Download The formation of urine

The formation of urine
Involves 3 functions:
• Filtration
• Reabsorption
• Secretion
From: http://wikis.lib.ncsu.edu/index.php/Group_1_A_Discussion_of_Kidney_Development_and_Abnormalities
Filtration
Definition: Blood passes through a selectively permeable
membrane.
• Each nephron has an independent blood supply
• Blood moves through afferent arteriole into glomerulus
(high pressure filter) then out through the efferent
arteriole
• Dissolved solutes (ex: ions, glucose, amino acids,urea)
pass through the walls of the glomerulus into the
Bowman’s capsule moving from highlow pressure
• The following are too large to move through the walls of
the glomerulus: plasma proteins, erythrocytes, platelets
http://www.hcc.uce.ac.uk/physiology/renalsystem.htm
Reabsorption (…don’t copy this)
• 600mL of fluid flows through human kidneys every
minute!.. 120mL of that is filtered in the nephrons…if
none of that were reabsorbed, you would create 120mL
of urine each minute and would have to drink 1L of water
every 10minutes to maintain homeostasis!!!
• Only 1mL of urine is formed for each 120mL filtered
through the nephron… the rest is reabsorbed
Reabsorption- continued!
Definition: transfer of glomerular filtrate from the nephron (proximal
tubule) back into the capillaries (in the cortex or medulla of the
kidney)
• Occurs by active AND passive transport (numerous mitochondria
supply energy for active transport)
• Carrier molecules move Na+ ions across membranes of the cells
lining the nephron
• Negative ions (Cl-, HCO3) follow positive ions by charge attraction
• Reabsorption occurs until the threshold level of a substance is
reached (max. amount of material that can be moved across the
nephron)
• Excess (NaCl for example) remains in the nephron and is excreted
with the urine.
Reabsorption continued:
• Glucose and amino acids use carrier molecules to
shuttle them out of the nephron into the blood.
• Threshold level applies, so high blood glucose causes
excretion of sugar in urine
• High amount of solutes being transported out of the
nephron (and proteins remaining in bloodstream) creates
osmotic gradient drawing water out of the nephron.
(occurs in Loop of Henle)
• As water is reabsorbed from the nephron along Loop of
Henle, remaining solutes become more concentrated.
Then more NaCl is lost from the filtrate.
Secretion:
• The movement of wastes from the blood into the
nephron
• These include: nitrogen-containing wastes, H+
ions, some drugs (penicillin)
• Cells loaded with mitrochondria line the distal
tubule and tubular secretion occurs by active
transport… but unlike reabsorption, molecules
are shuttled from the blood into the nephron.
From: http://www.colorado.edu/intphys/Class/IPHY3430-200/017kidneys.htm