Chapter 4: Excretory Products and their Elimination

Introduction

Forms of Nitrogenous Wastes

• Excretion removes metabolic wastes like ammonia, urea, uric acid, CO₂, water, and ions (Na⁺, K⁺, Cl^-, phosphate, sulphate).
• Major nitrogenous wastes: ammonia, urea, uric acid.
• Animals differ in waste excretion: ammonotelic (ammonia), ureotelic (urea), uricotelic (uric acid).

Memory Aids

• Ammonotelic = Ammonia excretion, aquatic animals, high water need.
• Ureotelic = Urea excretion, mammals, moderate water need.
• Uricotelic = Uric acid excretion, birds/reptiles, low water need.

Quick Checkpoints

• What are the three main nitrogenous wastes?
• Which animals are ammonotelic, ureotelic, and uricotelic?
• How does water requirement vary among these types?

Excretory Organs

Types of Excretory Organs

• Protozoans: contractile vacuoles for waste removal.
• Insects: Malpighian tubules.
• Earthworms: nephridia.
• Vertebrates: kidneys.

Memory Aids

• Protozoans = Contractile vacuole.
• Insects = Malpighian tubules.
• Earthworms = Nephridia.
• Vertebrates = Kidneys.

Quick Checkpoints

• Which excretory organ is found in protozoans?
• What organ do insects use for excretion?
• What is the excretory organ in earthworms?
• Which organ do vertebrates use?

Human Excretory System

Components

• Pair of kidneys, pair of ureters, urinary bladder, urethra.

Memory Aids

• Kidneys filter blood and produce urine.
• Ureters transport urine to bladder.
• Bladder stores urine.
• Urethra expels urine.

Quick Checkpoints

• What are the main parts of the human excretory system?
• What is the function of ureters?
• Where is urine stored?
• Which tube expels urine?

Kidneys

Structure and Location

• Bean-shaped, reddish brown organs located between last thoracic and third lumbar vertebrae.
• Right kidney is slightly lower than left.
• Size: 10-12 cm length, 5-7 cm width, 2-3 cm thickness; weight: 120-170 g.
• Hilum: notch on inner concave surface for ureter, blood vessels, nerves.

Memory Aids

• Hilum is the entry/exit point for vessels and ureter.
• Right kidney lower due to liver position.

Quick Checkpoints

• Where are kidneys located?
• What is the hilum?
• Which kidney is lower and why?

Kidney Internal Structure

Zones and Parts

• Outer tough capsule covers kidney.
• Two zones: outer cortex and inner medulla.
• Medulla has medullary pyramids projecting into calyces.
• Cortex extends between pyramids as renal columns (columns of Bertin).
• Major calyces open into renal pelvis leading to ureter.

Memory Aids

• Medullary pyramids collect urine.
• Columns of Bertin separate pyramids.
• Renal pelvis funnels urine to ureter.

Quick Checkpoints

• What are the two main zones of the kidney?
• What are medullary pyramids?
• What is the function of renal pelvis?

Structure of Nephron

Components

• Nephrons are functional units; ~1 million per kidney.
• Two parts: glomerulus (capillary tuft) and renal tubule.
• Renal tubule starts with Bowman's capsule enclosing glomerulus (Malpighian body).
• Renal tubule segments: proximal convoluted tubule (PCT), Henle's loop (descending and ascending limbs), distal convoluted tubule (DCT), collecting ducts.
• Collecting ducts converge into renal pelvis via medullary pyramids.

Memory Aids

• Malpighian body = glomerulus + Bowman's capsule.
• Henle's loop has descending (thin, water permeable) and ascending (thicker, cuboidal cells) limbs.
• Collecting ducts collect urine from many nephrons.

Quick Checkpoints

• What are the two main parts of a nephron?
• What is the Malpighian body?
• Describe the segments of the renal tubule.
• Where does urine collect before entering the renal pelvis?

Types of Kidneys

Classification

• Archinephric kidney: basic ancestral form, in larvae of cyclostomes (Myxine).
• Pronephric kidney: primitive, embryonic stage in some vertebrates.
• Mesonephric kidney: main excretory organ in aquatic vertebrates, temporary in higher vertebrates.
• Metanephric kidney: advanced type with loop of Henle, functional in reptiles, birds, mammals.

Memory Aids

• Archinephric = ancestral.
• Pronephric = embryonic.
• Mesonephric = middle, temporary.
• Metanephric = mature, advanced.

Quick Checkpoints

• What is the ancestral kidney type?
• Which kidney is embryonic?
• Which kidney is temporary in higher vertebrates?
• Which kidney has loop of Henle?

