Table of Contents
Definition of the Human Blood Circulation
In the human circulatory system oxygenated blood is conducted away from the heart by the arteries. These arteries branch into smaller vessels, the so-called arterioles. Where they end, the capillaries, or the capillary network, begins, and this is where substance exchange between the blood and the interstitium occurs. In turn, the capillaries lead to larger vessels, the venules, which then lead to the veins. The venules then transport the deoxygenated blood from the periphery back to the heart.
General Structure and Functions of the Vessels
All larger vessels have the same general structure, which can vary according to localization and function (from inside to outside):
- The tunica intima (‘intima’ or ‘interna’)
- The tunica media (‘media’ or ‘muscularis’)
- The tunica externa adventitia (‘adventitia’)
Comparison of the layers in arteries and veins
|General appearance||Thick walls with small lumens
Generally appear rounded
|Thin walls with large lumens
Generally appear flattened
|Tunica intima||Endothelium usually appears wavy due to constriction of smooth muscle
Internal elastic membrane present in larger vessels
|Endothelium appears smooth
Internal elastic membrane lacking
|Tunica media||Usually, this is the thickest layer in arteries.
Smooth muscle cells and elastic fibers dominate (their proportion varies in keeping with the distance to the heart).
In larger vessels, there is an external elastic membrane.
|Normally thinner than the tunica externa
Smooth muscle cells and collagenous fibers predominate
Nervi vasorum and vasa vasorum present
The external elastic membrane is lacking.
|Tunica externa||In all but the larger arteries, this layer is usually thinner than the tunica media.
Collagenous and elastic fibers
Nervi vasorum and vasa vasorum present
|Normally the thickest layer in veins
Collagenous and smooth fibers predominate
Some smooth muscle fibers
Nervi vasorum and vasa vasorum present
Histology and differentiation of arteries
Arteries transport oxygenated blood from the heart to the periphery of the body, which explains the presence of the arterial high-pressure system.
The arterial part of the circulatory system can be subdivided histologically into 2 types:
|Elastic type arteries||Muscular type arteries|
|Distribution||All arteries close to the heart:
||All arteries distant from the heart|
|Structural Features||Wide intima, strong stratum subendotheliale to compensate for the mechanical strain. Many elastic fiber networks in the media.||Strong media rich in muscle cells|
During diastole, blood is conducted to the periphery by the elastic retracting forces of the artery wall
|Distribution of the blood to organs and tissue: regulation of pressure within the media|
Arteries narrow towards the capillary network to become so-called arterioles. Arterioles have a diameter of 10-20 µm due to the absence of the stratum subendotheliale of the intima.
They are designated as resistance vessels as they can regulate flow velocity by means of their respective wall muscles (~ 120 mmHg).
The following part is the capillary exchange system, which is located in the peripheral body regions and has a length of tens of thousands of kilometers, resulting in a large exchange surface. The conditions for gas and substance exchange between blood and the insterstitium are optimal as the cross-sectional area is very small (6-12 µm) and thus the correlating flow velocity of the blood is very slow (0.3 mm/s; for comparison – the flow velocity in arteries amounts to ca. 300 mm/s). Another important task performed by the capillaries is the elimination of by-products.
The capillary wall generally consists of an endothelial layer, a basal membrane, and the pericytes (contractile cells, which surround the endothelial cells).
Under the electron microscope, 3 different types of capillaries are distinguished:
|Continuous, non-fenestrated capillary||Fenestrated capillary||Discontinuous capillary|
|Structure||Endothelial cells closely connected with each other via tight junctions||Gaps between the endothelial cells (60-80 nm), which are closed by means of diaphragms: gapless basal membrane||Perforated endothelial layer (pores of up to 0.5 µm) corresponding to partial absence of intercellular contacts: incomplete or absent basal membrane|
|Distribution||(Mostly having a barrier function) nervous system, lung, heart, skeletal muscles||(Locations with high-rate metabolism) intestine, kidney, adenoid tissue||(Locations with high blood flow) sinusoids of the liver, the spleen, and the bone marrow|
Veins and Their Special Features
Veins are the vessels which transport the deoxygenated blood from the periphery back to the heart. This is the means by which the venous blood from the head, the neck, the arms, and the breast gathers in the superior vena cava, and the blood from the abdomen, the legs, and the pelvic organs gathers in the inferior vena cava. Both veins lead to the right atrium, from which point the blood is transported into the pulmonary circulation. Here, the blood is re-oxygenated.
The anatomy of veins is similar to that of arteries. However, the difference is that the walls of the veins are significantly thinner, which is why the blood pressure is markedly lower. This results in the venous low-pressure system, which contains 85% of blood volume.
In histological specimens the individual wall layers of the veins cannot be separated from each other as easily as with arteries.
