Question

Explain the physiology of urine formation.

Accepted Answer

Urine is a liquid by-product of the body secreted by the kidneys through a process called urination and excreted through the urethra. The normal range of urine output is 800 to 2000 millilitres per day. Normal urine colour ranges from pale yellow to deep amber. The urinary system consists of the kidneys, ureters, urinary bladder, and urethra.

The system produces urine by process of filtration, reabsorption, and tubular secretion.

Ultrafiltration: It is the filtration of blood under very high pressure from the glomerulus to the Bowman's capsule. The filtrate that comes in the PCT of the nephron is called nephric filtrate, also called glomerular filtrate. The nephric filtrate contains a large amount of water, a number of small-sized and soluble useful substances like glucose, amino acids, vitamin C, and a number of harmful substances like urea, uric acid, creatinine, ammonium salts, pigments, etc. The blood cells, proteins, fats etc., are not filtered out. Thus, the composition of primary urine is the same as blood minus cells and proteins. It is isotonic to the blood plasma.

Selective reabsorption: In this process, selective filtrates are reabsorbed. There are two processes involved in this step: passive transport or osmosis and active transport. Active transport uses ATP molecules since it is against the concentration gradient. When the filtrate passes through the renal tubule, there is an exchange of substances between the surrounding blood, and renal tubular fluid occurs. High threshold substances, e.g. glucose, amino acids, potassium and calcium ions, etc., are completely reabsorbed. Urea and uric acid are low threshold substances. They are reabsorbed negligibly. Water is reabsorbed everywhere except in the ascending loop of Henle. This absorption occurs by osmosis and is known as obligatory absorption of water. Amino acids and ions are pumped out from PCT. Glucose is maximally reabsorbed in healthy persons so that no glucose molecules remain in the urine.

Tubular secretion: Some unwanted substances which have passed into peritubular capillaries are secreted back in DCT during this process. Nitrogenous substances like creatinine and ions like potassium and hydrogen are also taken back in the lumen of the uriniferous tubule through the peritubular capillaries. Secretion of hydrogen ions is an important homeostatic mechanism to keep the balance of the acidity of the blood. Sodium ion concentration is maintained by aldosterone hormone, while calcium ion concentration is maintained by calcitonin and parathormone. If any toxic and abnormal constituent is present in the body, it is secreted back into the urine in this stage.

	List I		List II		List III
(i)	High threshold substances	(a)	Not at all reabsorbed	(p)	Creatinine
(ii)	Low threshold substances	(b)	Efficiently reabsorbed	(q)	Glucose
(iii)	Athreshold substances	(c)	Reabsorbed in very little amounts	(r)	Urea

	Column – I		Column – II (Part
	Function		Part of Excretory System)
a.	Ultrafiltration	i.	Henle’s loop
b.	Concentration of urine	ii.	Ureter
c.	Transport of urine	iii.	Urinary bladder
d.	Storage of urine	iv.	Malpighian corpuscle
		v.	Proximal convoluted tubule

Explain the physiology of urine formation.

Important Questions on Excretory Products and their Elimination

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