Nephron Zones

Location of Parts

• Malpighian corpuscle, PCT, DCT in renal cortex.
• Loop of Henle, collecting duct, duct of Bellini in renal medulla.
• PCT cells have brush border microvilli for surface area.
• Descending limb: thin, squamous epithelium, permeable to water.
• Ascending limb: thicker, cuboidal epithelium, impermeable to water.
• Efferent arteriole forms peritubular capillaries and vasa recta around tubules.

Memory Aids

• PCT: microvilli increase absorption.
• Descending limb: water permeable.
• Ascending limb: water impermeable.
• Vasa recta supports medullary function.

Quick Checkpoints

• Where are Malpighian corpuscle and PCT located?
• What epithelium lines the descending limb?
• What is the role of vasa recta?

Types of Nephrons

Characteristics

• Cortical nephrons (85%): short loop of Henle, extend little into medulla, few/no vasa recta.
• Juxtamedullary nephrons (15%): long loop of Henle, extend deep into medulla, associated with vasa recta.

Memory Aids

• Cortical = short loop, mostly cortex.
• Juxtamedullary = long loop, deep medulla.

Quick Checkpoints

• What percentage of nephrons are cortical?
• Which nephron type has a long loop of Henle?
• Which nephron type is associated with vasa recta?

Urine Formation

Processes

• Glomerular filtration (ultrafiltration) of blood plasma by glomeruli.
• Tubular reabsorption of useful substances back to blood.
• Tubular secretion of wastes and excess ions into filtrate.

Memory Aids

• Filtration: blood plasma filtered in glomerulus.
• Reabsorption: nutrients and water reabsorbed.
• Secretion: wastes secreted into tubule.

Quick Checkpoints

• What are the three main steps in urine formation?
• Where does filtration occur?
• What is reabsorbed during tubular reabsorption?
• What substances are secreted during tubular secretion?

Glomerular Filtration

Mechanism

• Blood filtered by glomerulus through endothelium, basement membrane, and Bowman's capsule epithelium.
• Podocytes form filtration slits allowing plasma except proteins to pass.
• Filtrate formed is called ultrafiltrate.
• Glomerular filtration rate (GFR) ~125 mL/min (180 L/day).
• Filtration driven by blood hydrostatic pressure; no energy required.

Memory Aids

• Filtration layers: endothelium, basement membrane, podocytes.
• Filtrate excludes proteins.
• GFR is a measure of kidney filtration efficiency.

Quick Checkpoints

• What are the three layers involved in filtration?
• What cells form filtration slits?
• What is the normal GFR value?

Regulation of Glomerular Filtration Rate

Mechanism

• Juxtaglomerular apparatus (JGA) regulates GFR.
• JGA formed by distal convoluted tubule and afferent arteriole contact.
• Drop in GFR triggers renin release from JG cells.
• Renin increases glomerular blood flow, restoring GFR.

Memory Aids

• JGA senses GFR changes.
• Renin release adjusts filtration rate.

Quick Checkpoints

• What is the role of JGA?
• How does renin affect GFR?

Effective Filtration Pressure

Components

• Glomerular blood hydrostatic pressure (GBHP): ~60 mmHg, promotes filtration.
• Blood colloidal osmotic pressure (BCOP): ~32 mmHg, opposes filtration.
• Capsular hydrostatic pressure (CHP): ~18 mmHg, opposes filtration.
• Effective filtration pressure (EFP) = GBHP - (BCOP + CHP) = 10 mmHg.
• EFP drives plasma filtration into Bowman's capsule.
• Renal plasma flow (RPF): 650-700 mL/min out of 1100-1200 mL blood/min.

Memory Aids

• EFP is net pressure for filtration.
• GBHP is main driving force.
• BCOP and CHP resist filtration.

Quick Checkpoints

• What pressures oppose filtration?
• How is EFP calculated?
• What is the typical value of EFP?

Tubular Reabsorption

Process

• Filtrate substances reabsorbed into blood via peritubular capillaries.
• About 99% of filtrate reabsorbed.
• Active reabsorption: glucose, amino acids, Na⁺.
• Passive reabsorption: nitrogenous wastes, water.

Memory Aids

• Most filtrate is reabsorbed to prevent loss.
• Active transport for nutrients and ions.
• Water reabsorbed passively.

Quick Checkpoints

• What percentage of filtrate is reabsorbed?
• Which substances are actively reabsorbed?
• How is water reabsorbed?

