Urine Formation

urine contains
waste, excess water, and electrolytes
Glomerular Filtration
-first step in filtration for urine
- water and other small molecules and ions are filtered out of the glomerular capillary plasma into the glomerular capsule
-its an autoregulated system
-certain conditions can override this system like the ANGIONTENSIN SYSTEM
Glomerular Filtration produces how much filtrate
180 L of fluid every 24 hours.
125mL/ min
all plasma is filtered about to 6 times per day
Tubular Reabsorption
process by which kidneys reclaim the just the right amount of water, electrolyte and glucose that the body requires
-this substances that are transported from the tubular fluid (fluid flowing down the renal tube) and flows into the peritubular capillary
-occurs in the proximal convoluted portion of the renal tube
Tubular Secretion
process by which the kidneys eliminates substance like hydrogen ions, certain toxins
-substances move from the plasma of the peritubular capillary into the fluid of the renal tubule
Proteins do not get filtered in the glomerular filtration process because:
they are to big to pass through
what makes the walls of the glomerular capillaries so permeable:
Fenestrae- they are tiny openings
What type of pressure moves substances through the glomerular capillary
hydrostatic pressure of the blood inside
what influences glomerular filtration:
-colloid osmotic pressure of the blood plasma in the glomerulus (inward)
-hydrostatic pressure inside the glomerular capsule (inward)
-glomerular hydrostatic pressure (outward)
Colloid Osmotic Pressure of Plasma
-is caused by the plasma proteins (will always be higher then the glomerular filtrate)
-it draws water back into the glomerular capillaries opposing filtration
Hydrostatic Pressure of the Glomerular Capsule
- when it increases in pressure it opposes filtration of fluid keeping it in the glomerular capillaries
Glomerular hydrostatic pressure
-moves the substances through the capillary wall and into the glomerular capsule
-most important factor determining net filtration pressure
-its the highest hydrostatic pressure in any capillary
Net Filtration Pressure
-is positive
-favors filtration at the glomerulus
- directly porpotional to Glomerular filtration rate
How can afferent and efferent arterioles affect Net Filtration Pressure?
- a change in the diameter size of these arterioles will change the glomerular hydrostatic pressure thus affecting the NFP
Vasoconstriction of Afferent Arteriole in response to the sympathetic nerve stimulation causes:
-NFP in the glomerulus to decrease and thus filtration rate goes down since there is not enough blood entering the glomerulus
-there is an appropriate decrease in rate of urine fomration when the body must conserve water
Vasoconstriction of the Efferent Arteriole causes:
-NFP in the glomerulus to increase, the rate increases since there is no blood leaving the glomerulus (blood is stagnant)
the filtration rate increases
What may physical factors may affect the Glomerular Capsule Hydrostatic Pressure?
-obstruction like a stone in a ureter or an enlarged prostate gland pressing on the urethra
-this factors will cause fluid back up into the renal tubules and raise the hydrostatic pressure in the glomerular capsule opposing filtration
What happens when BP and Volume of Blood drops?
-There is Vasoconstriction of the Afferent Arterioles
What happens when there is excess body fluid?
there is vasodilation of the afferent arterioles increasing filtration pressure and GFR
-its an enzyme released by the Juxtaglomerular cells of the afferent arterioles
1.its released in response to sympathetic nerves and RENAL BARORECEPTORS
- all of this factors stimulate the secretion of renin in response to a decrease in BP
2. Macula Densa also controls renin secretion
-cells in this region sense the [Na, K, Cl] ions in the distal renal tubule
-when the levels of these ions decreases it stimulates the secretion of renin
Renal Baroreceptors
-are found in the afferent arterioles
Renin + Angiotensinogen =
Angiotensin I
ACE (angiotensin-converting enzyme)
converts angiontensin I into ANGIOTENSIN II
Angiotensin II
-is a vasocontrictor thus affecting both the afferent and efferent arterioles
-it aids in balancing Na, water and BP
-stimulate increase in aldosterone- it stimulates Na reabsorption in the Distal Convoluted Tube
-stimulates ADH secretion helping retain water
Atrial Natriuretic Peptide (ANP)
-its secreted when the atria of the heart stretches due to increase in Blood Volume
-its stimulates Na excretion
Peritublar Capillary Blood Pressure is?
