EXCRETORY SYSTEM

Regulation of water balance and other blood substances
Elimination of cellular (metabolic wastes)
Wastes include: CO2 and H2O (cell respiration) and nitrogenous compounds (protein digestion)

Lungs remove CO2 and some water

Skin secretes (perspiration) some water and minerals
Diagram -- Excretion- skin structure
Kidneys remove most water and nitrogenous wastes
Nitrogenous wastes include ammonia
Liver converts blood ammonia to urea
Urea removed by kidneys
Stored in bladder as urine
Urinary System
Diagram -- Excretion- urinary system
Diagram -- 1--Urinary System
Diagram -- 2--Urinary System

Kidneys

2 bean-shaped excretory organs
Located small of back (behind stomach and liver)
3 main parts
Cortex (outermost) - 1/3 tissue mass
Medulla - innermost 2/3 of kidney
Renal pelvis - funnel-shaped structure in center of kidney
Kidney
Diagram -- Excretion- kidney function
Diagram -- Excretion- kidney structure 1
Diagram -- Excretion- kidney structure 2

Nephrons

Functional unit of kidney (more than a million)
Bowman's capsule
Cup shaped structure
Encloses a bed of capillaries (glomerulus)
Receives blood from renal artery
Blood forced through capillary wall into Bowman's capsule under high pressure
Filtrate flows through renal tubule (permeable walls)
Network of capillaries wrap around renal tubules
Composition of blood and filtrate modified at various points along the length (proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct) involving filtration, reabsorption and secretion
Filtration
Blood plasma (1/5) forced out of glomerulus into Bowman's capsule
Pressure forces water, nitrogenous wastes, glucose and minerals -> walls of tubules -> renal tubule
Reabsorption
Filtrate -> capillaries
Most reabsorption occurs in proximal convoluted tubule
75% water returned by osmosis
Glucose, Na+, K+ and Ca++ by active transport
Additional reabsorption occurs in distal convoluted tubule
Secretion
Some substances in blood -> filtrate in distal convoluted tubule
Blood pH adjusted by secretion of H+ ions into filtrate
Loop of Henle
Maintains high concentration of NaCl in extracellular spaces of medulla
Cl- ions actively transported from filtrate
Na+ follows negatively charged Cl- ions
Promotes reabsorption of water
Concentration
Urine from several tubules flow into collecting ducts
Urine concentrated by osmosis in collecting duct due to high NaCl concentration in extracellular fluid
99% of water returned to filtrate
Urinary system
Collecting ducts -> ureter-> urinary bladder -> urethra

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Body Quest - Excretory
Innerbody -Excretory -1
Innerbody- Excretory- 2
St. Mary's Biology - Excretory
The Mississippi School for Mathematics and Science - Urinary System
On-Line Biology Book - The Excretory System
The Garbage Collector - The Excretory System
Kidney Information Center - A guide to kidney disease, dialysis, transplantation, and donor issues
Emily's Road Map to the Human Body - Excretory System
The National Kidney and Urologic Diseases Information Clearinghouse (NKUDIC)  
Kidney & Urologic Health

 

  Encarta Intro Edition     Encarta (MSN) used as a reference for many of the articles 

Diagrams modified from Science on File CD