19 Homeostasis Introduction

Homeostasis relates to dynamic physiological processes that help us maintain an internal environment suitable for normal function. Homeostasis is not the same as chemical or physical equilibrium. Such equilibrium occurs when no net change is occurring: add milk to the coffee and eventually, when equilibrium is achieved, there will be no net diffusion of milk in the coffee mug. Homeostasis, however, is the process by which internal variables, such as body temperature, blood pressure, etc., are kept within a range of values appropriate to the system. When a stimulus changes one of these internal variables, it creates a detected signal that the body will respond to as part of its ability to carry out homeostasis.


(Definition) Homeostasis is the tendency of biological systems to maintain relatively constant conditions in the internal environment while continuously interacting with and adjusting to changes originating within or outside the system.

Consider that when the outside temperature drops, the body does not just “equilibrate” with (become the same as) the environment. Multiple systems work together to help maintain the body’s temperature: we shiver, develop “goose bumps”, and blood flow to the skin, which causes heat loss to the environment, decreases.

Many medical conditions and diseases result from altered homeostasis. This section will review the terminology and explain the physiological mechanisms that are associated with homeostasis. We will discuss homeostasis in every subsequent system. Many aspects of the body are in a constant state of change—the volume and location of blood flow, the rate at which substances are exchanged between cells and the environment, and the rate at which cells are growing and dividing, are all examples. But these changes actually contribute to keeping many of the body’s variables, and thus the body’s overall internal conditions, within relatively narrow ranges. For example, blood flow will increase to a tissue when that tissue becomes more active. This ensures that the tissue will have enough oxygen to support its higher level of metabolism.

Maintaining internal conditions in the body is called homeostasis(from homeo-, meaning similar, and stasis, meaning standing still). The root “stasis” of the term “homeostasis” may seem to imply that nothing is happening. But if you think about anatomy and physiology, even maintaining the body at rest requires a lot of internal activity. Your brain is constantly receiving information about the internal and external environment, and incorporating that information into responses that you may not even be aware of, such as slight changes in heart rate, breathing pattern, activity of certain muscle groups, eye movement, etc. Any of these actions that help maintain the internal environment contribute to homeostasis.

Example: Physical Exercise

We can consider the maintenance of homeostasis on a number of different levels. For example, consider what happens when you exercise, which can represent challenges to various body systems. Yet instead of these challenges damaging your body, our systems adapt to the situation. At the whole-body level, you notice some specific changes: your breathing and heart rate increase, your skin may flush, and you may sweat. If you continue to exercise, you may feel thirsty. These effects are all the result of your body trying to maintain conditions suitable for normal function:

Your muscle cells use oxygen to convert the energy stored in glucose into the energy stored in ATP (adenosine triphosphate), which they then use to drive muscle contractions. When you exercise, your muscles need more oxygen. Therefore, to maintain an adequate oxygen level in all of the tissues in your body, you breathe more deeply and at a higher rate when you exercise. This allows you to take in more oxygen. Your heart also pumps faster and harder, which allows it to deliver more oxygen-rich blood to your muscles and other organs that will need more oxygen and ATP.

As your muscles carry out cellular respiration to release the energy from glucose, they produce carbon dioxide and water as waste products. These wastes must be eliminated to help your body maintain its fluid and pH balance. Your increased breathing and heart rates also help eliminate a great deal of carbon dioxide and some of the excess water.

Your muscles use the energy stored in ATP molecules to generate the force they need to contract. A byproduct of releasing that energy is heat, so exercising increases your body temperature. To maintain an appropriate body temperature, your body compensates for the extra heat by causing blood vessels near your skin to dilate and by causing sweat glands in your skin to release sweat. These actions allow heat to more easily dissipate into the air and through evaporation of the water in sweat.

As you exercise for longer periods of time, you lose more and more water and salts to sweat (and, to a smaller extent, from breathing more). If you exercise too long, your body may lose enough water and salt that its other functions begin to be affected. Low concentrations of water in the blood prompt the release of hormones that make you feel thirsty. Your kidneys also produce more concentrated urine with less water if your fluid levels are low. These actions help you maintain fluid balance.



Share This Book