Excretory Products and Their Elimination

Excretion Process & Structures

Introduction

• Animals produce ammonia, urea, uric acid, carbon dioxide, water, and ions (Na+, K+, Cl-, phosphate, sulphate).
• These substances need to be removed to maintain health.
• Focus on nitrogenous wastes: ammonia, urea, uric acid.

Nitrogenous Wastes

• Ammonia: Most toxic, needs lots of water to remove.
• Urea: Less toxic, needs less water.
• Uric Acid: Least toxic, minimal water needed.

Excretion Processes

• Ammonotelism: Excretion of ammonia.
  • Seen in many bony fishes, aquatic amphibians, aquatic insects.
  • Ammonia excreted through body or gill surfaces as ammonium ions.
  • Kidneys play a minor role.
• Ureotelism: Excretion of urea.
  • Seen in mammals, many terrestrial amphibians, marine fishes.
  • Ammonia converted to urea in the liver, excreted by kidneys.
• Uricotelism: Excretion of uric acid.
  • Seen in reptiles, birds, land snails, insects.
  • Uric acid excreted as pellet or paste to conserve water.

Excretory Structures in Animals

• Invertebrates: Simple tubular forms.
• Vertebrates: Complex organs called kidneys.

Examples of Excretory Structures

• Protonephridia (Flame Cells): Found in flatworms, rotifers, some annelids, cephalochordates (e.g., Amphioxus).
  • Function: Osmoregulation.
• Nephridia: Found in earthworms and other annelids.
  • Function: Remove nitrogenous wastes, maintain fluid and ionic balance.
• Malpighian Tubules: Found in insects (e.g., cockroaches).
  • Function: Remove nitrogenous wastes, osmoregulation.
• Antennal (Green) Glands: Found in crustaceans (e.g., prawns).
  • Function: Excretion.

Human Excretory System

The human excretory system includes:

• Kidneys: Two reddish-brown, bean-shaped organs.
• Ureters: Two tubes connecting kidneys to the bladder.
• Urinary Bladder: Stores urine.
• Urethra: Tube for urine to leave the body.

Kidneys

• Location: Between the last thoracic and third lumbar vertebrae.
• Size: About 10-12 cm long, 5-7 cm wide, and 2-3 cm thick.
• Weight: 120-170 grams.
• Hilum: A notch where blood vessels, nerves, and ureter enter.
• Renal Pelvis: A funnel-shaped space inside the hilum with calyces.
• Layers: Outer tough capsule, inner cortex, and medulla.
• Medulla: Contains medullary pyramids.
• Cortex: Extends as renal columns (Columns of Bertini) between pyramids.

Nephrons

• Number: About one million per kidney.
• Parts: Glomerulus and renal tubule.
  • Glomerulus: Tuft of capillaries from afferent arteriole.
  • Bowman’s Capsule: Cup-like structure around the glomerulus.
  • Proximal Convoluted Tubule (PCT): Coiled tubule after Bowman’s capsule.
  • Henle’s Loop: Hairpin-shaped loop with descending and ascending limbs.
  • Distal Convoluted Tubule (DCT): Coiled tubule after Henle’s loop.
  • Collecting Duct: Straight tube where DCTs open.

Types of Nephrons

• Cortical Nephrons: Short Henle’s loop, extends slightly into medulla.
• Juxta Medullary Nephrons: Long Henle’s loop, deep into medulla.

Blood Supply

• Efferent Arteriole: Forms capillary network around the tubule.
• Peritubular Capillaries: Network around the renal tubule.
• Vasa Recta: ‘U’ shaped vessel parallel to Henle’s loop (absent or reduced in cortical nephrons).

Urine Formation

Urine formation happens in three main steps:

1. Glomerular Filtration
2. Reabsorption
3. Secretion

1. Glomerular Filtration

• Process: Blood is filtered by the glomerulus.
• Amount: 1100-1200 ml of blood filtered per minute.
• Filtration Pressure: Blood pressure in glomerular capillaries causes filtration.
• Layers Involved:
  • Endothelium of glomerular vessels
  • Epithelium of Bowman’s capsule
  • Basement membrane between the two
• Podocytes: Special cells in Bowman’s capsule with filtration slits.
• Ultra Filtration: Almost all plasma components except proteins pass through.
• Glomerular Filtration Rate (GFR): 125 ml/min or 180 liters/day.

Regulation of GFR

• Juxta Glomerular Apparatus (JGA): Sensitive region formed by DCT and afferent arteriole.
• Function: Releases renin when GFR falls, normalizing blood flow and GFR.

