1.0 Excretion and the Urinary System
Excretion is the process of eliminating metabolic nitrogenous wastes (like urea and uric acid) from the body. It is vital for maintaining Homeostasis—the steady internal state of the body.
Osmoregulation: The process of regulating the water and salt content in the blood and body fluids. The kidney is the primary organ for this.
Organs of the Urinary System
The human urinary system consists of a specific set of organs. Sequence-based questions on these are common in Section A.
- 🛡️ Kidneys: Bean-shaped organs that filter blood.
- 🛡️ Ureters: Narrow tubes that carry urine from the kidneys to the bladder.
- 🛡️ Urinary Bladder: A muscular sac that stores urine temporarily.
- 🛡️ Urethra: The duct through which urine is discharged from the body.
Internal Structure of the Kidney
If asked to identify the vessel carrying blood away from the kidney with the lowest concentration of urea, it is the Renal Vein. The Renal Artery carries the most waste into the kidney.
The right kidney is slightly lower than the left one to accommodate the large space occupied by the liver on the right side!
2.0 The Nephron: Structural & Functional Unit
Each kidney contains over a million microscopic filtration units called Nephrons. Understanding the structure of a nephron is essential for identifying diagrams in ICSE Section B.
1. Malpighian Corpuscle (Renal Corpuscle)
- Bowman’s Capsule: A double-walled cup-like structure.
- Glomerulus: A knot of capillaries inside the capsule. The Afferent arteriole (wider) enters, and the Efferent arteriole (narrower) leaves.
2. Renal Tubule
- PCT (Proximal Convoluted Tubule): Closest to Bowman’s capsule.
- Loop of Henle: A U-shaped hair-pin loop (Descent and Ascent).
- DCT (Distal Convoluted Tubule): Farther from the capsule.
- Collecting Duct: Receives urine from multiple nephrons.
The Afferent arteriole is wider than the Efferent arteriole. This difference in diameter creates a high hydrostatic pressure in the glomerulus, which is the driving force for Ultrafiltration.
Location of Nephron Parts
Do not confuse Malpighian Body (in Kidney) with Malpighian Layer (in Skin) or Malpighian Tubules (in Cockroach). This is a common "odd-one-out" or "naming" question!
The total length of nephrons in both kidneys is nearly 60 kilometers! This provides an enormous surface area for reabsorbing water and nutrients.
3.0 Physiology of Urine Formation
The production of urine happens in three distinct phases. The kidney must ensure that waste is removed while precious nutrients like glucose and water are kept in the body.
1. Ultrafiltration (Glomerular Filtration)
Occurs in the Malpighian Corpuscle. High pressure forces liquid out of the blood into the Bowman's capsule. The resulting fluid is called Glomerular Filtrate.
What is NOT filtered? Blood cells and large proteins (they are too big to pass through).
2. Selective Reabsorption
As the filtrate passes through the PCT, Loop of Henle, and DCT, the body "reclaims" useful substances. 100% of Glucose is reabsorbed in a healthy person.
3. Tubular Secretion
Certain substances like K+ and H+ ions are actively secreted from the blood into the tubule to maintain the body's pH and ionic balance.
4.0 Regulation & Constituents
Antidiuretic Hormone (ADH)
ADH (Vasopressin) controls the amount of water reabsorbed by the kidney. If ADH is low, the body produces large amounts of dilute urine—a condition known as Diabetes Insipidus.
Be careful with terms: Diabetes Mellitus (sugar in urine due to Insulin deficiency) vs. Diabetes Insipidus (watery urine due to ADH deficiency). ICSE often uses these in "Give Reason" questions.
We produce about 150-180 liters of glomerular filtrate every day, but we only excrete about 1-1.5 liters of urine. The rest is all reabsorbed!
4.0 Artificial Kidney (Dialysis)
When both kidneys fail to work (Renal Failure), urea and other wastes accumulate in the blood, which can be fatal. In such cases, an Artificial Kidney or Dialysis Machine is used.
Dialysis: The process of cleaning the blood by separating the waste substances (urea) from it using a semi-permeable membrane in an external machine.
Common Excretory Disorders
ICSE often asks for the technical names of these conditions in "Name the following" sections:
5.0 Osmoregulation
The kidney acts as a "tap" to balance the body's water level. This is achieved through the feedback mechanism involving the hormone ADH.
- 💧 During Summer: We lose water through sweat. The blood becomes concentrated. The Pituitary gland releases more ADH. Result: More water is reabsorbed, and urine is thick/concentrated.
- ❄️ During Winter: We sweat very little. Blood is dilute. Less ADH is released. Result: Less water is reabsorbed, and urine is dilute/voluminous.
"Why do we urinate more frequently in winter?"
Ans: In winter, perspiration is negligible. To maintain the water balance (Osmoregulation), the excess water is excreted by the kidneys, resulting in increased frequency.
The yellow color of urine is due to a pigment called Urochrome, which is formed by the breakdown of hemoglobin from old dead red blood cells in the liver!