What is a starting point in sinusoids

Structure and function of blood vessels


The vessels of the human body have two basic tasks: on the one hand, they are supposed to transport the blood from the heart to the organs and back, on the other hand, they enable the exchange of substances between the cells and the blood in the organs. The larger vessels such as arteries and veins are primarily used to transport the blood. As soon as they have reached an organ, they branch out into a multitude of small capillaries and venules (so-called end flow path). This is where the actual exchange of substances takes place.

These different functions are also reflected in a different structure of the vessels: veins and arteries as transport vessels have a fundamentally similar, three-layer structure (intima, media, adventitia), which is used for the safe and loss-free transport of the blood. The vessels of the terminal flow path, on the other hand, are supposed to enable an exchange of substances, so their wall is only single-layered and for the most part particularly permeable.

The exchange of substances in the end stream must take place in both directions, i.e. from the capillary to the tissue and from the tissue to the capillary. In the course of the exchange of substances, there is therefore a change in the forces acting between the capillary and tissue. The so-called Starling formula contrasts these forces in order to determine the filtered volume on the one hand and its flow direction (out of or into the blood vessel) on the other.

By the way, you can also find a Histo-Trainer series on the histology of blood and lymph vessels in the section on the wall layers of large blood vessels.

Circulatory elements

The heart is the motor of the lungs and body circulation, the vessels are the transport and exchange system. The capillaries are the finest branches of this transport system and are used for the exchange of substances. Since fluid escapes into the tissue during the exchange of substances, a second transport system (the lymphatic vessel system) is required, which returns this tissue fluid to the bloodstream.

Anatomical components of the circulation

  1. Heart: Each heart chamber serves as the motor of "their" circulation
  2. Blood vessels
    1. Arteries
      • Function: Transport the arterial (oxygen-rich) blood away from the heart
      • Course: Becoming gradually smaller (arteries → arterioles → metarterioles → capillaries)
      • Special feature: arterioles serve as so-called resistance vessels
    2. Blood capillaries
      • Function: Mediate the gas and mass exchange
      • Course: merging into venules (capillary → venules)
      • Special feature: extremely high cross-sectional area with low flow velocity and short diffusion distance
    3. Veins
      • Function: Transport the venous (low-oxygen) blood from the periphery to the heart
      • Course: Form after the capillary bed → become gradually larger (→ capillary → venole → vein → vena cava)
      • Special feature: Can store a large volume of blood (are therefore referred to as so-called capacity vessels)
  3. Lymph vessels
    • Function: Among other things, they serve to drain fluid from the tissue
    • Course: Lymph vessels gradually become larger and ultimately flow into the venous system (→ vein angle → superior vena cava) → no independent circulation

Functional division of the vascular sections

Depending on the function, the vessel sections can be assigned to a transport or exchange system.

Transport systemVessel section that is primarily used for the transport and storage of blood volume and the regulation of blood pressureConsists of large vessels (especially arteries and veins)
Exchange systemSection of the circulatory system that is responsible for the exchange of substances between tissue and bloodMainly consists of capillaries and postcapillary venules

Structure of large vessels

All large vessels have a similar structure and consist of three layers (from the inside out): intima, media and adventitia. The exact composition of these vessel layers varies depending on the specific function of the vessel. Arteries, for example, have a very muscular media to withstand high arterial blood pressure. Veins only have to withstand the low venous blood pressure, which is why their media contains significantly fewer muscles. Due to the sparsely developed muscles, veins have a larger lumen than arteries and are able to store larger amounts of blood.

Large vessels have a three-layer structure, whereas very small vessels such as capillaries and venules only consist of one endothelial cell layer!

The term arteriosclerosis describes a pathological change in arterial walls, which can lead to the stiffening and narrowing of the lumen of arteries. The most common form of arteriosclerosis is atherosclerosis, in which the vascular intima is abnormally changed. The starting point is usually an endothelial damage, which leads to the deposition of lipids circulating in the blood (e.g. LDL). As a result, inflammatory cells from the blood and muscle cells from the vascular media migrate into the subendothelial stratum of the intima. The interplay of local inflammation, progressive lipid deposition and the production of extracellular substance by the immigrated muscle cells leads to the formation of atheromatous plaques in the vascular intima. These protrude into the vessel lumen and thus narrow it. This narrowing of the lumen alone can cause a reduced perfusion of the subsequent tissue due to the greatly increased vascular resistance (Hagen-Poiseuille law). This can lead to the development of chronic diseases such as coronary artery disease (CHD) and peripheral arterial occlusive disease (PAD). In addition, several layers of platelet thrombi can be deposited on the atheromatous plaque, making the plaque more unstable. A rupture of the plaque can acutely occlude the vessel. This is the cause, for example, of acute ischemic events such as a stroke or a heart attack. The main risk factors for developing atherosclerosis are tobacco consumption, diabetes mellitus and arterial hypertension.

Peculiarities of arteries

In contrast to veins, arteries are part of the so-called high pressure system. This means that they have to withstand arterial blood pressure, which is why they have significantly more smooth muscles than veins.

Types of arteries and their characteristics
Elastic type arteriesMuscular type arteries
OccurrenceVessels close to the heart (aorta, pulmonary trunk and their large branches)Vessels distant from the heart (e.g. brachial artery