2. Reabsorption

• Volume: 99% of the filtrate is reabsorbed.
• Mechanisms: Active and passive.
  • Active: Reabsorbs glucose, amino acids, Na+.
  • Passive: Reabsorbs nitrogenous wastes and water.

3. Secretion

• Process: Tubular cells secrete H+, K+, and ammonia into the filtrate.
• Importance: Maintains ionic and acid-base balance of body fluids.

Function of the Tubules

1. Proximal Convoluted Tubule (PCT)

• Lining: Simple cuboidal brush border epithelium.
• Reabsorption: 70-80% of electrolytes and water, nearly all essential nutrients.
• Functions:
  • Maintains pH and ionic balance.
  • Secretes hydrogen ions, ammonia, and potassium ions.
  • Absorbs bicarbonate (HCO3–).

2. Henle’s Loop

• Descending Limb:
  • Permeable to water.
  • Concentrates filtrate as it moves down.
• Ascending Limb:
  • Impermeable to water.
  • Allows active and passive transport of electrolytes.
  • Dilutes filtrate as it moves up.

3. Distal Convoluted Tubule (DCT)

• Reabsorption: Conditional reabsorption of sodium (Na+) and water.
• Functions:
  • Absorbs bicarbonate (HCO3–).
  • Secretes hydrogen and potassium ions, ammonia.
  • Maintains pH and sodium-potassium balance.

4. Collecting Duct

• Structure: Extends from the cortex to the medulla.
• Reabsorption: Large amounts of water, some urea.
• Functions:
  • Produces concentrated urine.
  • Maintains pH and ionic balance by secreting hydrogen and potassium ions.

Mechanism of Concentration of the Filtrate

Counter Current Mechanism

• Henle’s Loop and Vasa Recta: Create a counter current system.
  • Henle’s Loop: Filtrate flows in opposite directions in the two limbs.
  • Vasa Recta: Blood flows in opposite directions in its two limbs.
• Osmolarity Gradient: Increases from 300 mOsmol/L in the cortex to 1200 mOsmol/L in the medulla.
• Role of NaCl and Urea:
  • NaCl transported by the ascending limb of Henle’s loop, exchanged with the descending limb of vasa recta.
  • Urea enters ascending limb of Henle’s loop, transported back by collecting tubule.
• Purpose: Maintains concentration gradient, aids water reabsorption from collecting tubule, concentrates urine.
• Result: Human kidneys can produce urine four times more concentrated than the initial filtrate.

Regulation of Kidney Function

Hormonal Feedback Mechanisms

• Key Players: Hypothalamus, JGA (Juxta Glomerular Apparatus), Heart.

Antidiuretic Hormone (ADH) / Vasopressin

• Trigger: Activated by osmoreceptors due to changes in blood volume, fluid volume, or ionic concentration.
• Function: Released from hypothalamus; helps reabsorb water, prevents diuresis (excessive urine production).
• Effects: Increases blood pressure by constricting blood vessels, boosts glomerular blood flow, and GFR (Glomerular Filtration Rate).

Renin-Angiotensin Mechanism

• Trigger: Activated by a fall in glomerular blood flow, blood pressure, or GFR.
• Process:
  • JG cells release renin.
  • Renin converts angiotensinogen to angiotensin I, then to angiotensin II.
  • Angiotensin II increases blood pressure, stimulates adrenal cortex to release aldosterone.
  • Aldosterone reabsorbs Na+ and water, increasing blood pressure and GFR.

Atrial Natriuretic Factor (ANF)

• Trigger: Increased blood flow to the atria of the heart.
• Function: Causes vasodilation, lowers blood pressure, checks renin-angiotensin mechanism.

Micturition (Urine Release)

Process

• Storage: Urine formed in nephrons is stored in the urinary bladder.
• Signal: CNS receives signal when the bladder stretches.
• Action: CNS sends messages to contract bladder muscles and relax urethral sphincter, releasing urine.

Micturition Reflex

• Definition: Neural process causing urine release.
• Average Excretion: 1 to 1.5 liters per day.

Characteristics of Urine

• Appearance: Light yellow, watery, slightly acidic (pH 6.0).
• Odor: Characteristic smell.
• Urea: 25-30 grams excreted per day.

Clinical Importance

• Diagnosis: Urine analysis helps diagnose metabolic disorders and kidney problems.
  • Examples:
    • Glycosuria: Presence of glucose, indicates diabetes.
    • Ketonuria: Presence of ketone bodies, indicates diabetes.

