Human Excretory System Overview

Overview

This lecture introduces the human excretory system, focusing on the kidneys and the nephron, explaining their structure and function in waste removal and osmotic regulation.

Introduction to the Excretory System

• The excretory system removes metabolic wastes and maintains osmotic balance (water and solute levels) in the body.
• Major metabolic wastes include carbon dioxide and nitrogenous wastes from protein breakdown.
• Organs involved include skin, liver, lungs, and especially the kidneys.

Organs of the Excretory/Urinary System

• Kidneys (two) filter blood and produce urine; located in the lower back.
• Ureters (two) transport urine from kidneys to bladder.
• Bladder (one) stores urine.
• Urethra (one) expels urine from the body.

The Nephron: Structure and Function

• Nephron is the functional unit of the kidney (about a million per kidney).
• Blood enters the nephron through the glomerulus, a capillary cluster, surrounded by the Bowman’s Capsule.
• Blood pressure forces fluid (filtrate) into Bowman’s Capsule containing water, salts, glucose, amino acids, H+, bicarbonate, vitamins, medications, and urea.

Filtrate Processing in the Nephron

• Proximal Tubule: NaCl, water, glucose, amino acids, K+, and bicarbonate are reabsorbed; H+ and NH4+ are secreted for pH regulation.
• Loop of Henle: Descending limb allows water reabsorption (via aquaporins); ascending limb reabsorbs NaCl—first by diffusion, then by active transport.
• Distal Convoluted Tubule: Further secretion (H+, NH4+, K+) and reabsorption (NaCl, water, bicarbonate) occurs, also regulating pH.
• Collecting Duct: NaCl and water are reabsorbed (hormonal control affects water); urea also partly reabsorbed; filtrate becomes urine.

Urine Formation and Excretion

• Urine travels from collecting ducts through ureters to the bladder, then out via the urethra.
• The process is complex and regulated by hormones and osmotic gradients.

Clinical Connections

• Diuretics increase water in urine, used to treat high blood pressure and heart failure.
• Dialysis (hemodialysis or peritoneal) is used when kidneys fail to filter blood.
• Nephrologists specialize in kidney health and disorders.

Key Terms & Definitions

• Osmotic balance — regulation of water and solute levels in the body.
• Metabolic waste — unwanted byproducts of metabolism, such as urea and CO2.
• Nephron — kidney's functional unit responsible for filtering blood and forming urine.
• Filtrate — fluid filtered from blood in the nephron.
• Reabsorption — substances move from filtrate back to body fluids.
• Secretion — substances added from body fluids to filtrate.
• Diuretic — medication that increases urine production.
• Dialysis — artificial blood filtration for kidney failure.

Action Items / Next Steps

• Review nephron structure and functions of each part.
• Explore types and effects of transport (diffusion, active transport) in nephron processing.
• Consider reading more on kidney diseases and nephrologist careers.