● In humans, the `color{violet}("excretory system")` consists of a `color{brown}("pair of kidneys, one pair of ureters, a urinary bladder")` and a `color{brown}("urethra.")`

● `color{brown}("Kidneys")` are `color{violet}("reddish brown, bean shaped structures situated")` between the levels of last `color{violet}("thoracic and third lumbar vertebra")` close to the `color{violet}("dorsal inner wall")` of the `color{violet}("abdominal cavity.")`

● Each `color{violet}("kidney")` of an adult human measures 10-12 cm in length, 5-7 cm in width, 2-3 cm in thickness with an average weight of 120- 170 g.

● Towards the centre of the inner concave surface of the `color{violet}("kidney")` is a notch called `color{brown}("hilum")` through which ureter, `color{violet}("blood vessels")` and `color{violet}("nerves enter.")`



● Inner to the `color{violet}("hilum")` is a broad funnel shaped space called the `color{brown}("renal pelvis")` with projections called `color{brown}("calyces.")`

● The outer layer of `color{violet}("kidney")` is a `color{brown}("tough capsule.")`

● Inside the `color{violet}("kidney,")` there are two zones, an outer `color{brown}("𝘤𝘰𝘳𝘵𝘦𝘹")` and an inner `color{brown}("𝘮𝘦𝘥𝘶𝘭𝘭𝘢.")`

● The `color{violet}("medulla")` is divided into a few conical masses (`color{brown}("medullary pyramids")`) projecting into the `color{violet}("calyces (sing.: calyx)")`.

● The `color{violet}("cortex extends")` in between the `color{violet}("medullary pyramids")` as `color{violet}("renal columns")` called `color{brown}("Columns of Bertini.")`

● Each `color{violet}("kidney")` has nearly one million `color{violet}("complex tubular structures")` called `color{brown}("nephrons")` , which are the `color{violet}("functional units.")`

● Each nephron has two parts – the `color{brown}("glomerulus")` and the `color{brown}("renal tubule.")`

● `color{brown}("Glomerulus")` is a tuft of capillaries formed by the `color{brown}("afferent arteriole")` – a fine branch of `color{violet}("renal artery.")`

● `color{violet}("Blood")` from the `color{violet}("glomerulus")` is carried away by an `color{brown}("efferent arteriole.")`

● The `color{violet}("renal tubule")` begins with a double walled cup-like structure called `color{brown}("Bowman’s capsule")`, which encloses the `color{violet}("glomerulus")`

● `color{violet}("Glomerulus alongwith Bowman’s")` capsule, is called the `color{brown}("𝘮𝘢𝘭𝘱𝘪𝘨𝘩𝘪𝘢𝘯 𝘣𝘰𝘥𝘺 𝘰𝘳 𝘳𝘦𝘯𝘢𝘭 𝘤𝘰𝘳𝘱𝘶𝘴𝘤𝘭𝘦")`.

● The tubule continues further to form a highly coiled network – `color{brown}("proximal convoluted tubule (PCT)")`.

● A hairpin shaped `color{brown}("Henle’s loop")` is the next part of the `color{violet}("tubule")` which has a descending and an ascending limb.

● The `color{violet}("ascending limb")` continues as another highly `color{violet}("coiled tubular region")` called `color{brown}("distal convoluted tubule")` (DCT).

● The DCTs of many nephrons open into a straight tube called `color{brown}("collecting duct,")` many of which converge and
open into the `color{violet}("renal pelvis")` through `color{violet}("medullary pyramids")` in the calyces.

● The Malpighian corpuscle, PCT and DCT of the nephron are situated in the `color{brown}("cortical region")` of the `color{violet}("kidney")` whereas the loop of Henle dips into the `color{brown}("medulla.")`

● In `color{violet}("majority of nephrons")`, the `color{violet}("loop of Henle")` is too short and extends only very little into the medulla.

● Such `color{violet}("nephrons")` are called `color{brown}("cortical nephrons.")`

● In some of the `color{violet}("nephrons, the loop of Henle")` is very long and runs deep into the medulla.

● These `color{violet}("nephrons")` are called juxta `color{brown}("medullary nephrons.")`

● The efferent arteriole emerging from the `color{violet}("glomerulus forms")` a fine capillary network around the `color{violet}("renal tubule")` called the `color{brown}("peritubular capillaries.")`

● A minute vessel of this network runs parallel to the `color{violet}("Henle’s loop forming")` a ‘U’ shaped `color{brown}("𝘷𝘢𝘴𝘢 𝘳𝘦𝘤𝘵𝘢.")`

● `color{violet}("𝘷𝘢𝘴𝘢 𝘳𝘦𝘤𝘵𝘢")` is absent or highly reduced in `color{violet}("cortical nephrons.")`