B.5.6. Special Circulations

Introduction: In several organs or tissues, local conditions may strongly modify or influence the behaviour of the blood circulation

A. Circulation in the Skeletal Muscles:
1. What is the problem?
At rest, a skeletal muscle requires very little energy but during exercise the muscle requires much more energy to contract. Therefore, the blood flow, which is very small at rest, has to increase 5-20 times to accommodate this demand for oxygen.
diagram of a closed capillary distribution
2. Solution:
Opening more capillaries inside the muscle essentially solves this problem. At rest, most capillaries are closed (i.e. are not necessary) and only 10-20% are open. But with exercise, the number of capillaries opens up to 100%.

diagram of an opened capillary distribution
3. Diffusion distance
This, as such, will increase the blood flow to the muscle but it will also have another additional benefit; the average distance of a muscle cell to the nearest opened capillary will decrease (see diagram). And, as you may remember, a decrease in diffusion distance will increase diffusion! (link)
4. But now there is a new problem!
Contractions are quite strong and the pressure inside the muscle will increase during the contraction. If this pressure is higher then the blood pressure, then the blood flow will stop! This means that no oxygen will arrive to the cells and no waste is removed from the cells. Essentially, the metabolism of the cell is stopped and this will limit the duration of the contraction.

effects of a Tonic Contraction
As you can see in this diagram, during the contraction (blue line), the blood flow has virtually stopped. This means that no oxygen and no nutrients are reaching the muscle cells. However, when the contraction has stopped (dashed line), perfusion is immediately restored. In fact, for some time, it is even stronger than normal, in order to replenish the oxygen shortage in the cells and to get rid of the accumulated waste. This is called ‘reactive hyperaemia’ (reactive = reaction to the contraction; aemia = blood and hyper = more).

effects of a phasic Contraction
6. Tonic and Phasic Contractions:
In the previous diagram, the contraction was tonic (long lasting). Some contractions however are phasic and that is very good. Because in that situation, when there is contraction, the blood flow will stop, but when the contraction relaxes (see dotted blue line "a"), the blood will flow again! In other words, during phasic contractions blood is able to reach the cells. That is why it is possible to perform phasic contractions much longer than tonic contractions.

B. Circulation in the Cardiac Muscle:


1.What is the problem?

The situation here is similar to that in the skeletal muscle. During contraction, the vessels are closed due to the high pressure caused by the contraction.

2.Systolic and diastolic perfusion.
This is not as bad as it sounds, as the cardiac contractions are phasic. In other words, perfusion through the coronary vessels takes place during diastole and less or not at all during systole. Actually, this is the only organ that is not perfused during systole but during diastole; all other organs have a stronger blood flow during systole (= the pulse).
3. Really?
Ah!! The previous statements were somewhat exaggerated. In fact, in the atria, contraction pressures are very low; maybe 5-10 mmHg. Blood pressure in the perfusing arterioles and capillaries are much higher. So, in the atria, even during systole, perfusion (= blood flow) does not actually stop.
4. What about the ventricles?
Also in the right ventricle, maximum pressure achieved is 25 mmHg (remember the systolic pressure in the pulmonary arteries?). Therefore, the coronaries in the right ventricle are hardly influenced by the contraction.

pressures in the four cardiac chambers
5. Left Ventricle:
But the situation is very different in the left ventricle. The maximum pressure achieved during systole is much higher than anywhere else; normally 120 mmHg. This is more than enough to stop perfusion in the left ventricular wall.

pressures across the left ventricular cardiac wall
This story explains why practically all heart infarcts occur in the left ventricle. This muscle has to work the hardest but is most impeded in its perfusion.

7. Pressure Gradient

In fact, there is a pressure gradient across the left ventricular wall during systole. The pressure inside the left ventricle is, during systole, 120 mmHg but outside, in the chest, it is 0 mmHg.

So, the pressure in the muscle close to the endocardium (=inside) is close to 120 mmHg while the pressure in the muscle fibers close to the epicardium is close to 0 mmHg. Therefore, during systole, the endocardium will get less blood then the epicardium. That is why many infarcts occur along the endocardium and less along the epicardium.

C. Circulation in the Brain:


1. What is the problem?
The brain is protected from its environment by the skull. This is a hard and bony structure, which cannot expand. Normally that is good because it offers a protection against accidental bumps.

2. Cerebral Oedema
But in a situation when oedema occurs (cerebral oedema), then tissue swelling may occur. In any other tissue or organs, this would not really be a problem. Only maybe inconvenience or maybe even some pain because of the swelling.
3. Intra-cranial pressure
But in the brain, if there is a swelling of the brain tissue inside the skull, the pressure inside the skull may increase. If this pressure becomes too high, then this may start to stop blood perfusion.

4. Vicious Circle:
This could easily lead to a vicious circle: increased swelling -> increased pressure -> reduced blood flow -> more ischemia (=lack of blood) -> more damage to the tissue and the blood capillaries -> more swelling -> etc -> etc
5. Solution?

The body will try to solve this problem by increasing the arterial blood pressure (through a reflex).

6. Medical solution:
But if this does not help, then the patient will gradually develop symptoms of brain dysfunction, become unconscious, and needs to be admitted urgently into the hospital. The solution then is to drill a hole through the skull to relieve the intra-cranial pressure!

D. Circulation in the Skin:


1. Cutaneous circulation:
Circulation in the skin (=cutaneous) is in some respects very different from that of other circulations.

2. Regulation:
The difference is that in this circulation, local blood flow control is not very relevant. Instead the nervous regulation of the circulation is very important. This is mainly the task of the sympathetic autonomic nervous system.
3. Why?
Why is the control of the circulation of the skin regulated by the brain? Because one of the function of the skin is controlling the body temperature .

By increasing or decreasing the perfusion of the skin, the body can allow more or less heat to disappear, thereby keeping the temperature inside the body constant at 36 oC. (98 oF).

E. The Portal Venous Circulation:


Sometimes, between the arteries and the veins there is not one but two capillary beds. This is the case in the hypophyse (a gland in the brain), in the kidneys and, most famously, in the liver. This has to do with the function of the intestines.
As you know, the intestine is where the food is ingested, broken down into many molecules and absorbed in the blood capillaries. These capillaries then drain into the intestinal veins. These veins do not go directly to the veins and to the heart. They first go to the liver!
The Portal System (or Circulation)
3. Why?
Why does the blood, from the intestines, first flow to the liver? This is because the liver is the major metabolic organ in our body. Therefore, our food first gets processed in the liver.
The liver is also important to detoxify possible toxins that have been ingested with your food. So, in a sense, it also offers additional protection.

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