A special feature of veins is the so-called venous valve, which can be found in the wall of the torso and in the extremities. Backflow of the blood into the periphery has to be inhibited by venous valves (intima duplications) since the low blood pressure found in the veins is not sufficient for transportation of the blood back to the heart. This occurs via rhythmic closure of the valves, which is further supported by the muscle pump.
The veins usually run parallel to the arteries. However, the number of venous vessels is greater than the number of arterial vessels, due to the presence of both deep and superficial veins. The latter lie directly under the surface of the skin. The deep venous system is connected with the superficial system via so-called perforating veins.
The venous system also contains smaller vessels, the venules, which correspond to the smaller vessels in the arteries. Their location is post-capillary, and they transport the blood from the capillary network to the veins. Their diameter increases constantly from the end of the capillaries to the veins, and this is accompanied by an increasing coat of muscle cells.
Blood Circulatory System
Despite the differences in structure and function, close interaction between arteries and veins occurs in the circulatory system in order to ensure optimal gas and substance exchange and transport.
The circulatory system is a closed system which can be divided into the ‘greater’ and the ‘lesser’ systems. Besides these two systems, the portal vein system plays as enormously important role as a ‘sub-branch,’ and it should not be disregarded when preparing for exams. A brief overview of this system therefore follows.
The greater circulation, also referred to as systemic circulation, performs the task of supplying the organs with oxygenated blood.
The Direction of Blood Flow
Left atrium of the heart → mitral valve → left ventricle of the heart → aortic valve → aorta → body arteries → arterioles → capillaries (location of gas and substance exchange) → venules → veins → superior/inferior vena cava
The lesser circulation or pulmonary circulation connects directly to the systemic circulation and it has the task of re-oxygenating the blood and conducting it back to the greater circulation, which in turn supplies the organs.
The direction of blood flow
Right atrium of the heart → tricuspid valve → right ventricle of the heart → valve of the pulmonary trunk → pulmonary trunk → pulmonary arteries → lung capillaries → pulmonary veins → left atrium of the heart
Note: The vessels that conduct blood away from the heart are arteries, and those that conduct blood towards the heart are veins, irrespective of oxygen content.
As mentioned, another important part of the circulatory system is the portal vein system. The venous return of the unpaired abdominal organs (the gastro-intestinal tract, the spleen, and the pancreas) occurs via the liver or via a common venous stem before the nutritious blood is conducted back to the systemic circulation by the inferior vena cava.
The portal vein collects the venous blood and conducts it to the liver, where it branches into another capillary system, the rete mirabile venosum (wonderful net). The substrates that were absorbed in the gastro-intestinal tract are here metabolized and any potentially poisonous substances are eliminated. The ‘detoxified’ blood then passes through the hepatic veins to reach the inferior vena cava.
Note: The so-called first-pass-effect can occur at this location due to metabolic processes; medications are thereby partially or completely degraded, meaning they are no longer able to act via the blood.
Cardinal Angiological Symptoms and Clinical Aspects
A short overview of the most important cardinal symptoms and clinical pictures that the prospective physician should know about follows, on account of the relevance and incidence of angiological problems.
- Pain (especially in the lower extremities, caused by ischemia)
- Paresthesia (evidence of circulatory disorders)
- Paleness (veins not filling)
- Cyanosis (reduction in venous drainage)
- Edema (especially on the lower leg and the ankle)
- Slow healing processes (due to reduced circulation)
- Degenerative vascular disease (arteriosclerosis, PAOD, aneurism, embolism)
- Inflammations (e.g. vasculitis)
- Neuro-vascular compression syndromes
- Function-related caused diseases (e.g. blood pressure problems)
|Superficial Venous System||Deep Venous System|
|CVI (may be a consequence of varicosis)||Retrograde backflow disorder|
|Thrombophlebitis||Antegrade disorder (‘venous block’)|
Possible Exam Questions About Arteries and Veins
The answers can be found below the references.
1. Which statements are correct in relation to the structure of blood vessels?
- The tunica interna consists of simple squamous epithelium.
- In elastic-type arteries, the stratum subendotheliale is only formed to a certain degree, or not present at all.
- Tight junctions may be found in non-fenestrated capillaries
- The flow velocity of the blood can be regulated via the wall muscles of the venules, which is why they are referred to as resistance vessels.
2. Which statement concerning the capillary exchange system is false?
- The flow velocity in the arteries is 1000 times higher than in the capillaries.
- Erythrocytes can only pass though capillary vessels by deforming themselves, due to the small cross-section of the latter.
- The walls of all capillaries consist of an endothelial layer, pericytes, and a basal membrane.
- Fenestrated capillaries are located in areas of high blood circulation.
2. Which of the following statements represents a correct deduction?
- If veins are insufficiently filled, or not filled at all, paleness can be observed, which can typically be found at PAOD.
- Impairment of venous return can lead to cyanotic changes.
- Paresthesia is always caused by diseases or lesions of the blood vessel system.
- The first-pass-effect causes a decrease in circulation, leading to slower healing processes.