Revision Notes: The Excretory System [Elimination of Body Wastes] | Biology for SSS 2 PDF Download

Excretion

Excretion refers to the process of eliminating harmful and unwanted substances from the body, particularly nitrogenous wastes.

Substances to Rid Off

• Carbon dioxide is expelled from the body through the lungs.
• Water, after being processed, becomes part of the body's water balance.

• Urea, a nitrogenous waste, is excreted by the kidneys.

• Excess salts are also excreted through the kidneys.

• The surplus water is removed from the body by the kidneys.

• Some bile pigments are excreted in the urine.

The Excretory Organs

1. The Kidneys: These are the primary organs responsible for excretion.
2. Sweat Glands: These glands excrete sweat, which contains nitrogenous waste. Sweat is produced and excreted only when necessary, such as for cooling the body.
3. The Lungs: The lungs expel carbon dioxide from the body.

The Urinary System

The human urinary system consists of:
(i) Two kidneys,
(ii) Two ureters,
(iii) A urinary bladder,
(iv) A urethra.
(a) A Pair of Kidneys

• The kidneys are dark red, bean-shaped organs, each measuring about 10 cm in length and 6 cm in width.
• The right kidney is positioned slightly lower than the left due to the presence of the liver.
• The renal artery supplies oxygenated blood to the kidneys, while the renal veins carry deoxygenated blood away from them.

(b) A Pair of Ureters

• The ureters are tube-like structures that emerge from the hilum, or notch, of each kidney.
• They connect to the urinary bladder from behind and are responsible for carrying urine from the kidneys to the bladder.

(c) Urinary Bladder

• The urinary bladder is a muscular, sac-like organ that temporarily stores urine.

(d) Urethra

• The urethra is a short muscular tube that expels urine from the body.
• It is longer in males and shorter in females.
• The opening of the urethra is controlled by sphincter muscles that relax during urination.

Internal Structure of the Kidneys

• The longitudinal section of a kidney reveals two distinct regions:
  (i) the outer dark cortex, and
  (ii) the inner lighter medulla.
• The medulla is made up of conical structures called pyramids.
• The apex, or tip, of each pyramid, known as the papilla, projects into the renal pelvis.

• The kidneys contain a vast number of uriniferous tubules, also called nephrons, renal tubules, or kidney tubules.
• These tubules are the basic structural and functional units of the kidneys.

Malpighian Tubule

• Bowman’s Capsule : This is a thin-walled, cup-like structure that encloses a network of tiny blood vessels called the glomerulus.
• The Bowman’s capsule and the glomerulus together form the Malpighian tubule or renal tubule.
• Proximal Convoluted Tubule (PCT) : This is the first part of the convoluted region of the tubule, located in the cortex of the kidney.
• Loop of Henle : This is the U-shaped middle part of the tubule that plays a crucial role in urine concentration.
• Distal Convoluted Tubule (DCT) : This is the end part of the kidney tubule, which opens into the collecting duct.

Blood Supply to the Kidneys
The blood supply to the kidneys follows this pathway:

• Dorsal aorta
• Renal artery
• Afferent arteriole
• Glomerulus
• Efferent arteriole
• Secondary capillary network (vasa recta)
• Renal vein
• Posterior vena cava

Formation of Urine

The process of urine formation involves two main steps: ultrafiltration and reabsorption.
Ultrafiltration

• Due to the hydrostatic pressure in the glomerulus, the liquid part of the blood filters out from the glomerulus and enters Bowman’s capsule.
• This filtration under high pressure is called ultrafiltration.
• The filtrate obtained is known as glomerular filtrate.

Reabsorption

• The glomerular filtrate then passes down the tubule, where water and other essential substances are reabsorbed by the body.
• This process of selective reabsorption is crucial for maintaining the body’s fluid and electrolyte balance.
• Potassium ions and certain substances, such as penicillin, are secreted into the urine through the distal convoluted tubule (DCT).
• The cells lining the DCT play a role in reabsorbing potassium ions and other substances back into the renal tubule, a process known as tubular secretion.

Part of the Renal Tubule and Their Functions

Urine Excretion

• The filtrate that remains after reabsorption and tubular secretion is called urine.
• Urine passes from the collecting duct to the renal pelvis and is then transported to the urinary bladder through the ureters.
• Urine is expelled from the body by relaxing the sphincters at the opening of the urethra, a process known as micturition or urination.

Physical Properties of Urine

• Colour: Yellow, due to the presence of urochrome.
• Volume: Typically ranges from 1 to 1.5 litres, but can vary based on liquid intake.
• pH: Ranges from 5 to 8, making urine slightly acidic.
• Odour: Faint ammonia-like smell due to bacterial activity.
• Specific Gravity: Ranges from 1.003 to 1.035.
• Urine is composed of approximately 95% water and 5% solid waste.

Abnormal Constituents in Urine

Haematuria

• Haematuria refers to the presence of blood in the urine.
• It can occur due to various reasons, including:
• Infection in the urinary tract
• Kidney stones
• Tumours in the urinary tract or kidneys
• In such cases, blood passes out with urine, indicating an underlying health issue.

Glycosuria: 

• In diabetes mellitus, excess glucose is excreted in urine.
• When bile is present in urine, it may be due to conditions like anaemia, hepatitis, or liver cirrhosis.

Regulation of Urine Output: 

• The water content in urine is regulated by the Anti-diuretic Hormone (ADH).
• A decrease in ADH secretion leads to increased urine production, a condition known as diuresis. Substances that promote urine production are called diuretics.

Osmoregulation: 

• While filtering urea from the blood, the kidneys also regulate the blood's water and salt concentration, a process known as osmoregulation. Drinking sufficient water is essential for proper kidney function.
• During summer, the body loses a significant amount of water through perspiration, resulting in thicker, more concentrated urine. In such cases, the kidneys need to reabsorb more water from the urine.

Artificial Kidney

Kidneys and Their Function

• If one kidney is damaged or removed, the other kidney can still take care of the body’s waste removal.
• However, if both kidneys fail, it can be life-threatening.

Dialysis and Artificial Kidneys

• When both kidneys fail, patients need dialysis to survive.
• A dialysis machine acts as an artificial kidney.
• During dialysis, blood is taken from the patient’s radial artery and passed through the machine.
• The machine removes excess salts and urea from the blood.
• After purification, the blood is returned to a vein in the same arm.