How Does the Body Maintain Water Balance

Water accounts for two-thirds of the body weight in adults. A 70 kg man has 42 litres of water in his body. Therefore, the maintenance of water in the body is essential for the body to protect against dehydration. Hence, water intake must balance the water loss. Urinary water loss through kidneys is the primary method of water loss from the body. In addition, water may be lost through feces and sweat production. The main water uptake of the body occurs through dietary intakes. A small amount of water is produced during metabolism as well. The water balance of the body can be maintained through the regulation of fluid intake and regulation of water excretion by kidneys.

Key Areas Covered

1. What is Water Balance
      – Definition, Water Intake, Water Loss
2. How Does the Body Maintain Water Balance
     – Regulation of Water Intake, Regulation of Water Loss
3. What are the Impairments of Water Balance
     – Hypotonicity, Hypertonicity

Key Terms: Dietary Intake, Dehydration, Hyperhydration, Hypohydration, Osmoregulation, Physiological Thirst, Water Intake, Water Loss

What is Water Balance

Water balance refers to an aspect of homeostasis of a particular organism in which the amount of water in the body is controlled through osmoregulation and behavior. The maintenance of water balance in the body is essential to avoid dehydration. It also prevents the formation of kidney stones and other medical implications. The water balance of the body can be maintained through the regulation of both water intake and water loss.

Water Intake

The main water intake of the body occurs through the fluid intake. Water is consumed by the body through drinking water, fluids, and food moisture. A healthy adult must drink around 2 litres of water per day. Fluids and water itself represent 70-80% of water intake while food moisture represents 20-30% of it. In addition, the oxidation of hydrogen-containing substrates produces water during metabolism. The metabolic water production by nutrients is shown in table 1.

Metabolic Water Production

Therefore, metabolic water production is proportional to the intake of nutrients.

Water Loss

The main method of water loss from the body is the excretion through kidneys. From 500 mL to several litres of water are excreted through urine. Osmoregulation is one of the functions of the body, involving in the regulation of body fluid volumes. Nephron is the functional unit of the kidney that is involved in the production of urine in order to eliminate excess water in the body fluids. Each kidney contains around one million nephrons. The blood is filtered through three steps: filtration, reabsorption, and secretion. A small amount of water passes out from the body through feces and sweat production. About 200 mL of water is lost through feces per day in a healthy adult. Sweat is produced by the sweat glands in the dermis. Around 0.3-2.6 L of sweat is produced per day in adults. In high ambient temperature, high humidity or during intense physical activity, the production of sweat is increased.

How Does the Body Maintain Water Balance

The body maintains the water balance through the regulation of the water intake and water loss from the body. Organisms can maintain a tight constancy of water in the body through these two regulation mechanisms.

Regulation of Water Intake

The body regulates the water intake mainly through physiological thirst. The thirst is defined as the conscious desire to drink water. The increased plasma osmolality is the main stimulus for thirst. The osmoreceptors in the hypothalamus detect the increased osmolality in the blood plasma, resulting in the sensation of thirst. when there is a slight increase in the plasma osmolality, the anti-diuretic hormone (ADH) is secreted to increase the water reabsorption in renal tubules. The decreased blood volume or blood pressure and the increase of circulating angiotensin also result in the sensation of thirst. Social and environmental factors such as social influence, habits, and dry mouth also increase the water intake.

Regulation of Water Loss

The body regulates the water loss mainly through excretion by kidneys. Depend on the hydration state of the body and the amount of fluid intake, kidneys are capable of regulating the amount of water excreted through urine. Therefore, kidneys maintain a constant composition of solutes in the extracellular fluid. The hormone involved in this feedback mechanism is the ADH.

In a water deficit, the osmolality of plasma is increased above to its regular value, 280 mOsmol/kg H₂O. This is detected by osmoreceptors, stimulating the release of ADH. This causes the dilution of the extracellular fluid. When there is excess water in the body, the ADH secretion is reduced, decreasing the water reabsorption. Maintenance of water balance of the body is shown in figure 1.

Figure 1: Maintenance of Water Balance

What are the Impairments of Water Balance

The impairment of water balance may occur due to conditions such as hypohydration or hyperhydration. Hypohydration is a result of the loss of body water by dehydration. It results in hypertonic extracellular fluids. Hyperhydration is a result of the excessive consumption of water over a short period of time. It results in hypotonic extracellular fluids. Isotonic extracellular fluid can be identified when the amounts of water is well balanced in the body. The causes for different types of conditions in the extracellular fluid are shown in table 2.

Causes for Different Types of Conditions in Extracellular Fluid

Conclusion

Water accounts the most of the body weight of most organisms. The body maintains the amount of water in the body either by regulating the water intake or water loss from the body. Water intake mainly occurs through the diet. It can be increased by the sensation of physiological thirst. Water loss mainly occurs through the excretion by kidneys. The kidney can regulate the amounts of water in the body based on the tonicity of the extracellular fluid.

Reference:

1. “Water Balance.” Lumen / Boundless Anatomy and Physiology, Available here.