Excretory Products and Their Elimination

Excretion Process & Structures

Introduction

• Excretion is the process of removal of metabolic waste products from the body.
• Animals produce nitrogenous wastes such as ammonia, urea and uric acid, along with carbon dioxide, excess water and ions like Na⁺, K⁺, Cl⁻, phosphate and sulphate.
• Accumulation of these wastes is harmful, therefore they must be eliminated to maintain internal balance (homeostasis).
• Among these, nitrogenous wastes are the most toxic and physiologically important.

Nitrogenous Wastes

Nitrogen metabolism produces three major types of wastes:

1. Ammonia:
   • Most toxic nitrogenous waste
   • Highly soluble in water
   • Requires a large amount of water for excretion
2. Urea:
   • Moderately toxic
   • Requires less water for excretion compared to ammonia
3. Uric Acid:
   • Least toxic
   • Almost insoluble in water
   • Requires minimal water for excretion

Modes of Excretion

Based on the type of nitrogenous waste excreted, animals are classified as:

1. Ammonotelism:
   • Excretion of ammonia
   • Ammonia is excreted by diffusion across body surface or gills as ammonium ions
   • Requires large quantity of water
   • Kidneys play a minor role
   • Seen in many bony fishes, aquatic amphibians and aquatic insects
2. Ureotelism:
   • Excretion of urea
   • Ammonia produced during metabolism is converted into urea in the liver
   • Urea is transported by blood and excreted by kidneys
   • Requires less water than ammonotelism
   • Seen in mammals, marine fishes and many terrestrial amphibians
3. Uricotelism:
   • Excretion of uric acid
   • Uric acid is excreted as semi-solid pellets or paste
   • Helps in maximum conservation of water
   • Seen in reptiles, birds, insects and land snails

Excretory Structures in Animals

Different animals possess different excretory organs based on their complexity:

• Protozoans: Contractile vacuoles and plasma membrane
• Sponges and Hydra: General body surface
• Flatworms: Protonephridia (flame cells or solenocytes) – mainly for osmoregulation
• Annelids (earthworm): Nephridia and chloragogen cells – excretion and ionic balance
• Insects (cockroach): Malpighian tubules – nitrogenous waste removal and osmoregulation
• Crustaceans (prawn): Antennal or green glands
• Vertebrates: Kidneys

Human Excretory System

The human excretory system consists of:

1. A pair of kidneys
2. A pair of ureters
3. Urinary bladder
4. Urethra

Kidneys

External Structure of Kidney

• Appearance: Reddish-brown, bean-shaped organs
• Location: Located between the last thoracic and third lumbar vertebrae
  • Right kidney lies slightly lower than the left
• Size: 10–12 cm long, 5–7 cm wide, 2–3 cm thick
• Weight: 120-170 grams.
• Hilum:
  • A concave notch on the inner surface
  • Entry and exit point for renal artery, renal vein, nerves and ureter

Internal Structure of Kidney

• Outer tough fibrous capsule
• Cortex: Outer region
• Medulla: Inner region containing medullary pyramids
• Cortex extends between pyramids as renal columns (Columns of Bertini)
• Calyces collect urine from pyramids and open into renal pelvis
• Renal pelvis leads into the ureter

Nephrons

• Structural and functional units of kidneys
• Number: About one million nephrons per kidney

Structure of Nephron

• Each nephron consists of:
• a. Renal Corpuscle (Malpighian Body)
  • Glomerulus: Tuft of capillaries formed by afferent arteriole
  • Bowman’s capsule: Double-walled cup enclosing the glomerulus
• b. Renal Tubule
  • Proximal Convoluted Tubule (PCT): Highly coiled, lined with brush-border microvilli for reabsorption
  • Loop of Henle: Hairpin-shaped loop with descending and ascending limbs
  • Distal Convoluted Tubule (DCT): Involved in selective secretion and ionic balance
  • Collecting Duct: Receives urine from many nephrons and opens into renal pelvis

