Question

Give reasons for this sentence:
Urine is formed from alkaline blood but it is acidic in nature.

Answer 1

Hint: Formation of urine includes three main steps that are glomerular filtration, reabsorption and secretion. These processes take place within the different parts of nephrons. The urine helps to excrete out nitrogenous wastes and toxic substances from the body of humans.

Complete answer:
1.Glomerular Filtration
The first step in urine formation is that the filtration of blood, which is administered by the glomerulus and is named glomerular filtration. On an average amount 1200 mL of blood is filtered by the kidneys per minute which constitute roughly $\dfrac{1}{5}$th of the blood pumped out by each ventricle of the guts during a minute.
The glomerular capillary vital sign causes filtration of blood through three layers, that us
The endothelium of glomerular blood vessels,
The epithelium of Bowman's capsule, and
A basement membrane between these two layers.
These three structures form the filtration membrane. The epithelial cells of Bowman's capsule called podocytes are arranged in an intricate manner so as to leave some minute spaces called filtration slits or slit pores.

2.Tubular Reabsorption:
This is the second step. A comparison of the quantity of the filtrate formed per day (180 litres per day) thereupon of the urine released (1.5 litres), suggest that almost 99 percent of the filtrate has got to be reabsorbed by the renal tubules. This process is named reabsorption. The tubular epithelial cells in several segments of nephron perform this either by active or passive mechanisms. for instance, substances like glucose, amino acids, Nat, etc., within the filtrate are reabsorbed actively, whereas the nitrogenous wastes are absorbed by transport. Reabsorption of water also occurs passively within the initial segments of the nephron.

3.Tubular Secretion
The phenomenon of secretion of metabolic wastes by tubular cells into the filtrate is understood as tubular secretion. It includes secretion of $\mathop K\nolimits^ + $, $\mathop H\nolimits^ + $ , ammonia and a few wastes like hippuric acid, creatinine, drugs, pigments, toxins etc.
So, urine excretes out $\mathop H\nolimits^ + $ ions so show basic that is alkaline in nature in spite of formation from blood.

Note: It is the sole mode of excretion in animals which don't have glomeruli, e.g., desert amphibians, marine fishes. Tubular secretion may be a crucial step in urine formation because it helps within the maintenance of ionic and acid-base equilibrium of body fluids.