FORMATION OF URINEThe formation of urine occurs in three separate steps.

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FILTRATIONThe movement of fluids from the blood into the Bowman's capsule of the nephron.

Blood moves from the afferent arteriole into the glomerulus (a high-powered filter, acting at 65 mm of Hg, ~1.5 times the normal pressure of a capillary).Dissolved solutes pass from the blood into the Bowman's capsule along the pressure gradient. Not all solutes make it through.

DO: DON'T (too large)Water -Plasma proteins

Sodium chloride -Red blood cells

Glucose -Platelets

Amino acids

protons

REABSORPTION The transfer of essential solutes and most water back into the surrounding cells

and the blood stream. For every 120 mL of fluid (amount filtered by kidneys each minute), 1 mL of urine

is formed, 119 mL of fluid and solutes is reabsorbed. Occurs throughout the nephron, mainly in the proximal tubule and loop of Henle Occurs by active and passive transport.

Passive: water by osmosis (*more on this on next slide), K+ by diffusion (proximal tubule)

Active: NaCl (Na+ is pumped from the nephron into the surrounding cells, Cl-

follows by diffusion, major site of NaCl reabsorption is the loop of Henle, then the distal tubule and collecting ducts),

HCO3

- (passive initially in proximal tubule, active by the distal tubule) Glucose, Amino acids (proximal tubule) There is maximum amount that can be reabsorbed by active transport

due to the amount of energy available, this is called the threshold level – the rest will be excreted in the urine.

**NOT reabsorbed: uric acid, urea, excess solutes above the threshold level.

Water is automatically reabsorbed back into the cells and bloodstream for 2 reasons:

The blood that left the glomerulus by the efferent arteriole travels through the peritubular capillaries. It has a very high concentration in plasma proteins and is very low in water, therefore it draws water from the nephron and surrounding cells into the blood.

Then, as solutes (K+, NaCl, etc) are drawn out of the nephron into the cells surrounding the nephron they create an osmotic gradient. Their concentration becomes high in the cells and low in the filtrate.

Therefore, water moves out of the nephron into the surrounding cells once more, again by osmosis.

As water gets reabsorbed, the solutes left in the nephron become quite concentrated and small amounts of some, like urea and uric acid will diffuse back into the blood, however most of the urea and uric acid will be left to form urine.

Water being reabsorbed in the proximal tubule (65%) and descending loop of Henle (25%) is said to be obligatory because it is due to osmosis.

Water reabsorption continues in the collecting duct and is called facultative because it is under control of the hormone ADH (next class).

The stuff left in the nephron that is not reabsorbed is the filtrate.

WATER REABSORPTION

SECRETIONThe release of substances into the filtrate from the blood and cells surrounding the nephron.Occurs mainly in the distal tubules and in the collecting duct, but some also occurs in the proximal tubule.Purpose:

to release any toxins and drugs that have not been filteredMaintain the electrolyte balance of the body (if positive sodium ions are reabsorbed then positive ions like potassium must be secreted to keep the balance – even though reabsorbed intially).Acid-base balance (usually it is an acid being secreted, essentially a proton plus whatever it is attached to).

The bicarbonate ion is never secreted since it is used as a buffer in the maintenance of our blood pH.Acidic juices like cranberry cause our urine to be quite acidic which helps protect against UTIs and prevent kidney stones. Hmwk: Read section 7.5 and answer questions on p 352 #2-7