MECHANISM URINE FORMATION

The kidneys form urine, and while carrying out this process, perform its main functions, namely:

(i) The excretion of nitrogenous waste products and

(ii) "Homeostasis" or maintaining stable, constant conditions of internal environment. Some of the homeostatic functions of the kidney are: 

* Maintenance of  pH (or acid-base balance). It is brought about mainly by phosphate buffers (you must have learnt about buffers in your chemistry section.)

** Maintenance of ion concentration (Na+, K+, H+, C1₋, HPO2/4-, etc), and 

*** Maintenance of Osmotic pressure  

   The figure shows a diagrammatic representation of the relationship between the vascular and tubular comportment of a nephron.  If the direction of the movement of fluid and dissolved solutes is from the tubule to blood stream ( capilary network), it is called as tubular reabsorption and if it occurs in the opposite direction, i.e from the blood stream to tubular lumen, then it is known as tubular secretion. These two processes help in urine formation.

The process of urine formation by kidneys essentially involves three basic processes:

(i)  Glomercular Filtration : This refers to be movement of fluid and dissolved solutes by force or pressure (blood pressure) from the glomerulus to Bowan's capsule, through small pores or openings called as "filtration slits".

the process is somewhat similar to "straining of tea" through "tea strainer". Here, tea is equivalent to "blood";

Pores in the strainer =filtration slits.

Strainer = Bowman's capsule

& tea leaves  are = proteins &  blood cells which are not allowed to pass. 

Only small molecules of substances present in the blood viz. glucose, amino acids, urea, water, salts, etc. can pass through the openings  of the glomerulus and collect as filtrate in the bowman's capsule. The fluid finally resent in the capsule after filtration is known as ultra -filtrate and the process is known as Glomerular Filtration. the glomerular filtration rate (GFR) in adult man is 120ml/ mt or 180 liters/ day. 

(ii) Tubular reabsorption: The glomerular filtration filtration rate of human kidney is 180 litres/ day, whereas if the rate of urine formation is measured, it is found to be only 1-2 litres/day" the indicate that more than 99% of eater that is filtered through glomerulus re-enters the blood stream (i.e., reabsorbed), as it passes through the various tubules. Not only this, if we compare the volume and composition of final urine that is formed with that of the glomerular  filtrate, it shows that many of the components that were originally present in the filtrate, are either missing or present in much lower concentration in the urine. This indicates that water and these components are subjected to process known as tubular reabsorption . some solutes like glucose, amino acids, vitamins, NA+, HCO₃₋, etc, are reabsorbed by active reabsorption  involving ₋utilization of energy whereas chloride icons, urea, etc. are reabsorbed by passive reabsorption, i.e, without the use of energy.

Reabsorption of water is known as obligatory or compulsory when it's linked to the movement of sodium/Chloride ions. Where as it is called facultative or optional, when it 's influenced by the presence or absence of Anti Diuretic Hormone (ADH). The name is derived from two words- "anti" meaning opposing and "diuresis" meaning urine formation. Thus, this hormone opposes urine formation, when present. It is released by posterior pituitary gland. In fact. this hormone helps kidneys to excrete a concentrated urine and hence conserve water in the conditions of scarcity, such as, when a person is present in a desert! Formation of concentrated urine is further brought about by a unique process called as countercurrent mechanism, carried out by the human kidneys.

(iii)Tubular Secretion: some substances are not useful for the body and therefore have to be removed by the kidneys. This is done by tubular secretion, e.g., potassium ions (K+), hydrogen ions (H+), creatinine, etc. These substances are present in greater concentration in urine than in the filtrate, because of tubular secretion.

Dialysis

If a person's kidneys are diseased or injured they're unable to function normally, and therefore blood of the patient needs to be  cleansed artificially by dialysis. Dialysis separates large solutes from smaller ones by using a  a selectively permeable membrane.

Clinical disorders of Excretory System

Renal calculi or kidney stones

The crystals  of salts present in urine occasionally precipitate and solidify into insoluble stones of calcium oxalate, uric acid or calcium phosphate, blocking a narrow passage viz. ureters causing intense pain.