Chapter 2. Homeostasis

Part 1: Homeostasis

Homeostasis refers to a relatively stable set of conditions within an organism’s internal environment. Within the human body, maintaining a healthy environment for living cells requires maintaining appropriate conditions in the extracellular fluids—including the interstitial fluid and blood plasma—for each living cell to be able to function properly. The constancy of an internal environment is important in allowing chemical reactions to take place at rates necessary to support the body.

Conditions Maintained by Homeostasis

There are three important conditions that must be met in order for the chemical reactions in the body to occur at the rates necessary for homeostasis. The first condition is that there must be a proper concentration of gases, nutrients, water, and salts. The gases involved are oxygen, which is necessary for the process of cellular respiration to produce energy for the body, and carbon dioxide, which is a waste product of this process but also must be present in certain amounts for other processes to occur efficiently. Nutrients provide reactants, enzymes, cofactors, and energy for chemical reactions, while water and salts determine the fluid balance, electrolyte balance, and the pH of the body.

The second important condition for homeostasis is an optimum temperature that is around 37°C. The rates of chemical reactions are temperature dependent, typically occurring slower at cooler temperatures and faster at warmer temperatures. Additionally, the shapes of proteins (remember that proteins fulfill many roles in the body, including providing structure, muscle contraction, and enzymatic function, among others) are temperature dependent—if they overheat, they may denature and be rendered non-functional.

The third and final condition is an optimum pressure, for example, blood pressure. The concentrations of various substances in the body and the rates at which these substances move through the body are both dependent on blood pressure. Blood pressure is influenced by the volume of blood in the cardiovascular system, the diameter of the blood vessels and the strength of cardiac muscle contraction.

The Internal Environment

Homeostasis is maintained within the internal environment of the body. The human body is composed of cells surrounded by extracellular material, which is mostly fluid. Cells are responsible for controlling bodily activities and also controlling the composition of the environment around them. The internal environment is defined as the fluid inside the body but outside of cells.

The fluid inside the cells in the body is the intracellular fluid, also called cytoplasm (cytosol and the fluid inside organelles). Intracellular fluid makes up approximately 2/3 of the total body fluids.

The fluid outside the cells in the body is the extracellular fluid and is synonymous with the internal environment. Extracellular fluid is found in blood (i.e. plasma), in tissues (i.e. interstitial fluid), in the lymphatic system (i.e., lymph), in joints (i.e., synovial fluid), in the eyes (i.e., aqueous and vitreous humors), and in the central nervous system (i.e., cerebrospinal fluid). Extracellular fluid amounts to about 1/3 of the fluids in the body as a whole.

The three main fluid types within the extracellular compartment are plasma, interstitial fluid and lymph. Plasma is the liquid portion of blood and functions to move red and white blood cells, platelets, nutrient molecules, gases, electrolytes, and wastes throughout the circulatory system. Materials are exchanged at capillary beds where the blood vessel walls are thin enough to allow materials to move between the blood and the tissues.

The fluid that surrounds living cells within tissues is called interstitial fluid, which has a unique composition relative to both intracellular fluid and blood plasma. Components of plasma move into the tissues, thus the interstitial fluid is partly derived from the plasma of blood. Interstitial fluid is also in contact with cells and is thus partly derived from the intracellular fluid.

Interstitial fluid must be returned to the circulatory system in order to maintain appropriate pressure within the cardiovascular system and for waste removal from cells. However, the interstitial fluid is under lower pressure than the plasma in the capillaries, and so only 85% of the interstitial fluid is returned directly back to the circulatory system at the capillaries (mostly as a result of the osmotic gradient; the higher concentration of solutes inside the capillaries “sucks” the water back in). The remaining 15% is returned indirectly through the vessels of another system called the lymphatic system. The fluid in these vessels, now called lymph, is derived from tissue fluid but is different from it in composition. Lymph is collected into larger lymphatic vessels that eventually drain into veins close to the heart.

Although the extracellular and intracellular compartments are distinct from one another, there is a continual exchange of materials between them. Cells must receive nutrients, have waste materials taken away, and exchange gases. Refer to Chapter 7, Cell Biology: Membrane Transport, to review how substances move between intracellular and extracellular compartments.

Stressors

A constant internal environment is impossible to maintain. The goal of homeostasis is to ensure that the constant changes to the internal environment are kept within a stable range. The internal environment is continually subjected to disturbances that, unless counteracted, would quickly cause a new set of conditions in the body that could result in illness or death. A stressor is any stimulus that causes an imbalance in the internal environment. Stressors may be factors external to the body or from within the body itself.

An example of an external stressors is a lack of environmental oxygen, which may occur at high altitudes or extreme environmental temperatures. Examples of internal stressors are rapid changes in blood pressure—hemorrhaging can cause a drop in pressure, while unpleasant thoughts may raise blood pressure—or changes in levels of nutrients, such as an increase or decrease in blood glucose levels.

