Homeostasis:What is homeostasis? In biology, the term homeostasis is used a lot and it can refer to many different biological processes, but at its core, homeostasis is actually a very simple concept. Homeostasis is the maintenance of a stable internal environment within an organism. Pretty simple, right? Just maintain the status quo and you'll be fine.
But that's a lot easier said than done. Maintaining a stable internal environment in a person means having to carefully regulate countless parameters which include:
- body temperature
- internal pH
- internal concentrations of various ions
- blood pressure
- glucose levels in the blood.
Endotherms and EctothermsHumans, along with all other birds and mammals are endotherms which means that they are animals that actively maintain a stable body temperature by generating heat. People sometimes call endotherms warm-blooded animals. In contrast, most other animals like reptiles, fish, and insects are ectotherms which are animals whose body temperature changes with their environment. People usually call ectotherms cold-blooded animals.
You can remember the difference between endotherms and ectotherms if you remember that endo- means inside endo- = inside and ecto- means outside ecto- = outside, so endotherms generate their own heat from within, and ectotherms get their heat from the outside. In general, ectotherms allow their body temperature to vary with the environment because it does not need to be maintained at a specific temperature for the animal to live. However, for most endotherms, body temperature must be maintained within a very narrow range at all times in order for the animal to live, with some exceptions for certain animals that lower their body temperature when they hibernate. Therefore, a stable body temperature of 98.6 degress Fahrenheit, give or take only a few degrees, is one aspect of homeostasis in humans.
The main advantage of having a stable body temperature is that it allows optimization of normal body functions, metabolism and enzymes to a specific set temperature. This ensures that no matter what the environmental conditions, as long as the animal maintains temperature homeostasis, it will still be able to perform all normal body functions at or near optimal levels. However, there is a price to pay for always operating with full functionality, and that price is energy efficiency. Endotherms ensure a stable body temperature mainly by generating their own heat by burning calories. If they didn't have to maintain a high body temperature, they wouldn't burn as many calories, and therefore would be able to survive on less food, or would be able to grow at a faster rate on the same amount of food.
Thermoregulation:So, now that we know why people maintain a stable body temperature, let's take a look at thermoregulation or the regulation of internal body temperature. As stated before, our high metabolic rate is the major source of heat in our bodies. Most of the time, our body temperature exceeds the temperature of the environment, which means that we are continuously losing heat to the environment. The rate at which we lose heat is therefore a key determinant of body temperature and many of the most effective methods of maintaining body temperature are based on either increasing or decreasing the rate of heat loss.
Long ago, humans developed a unique method for controlling the rate of heat loss called clothing. You may laugh but the most basic purpose of clothing is to provide insulation which reduces heat loss. How much of a person's skin is covered by the clothing and how thick the clothing is determines how much insulation it provides and how much heat it conserves.
Animals in the wild don't have clothing to increase their insulation. However, most endothermic animals do have a layer of insulation in the form of fur, feathers or a thick layer of fat. Many animals experience changes in the thickness of these insulating layers to respond to seasonal changes in temperature. In addition when mammals get cold, they immediately respond by raising their fur which increases the thickness of their insulation.
Even though humans generally don't have enough hair to serve as insulation, we still respond to cold by raising the hair on our bodies which is why we get goosebumps when we are cold. This is a leftover response from our evolutionary ancestors who did have enough hair to serve as insulation.
Yet another way to regulate heat loss is through the use of the circulatory system. Although we think of human body temperature as being 98.6 degrees Fahrenheit, the actual temperature varies throughout the body. Temperatures in the very center of our bodies, like around the heart or in our gut, are closer to 100 degrees Fahrenheit, but temperatures at the skin surface and especially in our extremities like our hands and feet are significantly cooler and fluctuate quite a bit with the outside temperature. You have probably noticed that at times your hands or feet will be cold to the touch, and this is why.
Blood that is pumped from the heart in our core, out to all the tissues of the body, and then back to the heart unavoidably transfers heat from the core to wherever it is pumped. Blood that is pumped through capillaries at the surface of the skin will lose a lot more heat than blood that is pumped through capillaries that are located deeper within our tissues. Our bodies take advantage of this and adjust blood flow through various tissues to help the body either conserve or dissipate heat.
When the body needs to conserve heat, capillaries at the skin surface constrict and limit the amount of blood that flows through them, reducing heat loss. When the skin is exposed to extremely cold temperatures, the body will even completely shut down the blood flow to the exposed area as a survival mechanism. If blood flow is shut down for too long, cells in the area will die. This phenomenon is commonly called frostbite. Capillaries at the skin surface can also be used in the opposite manner to help cool us down. When our bodies are too warm, capillaries at the skin surface are dilated, increasing blood flow to the surface and greatly increasing the amount of heat lost from the blood.