Tubular Secretion

Process

• Tubular cells secrete H⁺, K⁺, ammonia into filtrate.
• Maintains ionic and acid-base balance.
• Removes substances not filtered by glomerulus.
• Secretion via active and passive transport from peritubular capillaries.
• If filtration stops due to low pressure, urine forms by secretion only.

Memory Aids

• Secretion removes excess ions and wastes.
• Important for acid-base balance.

Quick Checkpoints

• Which ions are secreted into filtrate?
• Why is tubular secretion important?
• What happens when filtration stops?

Function of the Tubules

Proximal Convoluted Tubule (PCT)

• PCT lined by simple cuboidal brush border epithelium to increase surface area for reabsorption.
• Reabsorbs nearly all essential nutrients and 70-80% of electrolytes and water.
• Maintains pH and ionic balance by secreting H⁺, NH₃, K⁺ into filtrate and absorbing HCO₃^-.
• Glucose and amino acids reabsorbed by active transport; water by osmosis.
• Maximum reabsorption occurs in PCT.

Memory Aids

• PCT = "Primary Catch Tubule" for nutrients and water.
• Brush border = More surface, more absorption.
• Secretes acids and bases to balance pH.

Quick Checkpoints

• What epithelium lines the PCT?
• Which substances are actively reabsorbed in PCT?
• How does PCT maintain pH?

Henle's Loop

• Ascending limb has minimal reabsorption; maintains high osmolarity in medulla.
• Descending limb permeable to water, impermeable to electrolytes; concentrates filtrate.
• Ascending limb impermeable to water; actively/passively transports electrolytes out.
• Filtrate dilutes as electrolytes leave ascending limb to medullary fluid.

Memory Aids

• Descending limb = "Water leaves, salts stay".
• Ascending limb = "Salts leave, water stays".
• Loop creates medullary osmotic gradient.

Quick Checkpoints

• Which limb is permeable to water?
• What happens to filtrate concentration in descending limb?
• How does ascending limb affect filtrate?

Distal Convoluted Tubule (DCT)

Functions

• Conditional reabsorption of Na⁺ and water.
• Reabsorbs HCO₃^- and selectively secretes H⁺, K⁺, and NH₃ to maintain pH and ionic balance.

Memory Aids

• DCT = "Dynamic Control Tubule" for ions and pH.
• Selective secretion keeps blood balanced.

Quick Checkpoints

• What ions are reabsorbed in DCT?
• Which ions are secreted to maintain pH?

Collecting Duct

Functions

• Extends from cortex to medulla; reabsorbs large amounts of water to concentrate urine.
• Allows small amounts of urea to pass into medullary interstitium to maintain osmolarity.
• Maintains pH and ionic balance by selective secretion of H⁺ and K⁺.

Memory Aids

• Collecting duct = "Final water adjustment".
• Urea recycling helps osmolarity.
• Secretion controls blood pH and ions.

Quick Checkpoints

• What is the main function of the collecting duct?
• How does urea contribute to urine concentration?
• Which ions are secreted here?

Image Explanation

• Nephron filters blood and removes waste.
• PCT reabsorbs nutrients, water, NaCl, and HCO₃^-.
• H⁺ and NH₃ secreted into tubule.
• Descending limb reabsorbs water, concentrating filtrate.
• Ascending limb reabsorbs NaCl, diluting filtrate.
• DCT selectively reabsorbs and secretes ions.
• Collecting duct reabsorbs water and urea to concentrate urine.
• Process maintains water, salt, and acid-base balance.

Mechanism of Concentration of the Filtrate

Counter Current System in Kidney

• Mammalian kidneys produce concentrated urine via loop of Henle and vasa recta in the medulla.
• Filtrate flows in opposite directions in the two limbs of loop of Henle, forming a counter current.
• Blood flow in the two limbs of vasa recta is also opposite, creating a counter current pattern.
• This arrangement maintains an osmolarity gradient from cortex (300 mOsmol/L) to inner medulla (1200 mOsmol/L).
• NaCl is actively transported by the ascending limb of Henle's loop and exchanged with descending vasa recta.
• NaCl returns to medullary interstitium via ascending vasa recta.
• Urea enters thin ascending limb and is recycled via collecting tubule, aiding concentration.
• Counter current mechanism conserves water and concentrates urine up to four times the initial filtrate.
• Longer loop of Henle in mammals correlates with higher urine concentration due to more water reabsorption.

Memory Aids

• Counter current = Opposite flows in loop of Henle and vasa recta.
• Osmolarity gradient: Cortex 300 → Inner medulla 1200 mOsmol/L.
• NaCl pumped up ascending limb; water moves down descending limb.
• Urea recycling enhances medullary concentration.
• Longer loop = More water conserved = More concentrated urine.

