Water is the single largest component of the human body, making up an estimated 45 to 65% of an adult’s total body weight. This molecule acts as the universal solvent, facilitating countless chemical reactions and transportation processes that sustain life. Water is present in every tissue, comprising about 73% of the brain and heart and 79% of the muscles and kidneys. Understanding how the body processes water from intake to its eventual exit reveals a complex, highly regulated physiological system.
Absorption in the Digestive System
The journey of water begins in the digestive tract, where ingested fluids and food are prepared for entry into the body. While water passes quickly through the stomach, only a small amount of absorption occurs there. The stomach’s primary role is to regulate the rate at which fluid is delivered to the small intestine, which significantly impacts the speed of overall water uptake.
The vast majority of water absorption takes place rapidly in the small intestine, which receives both ingested fluid and large volumes of digestive secretions. By the time the contents reach the large intestine, approximately 80 to 90% of the original fluid volume has already been absorbed. This process relies entirely on osmosis, where water moves across the intestinal lining in response to an osmotic gradient.
This osmotic gradient is established by the active absorption of solutes, especially sodium and glucose, into the cells lining the small intestine. As these particles are pumped out, they create a high concentration of dissolved material. Water then follows this concentration gradient, diffusing from the gut lumen into the bloodstream. The large intestine absorbs remaining water and electrolytes, helping to solidify waste before elimination.
Transport Through the Body and Cellular Uptake
Once absorbed, water enters the portal circulation and is quickly distributed throughout the body via the bloodstream. Blood plasma, which is approximately 90% water, is the primary medium for transport. This plasma carries dissolved nutrients, oxygen, hormones, and waste products to every tissue.
The next step involves water moving from the blood vessels into the tissues surrounding the cells. As blood reaches the tiny capillaries, hydrostatic pressure pushes some water and small dissolved substances out. This fluid, now called interstitial fluid, bathes the cells and acts as the immediate environment for exchange.
Water then enters the cells themselves, moving across the cell membranes via osmosis, driven by the differing concentrations of solutes inside and outside the cell. Inside the cell, water is termed intracellular fluid, which constitutes about two-thirds of the body’s total water. Within the cells, water serves as the medium for all metabolic chemical reactions and helps maintain the cell’s structure and proper shape. Water’s high specific heat capacity also allows the body to absorb and transfer heat, helping maintain a stable internal temperature.
Filtration, Regulation, and Elimination
The body maintains a stable water balance through a sophisticated process of filtration and regulation primarily controlled by the kidneys. The kidneys constantly filter the blood, removing waste and recovering necessary water and substances. Filtration occurs in the glomerulus, where approximately one-fifth of the blood is filtered into a preliminary fluid called filtrate.
About 99% of the water in this filtrate is reabsorbed back into the bloodstream to prevent excessive loss. This reabsorption is tightly controlled by hormones, most notably antidiuretic hormone (ADH), also known as vasopressin. When the body senses an increase in blood solute concentration, receptors in the hypothalamus trigger the release of ADH from the pituitary gland.
ADH acts on the kidney tubules by increasing the insertion of water channels, called aquaporins, into the cell membranes. This action makes the tubules more permeable to water, allowing more water to move out of the filtrate and back into the blood, resulting in more concentrated urine. Conversely, if the blood is too dilute, ADH release is suppressed, leading to less water reabsorption and the production of a larger volume of dilute urine.
Water not recovered by the kidneys is eliminated through various exit routes. Urination is the most significant route for controlled water loss, with an average adult excreting about 1.5 liters of urine daily. The body also experiences insensible water loss, including evaporation from the skin and water vapor lost through exhaled air, accounting for roughly 900 milliliters daily. A small amount of water is also lost through the feces.