of all blood vessels in the body (veins: 75 percent, arteries: 20 percent). The wafer-thin vessels form a finely branched, closed capillary network (rete capillare) over a total length of an estimated 100,000 kilometers. The more oxygen a tissue needs and the more metabolically active it is, the denser its network of capillaries. The brain, lungs, skeletal muscles and heart are criss-crossed by numerous of these wafer-thin vessels. Tissues with a slow metabolism, such as tendons and ligaments, on the other hand, have only a few capillaries. There are also areas in our body that have no capillaries at all and are only supplied with nutrients by diffusion from surrounding tissue, such as the joint cartilage in the knee, the heart valves and the lenses of the eyes.
Structure of the capillaries
With a diameter of only five to ten micrometers (µm), the capillaries are sometimes smaller than the red blood cells (seven to eight µm), which therefore have to deform somewhat in order to pass through the fine vessels.
Three types of capillaries can be distinguished depending on the fine structure of the wall:
- Continuous capillaries: closed endothelial layer, completely surrounded by basement membrane; occur: Skin, lungs, heart, brain, spinal cord, skeletal muscles
- Fenestrated capillaries: Endothelial layer with pores (20 to 80 nanometers, thin basement membrane; occurrence: Gastrointestinal tract, kidneys, endocrine glands
- Discontinuous capillaries (sinusoids): Gaps (two to five nanometers) in the endothelial cell layer and basement membrane; occurrence: Bone marrow, liver, spleen
The tasks of the capillaries
The walls of the capillaries are permeable to certain substances, gases and fluids – especially the discontinuous vessels. The widely branched capillary network therefore enables the exchange of gases and substances between the blood and the surrounding tissue. For example, oxygen, nutrients, metabolic products, water and inorganic ions can migrate from the blood into the space between tissues/cells (interstitium) and vice versa. Exceptions are blood cells and large proteins, for which the wall of the fine vessels is too dense.
In addition, the conditions on the capillary walls are extremely favorable: the substances have a lot of space and a lot of time. Due to its fine branching, the capillary network achieves a large overall cross-section (around 800 times larger than the cross-section of the aorta) and the blood flow slows down to 0.3 millimetres per second (aorta: 320 millimetres per second).
The walls of the fine vessels are therefore heavily filtered and reabsorbed. Around 20 liters are filtered into the interstitium every day, of which around 18 liters are reabsorbed into the capillaries and venules. The remaining two liters are returned to the blood via the lymphatic system.
Capillaries: Diseases and complaints
If the permeability of the capillaries is impaired, blood or blood components can leak out of the vascular system into the surrounding tissue. This results in edema and petechiae (punctiform bleeding of the skin and mucous membranes), for example.
Capillary leak syndrome is a rare, severe disease in which the permeability of the fine vessels is increased. Characteristic symptoms are episodes of low blood pressure, edema and low blood volume (hypovolemia). The cause of the disease is unknown, so it can only be treated symptomatically. The prognosis is poor.
Other health problems in the area of the capillaries include malformations, ruptures, thromboses and embolisms.