Quick Checkpoints

• What creates the counter current in the nephron?
• How does NaCl contribute to osmolarity gradient?
• Role of vasa recta in concentration mechanism?
• How does urea aid urine concentration?
• Effect of loop of Henle length on urine concentration?

Regulation of Kidney Function

Hormonal Feedback Mechanism

• Kidney function is regulated by hormones via feedback loops.
• Key regulators: Hypothalamus (ADH), Juxtaglomerular Apparatus (JGA), Heart (ANF).

Memory Aids

• ADH from hypothalamus controls water balance.
• JGA controls blood pressure via renin.
• ANF from heart opposes renin effects.

Quick Checkpoints

• Which hormones regulate kidney function?
• What triggers ADH release?
• How does JGA affect blood pressure?
• Role of ANF in kidney regulation?

Antidiuretic Hormone (ADH)

Function and Regulation

• Osmoreceptors detect changes in blood and body fluid volume and ionic concentration.
• Fluid loss activates osmoreceptors, stimulating hypothalamus to release ADH (vasopressin) from neurohypophysis.
• ADH promotes water reabsorption in distal kidney tubules, reducing urine output (prevents diuresis).
• Increased body fluid volume inhibits ADH release, completing feedback.
• ADH also constricts blood vessels, raising blood pressure and glomerular filtration rate (GFR).
• Diuretics increase urine production, preventing water retention and edema.
• Alcohol inhibits ADH release; caffeine blocks ADH action and sodium reabsorption, increasing urine loss.
• ADH deficiency causes diabetes insipidus with excessive urine output (20-25 L/day).

Memory Aids

• ADH = Anti-Diuresis Hormone.
• Osmoreceptors trigger ADH release on fluid loss.
• Alcohol and caffeine increase urine by affecting ADH.
• Diabetes insipidus = ADH deficiency disease.

Quick Checkpoints

• What activates osmoreceptors?
• How does ADH affect kidney tubules?
• Effect of alcohol and caffeine on ADH?
• What causes diabetes insipidus?

Juxtaglomerular Apparatus (JGA)

Renin-Angiotensin Mechanism

• Decrease in glomerular blood flow/pressure/GFR activates JG cells to release renin.
• Renin converts angiotensinogen to angiotensin I, then to angiotensin II.
• Angiotensin II effects:
• Powerful vasoconstrictor, raises glomerular blood pressure and GFR.
• Stimulates adrenal cortex to release aldosterone.
• Aldosterone promotes sodium and water reabsorption in distal tubules, increasing blood pressure and GFR.
• This is the Renin-Angiotensin mechanism controlling blood pressure and kidney function.

Memory Aids

• Renin released when blood flow drops.
• Angiotensin II = vasoconstrictor + aldosterone stimulator.
• Aldosterone = sodium & water reabsorption.
• Renin-Angiotensin system regulates blood pressure.

Quick Checkpoints

• What triggers renin release?
• Role of angiotensin II?
• How does aldosterone affect kidneys?
• What is the Renin-Angiotensin mechanism?

Atrial Natriuretic Factor (ANF)

Function and Regulation

• Increased blood flow to heart atria triggers ANF release.
• ANF causes vasodilation, lowering blood pressure.
• ANF inhibits renin release from JGA, opposing the renin-angiotensin mechanism.

Memory Aids

• ANF released by atria on high blood flow.
• ANF dilates vessels, reduces blood pressure.
• ANF blocks renin to balance blood pressure.

Quick Checkpoints

• What triggers ANF release?
• How does ANF affect blood vessels?
• What is ANF's role in renin regulation?

Micturition

Mechanism and Process

• Expulsion of urine from the urinary bladder is called micturition.
• Neural control of this process is the micturition reflex.
• Urine formed by nephrons is stored in the urinary bladder.
• Stretching of bladder walls by urine activates stretch receptors.
• Stretch receptors send signals to the central nervous system (CNS).
• CNS sends motor signals to contract bladder smooth muscles.
• Simultaneously, urethral sphincter relaxes to release urine.
• Average urine output in adults is 1 to 1.5 litres per day.

Urine Characteristics

• Urine is light yellow, watery, and slightly acidic (pH ~6.0).
• Colour is due to pigment urochrome from haemoglobin breakdown.
• About 25-30 g of urea is excreted daily.
• Standing urine develops ammonia smell due to bacterial urea breakdown.
• Urine analysis aids diagnosis of metabolic and kidney disorders.
• Glucose (glycosuria) and ketone bodies (ketonuria) indicate diabetes mellitus.
• Albumin presence is albuminuria; WBCs or pus presence is pyuria.

