Chapter 5.3: Excretory Products and Their Elimination

1. Homeostasis

• Definition: Homeostasis is the maintenance of a constant internal environment of the body, despite changes in the external environment. This includes maintaining constant body temperature, pH, and concentrations of water, salt, and other substances. The excretory system plays a crucial role in maintaining homeostasis, particularly in osmoregulation.

2. Modes of Excretion

Animals accumulate ammonia, urea, uric acid, carbon dioxide, water and ions like Na+, K+, Cl–, phosphate, sulphate, etc., either by metabolic activities or by other means like excess ingestion. The process of removal of these harmful metabolic wastes from the body is called excretion. Based on the type of nitrogenous waste excreted, animals are categorized as follows:

• Ammonotelism (Excretion of Ammonia):

  • Definition: The process of excreting ammonia is known as ammonotelism.
  • Explanation: Ammonia is the most toxic form of nitrogenous waste and requires a large amount of water for its elimination. Therefore, this mode of excretion is common in aquatic animals.
  • Examples: Many bony fishes, aquatic amphibians, and aquatic insects.

• Ureotelism (Excretion of Urea):

  • Definition: The process of excreting urea is known as ureotelism.
  • Explanation: Ammonia produced by metabolism is converted into less toxic urea in the liver of these animals and released into the blood which is filtered and excreted out by the kidneys. It requires a moderate amount of water for elimination.
  • Examples: Mammals, many terrestrial amphibians, and marine fishes.

• Uricotelism (Excretion of Uric Acid):

  • Definition: The process of excreting uric acid is known as uricotelism.
  • Explanation: Uric acid is the least toxic nitrogenous waste and can be excreted with a minimum loss of water, in the form of a paste or pellet. This is an adaptation for water conservation.
  • Examples: Reptiles, birds, land snails, and insects.

3. Human Excretory System

a) Structure of the Human Excretory System

The human excretory system consists of a pair of kidneys, a pair of ureters, a urinary bladder, and a urethra.

• Kidneys: Reddish-brown, bean-shaped structures situated between the levels of the last thoracic and third lumbar vertebra close to the dorsal inner wall of the abdominal cavity.
• Ureters: A pair of thin muscular tubes that emerge from the hilum of each kidney and carry urine to the urinary bladder.
• Urinary Bladder: A muscular sac-like structure that stores urine temporarily.
• Urethra: A membranous tube that extends from the neck of the bladder to the exterior, through which urine is discharged.

b) Structure of the Kidney

• External Structure: Each kidney is covered by a tough, fibrous capsule. Towards the centre of the inner concave surface of the kidney is a notch called the hilum through which the ureter, blood vessels, and nerves enter.
• Internal Structure (L.S.): Internally, the kidney is divided into two zones:
  • Cortex: The outer, darker region.
  • Medulla: The inner, lighter region. The medulla is divided into a few conical masses called medullary pyramids projecting into the calyces (singular: calyx). The cortex extends in between the medullary pyramids as renal columns called Columns of Bertini.

c) Structure of a Nephron

• Definition: Each kidney has nearly one million complex tubular structures called nephrons, which are the functional units of the kidney.
• Parts of a Nephron:
  1. Glomerulus: A tuft of capillaries formed by the afferent arteriole (a fine branch of the renal artery). Blood from the glomerulus is carried away by an efferent arteriole.
  2. Bowman's Capsule: A double-walled, cup-like structure that encloses the glomerulus. The glomerulus along with Bowman’s capsule is called the malpighian body or renal corpuscle.
  3. Renal Tubule: The tubule continues further to form a highly coiled network – Proximal Convoluted Tubule (PCT), a hairpin-shaped Henle’s Loop (which has a descending and an ascending limb), and another highly coiled tubular region called Distal Convoluted Tubule (DCT). The DCTs of many nephrons open into a straight tube called the collecting duct.

4. Physiology of Urine Formation

Urine formation involves three main processes: glomerular filtration, reabsorption, and secretion, that take place in different parts of the nephron.

a) Glomerular Filtration (Ultrafiltration)

• Process: The first step in urine formation is the filtration of blood, which is carried out by the glomerulus. On average, 1100-1200 ml of blood is filtered by the kidneys per minute. The glomerular capillary blood pressure causes filtration of blood through 3 layers, i.e., the endothelium of glomerular blood vessels, the epithelium of Bowman’s capsule, and a basement membrane between these two layers. Almost all the constituents of the plasma except the proteins pass into the lumen of the Bowman’s capsule. Therefore, it is considered a process of ultrafiltration.
• Glomerular Filtration Rate (GFR): The amount of filtrate formed by the kidneys per minute is called GFR. In a healthy individual, it is approx. 125 ml/minute, i.e., 180 litres per day.

b) Selective Reabsorption

• Process: A comparison of the volume of the filtrate formed per day (180 litres) with that of the urine released (1.5 litres), suggests that nearly 99 per cent of the filtrate has to be reabsorbed by the renal tubules. This process is called reabsorption.
• Regions of Reabsorption:
  • Proximal Convoluted Tubule (PCT): Nearly all of the essential nutrients, and 70-80% of electrolytes and water are reabsorbed by this segment.
  • Henle's Loop: Reabsorption is minimum in its ascending limb. However, this region plays a significant role in the maintenance of high osmolarity of medullary interstitial fluid.
  • Distal Convoluted Tubule (DCT): Conditional reabsorption of Na+ and water takes place in this segment.
  • Collecting Duct: A large amount of water could be reabsorbed from this region to produce concentrated urine.

c) Tubular Secretion

• Process: During urine formation, the tubular cells secrete substances like H+, K+, and ammonia into the filtrate. Tubular secretion is also an important step in urine formation as it helps in the maintenance of ionic and acid-base balance of body fluids.