Blood Supply of Nephron

• Afferent arteriole brings blood to glomerulus
• Efferent arteriole carries blood away
• Peritubular capillaries surround renal tubules
• Vasa recta: U-shaped vessels parallel to loop of Henle (well developed in juxtamedullary nephrons)

Types of Nephrons

1. Cortical Nephrons:
   • About 85% of nephrons
   • Short loop of Henle
   • Extend slightly into medulla
2. Juxta Medullary Nephrons:
   • About 15% of nephrons
   • Very long loop of Henle
   • Extend deep into medulla
   • Essential for urine concentration

Types of Kidneys in Vertebrate Evolution

• Archinephros: Primitive kidney in larval cyclostomes
• Pronephros: Embryonic kidney of vertebrates
• Mesonephros: Functional kidney of fishes and amphibians; temporary in higher vertebrates
• Metanephros: Most advanced kidney with loop of Henle; functional in reptiles, birds and mammals

Urine Formation

Urine formation is the process by which the kidneys remove nitrogenous wastes, excess salts and water from the blood. It occurs in the nephrons and involves three main steps:

1. Glomerular Filtration
2. Tubular Reabsorption
3. Tubular Secretion

1. Glomerular Filtration

Glomerular filtration is the first step of urine formation, where blood is filtered through the glomerulus into Bowman’s capsule.

• Process:
  • Blood is filtered due to high hydrostatic pressure in glomerular capillaries
  • About 1100–1200 mL of blood is filtered by kidneys per minute
  • This is roughly one-fifth of cardiac output
• Filtration Membrane (Three Layers)
  1. Endothelium of glomerular blood vessels
  2. Epithelium of Bowman’s capsule
  3. Basement membrane between the two
• Podocytes
  • Specialized epithelial cells of Bowman’s capsule
  • Have foot-like processes forming filtration slits (slit pores)
  • Ensure fine filtration
• Ultrafiltration
  • Almost all plasma components pass into Bowman’s capsule
  • Proteins and blood cells are retained in blood
  • No energy expenditure is required
• Glomerular Filtration Rate (GFR)
  • Volume of filtrate formed per minute
  • Normal GFR: 125 mL/minute (≈180 litres/day)

Regulation of Glomerular Filtration Rate (GFR)

• Juxta Glomerular Apparatus (JGA):
  • A special sensitive region formed by
    – Distal Convoluted Tubule (DCT)
    – Afferent arteriole
  • Acts as a regulatory unit
• Function:
  • A fall in GFR activates JGA
  • JGA cells release renin
  • Renin increases glomerular blood flow
  • GFR is restored to normal

Effective Filtration Pressure (EFP)

• EFP = Glomerular Blood Hydrostatic Pressure − (Blood Colloidal Osmotic Pressure + Capsular Hydrostatic Pressure)
  • GBHP ≈ 60 mmHg
  • BCOP ≈ 32 mmHg
  • CHP ≈ 18 mmHg
• EFP = 60 − (32 + 18) = 10 mmHg
• This pressure drives filtration.

2. Tubular Reabsorption

Tubular reabsorption is the second step of urine formation, in which useful substances are reabsorbed from filtrate back into blood.

• Extent:
  • About 99% of the filtrate is reabsorbed
  • Prevents loss of water, nutrients and ions
• Mechanisms: Active and passive.
  • Active Reabsorption:
    • Glucose
    • Amino acids
    • Sodium ions (Na⁺)
  • Passive Reabsorption:
    • Water
    • Nitrogenous wastes

Reabsorbed substances enter blood through peritubular capillaries.

3. Tubular Secretion

Tubular secretion is the third step of urine formation, where substances are actively or passively secreted into the filtrate.

• Substances Secreted
  • Hydrogen ions (H⁺)
  • Potassium ions (K⁺)
  • Ammonia (NH₃)
• Importance
  • Maintains acid–base balance
  • Maintains ionic balance
  • Removes substances not filtered at glomerulus

When filtration pressure falls significantly, urine formation continues mainly by tubular secretion.

