Overview
This lecture introduces the human excretory system, emphasizing the kidneys and the nephron's role in filtering blood and forming urine.
Introduction to the Excretory System
- The excretory system maintains osmotic balance and removes metabolic wastes from the body.
- Key metabolic wastes include carbon dioxide and nitrogenous wastes from protein breakdown, like urea.
- Several organs assist excretion: skin (water/substance loss), liver (detoxifies/produces urea), and lungs (remove COâ‚‚).
- The urinary system includes kidneys, ureters, bladder, and urethra.
Structure and Function of the Urinary System
- Kidneys filter blood and produce urine.
- Each kidney contains about one million nephrons, their functional units.
- Urine flows from kidneys through ureters to the bladder, and out via the urethra.
Nephron Anatomy and Filtration Process
- The nephron's first part, the glomerulus, is a capillary cluster surrounded by Bowman’s capsule.
- Blood pressure forces fluid (filtrate) from the glomerulus into Bowman’s capsule.
- Filtrate includes water, glucose, amino acids, salts, ions, medications, vitamins, and urea.
Substances Movement in the Nephron
- Some filtrate components are reabsorbed into the body from the nephron, while others are secreted into it.
- Transport occurs via diffusion, facilitated diffusion (no ATP), or active transport (requires ATP).
Nephron Segments and Functions
- Proximal tubule: reabsorbs NaCl, water, glucose, amino acids, potassium, and bicarbonate; secretes H+ and ammonium.
- Loop of Henle: descending limb reabsorbs water; ascending limb reabsorbs NaCl through passive (thin) and active (thick) transport.
- Distal convoluted tubule: reabsorbs NaCl, water, and bicarbonate; secretes H+, ammonium, and potassium.
- Collecting duct: reabsorbs NaCl and water (amount regulated by hormones), and some urea; final urine concentration is determined here.
Urine Elimination and Regulation
- Urine moves from nephron to ureters, bladder, and out via the urethra.
- Hormones adjust water reabsorption in the collecting duct, affecting urine concentration.
- Diuretics increase water in urine and are used for certain medical conditions.
- Dialysis mimics kidney function for people with renal failure.
Key Terms & Definitions
- Excretory system — system responsible for removing wastes and maintaining osmotic balance.
- Osmotic balance — equilibrium between water and solute concentrations in the body.
- Nephron — functional unit of the kidney, filters blood and forms urine.
- Filtrate — fluid filtered from blood into the nephron.
- Reabsorption — movement of substances from filtrate back into the body.
- Secretion — transfer of substances from blood into nephron filtrate.
- Diuretics — medications that increase urine production.
- Dialysis — process that artificially filters blood when kidneys fail.
Action Items / Next Steps
- Review nephron structure and the flow of filtrate through each segment.
- Study how hormones regulate water balance in the collecting duct.
- Further reading on types of transport across nephron membranes.