Describe the main phases of urine formation.

(A) Urine formation involves three main processes:
$(1)$ Glomerular Filtration,$(2)$ Reabsorption,$(3)$ Secretion.
These processes take place in different parts of the nephron.
$(1)$ Glomerular Filtration: Blood is filtered in the glomerulus. On average,$1100-1200 \text{ ml}$ of blood is filtered by the kidneys per minute,which constitutes roughly $1/5$ of the blood pumped out by each ventricle of the heart in a minute. The difference in the diameter of the afferent arteriole (wider) and efferent arteriole (narrower) creates pressure,facilitating filtration. Blood is filtered through three layers: the endothelium of glomerular blood vessels,the epithelium of Bowman's capsule (podocytes),and a basement membrane. The glomerular filtration rate $(GFR)$ in a healthy individual is approximately $125 \text{ ml/minute}$ $(180 \text{ liters/day})$. The juxtaglomerular apparatus $(JGA)$ regulates $GFR$.
$(2)$ Reabsorption: Nearly $99\%$ of the filtrate $(180 \text{ liters/day})$ is reabsorbed by the renal tubules,leaving only about $1.5 \text{ liters}$ of urine. Substances like glucose,amino acids,and $Na^{+}$ are reabsorbed actively,while nitrogenous wastes are absorbed by passive transport in the proximal convoluted tubule $(PCT)$. The Henle's loop helps in concentrating the filtrate.
$(3)$ Secretion: Tubular cells in the distal convoluted tubule $(DCT)$ secrete substances like $H^{+}$,$K^{+}$,and ammonia into the filtrate. These substances were not filtered during glomerular filtration. This process helps in maintaining the ionic and acid-base balance of body fluids. Finally,the urine is transported to the collecting duct,where water is reabsorbed to produce concentrated urine.