Function of the Tubules

1. Proximal Convoluted Tubule (PCT)

• Structure:
  • Lined by simple cuboidal brush-border epithelium
  • Microvilli increase surface area
• Functions:
  • Reabsorbs nearly all essential nutrients
  • Reabsorbs 70–80% of electrolytes and water
  • Maximum reabsorption occurs here
• Secretion:
  • Hydrogen ions
  • Potassium ions
  • Ammonia
• pH Regulation:
  • Absorbs bicarbonate (HCO₃⁻)
  • Helps maintain acid–base balance

2. Henle’s Loop

1. Descending Limb:
   • Permeable to water
   • Impermeable to electrolytes
   • Filtrate becomes concentrated
2. Ascending Limb:
   • Impermeable to water
   • Actively and passively transports electrolytes
   • Filtrate becomes diluted

• Significance
  • Maintains high osmolarity of medullary interstitial fluid
  • Essential for urine concentration

3. Distal Convoluted Tubule (DCT)

• Functions:
  • Conditional reabsorption of sodium (Na⁺) and water
  • Reabsorbs bicarbonate (HCO₃⁻)
• Secretion:
  • Hydrogen ions
  • Potassium ions
  • Ammonia
• Role:
  • Maintains pH balance
  • Maintains sodium–potassium balance

4. Collecting Duct

• Structure: Extends from the cortex to the medulla.
• Functions:
  • Reabsorbs large amounts of water
  • Produces concentrated urine
  • Allows limited diffusion of urea to maintain medullary osmolarity
• Secretion:
  • Hydrogen ions
  • Potassium ions
• Role:
  • Maintains ionic and acid–base balance

Mechanism of Concentration of the Filtrate

• Mammalian kidneys have a special ability to produce concentrated urine.
• This ability is due to a mechanism operating in the medulla of the kidney, involving the loop of Henle and vasa recta.

Counter Current Mechanism

Henle’s Loop and Vasa Recta together form a counter current system.

• Henle’s Loop:
  • Filtrate flows in opposite directions in its two limbs
  • Descending limb: filtrate moves downward
  • Ascending limb: filtrate moves upward
• Vasa Recta:
  • Blood flows in opposite directions in its two limbs
  • Acts as a counter current exchanger

Because fluids flow in opposite directions, this arrangement is called the counter current mechanism.

• Osmolarity Gradient in the Kidney:
  • Osmolarity increases progressively from cortex to medulla
  • Cortex: about 300 mOsmol/L
  • Inner medulla: about 1200 mOsmol/L
• This increasing osmotic gradient is essential for concentrating urine.

Role of NaCl and Urea

• Sodium Chloride (NaCl)
  • Actively transported out of the ascending limb of Henle’s loop
  • Enters medullary interstitial fluid
  • Exchanged with the descending limb of vasa recta
  • Returned back to medullary interstitium by ascending limb of vasa recta
• Urea
  • Small amount of urea enters the thin ascending limb of Henle’s loop
  • Urea is transported back into medullary interstitium by collecting tubule

Together, NaCl and urea maintain high osmolarity of the medullary interstitial fluid.

Significance of Counter Current Mechanism

• Maintains concentration gradient in medulla
• Facilitates reabsorption of water from collecting duct
• Produces concentrated urine
• Human kidneys can produce urine nearly four times more concentrated than the initial filtrate

Additional Info

• Degree of urine concentration is directly related to the length of loop of Henle
• Longer loop of Henle → more water reabsorption → more concentrated urine

Regulation of Kidney Function

• Kidney function is regulated by hormonal feedback mechanisms involving:
  • Hypothalamus (ADH)
  • Juxtaglomerular Apparatus (JGA)
  • Heart (Atrial Natriuretic Factor)

Antidiuretic Hormone (ADH / Vasopressin)

• Source
  • Released by hypothalamus through neurohypophysis
• Trigger
  • Activation of osmoreceptors due to:
    – Decrease in blood volume
    – Decrease in body fluid volume
    – Increase in ionic concentration
• Functions
  • Increases water reabsorption in distal tubules and collecting ducts
  • Prevents diuresis (excess urine production)
  • Maintains body fluid balance
• Additional Effects
  • Causes vasoconstriction
  • Increases blood pressure
  • Increases glomerular blood flow and GFR
• Feedback Control
  • Increase in body fluid volume switches off osmoreceptors
  • ADH release is reduced
• Clinical Relevance
  • Deficiency of ADH causes diabetes insipidus
  • Urine output may reach 20–25 litres/day

