Air Vessels
The large arteries such as the aorta and its branches are known as air vessels. They typically contain a high proportion of elastic fibres and are therefore of the elastic type. Due to the air vessel function, the pulsating flow generated by the irregular pumping action of the heart is increasingly converted into a continuous flow in the more distant arteries.
This is done by flowing only about half of the blood directly into the arteries during systole. The other half is initially stored in the enormously elastic aorta. The wall of the aorta has very good restoring forces due to the numerous elastic fibres, which then press the stored blood into the arteries during diastole. This compensates for pressure and flow peaks.
Resistance Vessels
Small arteries and the arterioles are called resistance vessels. They serve to reduce blood pressure before it enters the capillaries. Together they form 50% of the total resistance. This effect is based on the strong decrease of the individual diameters of the vessels. The total resistance is therefore very strongly influenced and exerts a great influence on the total peripheral (far from heart) resistance.
Capacity Vessels ̈ße
Capacity vessels are the parts of the venous system. The veins have a very good compliance. Compliance describes the property of a vessel to take up a certain volume due to elastic fibres, despite a slight increase in pressure. This means that the capacity vessels are able to store about 80% of the total blood volume. If necessary, this volume can be mobilised by increasing the tone of the smooth vascular muscles.
Sphincter vessels
Vessels of this type have a ring-shaped closure mechanism. This makes it possible to regulate the blood flow of the downstream arteries. For example, the arterioles control the flow of blood into the capillary system.
Capillary system
In the end, it remains to be said that capillaries are responsible for the mass transfer. While fat-soluble substances move freely through the wall, water-soluble substances must “diffuse” through the wall or fall back on other transport systems. As capillaries are of enormous physiological importance, it is useful to know their subdivision: Continuous capillaries Fenestrated capillaries Sinusoidal capillaries Continuous capillaries: With continuous capillaries the cells form a mostly completely closed wall.
Fenestrated capillaries: This type of capillary has pores in its inner layer, which are important for the exchange of low-molecular substances. They are mainly found in the intestinal tract, where the absorption capacity is relatively high. Sinusoidal capillaries: Sinusoidal or discontinuous capillaries have a significantly larger vessel diameter than the other two capillary types.
They also have very large pores. Even large molecules, such as proteins, can be absorbed through this wall. – Continuous capillaries
- Fenestrated capillaries
- Sinusoidal capillaries
Most vessels have a characteristic three-layer wall structure.
This can vary depending on the type of vessel and the conditions. In general, the higher the mean pressure, the thicker and more muscular is the middle layer of the vessel. The innermost layer is formed by a single-layer platelet of cells, also known as endothelium.
These cells are aligned longitudinally so that they can ensure a smooth blood flow through the vessels. The endothelium sits on a basal layer, the basal lamina. It anchors the endothelium to the underlying muscle cell-rich layer.
Beneath the endothelium lies the so-called subendothelial layer, which consists mainly of extracellular matrix, i.e. connective tissue, and contains hardly any cells. The veins have a special feature in this layer. A duplication of the intima is formed by the venous valves, which promote the return flow of blood to the heart and close like a valve when the flow is reversed.
The media is the thickest layer of the vascular wall and is separated from the intima by the so-called Membrana elastica interna, a thin layer rich in fibers that promotes mobility. It contains mainly smooth muscle cells and extracellular matrix with elastic and collagen fibres. The circular muscle cells serve to regulate the vascular width.
In larger vessels, the media is often followed by a so-called Membrana elastica externa. The outer layer is a layer of connective tissue that embeds the vessel in the surrounding tissue. Among other things, it contains fibroblasts, elastic fibres and collagen fibres. In addition, there are tiny vessels to supply the arteries (vasa vasorum) and lymph vessels.