Question

First step in urine formation is
a. Tubular secretion
b. Tubular reabsorption
c. Ultrafiltration
d. Selective secretion

Answer 1

The excretory system is responsible for the expulsion of unwanted waste out of the body. This process takes place in the nephron of the kidney through the filtration of blood. After the formation of urine it passes from the kidney to the urinary bladder through the ureter and then it is transported to the outside of the body through the urethra.

Complete answer:
The formation of urine takes place by three steps-
Ultrafiltration-
It is the filtration of the liquid part of blood under extraordinary force and tremendous pressure, in the Bowman’s capsule. The afferent arteriole is subjected to high hydrostatic pressure which makes the liquid pass through the podocyte cells. This filtrate is known as the glomerular filtrate which contains all the molecules and the blood plasma except bigger molecules like red blood cells, proteins, and other higher molecules. The volume of liquid which is filtered per unit time in all glomeruli is known as the glomerular filtration rate. The value of GFR is 120ml/min.

Tubular reabsorption-
The glomerular filtrate as it passes through the proximal convoluted tubule and Loop of Henle is reabsorbed. The molecules which are important to the body like Na, Cl and certain other drugs are reabsorbed back to the blood.

Tubular secretion-
The final step to urine formation is tubular secretion which occurs in the descending loop of Henle in the tubular cells. The ions, acid-base, and other drugs and potassium ions are secreted back to the glomerular filtrate. This final product is then passed to the collecting duct.

Hence, the correct answer is option (C).

Note: Sometimes there is an abnormality in the formation of urine which can cause fatal disorders. The excess secretion of proteins in urine is termed as Proteinuria and the excess secretion of glucose in the urine is termed as glycosuria. The excess of acetone and ketone body in the urine is termed as ketonuria. There are several abnormalities seen in urine but these are the most prominent ones.