Vasopressin, also called antidiuretic hormone (ADH), is part of the hormonal control of urine excretion and functions to enhance reabsorption of water, limiting the excretion of water in urine. Vasopressin is released when osmoreceptor cells in the hypothalamus detect a rise in the osmolarity (solute concentration) of the blood above a threshold level. Upon release, vasopressin reaches the kidney and binds to receptors on cells in the collecting duct, which stimulates release of aquaporin water channels from storage vesicles within the cells. Aquaporin channels are selectively permeable to water, and allow the flow of water out of the filtrate/urine. This water is then reclaimed by the body and used to increase blood volume, increase blood pressure, and reduce blood osmolarity.

One of the key adaptations of the mammalian kidney is the ability to conserve water through reabsorption and excretion of concentrated urine. This is accomplished by maintenance of an osmolarity gradient, suitable for extracting water from the filtrate. The two primary solutes are sodium, which is deposited in the renal medulla by the loop of Henle, and urea, which crosses the epithelium of the collecting duct in the inner medulla. The increased osmolarity of the interstitial fluid enables water to be extracted and conserved through aquaporin proteins in the collecting duct.

Antidiuretic hormone (ADH) is responsible for concentrating the urine. This is accomplished by making the collecting duct permeable to water, and allowing it to passively diffuse into the renal medulla. Alcohol will inhibit the function of ADH, which means that the urine will be less concentrated because water is unable to leave the collecting duct, thus also increasing the volume.

In diabetes insipidus, ADH is not available to reabsorb the water from the collecting tubule of the renal system. As a result, more fluid will be lost and frequent urination will occur. In diabetes mellitus the osmolarity of the blood is high due to a constant high concentration of glucose in the blood. Due to the increase in osmolarity, water will be drawn from the tissues and into the blood. When the blood reaches the kidneys, it will be filtered and result in more water in the urine. More water into the urine increases its volume and leads to frequent urination. Hyperglycemia and glucose in the urine only occurs in diabetes mellitus and not in diabetes insipidus. In diabetes insipidus, the problem is not the blood glucose level but the inability to secrete ADH. The term "diabetes" does not automatically mean a problem related to sugar.

ADH is a hormone that increases permeability to water in the collecting ducts and therefore increases water reabsorption from the urine. This decreases the volume of water in the urine. Additionally, it can induce vasoconstriction in high concentrations, which narrows blood vessels and thereby increasing blood pressure. 

Parietal cells in the stomach release hydrochloric acid, activating pepsin and aiding in digestion. This creates a highly acidic environment in the stomach that could be harmful to other regions of the body. When the stomach contents, or chyme, is transported out of the stomach and enters the small intestine it must be neutralized. The first segment of the small intestine is the duodenum, where digestive enzymes from the pancreas are secreted to help digest fats and proteins.

Along with these enzymes, bicarbonate is secreted by the pancreas into the small intestine. The bicarbonate reacts with the remaining acid, producing water, salt, and carbon dioxide.

The functional unit of the kidney is the nephron. Blood is filtered into the nephron to create filtrate. As the filtrate flows through the nephron tubules, its concentration is tightly regulated and ions and water are added and removed. The end result is a highly-concentrated filtrate that is transported to the bladder for excretion.

Neurons are the functional unit of the nervous system, not the kidney. Sarcomeres are the basic contractile unit of skeletal muscle, and chief cells are specialized stomach cells that secrete digestive enzymes, such as pepsinogen.

Ammonia is highly water-soluble and can be toxic to cells at low concentrations due to presence of its ammonium ion, which can interfere with oxidative phosphorylation. Ammonia is small and can easily diffuse through cell membranes, making it easy to excrete. Essentially, there is a trade off of easy excretion and toxicity levels.

For aquatic animals, however, toxicity is negligible due to the large volume of water available to dilute ammonia wastes. The high solubility of ammonia wastes and the abundance of water solvent allow for the ammonia to be transported out of cells in an very dilute concentration, without harming the organism. This allows aquatic organisms to conserve energy, compared to terrestrial organisms that must convert ammonia wastes to other forms.

Amphibians and mammals convert ammonia to urea, which can be excreted with less water, but must still be relatively dilute. These animals release liquid wastes from the body, resulting in water loss, but conserve energy compared to organisms that continue to convert urea into uric acid. Birds and reptiles excrete uric acid, which requires very little water waste, but uses a larger amount of energy in conversion. This is beneficial to animals that may not have ready access to fresh water.

There is a key trade-off between energy consumption and toxicity in the excretion of nitrogenous wastes. Ammonia is the simplest form of the waste product, and requires very little energy to produce; however, it is highly toxic and must be diluted to extremely low concentrations in order to be safe to the cells. Many aquatic animals excrete ammonia because of their proximity to water. Access to large amounts of water means that these organisms can safely excrete dilute ammonia without needing to use energy in conversions.

Terrestrial animals, with less access to water, excrete urea or uric acid. These wastes are derived from ammonia, but require an input of energy for the conversion. They are less toxic and require less water loss for dilution, making them ideal for animals that must conserve fluids. Uric acid is the least toxic of the nitrogenous wastes, but also requires the greatest energy investment.

Alcohol decreases the activity of antidiuretic hormone (vasopressin). A diuretic increases the production of urine and thus, inhibition of this antidiuretic hormone results in an increase in the production of highly diluted urine.

Alcohol does not block the flow of fluids through the kidney tubules.