Part 2: Feedback loops

Maintaining homeostasis requires that the body continuously monitor its internal conditions. From body temperature to blood pressure to nutrient levels, each physiological condition has a particular set point. A set point is the physiological value around which the normal range fluctuates. A normal range is the restricted set of values that is optimally healthy and stable. For example, the set point for normal human body temperature is approximately 37°C (98.6°F); however, physiological body temperature tends to fluctuate within a few degrees above and below that set point depending on factors such as activity, digestive function, stress, etc. Control centers in the brain and other parts of the body monitor and react to deviations from homeostasis using negative feedback. Negative feedback is a mechanism that reverses a deviation from the set point. Therefore, negative feedback maintains body parameters within their normal range. The maintenance of homeostasis by negative feedback occurs throughout the body at all times, and an understanding of negative feedback is thus fundamental to an understanding of human physiology.

Negative Feedback

Feedback systems have five basic components (Figure 2.1). A sensor, also referred to a receptor, detects when the stressor/stimulus produces a deviation in a physiological value away from the set point. This change in value is reported to the control center. The control center is the component in a feedback system that compares the value to the normal range. If the value deviates too much from the set point, then the control center activates an effector. An effector is the component in a feedback system that produces a response, which in the case of a negative feedback loop, causes a return of the physiological value to its normal range.

For example, in the control of blood glucose, specific endocrine cells (receptors) in the pancreas detect excess glucose (stimulus) in the bloodstream. These pancreatic beta cells (control center) respond to the increased level of blood glucose by releasing the hormone insulin into the bloodstream. The insulin signals skeletal muscle fibers, fat cells (adipocytes), and liver cells (effectors) to take up the excess glucose (response), removing it from the bloodstream. As the glucose concentration in the bloodstream drops, the decrease in concentration—the actual negative feedback (outcome)—is detected by pancreatic beta cells, and insulin release stops. This helps prevent blood sugar levels from continuing to drop below the normal range.

Humans have a similar temperature regulation feedback system that works by promoting either heat loss or heat gain (Figure 2.1b). When the brain’s temperature regulation center in the hypothalamus receives data from the sensors indicating that the body’s temperature exceeds its normal range, it stimulates a cluster of brain cells referred to as the thermoregulatory center. This stimulation has three major effects:

• Blood vessels in the skin begin to dilate, allowing more blood from the body core to flow to the surface of the skin and allowing heat to radiate into the external environment.
• As blood flow to the skin increases, sweat glands are activated to increase their output. As sweat evaporates from the skin surface into the surrounding air, it lowers the temperature of the skin’s surface.
• The depth of respiration increases, and a person may breathe through an open mouth instead of through the nasal passageways. This further increases heat loss from the lungs.

In contrast, activation of the thermoregulatory center by exposure to cold reduces blood flow to the skin, and blood returning from the limbs is diverted into a network of deep veins. This arrangement traps heat closer to the body core and restricts heat loss. If heat loss is severe, the hypothalamus triggers an increase in random signals to skeletal muscles, causing them to contract and producing the shivering response. The muscle contractions involved in shivering release heat while using ATP. The brain triggers the thyroid gland in the endocrine system to release the thyroid hormone, which increases metabolic activity and heat production in cells throughout the body. The hypothalamus also signals the adrenal glands to release epinephrine (adrenaline), a hormone that causes the breakdown of glycogen into glucose, which can be used as an energy source. The breakdown of glycogen into glucose also results in increased metabolism and heat production.

Positive Feedback

Positive feedback intensifies a change in the body’s physiological condition rather than reversing it. Positive feedback loops employ the same five basic components as negative feedback loops, with the difference being that a deviation from the normal range results in more change, and the system moves farther away from the set point. Positive feedback in the body is much more rare than negative feedback and often occurs when a dramatic change in normal physiological activity is desired. Childbirth, blood clotting, and micturition are some examples of physiological positive feedback loops (there are a number of other examples in immunity, in female reproductive cycles and at the level of cellular function).

Childbirth at full term is an example of a situation in which the maintenance of the existing body state is not desired. Enormous changes in the mother’s body are required to expel the baby at the end of pregnancy. And the events of childbirth, once begun, must progress rapidly to a conclusion, or the life of the mother and the baby are at risk. The extreme muscular work of labor and delivery are the result of a positive feedback system (Figure 2.2).

The first contractions of labor (the stimulus) push the baby toward the cervix (the lowest part of the uterus). The cervix contains stretch-sensitive nerve cells that monitor the degree of stretching (the sensors). These nerve cells send messages to the brain, which in turn causes the pituitary gland at the base of the brain (the control center) to release the hormone oxytocin into the bloodstream. Oxytocin causes stronger contractions of involuntary smooth muscles in the uterus (the effectors), pushing the baby farther down the birth canal. This causes even greater stretching of the cervix (the response). The cycle of stretching, oxytocin release, and increasingly more forceful contractions stops only when the baby is born. At this point, the stretching of the cervix halts, stopping the release of oxytocin.

A second example of positive feedback centers on reversing extreme damage to the body. Following a penetration wound, the most immediate threat is excessive blood loss. Less blood circulating means reduced blood pressure and reduced perfusion (distribution of blood) to the brain and other vital organs. If perfusion is severely reduced, vital organs will shut down, and the person will die. The body responds to this potential catastrophe by releasing substances in the injured blood vessel wall that begin the process of blood clotting. As each step of clotting occurs, it stimulates the release of more clotting substances. This accelerates the processes of clotting and seals off the damaged area. Clotting is contained in a local area based on the tightly controlled availability of clotting proteins. This is an adaptive, life-saving cascade of events.

Practice

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