Excretory System Overview

Overview

This lecture covers the human excretory system, focusing on its organs, functions, urine formation, disorders, and the importance of osmoregulation.

The excretory system removes waste products from the body.
Main excretory organs: kidneys (pair), ureters (pair), urinary bladder, and urethra.
Skin and lungs also help remove some waste, while the liver has a detoxification role.

Kidneys are reddish-brown, bean-shaped, located in the abdominal cavity, protected by floating ribs.
Each kidney has an outer cortex and inner medulla.
Medulla contains pyramids; their tips (papillae) connect to minor and major calyces, then to the renal pelvis, and finally to the ureter.
Nephrons are the structural and functional units of the kidney; each kidney contains about 1 million nephrons.

Skin removes some salts and water through sweat (mainly for cooling, not excretion).
Lungs expel carbon dioxide.
Liver detoxifies substances (e.g., converts ammonia to urea via the urea cycle).

Urine formation occurs in three steps: ultrafiltration, tubular reabsorption, and tubular secretion.
Ultrafiltration at the glomerulus forms glomerular filtrate (180L/day; only 1-1.5L excreted).
Reabsorption returns glucose, amino acids, and most water to the blood.
Secretion adds ions and waste (like H+, K+, and some urea) into nephron tubule.
Blood enters through afferent arteriole, filtered through glomerulus, exits via efferent arteriole and renal vein.

Normal urine: clear yellow (due to urochrome), pH 5–8, 1–1.5L/day, specific gravity ~1.015–1.025.
Main components: water (~95%), urea (2.3g/L), creatinine, uric acid, ions (Na+, K+, Cl-), trace ammonia.
Urine abnormalities can signify disease (e.g., glucose in urine indicates diabetes mellitus).

Albuminuria: protein (albumin) in urine, often due to high blood pressure or kidney infection.
Hematuria: blood in urine, due to infections, stones, or tumors.
Glycosuria: glucose in urine, common in diabetes mellitus.
Presence of bile pigments (bilirubin/biliverdin) can indicate jaundice or liver disorders.

Osmoregulation: maintaining blood’s water and salt balance, regulated by kidneys.
Antidiuretic hormone (ADH/vasopressin) from the posterior pituitary increases water reabsorption.
Deficiency of ADH leads to diabetes insipidus, causing excessive dilute urine.

When kidneys fail, hemodialysis (artificial kidney) removes waste from the blood.
Dialysate fluid must match blood osmotic pressure for effective diffusion.

Excretion — Removal of metabolic waste from the body.
Nephron — Structural and functional unit of the kidney that filters blood and forms urine.
Ultrafiltration — High-pressure filtering of blood at the glomerulus.
Tubular Reabsorption — Process of reclaiming needed substances (like glucose, water) from filtrate back to blood.
Tubular Secretion — Transport of additional wastes from blood into nephron tubule.
Osmoregulation — Regulation of water and electrolyte (salt) balance in the body.
ADH (Antidiuretic Hormone) — Hormone that increases water reabsorption in kidneys.
Dialysis — Artificial process to remove waste when kidneys fail.

Review textbook diagrams of kidney and nephron.
Complete PYQs (Past Year Questions) and sample papers on the excretory system.
Make a chart summarizing urine formation steps and nephron structure for revision.
Revise differences between excretory disorders and their symptoms.