11.3 – the kidney. excretion: the process of removing metabolic waste from the cells, tissue...
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Human Health & Physiology
11.3 – The kidney
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Kidney Structure & Function
Excretion: the process of removing metabolic waste from the cells, tissue fluid, and blood of living organisms The main organ of excretion in mammals
is the kidneyOsmoregulation: the control of
water balance of the blood, tissue or cytoplasm of a living organism
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Kidney Structure & Function Functions:
Produces urine Maintain water balance Maintain blood pH Maintain blood pressure
The functional unit of the kidney is the nephron
There are more than 1 million nephrons in a human kidney
On average 120mL/min of fluid passes through the kidney
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Kidney Structure & Function Roles of the nephron:
Ultrafiltration Reabsorption Secretion
For the kidney you should be able to label: Cortex Medulla Pelvis Ureter Renal blood vessels
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Nephron Structure & Function
Afferent arteriole: brings blood into the glomerulus from the renal artery
Efferent arteriole: takes blood out of the glomerulus into the surrounding capillary network and then into the renal vein
Glomerulus: a ball of capillaries that are fenestrated (have pores) and are surrounded by a basement membrane that filters what passes through into the filtrate
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Nephron Structure & Function
Bowman’s capsule: a cup shaped structure at the end of the nephron that surrounds the glomerulus and collects the filtrate
Proximal convoluted tubule (PCT): lined with microvilli to increase surface area and has many mitochondria to provide ATP for active transport
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Nephron Structure & Function
Loop of Henle: carries the filtrate from the PCT to the DCT The loop of Henle descends into the
medulla of the kidney The concentration gradient of salt
increases as you move down the medulla of the kidney
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Nephron Structure & Function
Distal convoluted tubule (DCT): conducts urine from the loop of Henle to the collecting duct It is the final place where blood pH and
ions are balancedCollecting duct: collects urine and
carries it into the renal pelvis to the ureter This is where the final water balance of
the blood occurs
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Formation of urine
1. Blood enters the glomerulus through the afferent arteriole under high pressure
2. This forces water, amino acids, small proteins, glucose, and ions into the Bowman’s capsule
The product is the filtrate that enters the nephron
3. Filtrate flows into the PCT Glucose, amino acids, and ions are reabsorbed
into the bloodstream through active transport Small proteins are reabsorbed by pinocytosis
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Formation of urine
4. Filtrate flows into the descending loop of Henle
The descending loop is permeable to water but not to salt
5. The loop of Henle is hypotonic (higher salt/urea concentration outside) to the medullary fluid
Water moves out of the nephron by osmosis Therefore the filtrate becomes more
concentrated and hypertonic
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Formation of urine
6. The ascending loop of Henle is permeable to salt, but not to water
As the filtrate moves up the ascending loop, salt (Na+ and Cl–) moves out passively at first, then is actively pumped out at the top of the loop
7. Filtrate passes into the DCT where it is adjusted to balance blood pH (by secretion of H+) and ion composition
More water is also reabsorbed
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Formation of urine
8. Filtrate moves into the collecting duct, where water may be further reabsorbed if needed
This is controlled by anti-diuretic hormone (ADH)
9. ADH is secreted by the pituitary gland and increases the permeability of the DCT and collecting duct to water
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Formation of urine
10.If blood is low in water content, ADH is secreted and more water is reabsorbed from the collecting duct into the blood
Concentrated urine is formed and water is conserved
11.If blood is high in water content, ADH is not secreted and no more water is reabsorbed
Dilute urine is formed
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Comparing Solute Concentrations
Molecule Amount in blood plasma (mg/100
mL)
Amount in
glomerular
filtrate (mg/100
mL)
Amount in urine (mg/100
mL)
proteins > 700 0 0
glucose > 90 > 90 0
urea 30 30 > 1800Damon, A., McGonegal, R., Tosto, P., & Ward, W. (2007). Higher Level Biology. England: Pearson Education, Inc.
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Diabetes & Glucose in Urine
People with uncontrolled diabetes can have a large amount of glucose in their blood
Glucose enters the glomerular filtrate and is reabsorbed by active transport There is a maximum rate at which
reabsorption can occur If there is too much glucose in the
blood, reabsorption of all glucose from the glomerular filtrate cannot be achieved
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Nephron Structure & FunctionGlomerulus Filtration
Glomerular blood pressure forces some of the water and dissolved substances from the blood plasma through the pores of the glomerular walls
Bowman’s capsule
Receives filtrate from glomerulus
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Nephron Structure & FunctionProximal tubule
ReabsorptionActive reabsorption of all nutrients, including glucose and amino acidsActive reabsorption of positively charged ions such as Na+, K+, Ca2+
Passive reabosrption of water by osmosisPassive reabsorption of negatively charged ions such as chloride and bicarbonate by electrical attraction to positively charged ionsSecretionActive secretion of hydrogen ions
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Nephron Structure & FunctionDescending loop of Henle
ReabsorptionPassive reabsorption of water by osmosis
Ascending loop of Henle
ReabsorptionActive reabsorption of Na+ ionsPassive reabsorption of Cl–, K+
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Nephron Structure & FunctionDistal tubule
ReabsorptionActive reabsorption of Na+ ionsPassive reabsorption of water by osmosisPassive reabsorption of negatively charged ions such as Cl– and bicarbonateSecretionActive secretion of hydrogen ionsPassive secretion of K+ ions by electrical attraction to chloride ions
Collecting duct
ReabsorptionPassive reabsorption of water by osmosis
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References
1. Damon, A., McGonegal, R., Tosto, P., & Ward, W. (2007). Higher Level Biology. England: Pearson Education, Inc.
2. Raven, P.H., Johnson, G.B., Losos, J.B., Mason, K.A., & Singer, S.R. (2008). Biology. (8th ed.). New York: McGraw-Hill Companies, Inc.