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 highlow 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