Formation of Urine7.5

Filtration: movement of fluids from blood into the Bowman’s Capsule

Reabsorption: transfer of essential solutes and water from nephron back into blood

Secretion: movement of materials from blood back into the nephron.

Three functions urine formation depends on:

Each nephron has independent blood supply.

Afferent arteriole glomerulus (high-pressure filter) selected dissolved solutes pass into Bowman’s capsule◦ Normal Capilary bed pressure: 25 mm Hg◦ Pressure in glomerulus: 65 mm Hg

Filtration

Plasma protein, blood cells, and platelets are too ___________.

Selectiveness of the Bowman’s Capsule

120 mL of fluid is filtered into the nephrons every minute◦ If reabsorption did not occur, we would form 120

mL of urine each minute. Need to consume at least 1 L of fluid every 10 minutes for homeostasis.

◦ Only 1/120 mL of fluid forms urine.◦ Remaining 119 mL of fluids and solutes

reabsorbed.

Reabsorption

PROXIMAL TUBULE Occurs by active transport.

◦ Carrier molecules: move Na+ across cell membrane (out of proximal tubule) Negative ions: Cl-, HCO3-, follow Na+ by charge

attraction. ◦ Glucose + AAs attach to specific carrier molecules,

shuttling them out of the nephron. Amount limited.

Threshold level: maximum amount of material that can be moved across a nephron (energy limitations/substrate saturation). ◦ Excess NaCl remains in nephron: excreted with urine.

Active Transport (selectivity)

Actively transported solutes (out of nephron) create osmotic gradient.◦ Draws water from nephron.

Proteins not absorbed by nephron (still in blood) create osmotic force◦ Draw water from interstitial fluid into the blood.

As water is reabsorbed FROM the nephron, solutes become more concentrated.

Urea and uric acid may diffuse back into blood stream, but less than was originally filtered.

Passive Transport (selectivity)

Movement of wastes from blood into nephron. ◦ Nitrogen-containing wastes, excess H+ ions, K+

ions. Cells loaded with mitochondria line the

distal tubule. ◦ Similar to reabsorption: tubular secretion occurs

by active transport◦ Different from reabsorption: molecules shuttled

from blood into nephron.

Secretion (distal tubule)

http://www.youtube.com/watch?v=vEXx5YLcGmQ&feature=related

Summary: Urine Formation

Page 352, #1-6.

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