Excretion Exercise Solutions | KalviGavani

| Feature | Ammonotelic | Ureotelic | Uricotelic | |---|---|---|---| | Waste product | Ammonia (NH₃) | Urea | Uric acid | | Toxicity | Most toxic | Moderately toxic | Least toxic | | Water needed | Very high | Moderate | Very little | | Energy needed | Least | Moderate | Most | | Examples | Fish, aquatic invertebrates | Humans, mammals, amphibians | Birds, reptiles, insects |

Glomerular filtration rate (GFR) is the volume of filtrate formed by both kidneys per minute. Normal GFR is about 125 mL/min or 180 L/day. - The kidneys filter about 180 L of blood per day, but only about 1.5 L of urine is produced per day - This means about 178.5 L of filtrate is reabsorbed back into the blood - GFR is regulated by blood pressure and hormones (ADH, aldosterone, ANF)

The loop of Henle creates a concentration gradient in the medulla that allows the kidney to produce concentrated urine: - Descending limb: Permeable to water, impermeable to salts → water moves out by osmosis → filtrate becomes more concentrated - Ascending limb: Impermeable to water, permeable to salts (Na⁺, Cl⁻) → salts move out → filtrate becomes dilute - This countercurrent mechanism concentrates the medullary interstitium, allowing water reabsorption in the collecting duct

Micturition (urination) is the process of expelling urine from the urinary bladder through the urethra. - The bladder can hold about 300-500 mL of urine - When the bladder is about half full, stretch receptors send signals to the brain - The internal urethral sphincter relaxes involuntarily - The external urethral sphincter relaxes voluntarily (under our control) - Urine is expelled from the body

Structure of a Nephron: Each nephron has two main parts: 1. Renal Corpuscle: - Bowman's capsule: Cup-shaped structure with two walls (parietal and visceral) - Glomerulus: Tuft of capillaries inside Bowman's capsule - Afferent arteriole brings blood in; efferent arteriole takes blood out - Filtration occurs here 2. Renal Tubule: - Proximal convoluted tubule (PCT): Highly coiled, in cortex; lined with brush border microvilli for reabsorption - Loop of Henle: U-shaped, extends into medulla; descending limb (permeable to water) and ascending limb (permeable to salts) - Distal convoluted tubule (DCT): In cortex; reabsorbs water and salts, secretes H⁺ and K⁺ - Collecting duct: Receives urine from multiple nephrons; final concentration of urine Urine Formation (Three Steps): Step 1: Glomerular Filtration: - Blood enters the glomerulus through the afferent arteriole - High blood pressure (hydrostatic pressure ~55 mmHg) forces water and small solutes through the filtration membrane into Bowman's capsule - The filtrate contains water, glucose, amino acids, urea, salts — everything except blood cells and large proteins - GFR ≈ 125 mL/min or 180 L/day Step 2: Tubular Reabsorption: - As filtrate passes through PCT, loop of Henle, and DCT, useful substances are reabsorbed into the blood: - PCT: 100% glucose, 100% amino acids, ~65% water, ~65% Na⁺, bicarbonate - Descending loop of Henle: Water reabsorbed by osmosis - Ascending loop of Henle: Na⁺, K⁺, Cl⁻ reabsorbed by active transport - DCT: Water (under ADH control) and Na⁺ (under aldosterone control) reabsorbed - Of the 180 L filtered, only ~1.5 L becomes urine; the rest is reabsorbed Step 3: Tubular Secretion: - Additional wastes are secreted from the blood into the tubule: - H⁺ (acid-base balance), K⁺, NH₃, creatinine, drugs, urea - This ensures waste removal and maintains blood pH

1. ADH (Antidiuretic Hormone / Vasopressin): - Source: Posterior pituitary gland - Trigger: High blood osmolarity (concentrated blood) detected by osmoreceptors in the hypothalamus - Action: Increases the permeability of DCT and collecting duct to water - Effect: More water is reabsorbed into the blood → concentrated urine, reduced urine volume - Deficiency: Diabetes insipidus (large volumes of dilute urine) 2. Aldosterone: - Source: Adrenal cortex - Trigger: Low blood Na⁺ or low blood pressure (detected by juxtaglomerular apparatus) - Action: Increases Na⁺ reabsorption in DCT and collecting duct - Effect: Na⁺ reabsorbed → water follows by osmosis → increased blood volume and blood pressure; also increases K⁺ secretion - Part of the Renin-Angiotensin-Aldosterone System (RAAS) 3. ANF (Atrial Natriuretic Factor): - Source: Heart atria - Trigger: High blood volume / increased blood pressure - Action: Decreases Na⁺ reabsorption in DCT and collecting duct - Effect: More Na⁺ and water excreted → increased urine volume → decreased blood volume and blood pressure - ANF opposes the RAAS system — it lowers blood pressure 4. Renin-Angiotensin-Aldosterone System (RAAS): - When blood pressure drops, the juxtaglomerular apparatus (JGA) releases renin - Renin converts angiotensinogen → angiotensin I → angiotensin II - Angiotensin II: (a) constricts blood vessels (raising BP), (b) stimulates aldosterone release → Na⁺ and water reabsorption → blood volume and pressure increase

Dialysis is the artificial process of removing waste products and excess fluid from the blood when the kidneys are unable to do so (kidney failure). When is dialysis needed? - When both kidneys are severely damaged (renal failure) - When GFR falls below 15 mL/min (normal: 125 mL/min) - When urea and creatinine levels in blood become dangerously high Haemodialysis (Artificial Kidney): 1. Blood is drawn from an artery (usually in the arm) 2. The blood is passed through a dialyser (artificial kidney) containing: - A semi-permeable membrane with tiny pores - Dialysing fluid on the other side of the membrane 3. Waste products (urea, creatinine, excess salts, excess water) diffuse from the blood through the membrane into the dialysing fluid 4. Useful substances (glucose, amino acids) are retained in the blood because the dialysing fluid has the same concentration as normal blood plasma 5. The purified blood is returned to the body through a vein 6. The process takes 3-4 hours and must be done 2-3 times per week Limitations of dialysis: - Does NOT cure kidney failure — only manages symptoms - Must be done regularly for life (or until kidney transplant) - Risk of infection and blood clots - Expensive and time-consuming Kidney transplant is the permanent solution for kidney failure, where a healthy kidney from a donor (living or deceased) is transplanted into the patient.