In this article we will discuss about the Regulation and Functions of Kidney of Human Beings.
Regulation of Kidney:
The functions of the kidney are controlled by antidiuretic hormone (ADH), juxtaglomerular apparatus (JGA) and Atrial Natriuretic Factor (ANF).
(i) Control by Antidiuretic Hormone (ADH):
ADH is secreted by hypothalamus of the brain and released into the blood from the posterior lobe of the pituitary gland. The release of ADH is triggered when osmoreceptors in the hypothalamus detect an increase in the osmolarity of the blood above a set point of 300 mos mL-1. In this situation the osmoreceptor cells also promote thirst. It increases the reabsorption of water in the distal convoluted tubule and collecting duct.
(ii) Control by Juxtaglomerular Apparatus (JGA):
JGA operates a multi hormonal Renin-Angiotensin- Aldosterone System (RAAS). Juxtaglomerular cells secrete an enzyme, renin into the blood stream. Renin changes plasma protein, called angiotensinogen to a peptide, called angiotensin II, which works as hormone.
Angiotesin II increases blood pressure by causing arterioles to constrict. It also increases blood volume in two ways: firstly, it induces the proximal convoluted tubules to reabsorb more NaCl and water and secondly it stimulates the adrenal glands to release a hormone, called aldosterone that induces the distal convoluted tubule to absorb more Na+ and water.
(iii) Control by Atrial Natriuretic Factor (ANF):
There is an another hormone, a peptide called Atrial Natriuretic Factor (ANF) which opposes the regulation by RAAS. The walls of the atria of the heart release ANF in response to an increase in blood volume and pressure.
ANF inhibits release of renin from the JGA and thereby inhibits NaCl reabsorption by the collecting duct and reduces aldosterone release from the adrenal gland. Thus ADH, RAAS and ANF regulate the functions of kidneys. As a result they control body fluid osmolarity, salt concentration, blood pressure and blood volume.
Micturition:
The expulsion of urine from the urinary bladder is called micturition. It is a reflex process, but in grown up children and adults, it can be controlled voluntarily to some extent.
Nerve Supply to urinary bladder and sphincters:
The urinary bladder and the internal sphincter are supplied by both sympathetic and parasympathetic nervous systems of autonomic nervous system whereas, the external sphincter is supplied by the somatic nerve.
Function of sympathetic nerve:
The stimulation of nerve causes relaxation of detrusor muscle of the urinary bladder and constriction of the internal sphincter. So, it causes filling of the urinary bladder and the sympathetic nerve is called nerve of filling.
Function of para sympathetic nerve:
The stimulation of this nerve causes contraction of detrusor muscle and relaxation of the internal sphincter leading to emptying of the urinary bladder. So the parasympathetic nerve is called the nerve of emptying or nerve of micturition.
Function of somatic (pudendal) nerve:
It maintains the tonic contraction of the skeletal muscle fibres forming external sphincter so that, the external sphincter is constricted always. During micturition, this nerve is inhibited, thus the somatic (pudendal) nerve is responsible for voluntary control of micturition. The urine flows out from the urinary bladder through the urethra.
Constituents of Urine:
Physical Properties:
Urine is a transparent, light yellow liquid with a slightly acid pH (average pH 6.0). The colour of urine is caused by the pigment urochrome, which is a breakdown product of haemoglobin from worn out red blood corpuscles.
The colour of the urine may be affected by foods. The pH range of urine is normally between 4.5 and 8.2 depending upon the amount of acidic and basic foods in the diet. Fruits increase the acidity and vegetables increase the alkalinity of the urine.
A high-protein diet also produces an acid urine because of acidic products from amino acid metabolism. A normal adult person secretes about 1.5 litres of urine in 24 hours. Substances that increase the formation of urine are called diuretics.
Tea, coffee and alcoholic beverages have diuretic effects. The urine is hypertonic (i.e., it has a higher osmotic pressure than the blood plasma). When the urine is allowed to stand for some time it smells strongly of ammonia due to bacterial degradation of urea to ammonia. The specific gravity of urine is usually between 1.015 and 1.025.
Chemical Composition:
About 95% of the volume of urine is water; other substances are only about 5%. Organic substances include nitrogen, urea, creatine, creatinine, ammonia, uric acid, hippuric acid, oxalic acid, amino acids, allantoin, vitamins, hormones and enzymes.
The inorganic substances include chloride, phosphate, sulphate, potassium, sodium, calcium, magnesium, iodine, arsenic and lead. It is important to note that no glucose is normally found in the urine.
Abnormal Urine Conditions:
(i) Albuminuria:
Presence of albumin in urine is called albuminuria. It usually occurs in nephritis (inflammation of glomeruli). In this condition the size of the filtering slits enlarges.
(ii) Glycosuria:
Presence of glucose in urine is known as glycosuria. It occurs in Diabetes mellitus.
(iii) Hematuria:
Presence of blood or blood cells in urine is called hematuria.
(iv) Ketonuria:
Presence of abnormally high ketone bodies in urine is termed as ketonuria.
(v) Haemoglobinuria:
Presence of haemoglobin in urine is called hemoglobinuria.
(vi) Uremia:
Presence of excess urea in urine is known as uremia.
(vii) Pyuria:
The presence of WBCs or pus in the urine is called pyuria.
Deficiency of ADH causes Diabetes insipidus which is characterised by excessive dilute Urine.
Functions of Kidney:
1. Osmoregulation:
Kidney removes excess of water from the body.
2. Elimination of Nitrogenous wastes:
Kidney removes nitrogenous wastes such as urea, and uric acid from the blood.
3. Maintenance of pH:
Kidney removes excess of acids and alkalies from the blood to maintain proper pH of blood (about 7.4).
4. Maintenance of Salt contents:
Kidney maintains proper amount of mineral salts such as sodium, potassium in the body.
5. Removal of other Substances:
Kidney eliminates toxic substances, drugs, pigments, excess vitamins from the blood.
6. Maintenance of Blood Pressure:
Kidney controls the fluid balance in the body, therefore, it maintains blood pressure.
7. Homeostasis:
Because kidney removes various unwanted materials from the blood, it helps in keeping the internal environment of the body constant.
8. Secretion of Renin:
Kidney secretes an enzyme (which acts as hormone), the renin which changes the plasma, protein, the angiotensinogen (produced by liver) into angiotensin II. The latter stimulates the adrenal cortex to secrete aldosterone (hormone) which increases the rate of reabsorption of Na+ in the nephrons.
9. Erythropoietin Production:
The kidney produces erythropoietin (hormone) that stimulates the formation of erythrocytes (RBCs).