In addition to its role in the transport of materials, the circulatory system is responsible for the distribution of heat throughout the body. This is true both of
- endotherms, animals — birds and mammals — that generate internally the heat needed to maintain their body temperature. Birds and mammals are "warm-blooded' or homeothermic, maintaining their body temperature within narrow limits, no matter what the ambient temperature.
- mesotherms, animals that generate heat internally but do not maintain a fixed body temperature. Tuna, some sharks, and the echidna are mesotherms.
- ectotherms, animals — the other vertebrates and the invertebrates — that secure their heat from their surroundings (e.g., by basking in the sun). Ectotherms are "cold-blooded" or poikilothermic.
The major source of heat for endotherms is the metabolism of their internal organs. Over two-thirds of the heat generated in a resting human is created by the organs of the thoracic and abdominal cavities and the brain (which contributes 16% of the total — about the same as all our skeletal muscles when they are at rest). There are several measures that an endothermic animal can take if it begins to lose heat to its surroundings faster than it can generate heat (i.e., it begins to grow cold). It can increase the metabolic rate of its tissues. Many small mammals and human infants do this as their surroundings get colder, but it is still uncertain whether adult humans can. The increase in metabolism, with the accompanying release of heat, occurs in brown adipose tissue. It can also increase its physical activity. At rest, muscles make only a small contribution (about 16%) to body heat. During vigorous exercise, this can increase greatly. In the absence of voluntary muscle action, the same effect is achieved by shivering. The greater the surface-to-volume ratio of a part of the body, the faster is can transfer heat to its surroundings. This is why you first notice cold in your hands and feet. The loss of heat from the extremities can be sharply reduced by diminishing their blood supply. In extreme cold, for example, the blood supply to the fingers can drop to 1% or so of its normal value.
Countercurrent heat exchanger
Many animals (including humans) have another way to conserve heat. The arteries of our arms and legs run parallel to a set of deep veins. As warm blood passes down the arteries, the blood gives up some of its heat to the colder blood returning from the extremities in these veins. Such a mechanism is called a countercurrent heat exchanger. When heat loss is no problem, most of the venous blood from the extremities returns through veins located near the surface.
Countercurrent heat exchangers can operate with remarkable efficiency. A sea gull can maintain a normal temperature in its torso while standing with its unprotected feet in freezing water. When you consider that the blood of fishes passes over the gills which are bathed in the surrounding water, it is easy to see why fishes are "cold-blooded". Nonetheless, some marine fishes (e.g., the tuna) are mesotherms — able to keep their most active swimming muscles warmer than the sea by using a countercurrent heat exchanger.
The above photograph on the right shows a cross section through a skipjack tuna. The dark muscle on either side of the vertebral column is maintained at a higher temperature than the rest of the fish thanks to its countercurrent heat exchanger. The cold, oxygen-rich arterial blood passes into a series of fine arteries that take the blood into the active muscles. These fine arteries lie side by side with veins draining those muscles. So as the cold blood passes into the muscles, it picks up the heat that had been generated by these muscles and keeps it from being lost to the surroundings. Thanks to this countercurrent heat exchanger, a tuna swimming in the winter can maintain its active swimming muscles 14°C warmer than the surrounding water. The photomicrograph on the left is of a cross section through the heat exchanger. Note the close, parallel packing of the arteries (thick walls) and veins (thin walls).
Countercurrent exchangers also operate in the kidney and are built into the design of artificial kidneys.
The circulatory system is also responsible for cooling an animal. If the animal's "core" body temperature gets too high, the blood supply to the surface and extremities is increased enabling heat to be released to the surroundings. If this is insufficient, the animal can evaporate water from the blood — in the form of sweat for those animals with sweat glands. The evaporation of 1 gram of water absorbs some 540 calories of heat.
Most endotherms cannot tolerate a rise in body temperature of more than 5°C or so. The brain is the organ most susceptible to damage by a high temperature. Some mammals, dogs for example, have a countercurrent heat exchanger located between the carotid arteries and the vessels that distribute blood to the brain. This heat exchanger transfers some of the heat of the arterial blood to the relatively cool venous blood returning from the nose and mouth. This cools their arterial blood before it reaches the brain.
The shifting of blood flow as needed to maintain homeothermy is controlled by temperature receptors in the hypothalamus of the brain. One set of receptors here responds to small (0.01°C) increases in the temperature of the blood. When triggered, all the activities such as shunting blood vessels to the skin and extremities and sweating by which the body cools itself are brought into play. It is this center that enables us to maintain a constant body temperature (homeothermy) during periods of extreme exertion or in hot surroundings.
A second region of the hypothalamus triggers warming responses such as shunting blood away from the skin and extremities and shivering when the body becomes chilled. It is the hypothalamus that executes the fever response. In effect, the hypothalamus is the body's thermostat. The release of prostaglandins during inflammation increases the setting; that is, turns the thermostat "up". If the body temperature is not yet there, the body begins shivering violently — causing "chills" — to generate the heat needed. The result is fever when the new set point is reached.