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The Excretory System: Urine Formation
… eww
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The Excretory System
• Mammals must excrete many different wastes in order to remove toxic substance from the body
• The wastes form as a result of the metabolic processes of the body and degeneration of cells (ie. urea, ammonia, CO2, lactic acid, etc.)
• Much of the excretory system flows through the kidneys
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Kidneys
• Part of excretory system, controlled by nervous and endocrine system
• Weighing ~0.5 kg each, kidneys may hold as much as 25% of the body’s total blood volume at any given time
• Involved in the production of urine
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KidneysStabilize internal environment by:• regulating water content (osmoregulation)• eliminating poisonous by-products of biological reactions• regulating ion concentration
Mechanisms of Regulation:• diffusion/osmosis• forced filtration• active transport
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Nephron
• Each kidney contains over 1 million slender tubules called nephrons
• The functional units of the kidney
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Nephron Structure
Glomerulus:• High-pressure capillary
bed• The site of blood filtration
Bowman’s Capsule:• Funnel-like structure• ‘Beginning’ of the nephron
Each nephron has its own independent blood supply (renal arterioles and veins)
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Nephron Structure
Components not entering capsule:
• Blood proteins• Blood Cells• Platelets
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Nephron Structure
Proximal Tubule:• Re-absorption of
molecules important to the blood
Loop of Henle:• Concentration of
urine (solutes to be excreted)
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Nephron Structure
Distal Tubule:• Final re-absorption
and secretion of molecules
Collecting Duct• Collection of urine
from multiple nephrons
• Joins with the ureters; ultimately the bladder
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Urine Formation
Accomplished by 3 functions of the nephron:
Forced filtration of blood from the afferent arteriole into the glomerulus, and through Bowman’s capsule under high pressure
• proteins, blood cells and platelets are too large to pass through & stay in the blood
• sodium chloride, glucose, amino acids, ions enter the nephron
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Urine FormationRe-absorption of most fluids and solutes from the
filtrate back into the blood• for every 120 mL of fluids filtered, only 1 mL becomes
urine• many solutes are removed from the nephron into the
capillary beds surrounding the convoluted tubules• passive transport moves Na+, Cl- and HCO3
• active transport moves glucose and amino acids• an osmotic gradient is created by the solutes in the
capillary bed, drawing water out of the nephron (in all tubules but the ascending Loop of Henle)
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Urine Formation
Secretion moves wastes from blood into the distal tubule of the nephron
• Urea, uric acid, ammonia, excess ions, minerals, drugs, etc. moved by active transport
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How it all works:
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How it all works:
YouTube Link:
http://www.youtube.com/watch?v=vNvZaGcLzEo
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Lab Exercise
Comparing Solutes in the Plasma, Nephron & Urine
• Activity: 7.5.1 - p.363