● `color{violet}("Urine formation")` involves three main processes namely, `color{brown}("glomerular filtration, reabsorption and secretion")`, that takes place in different parts of the `color{violet}("nephron.")`

● The first step in `color{violet}("urine formation")` is the `color{violet}("filtration of blood")`, which is carried out by the `color{violet}("glomerulus")` and is called `color{brown}("glomerular filtration.")`

● On an average, `color{brown}("1100-1200 ml of blood")` is filtered by the `color{violet}("kidneys")` per minute which constitute roughly 1/5th of the `color{violet}("blood pumped")` out by each ventricle of the `color{violet}("heart")` in a minute.

● The `color{violet}("glomerular capillary blood pressure")` causes filtration of blood through 3 layers, i.e., the `color{brown}("endothelium")` of `color{violet}("glomerular blood vessels,")` the `color{brown}("epithelium")` of `color{violet}("Bowman’s capsule ")` and a `color{brown}("basement membrane")` between these two layers.

● The `color{violet}("epithelial cells of Bowman’s capsule")` called `color{brown}("podocytes")` are arranged in an intricate manner so as to leave some minute spaces called `color{brown}("filtration slits or slit pores.")`

● `color{violet}("Blood is filtered")` so finely through these membranes, that almost all the constituents of the `color{violet}("plasma")` except the proteins pass onto the `color{violet}("lumen")` of the `color{violet}("Bowman’s capsule.")`

● Therefore, it is considered as a process of `color{brown}("ultra filtration.")`

● The amount of the filtrate formed by the `color{violet}("kidneys")` per minute is called `color{brown}("glomerular filtration rate (GFR)")`. GFR in a healthy individual is approximately `color{brown}("125 ml/minute,")` i.e., 180 litres per day !

● The `color{violet}("kidneys")` have built-in mechanisms for the `color{violet}("regulation of glomerular filtration rate.")`

● One such efficient mechanism is carried out by `color{brown}("juxta glomerular apparatus (JGA).")`

● `color{violet}("JGA")` is a special sensitive region formed by `color{violet}("cellular modifications")` in the `color{violet}("distal convoluted tubule")` and the afferent arteriole at the location of their contact.

● A fall in GFR can activate the `color{violet}("JG cells")` to release `color{brown}("renin")` which can stimulate the `color{violet}("glomerular blood flow")` and thereby the `color{violet}("GFR back to normal.")`

● A comparison of the volume of the filtrate formed per day (`color{brown}("180 litres per day")`) with that of the urine released (`color{brown}("1.5 litres")`), suggest that nearly `color{brown}("99%")` of the filtrate has to be reabsorbed by the `color{violet}("renal tubules.")`

● This process is called `color{brown}("reabsorption.")`

● The `color{violet}("tubular epithelial cells")` in different segments of `color{violet}("nephron perform")` this either by active or passive mechanisms.

● For example, substances like `color{brown}("glucose, amino acids", Na^+)`, etc., in the filtrate are reabsorbed actively whereas the `color{brown}("nitrogenous wastes")` are absorbed by `color{violet}("passive transport.")`

● Reabsorption of `color{brown}("water")` also occurs passively in the initial segments of the `color{violet}("nephron.")`

● During `color{violet}("urine formation,")` the `color{violet}("tubular cells")` secrete substances like `color{brown}(H^+, K^+ "and ammonia")` into the filtrate.

● `color{violet}("Tubular secretion")` is also an important step in `color{violet}("urine formation")` as it helps in the maintenance of ionic and acid base balance of `color{violet}("body fluids.")`