The blood has to be warmed as it returns to the heart.
Heat is produced by muscular construction, if to much heat is produced it must be removed from the body so a healthy condition can be maintained. The water in the blood has a great heat carrying capacity, The superficial capillary's in the subcutaneous layer of the skin that are close to the surface, when blood passes through these some of the heat radiates to the surface of the skin the sweat glands take water from the blood and secrete it onto the skin where the heat evaporates. If the body becomes cold the heat loss must be reduced, at this point the superficial capillaries can be closed down, keeping the heat in. The body's core temperature must be maintained at a temperature that is within very narrow limits, this is controlled by the detectors in the hypothalamus which is in the forebrain steam, the detectors also monitor the blood flowing through the brain. The cardiovascular system has a counter currant mechanism, the venous blood returning to the trunk is warmed by the arterial blood flowing in the opposite direction. As we breath in through the nose the air is warmed by arteries with in the nasal chambers. The brain and the liver have a high metabolic rate producing excessive heat, part of the blood supply to them is designed to remove excess heat. Redness at the sight of an injury or infection is called erythema and indicates that extra blood and heat are at the sight for healing purposes.
The blood plays a vital role in temperature control in the body.
Elimination of Excess Heat
Heat is produced as a by-product by various activities of the human body, particularly muscular contractions. When excess heat is accumulated, it must be eliminated from the body to maintain a healthy condition.
The water of the blood has a great heat-carrying capacity.
There are superficial capillary beds in the subcutaneous layer, close to the surface of the body. When the blood flows through these beds, some of its heat can radiate directly to the surrounding environment.
The sweat glands take water from the blood and secrete it onto the surface of the skin. Here, even more calories of heat are lost during the evaporation of the water.
Conservation of Body Heat
On the other hand, if the body has an insufficient amount of heat, heat loss must be reduced.
For this purpose, the superficial capillary beds can be closed down. Then, the fat in the subcutaneous layer serves as insulation.
Core Temperature Control
Unlike the peripheral portions of the body, whose temperatures may vary considerably, the center of the body must be maintained at a certain temperature within very narrow limits.
There are special temperature detectors in the hypothalamus of the forebrain stem. These continuously monitor the temperature of the blood flowing through the brain.
The blood system uses a counter current mechanism to maintain core temperature as follows:
The peripheral blood in the limbs is several degrees cooler than the blood in the center of the body. Therefore, it must be warmed as it returns toward the heart.
The arteries and veins of the limbs are located side by side as they extend from the trunk and through the length of the limbs. As it returns to the trunk, cool venous blood is gradually warmed by the arterial blood flowing in the opposite direction.
Cooling of Organs with a High Metabolic Rate
Certain organs of the body, such as the brain and the liver, have a relatively high metabolic rate.
Because of this, they produce excessive heat.
Part of the blood supply to these organs is specifically designed to remove the excess calories of heat.
Warming of Inflowing Air
As blood flows through the arteries of the mucoperiosteum of the nasal chambers, the inflowing air is warmed.
At the site of an infection or injury, the most common reaction observed is redness (erythema).
This indicates that extra blood and heat are available for healing.
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