Excretory System

Excretory System

Introduction

o   The excretory system is responsible for removing metabolic waste products, maintaining water and electrolyte balance, and regulating blood pressure and pH. In humans, the urinary system is the main excretory system, though other organs like the lungs, skin, and liver also contribute.

o   The urinary system includes two kidneys, two ureters, one urinary bladder, and one urethra. Its functional unit is the nephron, where urine is formed.

Major functions of the excretory system

• Removal of nitrogenous waste (urea, uric acid, creatinine).
• Regulation of water and electrolyte balance.
• Regulation of blood pressure (via renin and other mechanisms).
• Maintenance of acid–base balance.
• Endocrine role (erythropoietin, renin, activation of vitamin D).

Urinary System – Structure & Functions of Organs

1. Kidneys

• Structure: Two bean-shaped, reddish-brown organs located retroperitoneally on either side of the vertebral column at T12–L3 level. Right kidney lies slightly lower due to the liver.
• Dimensions: ~10–12 cm long, 5–7 cm wide, 3 cm thick, weight 120–170 g.
• Coverings: Fibrous capsule, perirenal fat, and renal fascia.
• Hilum: Medial concavity where renal artery enters, renal vein exits, and ureter emerges.
• Function: Filtration of blood, urine formation, electrolyte & fluid balance, blood pressure regulation, hormone secretion.

2. Ureters

• Structure: Muscular tubes (25–30 cm long) carrying urine from renal pelvis to urinary bladder.
• Course: Descends retroperitoneally and opens obliquely into the posterior wall of the bladder.
• Function: Propel urine via peristalsis and gravity.

3. Urinary Bladder

• Structure: Muscular, hollow sac located in the pelvic cavity. Lined by transitional epithelium, wall has detrusor muscle.
• Capacity: ~300–500 mL in adults (urge at ~200–250 mL).
• Special part: Trigone – triangular area at the base between ureteric openings and internal urethral orifice.
• Function: Stores urine temporarily and expels it during micturition.

4. Urethra

• Structure: Muscular tube conveying urine from bladder to outside.
• Female: short (~4 cm).
• Male: long (~18–20 cm) with prostatic, membranous, and penile parts.
• Function: Passage of urine (and semen in males).

Internal Structure of Kidney

• Cortex: Outer granular region containing renal corpuscles (glomeruli + Bowman’s capsules), proximal and distal convoluted tubules.
• Medulla: Inner striated region with renal pyramids, loops of Henle, and collecting ducts.
• Renal Columns (of Bertin): Cortical tissue between pyramids.
• Papillae: Apex of pyramids opening into minor calyces → major calyces → renal pelvis → ureter.
• Nephrons: Microscopic functional units (~1–1.5 million per kidney).

Juxta-Glomerular Apparatus (JGA)
Components:

1. Juxtaglomerular (JG) cells: Modified smooth muscle cells of afferent arteriole, secrete renin.
2. Macula densa: Specialized epithelial cells in distal convoluted tubule, sensitive to sodium concentration in filtrate.
3. Extraglomerular mesangial cells (Lacis cells): Supportive role, communication between macula densa and JG cells.

Functions:

• Secretion of renin → activation of renin–angiotensin–aldosterone system (RAAS).
• Regulation of glomerular filtration rate (GFR).
• Maintenance of blood pressure and sodium balance.

Structure of Nephron

The structural and functional unit of the kidney. Each kidney has ~1–1.5 million nephrons.

Parts:

1. Renal Corpuscle:
• Glomerulus: Tuft of capillaries arising from afferent arteriole.
• Bowman’s capsule: Double-walled epithelial cup surrounding glomerulus; collects filtrate.
2. Renal Tubule:
• Proximal convoluted tubule (PCT): Highly coiled, lined with brush-border epithelium for absorption.
• Loop of Henle: Descending thin limb (water-permeable) and ascending thick limb (NaCl reabsorption).
• Distal convoluted tubule (DCT): Involved in selective secretion and absorption, regulated by hormones.
3. Collecting Ducts: Receive filtrate from many nephrons, final concentration of urine occurs here under ADH influence.

Types of Nephrons:

• Cortical nephrons (85%): Short loops of Henle, mainly in cortex.
• Juxtamedullary nephrons (15%): Long loops extending deep into medulla, crucial for urine concentration.

Urine Formation

Urine is formed through three main processes:

1. Glomerular Filtration
• Passive process driven by blood pressure.
• Plasma (except proteins and cells) filtered into Bowman’s capsule.
• Normal GFR: ~125 mL/min (180 L/day).
2. Tubular Reabsorption
• Selective reuptake of essential substances (glucose, amino acids, water, Na+, Cl–, HCO₃–).
• Occurs mainly in PCT (65–70%).
• Loop of Henle: reabsorbs water (descending limb) and NaCl (ascending limb).
• DCT & collecting duct: reabsorption regulated by hormones (aldosterone, ADH).
3. Tubular Secretion
• Active secretion of H+, K+, NH₄+, creatinine, and drugs into tubular fluid.
• Maintains acid–base balance and eliminates waste.

Final urine volume: ~1–1.5 L/day.

Normal Urine Constituents

Normal urine characteristics:

• Volume: 1–1.5 L/day.
• Color: Pale yellow (urochrome pigment).
• Reaction: Slightly acidic (pH 6).
• Specific gravity: 1.003–1.030.

Chemical composition:

1. Water – ~95%.
2. Solids (5%)
• Organic constituents:
• Urea (main nitrogenous waste, 25–30 g/day).
• Creatinine.
• Uric acid.
• Hippuric acid.
• Inorganic constituents:
• Na+, K+, Cl–, HCO₃–, phosphate, sulfate.
• Calcium, magnesium (traces).

Abnormal constituents (indicative of pathology):

• Glucose (glycosuria – diabetes mellitus).
• Protein (proteinuria – kidney disease).
• Ketone bodies (ketonuria – diabetes/starvation).
• Blood (hematuria).
• Bile salts/pigments (jaundice).

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