Human Excretion

Human Excretion

1. Nitrogenous Wastes & Human Excretory Anatomy

A. Modes of Excretion & Nitrogenous Wastes

B. Human Excretory System Components

• 1. Kidneys (Pair): Reddish-brown, bean-shaped organs (Length: 10–12 cm, Width: 5–7 cm, Thickness: 2–3 cm, Weight: 120–170 g).• Positioned between $T_{12}$ and $L_3$ vertebrae attached to dorsal abdominal wall. Covered by peritoneum on ventral side only $\to$ Retroperitoneal / Metanephric Kidney.

  • Outer Renal Cortex & Inner Renal Medulla (divided into medullary pyramids projecting into calyces). Cortex extending between pyramids forms Columns of Bertini.

• 2. Ureters (Pair): Hollow muscular tubes lined by stretchable Transitional Epithelium. Propels urine via peristaltic movements.
• 3. Urinary Bladder: Muscular sac containing smooth Detrusor Muscle (longitudinal-circular-longitudinal) and a triangular base called Trigone of Bladder. Stores up to 700–800 mL of urine.
• 4. Urethra: Canal guarded by Internal Urethral Sphincter (involuntary) and External Urethral Sphincter (voluntary).• Longer in males, shorter in females $\to$ Urinary Tract Infections (UTI) are more common in females.

2. Urea Cycle, Nephron Structure & Types

A. Urea / Krebs-Henseleit / Ornithine Cycle

Occurs inside Liver cells (Mitochondria & Cytoplasm) to convert highly toxic ammonia into less toxic urea:

$\text{Amino Acid} \to \text{Glutamate} \to \text{NH}_3 + \text{CO}_2 + 2\text{ATP} \xrightarrow{\text{Carbamoyl Phosphate Synthetase}} \text{Carbamoyl Phosphate} + \text{Ornithine} \to \text{Citrulline} \xrightarrow{\text{Aspartate}} \text{Argininosuccinate} \to \text{Arginine} \xrightarrow{\text{Arginase}} \mathbf{\text{Urea}} + \text{Ornithine}$

B. Structure of Nephron

Structural and functional unit of kidney ($\approx 1\text{ million}$ per kidney). Each nephron consists of two parts:

• 1. Glomerulus: Tuft of capillaries formed by Afferent Arteriole (branch of renal artery) and drained by Efferent Arteriole.
• 2. Renal Tubule: Begins with cup-like Bowman’s Capsule (Glomerulus + Bowman’s Capsule = Malpighian Body / Renal Corpuscle).• Followed by Proximal Convoluted Tubule (PCT), Loop of Henle (Descending & Ascending limbs), and Distal Convoluted Tubule (DCT).

  • DCT opens into Collecting Duct (Not a part of single nephron; collects from multiple nephrons).

C. Types of Nephrons

3. Urine Formation & Glomerular Filtration Mechanics

A. Bowman’s Capsule & Filtration Membrane

• Visceral Layer: Lined by specialized podocyte cells with foot projections forming Filtration Slits / Slit Pores (25 nm wide).
• Parietal Layer: Simple squamous epithelium.
• 3-Layered Filtration Barrier: Capillary Endothelium (Fenestrated) $+$ Basement Membrane $+$ Visceral Podocytes.

B. Net Filtration Pressure (NFP) Forces

• 1. Glomerular Hydrostatic Pressure (GHP): $+60\text{ mmHg}$ (Pushes fluid out; due to Diameter of Afferent Arteriole $>$ Efferent Arteriole).
• 2. Blood Colloidal Osmotic Pressure (BCOP): $-30\text{ mmHg}$ (Opposes filtration; due to plasma proteins like albumin).
• 3. Capsular Hydrostatic Pressure (CHP): $-20\text{ mmHg}$ (Opposes filtration; pressure of fluid inside Bowman’s capsule).
• $$\mathbf{\text{NFP} = \text{GHP} – (\text{BCOP} + \text{CHP}) = 60 – (30 + 20) = 10\text{ mmHg}}$$

C. GFR, Renal Blood Flow & Filtration Fraction

• Renal Blood Flow (RBF): Kidneys receive $\frac{1}{5}\text{th}$ of cardiac output $= 1100\text{–}1200\text{ mL/min}$.
• Renal Plasma Flow (RPF): Plasma volume in RBF $\approx 600\text{ mL/min}$.
• Glomerular Filtration Rate (GFR): Amount of filtrate formed by kidneys per minute $= \mathbf{125\text{ mL/min}}\ (180\text{ L/day})$.
• Filtration Fraction (FF): $\frac{\text{GFR}}{\text{RPF}} = \frac{125}{600} \approx 16\text{–}20\%$.
• Ultrafiltrate Nature: Deproteinized plasma (contains $\text{H}_2\text{O}$, glucose, amino acids, urea, uric acid, salts; no blood cells or proteins).

