September 24, 2014
Functions of the kidney
1. elimination of metabolic wastes (esp. nitrogen)
Why?
Your "metabolism" includes all the reactions that take place
in your body. In some of those reactions, the liver turns
excess or worn-out proteins into carbs we can "burn" for
energy. This process releases ammonia -- pretty toxic!!
Fortunately, the liver can turn ammonia into urea, which is
less toxic.
2. homeostasis balance of ions and pH
pH: Many processes depend on the right level of ions and pH.
Many of our metabolic reactions tend to lower pH so our kidneys
need to remove some H+ ions. If the blood pH gets too high (basic),
we secrete fewer H+ ions.
Also, acidic urine is good because it keeps bacteria from growing
and solutes from precipitating (kidney stones!!)
ions: We need a steady supply of ions such as Na+ and
and K+ to make our bodies work correctly. But if we have too high a
concentration of ions in our blood, it will pull water out of our cells by
osmosis and make our blood volume and pressure go up.
September 24, 2014
3. fluid balance -- Too little water leads to dehydration. Too much can
lead to dilution of vital ions to the point that a person dies.
4. helps maintain blood pressure homeostasis -- maintained through
level of blood volume
blood pressure too high -- damage to organs, threat of stroke or heart
trouble
blood pressure too low -- fainting, other effects of not enough blood
reaching the brain
How does the kidney's form help it carry out its function?
step 1: filtration
* happens at Bowman's capsule/glomerulus
* The glomerulus capillaries have twice the pressure of the average capillary. This high pressure
pushes out into the Bowman's capsule the blood's plasma and everything dissolved in it:
water
salts/minerals/vitamins
urea
glucose
amino acids
hormones
* By controlling the size of the artery that lets blood out of the glomerulus (see below), the body can
change the pressure in the capillaries and thus how much plasma is pushed out
* glomerulus surface area = surface area of your skin!!! That's a good thing because your 5 liters of
blood are filtered 60 times a day!
September 24, 2014
If the kidney stopped here we would be in trouble. We would lose all our plasma in a half
hour, along with all the good stuff dissolved in it. Fortunately, the kidney is good at
reabsorption of important substances back into the blood (peritubular capillary)
Step 2: Reabsorption
Most of this takes place in the proximal tubule.
The proximal tubule reabsorbs:
2/3 of water in filtrate
glucose, amino acids, vitamins
some urea
salts, minerals -- including potassium, which is important for muscles & nerves
How does it do this?
a. surface of tubule is folded (microvilli) -- more surface area for more reabsorption
b. sodium-glucose pump -- pulls back glucose and sodium (Na+) at same time,
Cl- follows the positive charge (facilitated diffusion and diffusion)
* There is a limit to how much glucose we can reabsorb. People with high levels of
blood glucose will not get it all back from the filtrate, so they will have glucose
in their urine. This allows for an easy test for diabetes.
* Most people do not reabsorb all the sodium from the filtrate, so we need a daily
supply of salt. The amount we reabsorb is genetically controlled, so some need more
salt than others. If you eat too much salt and recover most or all of it from the filtrate, the
blood's salt level will go up. This will pull water into the blood by osmosis. Blood volume will
increase and blood pressure will go up. If you eat too little salt or do not recover enough, the
opposite happens and blood pressure can go too low.
c. other active transport mechanisms recover rest of substances listed above
September 24, 2014
What happens at the Loop of Henle?
a. descending limb -- goes down into the medulla which has a high salt content
-- filtrate loses water by osmosis, NaCl diffuses in
b. ascending limb -- impermeable to water, pumps more salt into medulla (from
the filtrate) so that:
* medulla stays salty
* filtrate won't be so salty that it pulls in water by osmosis when it reaches
the distal convoluted tubule
What happens at the distal convoluted tubule and collecting duct?
* Hormones control how permeable the cells are to water, this allows body
to control how much water is reabsorbed from filtrate when collecting duct
takes it back down into the salty medulla
* The kidney also pumps Na+ from the filtrate in the distal tubule as needed
for electrolyte balance.
Step 3: secretion
* takes place throughout the nephron
* happens by active transport -- from blood to filtrate
at proximal tubule: capillary secretes H+ (to regulate pH), uric acid, drugs such as aspirin and
penicillin
at distal tubule:
capillary secretes K+ for electrolyte balance
at collecting duct: capillary secretes H+ for pH regulation