Urine Formation - JCE MSCE LESSONS

MSCE Biology for Form 4: Master Key Concepts

There are two processes involved in urine formation:

Blood Entry: Blood enters the kidney carrying both wastes and useful substances in its plasma.
Process: The body must retain useful substances while removing wastes. Plasma enters the nephron with wastes through ultrafiltration.
Location: Ultrafiltration occurs in the glomerulus.
Mechanism: The glomerulus is surrounded by capillaries that are wider at the entry point than at the exit, creating high pressure. This pressure forces water, mineral ions, and small molecules (e.g., glucose, amino acids, and urea) out of the glomerulus.
Filtrate Formation: These substances pass through tiny pores in the capillary walls into Bowman’s capsule, where they form the glomerular filtrate.
Molecular Retention: Larger molecules such as blood proteins, white blood cells, red blood cells, and platelets remain in the blood due to their size.

Reabsorption: Capillaries surrounding the nephron selectively reabsorb useful substances back into the body.
Active Transport: All amino acids and glucose are reabsorbed by active transport in the proximal convoluted tubules.
Water and Salts: Some salts and water are also reabsorbed, with most water being reabsorbed in the loop of Henle and the collecting duct.
Urea: Urea is not reabsorbed back into the blood.
Urine Formation: After reabsorption, the filtrate, now called urine, contains high concentrations of urea, salts, and water.
Collection: Several nephrons drain into the collecting duct, which leads to the renal pelvis and ureter. The ureter conveys urine to the urinary bladder for temporary storage. Before urination, sphincter muscles relax to empty the bladder.

Substances Excreted by the Kidney:

Urea
Uric acid
Excess water
Ammonia
Residues of drugs
Toxic chemical additives
Excess minerals (which can cause osmotic pressure, making cells absorb excess water and potentially burst)