Renin–Angiotensin Mechanism

• Trigger
  • Fall in:
    – Glomerular blood pressure
    – Blood pressure
    – Glomerular filtration rate (GFR)
• Juxtaglomerular Apparatus (JGA)
  • JG cells release enzyme renin
• Sequence of Events
  • Renin converts angiotensinogen into angiotensin I
  • Angiotensin I is converted into angiotensin II
• Actions of Angiotensin II
  • Powerful vasoconstrictor
  • Raises blood pressure
  • Increases glomerular blood pressure and GFR
  • Stimulates adrenal cortex to release aldosterone
• Role of Aldosterone
  • Increases reabsorption of Na⁺ and water in distal tubules
  • Raises blood volume and blood pressure
  • Restores GFR

This entire pathway is called the Renin–Angiotensin Mechanism.

Atrial Natriuretic Factor (ANF)

• Source:
  • Released by atrial walls of the heart
• Trigger:
  • Increased blood flow to atria
• Functions:
  • Causes vasodilation
  • Lowers blood pressure
  • Inhibits release of renin from JGA
  • Acts as a check on renin–angiotensin mechanism

Micturition (Urine Release)

• Definition:
  • Micturition is the process of expulsion of urine from the urinary bladder.
  • The neural mechanism controlling this process is called the micturition reflex.

Process of Micturition

• Storage:
  • Urine formed in nephrons is transported to the urinary bladder
  • Urinary bladder stores urine temporarily
• Signal Generation:
  • As the bladder fills, its walls stretch
  • Stretch receptors present in the bladder wall get activated
  • These receptors send signals to the central nervous system (CNS)
• Action:
  • CNS sends motor impulses
  • Smooth muscles of the urinary bladder contract
  • Urethral sphincter relaxes
  • Urine is released outside the body
• Average Urine Output:
  • An adult human excretes 1–1.5 litres of urine per day

Characteristics of Urine

• Appearance:
  • Light yellow coloured
  • Watery fluid
• Reaction:
  • Slightly acidic
  • pH ≈ 6.0
• Odour:
  • Characteristic smell
  • Due to pigment urochrome, formed from breakdown of haemoglobin of worn-out RBCs
• Composition:
  • About 25–30 g of urea excreted per day

Additional Info

• If urine is kept for some time, bacteria convert urea into ammonia
• Ammonia gives a strong pungent smell to stale urine

Clinical Importance of Urine Analysis

• Diagnosis: Urine examination helps in diagnosis of metabolic disorders and kidney malfunction.
• Examples:
  • Glycosuria: Presence of glucose in urine → indicates diabetes mellitus
  • Ketonuria: Presence of ketone bodies → indicates diabetes mellitus
  • Albuminuria: Presence of albumin in urine → kidney damage
  • Pyuria: Presence of pus or WBCs in urine → infection

Role of Other Organs in Excretion

Apart from kidneys, lungs, liver and skin also help in removal of waste substances.

Lungs

• Remove large amounts of carbon dioxide
• About 200 mL CO₂ per minute is excreted
• Also remove water vapour

Liver

• Largest gland of the body
• Secretes bile
• Bile contains:
  • Bilirubin
  • Biliverdin
  • Cholesterol
  • Degraded steroid hormones
  • Vitamins
  • Drugs

These substances are excreted from the body along with digestive wastes.