Memory Aids

• Micturition reflex = bladder stretch → CNS signal → muscle contract + sphincter relax.
• Urochrome = urine yellow pigment from RBC breakdown.
• Glycosuria and ketonuria = diabetes markers in urine.
• Albuminuria = albumin in urine; Pyuria = pus/WBCs in urine.

Quick Checkpoints

• What triggers the micturition reflex?
• Which muscles contract and relax during urination?
• What causes the yellow colour of urine?
• How does urine pH and smell change over time?
• What do glycosuria and ketonuria indicate?
• Define albuminuria and pyuria.

Role of Other Organs in Excretion

Lungs

• Remove large amounts of CO₂ (200 mL/min).
• Eliminate significant water daily through respiration.

Liver

• Secretes bile containing bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins, and drugs.
• These substances exit the body with digestive wastes.

Skin

• Sweat glands produce watery fluid with NaCl, small urea, lactic acid.
• Sweat cools body surface and removes some wastes.
• Sebaceous glands secrete sterols, hydrocarbons, waxes via sebum.
• Sebum protects skin with oily covering.
• Small nitrogenous wastes also eliminated via saliva.

Memory Aids

• Lungs: "CO₂ and water out" through breathing.
• Liver: "Bile removes waste pigments and toxins."
• Skin: "Sweat cools and clears; sebum protects."
• Saliva: Minor nitrogenous waste exit.

Quick Checkpoints

• What wastes do lungs primarily remove?
• Which substances does liver secrete in bile?
• What is the main function of sweat besides waste removal?
• What wastes are eliminated by sebaceous glands?
• Can saliva remove nitrogenous wastes?

Uremia

Explanation

• Malfunction of kidneys causes urea accumulation in blood.
• Leads to kidney failure if untreated.
• Hemodialysis removes urea from blood externally.
• Blood is taken from artery, anticoagulant (heparin) added.
• Blood passes through cellophane tube in dialysing fluid without nitrogenous wastes.
• Urea and wastes diffuse out by concentration gradient.
• Cleaned blood returned to body with anti-heparin added.

Memory Aids

• Hemodialysis = "Blood cleaning outside body"
• Heparin prevents clotting during dialysis.

Quick Checkpoints

• What causes uremia?
• How does hemodialysis work?
• Why is dialysing fluid free of nitrogenous wastes?

Kidney Transplantation

Explanation

• Used to treat acute renal failure.
• Transplant functioning kidney from compatible donor.
• Close relatives preferred to reduce rejection risk.
• Immune system may reject incompatible kidneys.

Memory Aids

• Transplant = "Replace failed kidney"
• Compatibility reduces rejection.

Quick Checkpoints

• When is kidney transplantation used?
• Why choose a close relative as donor?
• What causes rejection?

Renal Calculi

Explanation

• Stones formed by crystallized salts (oxalates) in kidney.
• Caused by dehydration, excess uric acid, high milk intake.
• Also due to metabolic imbalance, infection, hormonal issues.

Memory Aids

• Calculi = "Kidney stones"
• Dehydration and diet affect stone formation.

Quick Checkpoints

• What are renal calculi made of?
• List causes of renal calculi.
• How does dehydration contribute?

Glomerulonephritis

Explanation

• Inflammation of kidney's glomerulus.
• Impairs filtration function.

Memory Aids

• Glomerulonephritis = "Glomerulus inflammation"

Quick Checkpoints

• What part of kidney is inflamed?
• How does inflammation affect kidney function?

Human Excretory System

Explanation

• Includes kidneys, ureters, bladder, and urethra.
• Kidney has outer renal cortex and inner renal medulla.
• Contains renal pelvis and calyces for urine collection.
• Nephrons are filtering units in cortex and medulla.
• Nephron parts: Bowman's capsule, glomerulus, tubules, collecting ducts.
• Blood enters nephron via arterioles, filtered in glomerulus.
• Filtrate passes tubules; useful substances reabsorbed.
• Waste and excess fluid form urine, collected in ducts to renal pelvis.
• Urine flows through ureters to bladder, exits via urethra.

Memory Aids

• Nephron = "Kidney's filtering unit"
• Flow: Blood → Glomerulus → Tubules → Urine

Quick Checkpoints

• What are the main parts of the excretory system?
• Where are nephrons located?
• What is the function of Bowman's capsule?
• How is urine formed and transported?