Special Circulations and Temperature Regulation

Last post on the circulatory system!

Give the relative importance of pressure, metabolic activity and neural control for blood flow through skeletal muscle, heart and brain.
Explain how blood is directed to active muscle during exercise.

During rest, only around 15-20% of blood goes to skeletal muscles (this goes up to 80-85% during exercise). At rest, this blood flow is controlled by the actions of sympathetic nerves. During exercise, this is mainly controlled by local metabolites. This also explains why active muscles get more blood: they produce more metabolites than inactive muscles. (For more information, please see my previous post.)

The heart always has around 5% of the blood flowing towards it, as the heart is pretty damn important. Blood flow to the heart is not affected by sympathetic nerves or by hormones: blood flow is almost entirely controlled by active hyperaemia.

Blood flow to the brain is controlled mainly by autoregulation, though these vessels also respond well to local metabolites. (Like the vessels supplying the heart, they do not respond well to sympathetic stimulation.) Although the percentage of cardiac output that the brain receives actually drops during exercise, since the total cardiac output has risen, the brain does receive more blood during exercise. The processes just described (autoregulation and regulation via metabolites) work pretty well when the blood pressure is between 60-160mmHg. When the blood pressure drops below 60mmHg, the Cerebral Ischaemic Response kicks in, as described here. Above 160mmHg, the pressure pushes fluid out of the capillaries into the brain, causing cerebral oedema.

Describe how contraction alters blood flow in skeletal muscle and coronary circulation.

I'm not really sure what is meant here, but my interpretation is that it's to do with how blood is shunted around the body. As alluded to in previous posts, contraction of various blood vessels shunts blood away from those vessels and towards other blood vessels that need it, like skeletal muscles, heart and brain.

Define Atherosclerosis.

Atherosclerosis is the stiffening and narrowing of blood vessels due to the deposition of fatty plaques. This increases the resistance as the lumen is effectually narrowed due to the presence of the plaques.

Define autoregulation for blood flow and its role in control of cerebral blood flow.

I mentioned autoregulation earlier on, but what exactly is it and how does it work? Autoregulation is a way of keeping blood flow constant despite small fluctuations in pressure. As mentioned above, it is a major player in control of cerebral blood flow.

So how does autoregulation work, exactly? Well, there are several mechanisms. Aside from the use of local metabolites, as described earlier, there is another main mechanism called the myogenic mechanism. Essentially, when pressure stretches the smooth muscle, the smooth muscle will contract in response. This increases resistance and decreases flow back to normal.

Describe the role of skin blood flow in temperature control.
Know the normal value for core body temperature.
Explain how changes in skin and core temperature alter skin blood flow including the role of the hypothalamus, neural reflexes and direct actions.

The skin actually receives a lot more blood flow than it really needs during rest, which sounds like a waste of blood flow until you realise that that's how the body gets rid of heat. Blood is cooled down after being transported to the skin. The normal value for core body temperature is around 37°C. This temperature is maintained through increasing or reducing the flow of blood to the skin.

There are several mechanisms through which skin blood flow is increased or reduced:

1. Skin blood vessels dilate when warm and contract when cold.
2. Sensory nerves detect skin temperature. This information is sent to the hypothalamus, which then activates sensory nerve fibres.

Define frostbite, hypothermia, heat exhaustion, heat stroke.
Explain how exercise in hot conditions can produce heat exhaustion.

As I've just mentioned, less blood flows to the skin when it's cold. In extreme cold, reduced blood flow to the skin damages the tissue, resulting in a condition called frostbite. However, even larger drops in temperature increase blood flow to the skin. This is probably the reason why people with hypothermia (an extremely low core body temperature) feel warm.

When skin or core temperature rises above 37°C, people start to sweat. This is due to the activation of sweat glands by the sympathetic nervous system (random reminder here that acetylcholine, not adrenaline/noradrenaline, stimulates sweat glands). The sweat glands also release bradykinin, which is a vasodilator and thus increases blood flow to the skin and sweat glands even more.

The good thing about this is that it gets rid of heat. The bad thing is that sweating reduces blood volume, which decreases mean arterial pressure. The baroreceptor reflex can make up for this, but this puts a lot of stress on the cardiovascular system.

Heat exhaustion is a condition in which the cardiovascular system is no longer able to supply blood to both the muscles and the skin. It often happens during exercise in hot conditions, because as I just said sweating puts stress on the cardiovascular system, which is already stressed because of exercise. Heat stroke is a far more serious condition in which the body is unable to cool itself down. This causes core body temperature to continue to rise until death.

That's a pretty bad note to end a series of posts on, so here's a nicer note: We're done with the cardiovascular system! Yay! Next we'll be moving on to renal physiology!