Skin

• Sweat Glands: Secrete sweat containing:
  • NaCl
  • Small amounts of urea
  • Lactic acid
• Functions:
  • Primary: Cooling of body
  • Secondary: Excretion of wastes

• Sebaceous Glands:
  • Secrete sebum containing:
    – Sterols
    – Hydrocarbons
    – Waxes
• Functions:
  • Provides oily protective covering to skin

• Saliva:
  • Eliminates small amounts of nitrogenous wastes

Disorders of the Excretory System

Uremia

• Condition:
  • Accumulation of urea in blood due to kidney malfunction
  • Highly harmful and may lead to kidney failure
• Treatment: Hemodialysis.
• Process:
  • Blood withdrawn from an artery
  • Mixed with heparin (anticoagulant)
  • Passed through dialysing unit containing a cellophane tube
  • Dialysing fluid has same composition as plasma but no nitrogenous wastes
  • Wastes diffuse out by concentration gradien
  • Purified blood returned to body through vein after adding anti-heparin

Kidney Transplantation

• Use:
  • Ultimate treatment for acute renal failure
• Procedure:
  • Healthy functioning kidney transplanted from a compatible donor
  • Preferably a close relative to reduce immune rejection

Renal Calculi (Kidney Stones)

• Condition:
  • Formation of hard insoluble masses of crystallised salts (oxalates, etc.) in kidney
• Causes:
  • Dehydration
  • Excess uric acid formation
  • Metabolic imbalance
  • Infection
  • Hormonal disturbance

Glomerulonephritis

• Condition:
  • Inflammation of glomeruli of kidney
  • Affects filtration process

Chapter Summary:

• Many nitrogen-containing substances, ions, carbon dioxide, water and other metabolic wastes continuously accumulate in the body and must be eliminated to maintain homeostasis.
• The nature of nitrogenous waste excreted by animals mainly depends on their habitat and availability of water.
• The three major nitrogenous wastes are ammonia, urea and uric acid.

• Animals show different modes of excretion such as ammonotelism, ureotelism and uricotelism.
• Correspondingly, a variety of excretory organs have evolved, including protonephridia, nephridia, malpighian tubules, green (antennal) glands and kidneys.
• Apart from removing wastes, these organs also play an important role in maintaining 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 and a urethra.
• Each kidney contains more than one million microscopic tubular structures called nephrons, which are the structural and functional units of the kidney.

• Each nephron has two main parts:
• Glomerulus – a tuft of capillaries formed from the afferent arteriole
• Renal tubule – beginning with Bowman’s capsule and differentiated into proximal convoluted tubule (PCT), loop of Henle and distal convoluted tubule (DCT)

• The DCTs of many nephrons open into a common collecting duct, which passes through the medulla and opens into the renal pelvis via medullary pyramids.
• The glomerulus together with Bowman’s capsule forms the Malpighian or renal corpuscle.

• Urine formation involves three basic processes: filtration, reabsorption and secretion.
• Glomerular filtration is a non-selective process driven by blood pressure in the glomerular capillaries.
• About 1200 mL of blood is filtered per minute, producing nearly 125 mL of filtrate per minute, called the glomerular filtration rate (GFR).
• The juxtaglomerular apparatus (JGA) plays a crucial role in regulating GFR.

• Nearly 99% of the filtrate is reabsorbed along different parts of the nephron.
• The PCT is the major site of reabsorption and selective secretion.
• The loop of Henle plays a vital role in maintaining the osmolar gradient within the kidney interstitium, ranging from about 300 mOsmol L⁻¹ in the cortex to 1200 mOsmol L⁻¹ in the medulla.
• The DCT and collecting duct allow regulated reabsorption of water and electrolytes, contributing to osmoregulation.

• Tubular secretion of hydrogen ions, potassium ions and ammonia helps maintain ionic balance and acid–base equilibrium of body fluids.
• A counter current mechanism operates between the two limbs of the loop of Henle and the vasa recta.
• Due to this arrangement, the filtrate becomes concentrated while descending and diluted while ascending.
• Electrolytes and urea are retained in the medullary interstitium, facilitating water reabsorption and urine concentration.

• The filtrate is finally concentrated from about 300 mOsmol L⁻¹ to nearly 1200 mOsmol L⁻¹, conserving water.
• Urine is stored in the urinary bladder and released through the urethra by a voluntary neural process called micturition.
• In addition to kidneys, organs such as skin, lungs and liver also contribute to excretion.