● `color{violet}("Mammals")` have the ability to produce a concentrated urine.

● The `color{brown}("Henle’s loop and 𝑣𝑎𝑠𝑎 𝑟𝑒𝑐𝑡𝑎")` play a significant role in this.

● The flow of filtrate in the two limbs of `color{violet}("Henle’s loop")` is in opposite directions and thus forms a `color{brown}("counter current")`. The flow of blood through the two limbs of `color{violet}("𝑣𝑎𝑠𝑎 𝑟𝑒𝑐𝑡𝑎")` is also in a `color{violet}("counter current")` pattern.

● The proximity between the `color{violet}("Henle’s loop")` and `color{violet}("𝑣𝑎𝑠𝑎 𝑟𝑒𝑐𝑡𝑎,")` as well as the `color{violet}("counter current")` in them help in maintainingan increasing osmolarity towards the `color{brown}("inner medullary interstitium")`, i.e., from `color{brown}("300 mOsmolL" text()^(–1))` in the cortex to about `color{violet}("1200 mOsmolL" text()^(–1))` in the `color{violet}("inner medulla.")`

● This gradient is mainly caused by `color{violet}(NaCl)` and `color{violet}("urea.")`

● `color{violet}(NaCl)` is transported by the `color{violet}("ascending limb")` of `color{violet}("Henle’s loop")` which is exchanged with the `color{violet}("descending limb of 𝑣𝑎𝑠𝑎 𝑟𝑒𝑐𝑡𝑎")`.

● `color{violet}(NaCl)` is returned to the interstitium by the ascending portion of `color{violet}("𝑣𝑎𝑠𝑎 𝑟𝑒𝑐𝑡𝑎.")`

● Similarly, small amounts of `color{violet}("urea")` enter the thin segment of the `color{violet}("ascending limb of Henle’s loop")` which is transported back to the interstitium by the `color{violet}("collecting tubule.")`

● The above described transport of substances facilitated by the special arrangement of `color{violet}("Henle’s loop and vasa recta")` is called the `color{brown}("counter current mechanism")`

● This mechanism helps to maintain a concentration gradient in the `color{violet}("medullary interstitium.")`

● Presence of such interstitial gradient helps in an easy passage of water from the `color{violet}("collecting tubule")` thereby
concentrating the filtrate (urine).

● `color{violet}("Human kidneys")` can produce urine nearly four times concentrated than the initial filtrate formed.

● The functioning of the `color{violet}("kidneys")` is efficiently monitored and regulated by hormonal feedback mechanisms involving the `color{brown}("hypothalamus, JGA")` and to a certain extent, the `color{brown}("heart.")`

● `color{brown}("Osmoreceptors")` in the body are activated by changes in `color{violet}("blood volume, body fluid volume")` and `color{violet}("ionic concentration.")`

`star` `color{green}("Regulation by ADH:")`

● An excessive loss of `color{violet}("fluid")` from the body can activate these receptors which stimulate the hypothalamus
to release `color{brown}("antidiuretic hormone (ADH) or vasopressin")` from the `color{violet}("neurohypophysis.")`

● `color{violet}("ADH")` facilitates water reabsorption from latter parts of the `color{violet}("tubule, thereby preventing diuresis.")`

● An increase in body fluid volume can switch off the `color{brown}("osmoreceptors")` and suppress the `color{violet}("ADH")` release to complete the feedback.

● `color{violet}("ADH")` can also affect the kidney function by its constrictor effects on `color{violet}("blood vessels.")`

● This causes an increase in `color{violet}("blood pressure.")`

● An increase in `color{violet}("blood pressure")` can increase the `color{violet}("glomerular blood ")` flow and thereby the `color{violet}("GFR.")`

`star` `color{green}("Regulation by JGA:")`

● The `color{violet}("JGA ")`plays a `color{violet}("complex regulatory role.")`

● A fall in glomerular blood flow/glomerular blood pressure/GFR can activate the JG cells to release `color{brown}("renin")` which converts `color{brown}("angiotensinogen in blood")` to `color{brown}("angiotensin I")` and further to `color{brown}("angiotensin II.")`

● `color{brown}("Angiotensin II,")` being a powerful `color{brown}("vasoconstrictor,")` increases the `color{violet}("glomerular blood")` pressure and `color{violet}("thereby GFR.")`

● `color{violet}("Angiotensin II")` also activates the `color{brown}("adrenal cortex")` to release `color{brown}("Aldosterone.")`

● `color{violet}("Aldosterone")` causes `color{violet}("reabsorption of" Na^+)` and water from the distal parts of the tubule.

● This also leads to an increase in `color{violet}("blood pressure and GFR.")`

● This complex mechanism is generally known as the `color{brown}("Renin-Angiotensin")` mechanism.

`star` `color{green}("Regulation by ANF:")`

● An increase in `color{violet}("blood flow")` to the `color{violet}("atria of the heart")` can cause the release of `color{brown}("Atrial Natriuretic Factor")` (ANF).

● `color{violet}("ANF")` can cause `color{brown}("vasodilation")` (dilation of blood vessels) and thereby decrease the blood pressure.

● `color{violet}("ANF mechanism,")` therefore, acts as a check on the `color{violet}("renin-angiotensin mechanism.")`