Homeostasis refers to the ‘equilibrium (balance) in the body’s internal environment due to the consistent interaction of the body’s main regulatory processes.’ There are many types of homeostatic mechanisms regulating various aspects of the human body including the heart rate, breathing rate, body temperature, blood glucose levels as well as the water balance. This article will describe the homeostatic mechanisms that control the changes in water balance including the hormones that are involved in achieving the same.
How does the body gain and lose water?
Before discussing the homeostasis of body fluids, it is necessary to understand the methods in which the body gain and lose water. These methods would be the same processes that will be altered through the homeostatic mechanisms in order to achieve the right water balance. Among the methods of gaining water, oral intake in the form of drinking water and other fluids, as well as in the form of moist foods, is the most significant. In addition, cellular respiration, which produces carbon dioxide and water as by products also contribute towards gaining water in the body. When it comes to losing water, the main method is the excretion of urine while sweating and expulsion of feces would also contribute significantly. In addition, convection of moisture through the skin and during respiration is also considered important methods of losing water from the body.
Therefore, during homeostasis of water balance, these processes are either augmented or suppressed depending on the need.
What triggers the homeostatic control of water balance?
The triggering of the homeostatic mechanisms of water balance take place at the ‘thirst center’ located in the hypothalamus of the brain. The region contains receptors known as ‘osmoreceptors’ which hare sensitive to the osmotic changes that is taking place in the plasma (the fluid components of the blood). Thus, when there is a decline of 10 – 15% in the plasma volume or an increase in the plasma osmolality (the concentration of particle elements in the blood) by 1 – 2%, the osmoreceptors becomes activated and triggers the homeostatic mechanisms to correct the deviation. In addition, the thirst center is also receptive to the sensory signals coming from the baroreceptors (receptors that senses the changing pressures within the vascular compartment) located in various parts of the body as well as to certain hormones that are released in response to a depleting fluid volume.
What follows the triggering of the ‘thirst receptors’?
Following triggering the ‘thirst receptors’, a cascade of processes becomes activated. For instance, when the body water levels are detected of being too low, the hypothalamus would send a message to the cerebellum, which triggers the feeling of ‘thirst’. This would make a person drink water and therefore restore the fluid balance.
At the same time, the hypothalamus sends signals to the posterior pituitary of the brain to secrete a hormone known as ADH (Anti-diuretic hormone) which can promote absorption of more water in the distal convoluted tubules of the kidneys. As a result, the amount of water excreted through urine would be less while more water retains within the vascular compartment. Following these actions, the extra-cellular volume (the blood volume) would rise to its normal levels and the same would be detected by the osmoreceptors in the hypothalamus. The detection of a raised extracellular fluid volume would inhibit the thirst center, as well as the release of ADH from the posterior pituitary.
What signals inhibit the stimulation of the ‘thirst center’?
The moistening of the oral mucosa, as well as the signals received from the stomach and intestinal stretch receptors, are believed to inhibit the already stimulated ‘thirst center’ and thereby suppress its action to retain water.
What other hormones participate in the homeostatic mechanisms controlling the water balance?
In addition to ADH, two other hormones, namely aldosterone and natriuretic peptides, also participate in the homeostatic mechanisms depending on the intensity and the nature of the deviated water balance. Thus, the aldosterone, which is secreted by the adrenal cortex, promotes retention of sodium ions and therefore water within the vascular compartment. However, in order to achieve this, it promotes the excretion of potassium in the urine. Natriuretic peptides on the other hand are secreted by the atrium and the ventricles of the heart in response to the stretching of the atrial or ventricular muscles. Thus, the purpose of these peptides is to lower the pressure and the volume within the extracellular compartment, which can be achieved by increasing the urine output through inhibiting the actions of ADH, aldosterone as well as triggering sodium excretion by itself.
Thus, the homeostasis of water balance is a complex but efficient process that needs support from various elements belonging to several body systems.