Water makes up about 60% of an adult’s body weight, though this varies by age, sex, and body composition. It forms the environment where countless biological processes occur, sustaining life. Understanding water’s journey provides insight into its influence on overall health.
Absorption and Distribution
Once consumed, water travels through the digestive system. While some absorption occurs in the stomach, most water is absorbed in the small intestine, with the large intestine reabsorbing remaining amounts. Water moves from the intestines into the bloodstream.
After absorption, water enters the bloodstream and is distributed throughout the body into various fluid compartments. Approximately two-thirds of the body’s water resides within cells, known as intracellular fluid. The remaining one-third constitutes extracellular fluid, found outside cells in spaces like the blood plasma and interstitial fluid surrounding tissues.
Water’s Essential Functions
Water performs many essential roles within the body. It acts as a universal solvent, allowing many substances to dissolve and be transported throughout the body. This property allows blood to carry nutrients, oxygen, and hormones to cells, while also collecting metabolic waste products for removal.
Water also plays a role in regulating body temperature. Through sweating, water evaporates from the skin, carrying heat away and cooling the body. Water provides lubrication and acts as a shock absorber. It is a main component of fluids that lubricate joints, such as synovial fluid, enabling smooth movement. Water also cushions and protects organs and tissues, including the brain and spinal cord.
How Water Leaves the Body
The body continuously loses water through several pathways. The primary method of water excretion is through urination, where the kidneys filter waste products from the blood and combine them with water to form urine.
Water is also lost through the skin, primarily via sweating. Even without overt sweating, water evaporates from the skin surface and from air exhaled from the lungs, known as insensible water loss. A smaller amount of water is eliminated from the body through feces.
Maintaining Water Balance
The body maintains a balanced water level, a state referred to as fluid balance or homeostasis. Thirst signals the need for water intake, prompting individuals to drink. This response is triggered when the body detects changes in its fluid concentration or blood volume.
The kidneys are central to regulating water output, adjusting how much water is reabsorbed back into the bloodstream versus how much is excreted as urine. They also adjust the volume and concentration of urine based on the body’s hydration needs. This kidney function is influenced by hormonal signals, notably antidiuretic hormone (ADH), also known as vasopressin. ADH is released in response to increased blood solute concentration or decreased blood volume, signaling the kidneys to conserve water and produce more concentrated urine, helping to restore fluid balance.