The concentration gradient in medullary interstitium is established primarily by renal tubules of loop of Henle and the blood vessels surrounding them (vasa recta) in a process called countercurrent exchange.

The substances that exit from tubules for maintenance of such gradient are mainly sodium chloride $(\mathrm{NaCl})$, water and urea (contaning $\mathrm{H}^{+}, \mathrm{K}^{+}$and $\mathrm{NH}_3^{+}$).

	Organ	Excretory wastes
(a)	Kidneys	Urine
(b)	Lungs	CO$_2$
(c)	Liver	Urea
(d)	Skin	Sweat

Representing the structure of a renal corpuscle

Renin is released from the Juxta-Glomerular Apparatus (JGA) on activation by fall in the glomerular blood pressure/flow. Renin converts angiotensinogen in blood to angiotensin I and further to angiotensin II. Angiotensin II, being a powerful vasoconstrictor, increases the glomerular blood pressure and thereby Glomerular Filteration Rate (GFR). Angiotensin II also activates the adrenal cortex to release aldosterone. Aldosterone causes reabsorption of $\mathrm{Na}^{+}$and water from the distal parts of the tubule. This also, leads to an increase in blood pressure and GFR. This complex mechanism is generally known as Renin Angiotensin Aldosterone System or RAAS.

The process of excreting ammonia is called ammonotelism. many bony fishes, aquatic amphibians and aquatic insects are ammonotelic in nature. Ammonia, as its readily soluble, in water is excreted by diffusion across body surface or through gill surfaces (in fishes) as ammonium ions.

It is highly soluble in water and requires large amounts of water to be lost from the body. Such a mode of excretion is thus suitable for aquatic organisms which have a constant access to water.

Terrestrial adaptation requires the production of lesser toxic nitrogenous wastes like urea, as urea is less toxic and less soluble in water. This is important adaptation for water conservation in animals body. Mammals, many terrestrial amphibians and marine fishes mainly excrete urea and are called ureotelic animals.

In most insects land snails, reptiles etc., uric acid is excreted, hence they are called uricotelic animals. Conversion of ammonia to uric acid and its elimination requires less water, thus it is very essential for terrestrial forms that do not have constant water supply.