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Excretory System

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Outline

• The Mammalian Kidney• Transport Processes in Mammalian Nephron• Ammonia, Urea, and Uric Acid• Hormones Control Homeostatic Functions

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Vertebrate Kidney

• Kidney is made up of thousands of repeating units (nephrons), each with the structure of a bent tube.

– Blood pressure forces the fluid in blood past a filter, glomerulus, at the top of each nephron.

Water and small molecules pass through filter and into the nephron tube.

Sugars and ions are removed by active transport.

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Vertebrate Nephron Organization

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The Mammalian Kidney

• Each kidney receives blood from a renal artery, and produces urine.

– Urine drains from each kidney through a ureter which carries urine to urinary bladder.

– Within the kidney, mouth of ureter flares to form renal pelvis.

Divided into renal cortex and renal medulla.

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Urinary System of a Human Female

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The Mammalian Kidney

• Nephron structure and function– Blood is carried by an afferent arteriole to

the glomerulus. Blood is filtered as it is forced through

porous capillary walls.Glomerular filtrate enters Bowman’s

capsule. Moves to the proximal convoluted

tubule.

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The Mammalian Kidney

• Fluid then moves down the medulla and back into the cortex in a loop of Henle.

– After leaving the loop, the fluid is delivered to a distal convoluted tubule in the cortex that drains to a collecting duct.

merges with other collecting ducts to empty its contents into the renal pelvis

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Mammalian Kidney Nephron

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The Mammalian Kidney

• Reabsorption and secretion– Most of the water and dissolved solutes

that enter the glomerular filtrate must be returned to the blood.

– Reabsorption of glucose and amino acids, is driven by active transport carriers.

– Secretion of waste products involves transport across capillary membranes and kidney tubules.

• Excretion

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Fig. 49.18(TE Art)Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Renal tubuleExcretion

Filtration

Reabsorption to blood

Secretion from blood

Glomerulus

Bowman'scapsule

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Transport Processes in the Mammalian Nephron

• Some mechanism is needed to create an osmotic gradient between the glomerular filtrate and the blood, allowing reabsorption.

• Proximal convoluted tubule– Approximately two-thirds of NaCl and

water filtered in Bowman’s capsule is immediately reabsorbed across the walls of the proximal convoluted tube.

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Transport Processes in the Mammalian Nephron

• Loop of Henle– Descending limb is permeable to water, thus

water leaves via osmosis.– Water loss in the descending limb multiples

concentration achieved at each loop.– Ascending limb actively extrudes N+ and Cl

follows.– NaCl pumped out of ascending limb is

trapped within surrounding interstitial fluid.– countercurrent multiplier system

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Transport Processes in the Mammalian Nephron

• Distal tubule and collecting duct– Permeability of the collecting duct to water

is adjusted by antidiuretic hormone (ADH - vasopressin).

Kidneys also regulate the balance of electrolytes in the blood by reabsorption and secretion.

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Reabsorption of Salt and Water

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Hormones Control Homeostatic Functions

• Antidiuretic hormone– Stimulates reabsorption of water by the

kidneys.

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Hormones Control Homeostatic Functions

• Aldosterone– Promotes reabsorption of NaCl and water

across the distal convoluted tubule and the secretion of K+ into the tubule.

• Atrial natriuretic hormone– decreases NaCl reabsorption

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Fig. 49.22(TE Art)Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Low bloodpressure

Bowman'scapsule

Distalconvolutedtubule

Glomerulus

Afferentarteriole

Efferentarteriole

Loopof Henle

Increased NaCland H2O

reabsorption

Increasedblood

volume

Negativefeedback

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Low bloodflow

Proximalconvolutedtubule

Renin

Adrenalcortex

Angiotensinogen

Angiotensin II

3Juxtaglomerularapparatus

Kidney

Aldosterone

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