5. Counter-Current Mechanism

• Function: The ability of mammals to produce concentrated urine is mainly carried out by Henle’s loop and vasa recta (a capillary network running parallel to the Henle's loop).
• Mechanism: The flow of filtrate in the two limbs of Henle’s loop is in opposite directions and thus forms a counter-current. The flow of blood through the two limbs of the vasa recta is also in a counter-current pattern. This counter-current mechanism helps to maintain a concentration gradient in the medullary interstitium, which in turn helps in an easy passage of water from the collecting tubule thereby concentrating the filtrate (urine).

6. Regulation of Kidney Function

a) Antidiuretic Hormone (ADH) / Vasopressin

• Source: Produced by the hypothalamus and released from the posterior pituitary.
• Function: When there is an excessive loss of fluid from the body, ADH is released. It facilitates water reabsorption from the latter parts of the tubule (DCT and collecting duct), thereby preventing diuresis (excessive urine production). An increase in body fluid volume can switch off the osmoreceptors and suppress the ADH release to complete the feedback.

b) Renin-Angiotensin-Aldosterone System (RAAS)

• Activation: A fall in glomerular blood flow/glomerular blood pressure/GFR can activate the juxtaglomerular (JG) cells to release renin.
• Mechanism:
  1. Renin converts angiotensinogen in the blood to angiotensin I.
  2. Angiotensin I is further converted to angiotensin II by Angiotensin Converting Enzyme (ACE).
  3. Angiotensin II, being a powerful vasoconstrictor, increases the glomerular blood pressure and thereby GFR. It also activates the adrenal cortex to release Aldosterone.
  4. Aldosterone causes reabsorption of Na+ and water from the distal parts of the tubule. This also leads to an increase in blood pressure and GFR.

c) Atrial Natriuretic Factor (ANF)

• Source: The walls of the atria of the heart.
• Function: An increase in blood flow to the atria of the heart can cause the release of ANF. ANF can cause vasodilation (dilation of blood vessels) and thereby decrease the blood pressure. ANF mechanism, therefore, acts as a check on the renin-angiotensin mechanism.

d) Micturition

• Definition: The process of release of urine from the urinary bladder.
• Regulation: The stretching of the urinary bladder as it gets filled with urine sends signals to the central nervous system (CNS). The CNS passes on motor messages to initiate the contraction of smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter causing the release of urine.

e) Role of Erythropoietin

• Source: Juxtaglomerular cells of the kidney.
• Function: It is a hormone that stimulates erythropoiesis (formation of RBCs) in the bone marrow.

7. Role of Other Organs in Excretion

• Lungs: Remove large amounts of CO2 (18 litres/day) and also significant quantities of water every day.
• Liver: Secretes bile-containing substances like bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins, and drugs. Most of these substances ultimately pass out along with digestive wastes.
• Skin: The sweat and sebaceous glands in the skin can eliminate certain substances through their secretions. Sweat contains NaCl, small amounts of urea, lactic acid, etc. Sebaceous glands eliminate certain substances like sterols, hydrocarbons, and waxes through sebum.

8. Disorders of the Excretory System

• Uraemia: Accumulation of urea in the blood, which is highly harmful and may lead to kidney failure.
• Renal Failure: A condition where the kidneys stop functioning. In such patients, urea can be removed by a process called haemodialysis.
• Renal Calculi: Stone or insoluble mass of crystallized salts (oxalates, etc.) formed within the kidney.
• Nephritis: Inflammation of the kidneys, specifically the glomeruli (Glomerulonephritis).

a) Haemodialysis (Artificial Kidney)

• Process: Blood drained from a convenient artery is pumped into a dialyzing unit after adding an anticoagulant like heparin. The unit contains a coiled cellophane tube surrounded by a dialyzing fluid having the same composition as that of plasma except for the nitrogenous wastes. The porous cellophane membrane of the tube allows the passage of molecules based on a concentration gradient. As nitrogenous wastes are absent in the dialyzing fluid, these substances freely move out, thereby clearing the blood. The cleared blood is pumped back to the body through a vein after adding anti-heparin to it.

b) Kidney Transplant

• Process: It is the ultimate method in the correction of acute renal failure (kidney failure). A functioning kidney is used in transplantation from a donor, preferably a close relative, to minimize its chances of rejection by the immune system of the host.