4. Tubular Reabsorption, Secretion & Segmental Functions

A. Threshold Substances

• High Threshold: Maximally reabsorbed (e.g., Glucose, Amino acids, $\text{H}_2\text{O}$, $\text{Na}^+$).• Renal Threshold for Glucose: $180\text{ mg/dL}$ of blood. Exceeding this causes glucosuria.
• Low Threshold: Partially reabsorbed (e.g., Urea, Uric acid).
• Non-Threshold: Not reabsorbed at all; completely excreted (e.g., Inulin, Creatinine, Hippuric acid).

B. Functions of Tubular Segments

• 1. Proximal Convoluted Tubule (PCT):• Lined by simple cuboidal brush-border epithelium with dense microvilli and abundant mitochondria.

  • Reabsorbs 70–80% of total filtrate ($100\%$ Glucose & Amino acids via active transport; $\frac{2}{3}\text{rd}$ $\text{H}_2\text{O}$, $\text{Na}^+$, $\text{Cl}^-$, $\text{HCO}_3^-$ passively).

  • Osmolality remains Isotonic to blood plasma ($300\text{ mOsmol/L}$).

  • Selective secretion of $\text{H}^+$, $\text{NH}_4^+$, and $\text{K}^+$ to maintain pH and ionic balance.

• 2. Loop of Henle (LOH):• Descending Limb: Permeable to $\text{H}_2\text{O}$, impermeable to salts. Filtrate becomes concentrated $\to$ Hypertonic ($1200\text{ mOsmol/L}$) at hair-pin bend.

  • Ascending Limb: Impermeable to $\text{H}_2\text{O}$, permeable to salts ($\text{Na}^+, \text{Cl}^-$). Filtrate gets diluted $\to$ Hypotonic ($200\text{–}300\text{ mOsmol/L}$) as it enters cortex.

• 3. DCT & Collecting Duct: Conditional reabsorption of $\text{Na}^+$ and $\text{H}_2\text{O}$ under hormonal influence (Aldosterone & ADH).

5. Counter-Current Mechanism & Hormonal Control

A. Counter-Current Multiplier & Exchanger Mechanism

Operated by the Loop of Henle and Vasa Recta in Juxtamedullary nephrons to maintain an increasing osmotic gradient in medullary interstitium ($300\text{ mOsmol/L}$ in cortex to $1200\text{ mOsmol/L}$ in inner medulla).

• Gradient established primarily by $\text{NaCl}$ (transported by ascending LOH and exchanged with descending Vasa Recta) and Urea (recycled from collecting duct into thin ascending LOH).
• Allows human kidneys to produce urine that is 4 times more concentrated ($1200\text{ mOsmol/L}$) than initial filtrate ($300\text{ mOsmol/L}$).

B. Hormonal Feedback Mechanisms

• 1. RAAS System (Renin-Angiotensin-Aldosterone System):• Triggered by drop in Blood Pressure / Blood Volume / GFR.

  • Sensed by Juxtaglomerular (JG) Cells of JGA $\to$ Releases enzyme Renin into blood.

  • $\text{Angiotensinogen (from Liver)} \xrightarrow{\text{Renin}} \text{Angiotensin I} \xrightarrow{\text{ACE (Lungs)}} \text{Angiotensin II}$.

  • Angiotensin II Effects: Powerful vasoconstrictor (raises BP) $+$ Stimulates Adrenal Cortex to release Aldosterone (increases $\text{Na}^+$ and $\text{H}_2\text{O}$ reabsorption from DCT) $\to$ Restores GFR.

• 2. ADH / Vasopressin Control:• High blood osmolality / fluid loss triggers Hypothalamus osmoreceptors $\to$ Posterior Pituitary releases Anti-Diuretic Hormone (ADH).

  • Facilitates water reabsorption through aquaporins in DCT and Collecting Duct $\to$ Prevents Diuresis.

• 3. ANF Factor (Check on RAAS):• Increase in blood volume/pressure causes Atria of Heart to release Atrial Natriuretic Factor (ANF).

  • Causes Vasodilation and promotes $\text{Na}^+$ excretion (Natriuresis) $\to$ Decreases BP (acts as counter-check to RAAS).

6. Micturition, Urine Properties & Disorders

A. Micturition Reflex & Normal Urine Properties

• Micturition Reflex: Bladder filling stretches walls $\to$ Stretch receptors send signals to CNS $\to$ Motor signals induce contraction of Detrusor muscle and relaxation of urethral sphincters.
• Daily Excretion Volume: $1.0\text{ to } 1.5\text{ Litres}$ of urine per day ($25\text{–}30\text{ g}$ of Urea excreted daily).