Composition and Hormonal Regulation of Urine

Normal human urine is a pale-yellow, transparent liquid, its colour coming from the pigment urochrome; its exact composition varies with the food and fluid a person has consumed. The volume and concentration of urine are kept steady by two complementary regulatory pathways.

The first pathway centres on ADH (antidiuretic hormone). The hypothalamus contains specialised cells called osmoreceptors that continuously monitor the osmolarity of the blood (the total concentration of dissolved particles per litre). Whenever blood osmolarity rises — after eating salty food, for instance, or as a result of sweating and water loss — the osmoreceptors trigger the release of ADH from the neurohypophysis (posterior pituitary). ADH then increases the water permeability of the last part of the DCT and the entire collecting duct, so more water is reabsorbed there; this reduces urine volume and, in turn, brings blood osmolarity back down. As blood osmolarity returns to normal, osmoreceptor activity — and so ADH release — falls again; this self-correcting loop is a classic example of negative feedback. Haemorrhage and severe dehydration also stimulate osmoreceptors and trigger ADH release. When ADH is absent or insufficient, the collecting duct stays relatively impermeable to water, large volumes of dilute urine are produced (a state called diuresis), and the person tends to feel persistently thirsty — a combination of symptoms seen in the disorder diabetes insipidus.

The second pathway is the renin-angiotensin-aldosterone system (RAAS), run by the juxtaglomerular apparatus (JGA). Whenever blood supply to the afferent arteriole falls — for example because of low blood pressure or dehydration — the JG cells release the enzyme renin. Renin converts angiotensinogen (made by liver hepatocytes) into Angiotensin I, which is then converted by angiotensin-converting enzyme (ACE) into the active hormone Angiotensin II. Angiotensin II has a triple action: it constricts arterioles in the kidney, reducing blood flow through them and raising blood pressure; it directly stimulates PCT cells to reabsorb more Na+, Cl− and water; and it stimulates the adrenal cortex to release aldosterone, which in turn makes the DCT and collecting ducts reabsorb still more Na+ and water, further increasing blood volume and pressure. …