I. Parts of the Urinary System
A. Overall function of the system
1. Eliminates most of the
waste products of metabolism
a. must also retain (reabsorb) molecules essential for normal body function
(e.g. glucose and amino acids)
b.
this elimination of waste products plus reabsorption of essential materials
is
accomplished by nephrons
located in the kidneys
(each kidney has about one million nephrons).
2. regulation of body fluids
3. balance of the pH and electrolyte composition of body fluids.
4. production of hormones,
including renin, which is important in regulation
of blood pressure.
B. Kidney - has 3 main sections
1. Renal Cortex - outer
region (most of the nephron is located here).
2. Renal Medulla - inner
region
a. columns - contains blood vessels
b. pyramids - contain loops of henle and collection ducts.
3. Renal Pelvis - hollow
basin where the urine produced by the nephrons passes
before leaving the kidneys.
C. Nephron - the functional unit of the kidney: a
tiny coiled tube with a "bulb" at the end
1. Glomerulus: cluster of
capillaries in
Bowman's capsule of the nephron
a. occurs at the end of the afferent arteriole, which supplies the glomerulus with blood
b.
efferent arteriole: drains the blood from the glomerulus and carries it
to the capillaries
surrounding the coiled tube of the nephron.
2. Bowman's capsule: the "bulb"; hollow capsule that surrounds the glomerulus
3. Proximal Convoluted Tuble
(PCT): proximal coiled portion of tube leading away from
Bowman's capsule.
4. Loop of Henle: a thin tube that carries filtrate from the PCT to the DCT
5. Distal Convoluted Tubule
(DCT): coiled portion at the distal end of the Loop of Henle
that carries fluid from the Loop of Henle to the Collecting Duct.
6. The DCTs empty into Collecting Tubules, which carry the fluid from the nephron tubules down into the renal pelvis, where it is stored until it leaves the kidney through the ureter.
II. Production of Urine (Renal Physiology)
A. Glomerular Filtration
1. walls of glomerulus permit
the free flow of water
and soluble materials
(e.g. blood cells and large protein molecules remaining in the blood)
2. the pressure in the glomerulus is 3-4 times higher than in other capillaries.
3. increased pressure in the glomerulus "squeezes" material out of the blood into Bowman's capsule.
4. Glomerular filtrate: the
fluid that enters the Bowman's capsule (contains waste products
like urea, and needed materials like glucose).
B. Tubular Reabsorption
1. occurs in the PCT
2. as the filtrate travels
thru the nephron, water
and other needed substances, such as
glucose and amino acids, leave the tubule by diffusion and active transport
and enter
the tissue fluids surrounding the nephron.
3. they then enter the blood and return to the circulation.
C. Tubular Secretion
1. occurs in the DCT
2. as the modified filtrate
continues thru the Loop of Henle and the DCT, more waste
products, like urea and excess H+
ions can be transported from the
blood into the tubule thru the walls of the DCTs.
3. from the DCT the fluid
then passes into the collecting
duct. This fluid is
now called urine.
4. the kidneys help regulate
the pH of the
blood by increasing or decreasing the
number of hydrogen ions
that are secreted into the tubules to be eliminated
with the urine.
5. needed materials are reabsorbed and do not normally occur in the urine.
III. Concentration of Urine
A. The amount of water that is eliminated with the
urine is regulated by a mechanism
inside the nephron. This mechanism is influenced by a hormone from
the Pituitary
Gland, called antidiuretic hormone
(ADH).
B. Loop of Henle
1. descending limb - permeable
to water
2. ascending limb - not permeable to water
C. the walls of the ascending limb actively transport
sodium ions out
of the Loop
and into the surrounding tissue.
1. Sodium ions have a positive
charge, attracting negatively charged chloride ions. Thus,
the chloride ions follow the sodium ions out of the Loop and into surrounding
tissue.
2. This results in the tissue
outside the Loop to have a very high solute concentrations
(lots of sodium and chloride ions).
3. because the fluid in the ascending limb loses solutes that fluid has a low solute concentration.
D. water cannot pass from the area of low solute
concentration (inside) to the area of high
solute concentration
(outside) because the walls of the ascending limb are not permeable
to water. However,
water can flow out of the descending limb from an area of low solute
concentration to an
area of high solute concentration.
E. the fluid that passes out of the Loop of Henle
---> DCT ---> Collecting Duct has a low
solute concentration. This means the urine is dilute-
containing lots of water
and little solute.
F. If our intake of fluids is low then the body needs
to conserve water. Our body can vary
the amount of fluid eliminated by the ADH mechanism.
1. Water levels in our body
are monitored by the hypothalamus.
This monitors
water levels by monitoring the solute
concentration of the blood
2. If solute concentration is low, then kidneys can eliminate water in the diluted urine. However, if solute concentration is high, then kidneys need to conserve water.
3. When water levels in the body are low, the hypothalamus stimulates the release of ADH into the blood.
4. ADH makes the walls of the DCT and collecting tubule more permeable to water so that more water will be reabsorbed and less will be excreted with urine.
5. When the body is trying
to conserve water our urine is less dilute and more solute
concentrated.
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