Antidiuretic hormone water reabsorption controlled

Antidiuretic hormone (ADH)—which controls the reabsorption of water, which might cause a concentration or dilution of sodium. 

In order to make adjustments in the water load, the reabsorption of the remaining 20% of the filtered water from the distal tubule and the collecting duct is physiologically controlled by antidiuretic hormone (ADH), also referred to as vasopressin. Antidiuretic hormone, synthesized in the hypothalamus and released from the neurohypophysis of the pituitary gland, promotes the... 

The function of the loop of Henle is to enable production of a concentrated urine. It does this by generating a hypertonic interstitium, which provides a gradient for water reabsorption from the collecting duct. This, in turn, occurs under the control of antidiuretic hormone (ADH). There are several important requirements without which this mechanism would not work. These include the differential permeabilities of the two limbs to water and solutes and the presence of a blood supply that does not dissipate the concentration gradients produced. This is a simplified description to convey the principles. 

The smallest functional unit of the kidney is the nephron. In the glomerular capillary loops, ultrafiltration of plasma fluid into Bowman s capsule (BC) yields primary urine. In the proximal tubules (pT), approx. 70% of the ultrafiltrate is retrieved by isoosmotic reabsorption of NaCl and water. In the thick portion of the ascending limb of Henle s loop (HL), NaCl is absorbed unaccompanied by water. This is the prerequisite for the hairpin countercurrent mechanism that allows build-up of a very high NaQ concentration in the renal medulla In the distal tubules (dT), NaCl and water are again jointly reabsorbed. At the end of the nephron, this process involves an aldosterone-controlled exchange of Na+ against 1C or H. In the collecting tubule (C), vasopressin (antidiuretic hormone, ADH) increases the epithelial permeability for water, which is drawn into the hyperosmolar milieu of the renal medulla and thus retained in the body. As a result, a concentrated urine enters the renal pelvis. 

Vasopressin (antidiuretic hormone, ADH) plays an important role in the long-term control of blood pressure through its action on the kidney to increase water reabsorption. This and other aspects of the physiology of vasopressin are discussed in Chapters 15 and 37 and will not be reviewed here. 

Relatedly, malfunction of one of the sodium-water control mechanisms, such as a kidney that normally excretes excess water, can result in fluid retention and dilutional hyponatremia. The pituitary gland and hypothalamus function to release ADH (which controls water reabsorption), and the cortex of the adrenal gland seaetes aldosterone (which controls sodium reabsorption). An alteration in the function of either of these hormone systems will alter the body s regulation of sodium or water and can result in hyponatremia. 2 For example, in the syndrome of inappropriate antidiuretic hormone (SIADH), excessive ADH is produced (usually by a tumor or some pulmonary diseases such as tuberculosis or bacterial pneumonia), and the kidneys reabsorb excessive fluids, resulting in dilutional hyponatremia. Conditions causing decreased aldosterone secretion include... 

As mentioned before, the antidiuretic hormone is important in water reabsorption (the hormone s molecular properties and mechanism of action are discussed in the section on the hormones of the posterior lobe). In spite of the importance of the antidiuretic hormone in regulating water metabolism, the control it provides is not indispensable for water reabsorption. Twenty per cent of the water present in the glomerular filtrate can be reabsorbed even in absence of antidiuretic hormones. Consequently, in diabetes insipidus, no more than 80% of the water passing through the glomerulus is excreted. 

PGE2 is the main arachidonate metabolite in kidneys where it reduces ADH (antidiuretic hormone) - induced water reabsorption and may help to control renin release. It may also help to mediate the metabolism and interaction of macrophages with other cells. 

---