Overview
This lecture explains the human excretory system, details urine formation, excretory organs in various animals, mechanisms for urine concentration, kidney regulation, and discusses related disorders and exam-focused questions.
Excretory Products and Their Elimination
- Metabolic waste must be expelled from the body for health; this is called excretion.
- Main excretory products include ammonia, urea, uric acid, carbon dioxide, ions, and water.
- Accumulation of wastes in the body can cause toxicity and health issues.
- Excretory mechanisms differ among animal groups, depending on their habitat and physiology.
Excretory Forms in Animals
- Ammonotelic organisms excrete ammonia; common in bony fishes, aquatic amphibians.
- Ureotelic organisms excrete urea; found in mammals, terrestrial amphibians.
- Uricotelic organisms excrete uric acid; found in reptiles, birds, land snails.
- Humans are ureotelic, as they primarily excrete urea.
Human Excretory System Structure
- Consists of two kidneys, two ureters, a urinary bladder, and a urethra.
- Kidneys are bean-shaped, reddish brown organs located near the third lumbar vertebra.
- The hilum is the entry/exit site for blood vessels, nerves, and ureter.
- Kidney structure: outer cortex, inner medulla, renal pyramids, calyces, and pelvis.
The Nephron: Functional Unit of the Kidney
- Each kidney contains about one million nephrons.
- Nephron parts: glomerulus (capillary bundle), Bowman's capsule, proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct.
- Glomerulus and Bowman's capsule together form the renal corpuscle.
Urine Formation Process
- Urine is formed by three steps: glomerular filtration, tubular reabsorption, and tubular secretion.
- Filtration occurs in the glomerulus; blood pressure drives water and solutes into Bowman's capsule.
- Filtrate (about 125 ml/min) is mostly reabsorbed; only 1.5 liters of urine excreted daily.
- PCT reabsorbs 70–80% of electrolytes and water, and selectively secretes H+, ammonia, and potassium.
- Loop of Henle sets up osmotic gradient; descending limb is water-permeable, ascending is electrolyte-permeable.
- DCT and collecting duct help in fine-tuning ion and water balance, pH regulation, and final urine concentration.
Counter-Current Mechanism
- Helps produce concentrated urine.
- Involves the loop of Henle and vasa recta (capillaries running parallel).
- Water is reabsorbed in the descending limb; electrolytes are reabsorbed in the ascending limb.
- Osmolarity increases from cortex to inner medulla (300 mOsm/L to 1200 mOsm/L).
Regulation of Kidney Function
- Antidiuretic hormone (ADH) from the hypothalamus increases water reabsorption.
- Renin-angiotensin-aldosterone system (RAAS) raises glomerular blood pressure and increases sodium reabsorption when filtration rate drops.
- Atrial natriuretic factor (ANF) reduces blood pressure by dilating blood vessels and inhibiting RAAS.
Other Excretory Organs and Structures
- Skin (sweat glands) excretes water, salts, and some waste.
- Lungs expel carbon dioxide.
- Liver processes waste for excretion.
- Large intestine eliminates excess salts.
Excretory Disorders
- Uremia: accumulation of urea in blood; treated by dialysis.
- Renal failure: kidneys stop functioning due to infection, hypertension, or stones.
- Renal calculi (kidney stones): caused by precipitation of uric acid or oxalate.
- Pyelonephritis: bacterial infection leading to kidney inflammation.
- Glomerulonephritis: glomerulus inflammation due to toxins or drug reactions.
- Dialysis: artificial blood purification when kidneys fail.
- Kidney transplant: final option in end-stage renal failure.
Key Terms & Definitions
- Excretion — process of removing metabolic waste from the body.
- Ammonotelic — organisms excreting ammonia.
- Ureotelic — organisms excreting urea.
- Uricotelic — organisms excreting uric acid.
- Nephron — structural and functional unit of the kidney.
- Glomerular Filtration Rate (GFR) — rate of filtrate formed by kidneys per minute.
- Counter-current mechanism — process for urine concentration using the nephron and vasa recta.
- ADH — hormone promoting water reabsorption in the kidneys.
- Uremia — high urea levels in blood.
- Dialysis — artificial removal of waste from blood.
Action Items / Next Steps
- Review the structure of the nephron and steps of urine formation.
- Answer and practice important exam questions listed at the end of the lecture.
- Memorize key terms and their meanings for effective revision.
- Read the textbook chapter on excretory products and their elimination.