Define Glomerular Filtration Rate (GFR)

1. Excretion in organisms:

(i) Homeostasis is the maintenance of a constant internal environment.

(ii) Osmoconformers are animals that do not actively control the osmotic condition of their body fluids.

(iii) Osmoregulators are animals whose internal osmolarity remains constant and does not change according to the medium.

(iv) Excretion is the process in which substances of no further use or metabolic wastes are thrown out of the body.

(v) Contractile vacuoles, protonephridia, metanephridia, malpighian tubules, green glands, and the kidneys are the common excretory organs in different animals.

(vi) These organs not only eliminate nitrogenous wastes but also help in osmoregulation by the maintenance of ionic and acid-base balance of body fluids.

2. Human excretory system:

(i) Human excretory system or urinary system consists of: two kidneys, two ureters, the urinary bladder, and the urethra.

(ii) Kidneys are bean-shaped structures situated between the levels of last thoracic and third lumbar vertebra close to the dorsal inner wall of the abdominal cavity.

(iii) The nephron is the histological and functional unit of the kidneys.

(iv) Nephrons have two portions-renal corpuscles (Malpighian corpuscle) and renal tubule.

(v) Renal or Malpighian corpuscle consists of Bowman’s capsule and glomerulus.

(vi) Glomerulus is a tuft of capillaries formed from afferent arterioles, fine branches of renal artery.

(vii) The renal tubule starts with a double-walled Bowman’s capsule and is further differentiated into a proximal convoluted tubule ($\mathrm{PCT}$), Henle’s loop ($\mathrm{HL}$) and distal convoluted tubule ($\mathrm{DCT}$).

(viii) The $\mathrm{DCTs}$ join to a common collecting duct.

3. Nitrogenous waste:

(i) The type of nitrogenous wastes formed and their excretion vary among animals, mainly depending on the habitat, aquatic or terrestrial.

(ii) Nitrogenous wastes are produced during the catabolism of protein and nucleic acids.

(iii) The major nitrogenous wastes are ammonia, urea, and uric acid.

(iv) Ammonia is the chief by-product of protein, and amino acid metabolism and excretion of ammonia require a large amount of water, so carried out only by aquatic organisms.

(v) Terrestrial reptiles, birds, land snails, and most insects must conserve water in their bodies, hence they excrete uric acid.

4. Physiology of excretion:

(i) Physiology of excretion includes chemical and mechanical parts.

(ii) The chemical part includes deamination and the ornithine cycle.

(iii) Deamination takes place in all living cells, primarily in the liver cells.

(iv) Ornithine cycle, also called the urea cycle, is the detoxification of ammonia that takes place in the liver.

5. Urine formation:

(i) The mechanical part comprises the formation of urine.

(ii) Urine formation involves three main processes, i.e., glomerular filtration, tubular reabsorption, and tubular secretion.

(iii) Glomerular filtration is a non-selective process performed by the glomerulus using the glomerular capillary blood pressure.

(iv) Juxtaglomerular apparatus (JGA), a specialised portion of the nephrons, plays an important role in the regulation of Glomerular Filtration Rate.

6. Functions of the tubule:

(i) $\mathrm{PCT}$ is the major site of reabsorption and selective secretion.

(ii) Loop of Henle primarily helps to maintain osmotic gradient within the kidney interstitium.

(iii) $\mathrm{DCT}$ and collecting duct allow extensive reabsorption of water and certain electrolytes, which help in osmoregulation: ${\mathrm{H}}^{+}$, ${\mathrm{K}}^{+}$ and $N{H}_3$ could be secreted into the filtrate by the tubules to maintain the ionic balance and pH of body fluids.

7. Concentration of the filtrate:

(i) A counter-current mechanism operates between the two limbs of the loop of Henle and those of vasa recta (capillary parallel to Henle’s loop).

(ii) The filtrate gets concentrated as it moves down the descending limb but is diluted by the ascending limb.

(iii) $\mathrm{DCT}$ and collecting duct concentrate the filtrate about four times, an excellent mechanism for the conservation of water.

8. Regulation of kidney function:

(i) A polypeptide hormone, called atrial natriuretic hormone (ANH) is secreted by the right atrial myocardium of the heart in response to high blood pressure.

(ii) $\mathrm{ANH}$ increases urine and ${\mathrm{Na}}^{+}$ output and reduces blood volume and pressure.

(iii) Urine is stored in the urinary bladder until a voluntary signal from CNS releases it through the urethra, i.e.,  micturition.

(iv) Skin and lungs also assist in excretion.