Homeostatic Maintenance in the Body

You have read about general and specific examples of homeostasis, including positive and negative feedback, and have learned the terminology that is used to describe parts of the feedback loops. It is important to become comfortable with the terminology since it will be used to introduce new concepts in upcoming sections of this course.

Maintaining homeostasis within the body is important for proper physiological function. It is important to recognize the mechanisms of homeostasis in the body, as well as the consequences of homeostasis dysfunction.

In the following examples, you will learn to identify homeostasis at different levels of organization, such as how the body maintains tight control over small molecules, and the importance of maintaining cell number.

Homeostasis of Ions

Body functions such as regulation of the heartbeat, contraction of muscles, activation of enzymes, and cellular communication require tightly regulated calcium levels. Normally, we get a lot of calcium from our diet. The small intestine absorbs calcium from digested food.

The endocrine system is the control center for regulating blood calcium homeostasis. The parathyroid and thyroid glands contain receptors that respond to levels of calcium in the blood. In this feedback system, blood calcium level is the variable, because it changes in response to the environment. Changes in blood calcium level have the following effects:

• When blood calcium is low, the parathyroid gland secretes parathyroid hormone. This hormone causes effector organs (the kidneys and bones) to respond. The kidneys prevent calcium from being excreted in the urine. Osteoclasts in bones breakdown bone tissue and release calcium. When blood calcium levels are high, less parathyroid hormone is released. Parathyroid hormone is the main controller of blood plasma calcium levels in adults.
  
• Children have a second hormone that contributes to calcium regulation, called calcitonin. It is released from the thyroid gland when blood calcium levels are high. Calcitonin prevents bone breakdown and causes the kidneys to reabsorb less calcium from the filtrate, allowing excess calcium to be removed from the body in urine.

Calcium imbalance in the blood can lead to disease or even death. Hypocalcemia refers to low blood calcium levels. Signs of hypocalcemia include muscle spasms and heart malfunctions. Hypercalcemia occurs when blood calcium levels are higher than normal. Hypercalcemia can also cause heart malfunction as well as muscle weakness and kidney stones.

Homeostasis of Molecules

Glucose is an important energy source used by most cells in the body, especially muscles. Without glucose, the body “starves”, but if there is too much glucose, problems occur in the kidneys, eyes, and even with the immune response. Insulin is a hormone produced by the pancreas in response to increased blood glucose levels. When the pancreas releases insulin, it acts as a key to open passageways for glucose to enter all body cells, where it is used for energy production. The liver also plays an important role in this feedback loop. Excess glucose is used by liver and muscle cells to synthesize glycogen for storage. The pancreas also produces the hormone glucagon. Glucagon is released when blood glucose levels decrease and stimulates liver cells to catabolize glycogen back to glucose, which is then released into the blood to bring blood glucose levels back up.

Control of Cell Number

Although homeostasis is often carried out by a negative feedback loop with an identifiable receptor, control center and effectors, it more broadly means maintaining variables in a range suitable for optimal function. This includes the regulation of cell number in our tissues so that we don’t have too few or too many. It may be hard to identify specific components of a feedback loop, but it is clear that there are at least negative feedback cycles that help maintain cell numbers. This negative feedback is known to occur through cell-to-cell communications of neighboring cells and an ability to sense the levels of nutrients and matrix in the area they are growing in. Normally cells will stop dividing when there is an appropriate number of cells in a tissue or space. If a neighboring cell is lost or if there is an inadequate number of cells, cells may be stimulated to divide. Cells with too many neighbors trigger an internal response to die in a regulated programmed way called apoptosis. When cells sense they have no neighbors, signals in the nucleus cause division of the cell.