Levels of homeostasis regulation

The intracellular mechanism of regulation, that ensures homeostasis in the organism, is the first level of regulation. Signals to change the state of the cell are substances produced in the cell itself or received it from the outside. These substances may act in three ways:

• change the enzyme activity by inhibiting or activating;

• change the amount of enzymes and other proteins through the induction or repression of their synthesis or by altering their rate of degradation;

• change the rate of the transmembrane transport of substances interacting with the membrane.

The coordination of intraorganic metabolism is necessary for multicellular organisms with complex structure and differentiated organs that perform specific functions. For example, intensive work requires the inclusion of muscle glycogen mobilization processes in the liver or lipid mobilization in adipose tissue. Intraorganic coordination is provided by the transmission of signals in two ways: through the bloodstream with hormones (endocrine system) and through the nervous system.

The endocrine system is the second level of regulation. It is represented by glands (sometimes individual cells), synthesizing hormones – chemical signals. Hormones are released into the bloodstream in response to a specific stimulus. This stimulus can be a nerve impulse or a change in the concentration of a substance in blood that flows through the endocrine glands (e.g., reducing the concentration of glucose). The hormone is transported from the blood and reaches target cells. It modifies the metabolism through intracellular mechanisms, i.e. by changing the activity or amount of enzymes, or velocity of the transmembrane transport of substances. As a result of changes in metabolism stimulus, causing the release of hormones is eliminated (for example, increases the concentration of glucose in the blood). The hormone after performing its function is destroyed by special enzymes.

The third level of regulation is the nervous system with signal receptors of both external and internal environment. The signals are transformed into a wave of depolarization of the nerve fibers (nerve impulse), which in the synapse with the effector cell causes the release of neurotransmitter – a chemical signal. The mediator through the intracellular mechanisms of regulation causes a change in metabolism. Effector cells can be even some endocrine cells that respond to nerve impulses with synthesis and release of the hormone.

Three levels of regulation of metabolism are closely interrelated and function as a integrated system.

Changes in metabolism during starvation

Starvation may be incomplete (malnutrition), and complete. The main problem of incomplete starvation is protein deficiency. Complete starvation can be associated with digestive tract diseases or mental disorrdes.

There are three phases. Nitrogen balance in all phases of starvation is negative. First phase follows the postabsorbing period and lasts about 24 hours. Glycogen reserves are used, the concentration of insulin in the blood decreases as the concentration of glucagon and cortisol is increased. The rate of fat mobilization increases as well as the rate of gluconeogenesis from amino acids and glycerol. The concentration of glucose in the blood is decreased to the lower limits of normal. The second phase lasts about one week. Mobilization of fats is continued, the concentration of fatty acids and ketones in the blood increases. Acetone is not used and excreted primarily in expired air and through the skin. The concentration of insulin in the blood is low, and glucose can not penetrate into muscle cells. Sources of energy for muscles and other organs are fatty acids and ketone bodies. Consumers of glucose are the only insulin-independent cells, especially brain cells. Gluconeogenesis continues due to the degradation of tissue proteins. Metabolic rate is decreased. Third phase lasts for several weeks. The degradation rate of protein is stabilized (approximately 20 grams per day), respectively, the rate of gluconeogenesis decreases. In this phase, the main energy source for the brain is ketone bodies. If you continue to starve tissue atrophy will continue. After using up to one third or to half of all proteins the death comes up.