Role of Other Organs in Excretion

Lungs

• Function: Remove CO2 (about 200mL/minute) and water vapor.

Liver

• Function: Largest gland, secretes bile.
• Contents of Bile: Bilirubin, biliverdin, cholesterol, degraded hormones, vitamins, drugs.
• Pathway: These substances are excreted with digestive wastes.

Skin

• Sweat Glands: Produce sweat containing NaCl, urea, lactic acid.
  • Primary Function: Cooling effect.
  • Secondary Function: Removal of wastes.
• Sebaceous Glands: Secrete sebum containing sterols, hydrocarbons, waxes.
  • Function: Provides oily protective covering for the skin.
• Saliva: Can eliminate small amounts of nitrogenous wastes.

Disorders of the Excretory System

Uremia

• Condition: Accumulation of urea in blood, harmful and may lead to kidney failure.
• Treatment: Hemodialysis.
  • Process:
    • Blood taken from an artery, mixed with heparin (anticoagulant).
    • Blood passed through dialysing unit with cellophane tube.
    • Dialysing fluid outside tube has same composition as plasma (no nitrogenous wastes).
    • Wastes move out, cleaned blood returned to body with anti-heparin.

Kidney Transplantation

• Use: Treats acute renal failure.
• Procedure: Functioning kidney from donor, preferably a close relative.
• Success: Modern techniques increase success rates.

Renal Calculi

• Condition: Stones or insoluble mass of crystallized salts (oxalates) in kidney.

Glomerulonephritis

• Condition: Inflammation of glomeruli in the kidney.

Chapter Summary:

• Many nitrogen-containing substances, ions, CO2, water, etc., accumulate in the body and must be eliminated.
• The type of nitrogenous wastes and their excretion vary among animals, mainly depending on habitat (availability of water).
• Ammonia, urea, and uric acid are the major nitrogenous wastes excreted.
• Common excretory organs in animals include protonephridia, nephridia, malpighian tubules, green glands, and kidneys.
• These organs eliminate nitrogenous wastes and help maintain ionic and acid-base balance of body fluids.
• In humans, the excretory system consists of:
  • One pair of kidneys
  • A pair of ureters
  • A urinary bladder
  • A urethra
• Each kidney has over a million tubular structures called nephrons.
• Nephron is the functional unit of the kidney and has two parts:
  • Glomerulus
  • Renal tubule
• Glomerulus is a tuft of capillaries formed from afferent arterioles (fine branches of renal artery).
• The renal tubule starts with a double-walled Bowman’s capsule and is further divided into:
  • Proximal convoluted tubule (PCT)
  • Henle’s loop (HL)
  • Distal convoluted tubule (DCT)
• DCTs of many nephrons join to a common collecting duct, which opens into the renal pelvis through medullary pyramids.
• Bowman’s capsule encloses the glomerulus to form Malpighian or renal corpuscle.
• Urine formation involves three main processes:
  • Filtration
  • Reabsorption
  • Secretion
• Filtration is non-selective and performed by the glomerulus using glomerular capillary blood pressure.
• About 1200 ml of blood is filtered by the glomerulus per minute, forming 125 ml of filtrate in Bowman’s capsule per minute (GFR).
• JGA, a specialized part of nephrons, plays a significant role in regulating GFR.
• Nearly 99% of the filtrate is reabsorbed through different parts of the nephrons.
• PCT is the major site of reabsorption and selective secretion.
• HL helps maintain the osmolar gradient (300 mOsmolL–1 to 1200 mOsmolL–1) within the kidney interstitium.
• DCT and collecting duct allow extensive reabsorption of water and certain electrolytes, aiding in osmoregulation.
• H+, K+, and NH3 can be secreted into the filtrate by the tubules to maintain ionic balance and pH of body fluids.
• A counter current mechanism operates between:
  • Two limbs of the loop of Henle
  • Vasa recta (capillary parallel to Henle’s loop)
• The filtrate gets concentrated as it moves down the descending limb and diluted by the ascending limb.
• Electrolytes and urea are retained in the interstitium by this arrangement.
• DCT and collecting duct concentrate the filtrate from 300 mOsmolL–1 to 1200 mOsmolL–1, conserving water.
• Urine is stored in the urinary bladder until a voluntary signal from the CNS causes its release through the urethra (micturition).
• Skin, lungs, and liver also assist in excretion.