Discovering the importance of homeostasis

Maintaining a constant temperature:thermoregulation

All metabolic reactions in all organisms require that the temperature of the body be within a certain range. Because we humans are homeotherms or "warm-blooded", we maintain a relatively constant body temperature regardless of the ambient temperature. We do this by regulating our metabolic rate. The large number of mitochondria per cell enables a high rate of metabolism, which generates a lot of heat.

Regulating body temperature requires a steady supply of fuel (glucose) to the mitochondrial furnaces.

Another way we control our body temperature is by employing adaptations that conserve the heat generated by metabolism within the body in cold conditions or dissipate that heat out of the body in overly warm conditions. A few of the specific adaptations include the following:

• Sweating: Sweat glands in the skin open their pores to dissipate heat by evaporative cooling of water from the skin. They close to conserve heat. Sweat glands are opened and closed by the action of muscles at the base of the gland, deep under the skin. Refer to Chapter 4.
• Blood circulation: Blood vessels close to the skin dilate (enlarge) to dissipate heat in the blood through the skin. They constrict (narrow) to conserve heat. That’s why your skin flushes (reddens) when you’re hot: That’s the color of your blood visible at the surface of your skin. Refer to Chapter 9.
• Muscle contraction: When closing sweat pores and blood vessel constriction are not enough to conserve heat in cold conditions, your muscles will begin to contract automatically to generate more heat. This reaction is familiar as "shivering".
• Insulation: Regions of fatty tissue under the skin provide insulation, holding the warmth of the body in. Body hair aids in this, too (though not enough to keep us from needing a nice, warm winter coat).

 

Swimming in H2O: fluid balance

A watery environment is a requirement for a great proportion of metabolic reactions (the rest need a lipid, or fatty, environment). The body contains a lot of water: in your blood, in your cells, in the spaces between your cells, in your digestive organs, here, there, and everywhere. Not pure water, though. The water in your body is a solvent for thousands of different ions and molecules (solutes). The quantity and quality of the solutes change the character of the solution. Because solutes are constantly entering and leaving the solution as they participate in or are generated by metabolic reactions, the characteristics of the watery solution must remain within certain bounds for the reactions to continue happening.

• Changes in the composition of urine: The kidney is a complex organ that has the ability to measure the concentration of many solutes in the blood, including sodium, potassium, and calcium. Very importantly, the kidney can measure the volume of water in the body by sensing the pressure of the blood as it flows through (the greater the volume of water, the higher the blood pressure). If changes must be made to bring the volume and composition of the blood back into the ideal range, the various structures of the kidney incorporate more or less water, sodium, potassium, and so on into the urine. That’s why your urine is paler or darker at different times. This and other functions of the urinary system are discussed in Chapter 12.
• The thirst reflex: Water passes through your body constantly: mainly, in through your mouth and out through various organ systems, including the skin, the digestive system, and the urinary system. If the volume of water falls below the optimum level (dehydration), and the kidneys alone can’t regain the balance, the mechanisms of homeostasis intrude on your conscious brain to make you uncomfortable. You feel thirsty. You ingest something watery. Your fluid balance is restored and your thirst reflex leaves you alone.

 

Adjusting the fuel supply: blood glucose concentration

Glucose, the fuel of all cellular processes, is distributed to all cells dissolved in the blood. The concentration of glucose in the blood must be high enough to ensure that the cells have enough fuel. However, extra glucose beyond the immediate needs of the cells can harm many important organs and tissues, especially where the vessels are tiny, as in the retina of the eye, the extremities (hands and, especially, feet), and the kidneys. Diabetes is a disease in which there is a chronic overconcentration of glucose in the blood.

The amount of glucose in the blood is controlled mainly by the pancreas. Absorption by the small intestine puts the glucose from ingested food into the blood. Insulin is a hormone released into the blood from the pancreas in response to increased blood glucose levels. Most cells have receptors that bind the insulin, which allows glucose into the cells for cellular respiration. The cells of the liver, muscles, and adipose tissue (fat) take up the glucose and store it as glycogen (see Chapter 3). At times when your intestines aren’t absorbing much glucose, like hours after a meal, the production of insulin is suppressed and the stored glucose is released into the blood again. Refer to Chapter 8 for more information about the pancreatic control of blood glucose levels.

 

Measuring important variables

How does the pancreas know when to release insulin and how much is enough? How does the kidney know when the salt content of the blood is too high or the volume of the blood is too low? What tells the sweat glands to open and close to cool the body or retain heat? Well, read on!

Every homeostatic mechanism employs three parts: a receptor, an integrator, and an effector. Numerous receptors, or sensors, are strategically placed throughout your body. Some respond to chemical changes (like pH), others to mechanical ones (like blood pressure), and there are many more. These receptors are specialized nervous cells and communicate to the brain — the integrator — any changes from our balance. The brain processes all the incoming information and "decides" if a response is warranted. If it is, a message will be sent out via neurons or hormones to the effectors, which carry out the body’s response (the effect).

 

Feedback in physiology

In biology and other sciences, feedback is information that a system generates about itself or its effects that influences how its processes continue. Feedback mechanisms can be negative or positive. These terms do not mean that one is harmful and the other beneficial, and they are not opposites — that is, they do not counteract one another in the same system or process. Organisms use both types of feedback mechanisms to control different aspects of their physiology.

A negative feedback mechanism functions to keep things within a range. It tells the system to stop, slow down, decrease its output when the optimum quantity or range has been achieved or to speed up or increase its output when the quantity is below the optimum range. In other words, it tells a process to start doing the opposite of what it’s doing now. Negative feedback mechanisms maintain or regulate physiological conditions within a set and narrow range. Homeostasis depends on a vast array of negative feedback mechanisms.

A positive feedback mechanism tells a process to continue or increase its output. Positive feedback says, "Some is good; more would be better". It accelerates or enhances the output created by a stimulus that has already been activated. A positive feedback mechanism is usually a cascading process that increases the effect of the stimulus and pushes levels out of normal ranges, usually for a specific and temporary purpose. Because positive feedback can get out of control (think fire), there are relatively few positive feedback mechanisms. One example is the "clotting cascade" that occurs in response to a cut in a blood vessel, described in this chapter. Another is the release of oxytocin to intensify uterine contractions during childbirth (described in Chapter 14).