Q.The yellow colour of the urine is due to presence of ___________

Normal human urine is a pale-yellow, transparent fluid (its colour due to the pigment urochrome), whose exact composition depends on diet and fluid intake. Its volume and concentration are regulated by two coordinated hormonal systems.

ADH (antidiuretic hormone) is released from the posterior pituitary whenever hypothalamic osmoreceptors detect rising blood osmolarity (from water loss, salty food, haemorrhage or dehydration); it increases the water permeability of the late DCT and collecting duct, promoting water reabsorption, reducing urine volume, and restoring blood osmolarity through negative feedback. Insufficient ADH produces diuresis (large volumes of dilute urine) and persistent thirst — the disorder diabetes insipidus.

The renin-angiotensin-aldosterone system (RAAS), triggered by the juxtaglomerular apparatus when blood flow to the afferent arteriole falls, converts angiotensinogen to Angiotensin I (via renin) and then to the active Angiotensin II (via ACE); Angiotensin II constricts kidney arterioles (raising blood pressure), stimulates PCT reabsorption of Na+/Cl−/water, and stimulates the adrenal cortex to release aldosterone, which drives further Na+ and water reabsorption in the DCT and collecting duct. …