Low Pressure
What is the important of microvili in the renal tube?
-it increases the surface area which is exposed to the glomerular filtrate thus enhancing reabsorption
Glucose is reabsorbed by using what process and where ?
-takes place in the proximal convoluted tube
-active transport (which requires ATP) using a mechaninsm which has a LIMITED TRANSPORT CAPACITY
Water is reabsorbed by using what process and where?
-takes places in the proximal convoluted tube
-its transported by osmosis
-water moves from hypotonic to hypertonic
Why would portions of the distal convoluted tube and collecting duct be impermeable to water?
-its essential to the help regulate urine concentration and volume
Renal Plasma Threshold
this occurs when there is a really high concentration of glucose in the filtrate and its more then what the active transport mechanism can handle
-as a result some glucose will remain in the filtrate and be excreted in the urine
Diabetes Mellitus
-occurs in diabetic patients when they have to much glucose in the urine
glucose in the urine is glucosoria
increase in urine volume
osmotic diuresis
occurs when non-reabsorbed glucose in the tubular fluid increases the osmotic concentration of the tubular fluid which in turn reduces the volume of water reabsorbed by osmosis from the proximal tube and collecting duct
amino acids are reabsorbed where?
at first amino acids enter the glomerular filtrate but then they are reabsorbed while in the proximal convoluted tubule by the peritubular capillary
is so small that sometimes it enter the glomerular filtrate however through ENDOCYTOSIS this protein ire moved inot the peritubular capillary while in the proximal convoluted tube
How is Na reabsorbed and where does it occur?
-Reabsorption of Na occurs in the proximal convoluted region of the renal tube
-Its transported by the Sodium Pump Mechanism (which is a form of active transport)
What other molecules move through the tubular wall along with Na?
Negatively Charged ions move across the wall:
1. Cl (chloride ions)
2. HCO3 (bicarbonate ions)
3. PO4 (phosphate ion)
When is an Isotonic state reached in the renal tube?
this occurs at the end of the proximal convoluted tube
active transport and the rest of the tube
active transport continues to reabsorb Na as tubular fluid moves through the nephron loop and the distal convoluted tube and collecting duct
-about 97-99% of Na and water have been reabsorbed
The affect of Aldosterone on Na?
-its cause for Na to be reabsorbed at the Distal Convoluted Tube help keep constant balance of fluid
The affect of Anti-diuretic on water?
-its causes water to be retained
-allows water to be reabsorbed in the distal convoluted tubule and the collecting duct
Reabsorption of Na and water?
will always go hand in hand
-if Na is reabsorbed so will water be reabsorbed
Example of Tubular Secretion
Epithelial cell of the proximal convoluted tube secrete organic compounds like penicillin and histamine into the tubular fluid
Hydrogen Ions Secretion
-H ions are secreted through out the renal tube
-this aids in regulating the the pH level of body fluid
-so urine is acidic as a result of this secretion
K Secretion
-most K ions are reabsorbed in the proximal convoluted tube but
-some K ions are secreted in the distal convoluted tube and in the collecting duct
What makes possible the secretion of K into the Distal convoluted tube?
Active transport of Na out of the tubular fluid under the influence of Aldosterone produces a negative electrical charge in the tubule which then in turns attracts positively charged ions like K into the tubule
What happens when there is no ADH?
water is kept inside the renal tubule which is then excreted in the urine producing a dilute urine
Why is ADH released?
-ADH is released in response to decreasing concentration of water in the body fluid or to blood volume or low BP
What happens when there is ADH present?
its stimulates cells in the distal convoluted tube to be permeable to water so it can be reabsorbed into pertibular capillary causing urine to be concentrated
countercurrent mechanism
is found within the nephron loops specially in the the juxtamedullary nephrons
-this mechanism ensures that that medullary interstitial fluid becomes hypertonic
-its occurs as a result that the fluid descending down creates a current counter to the current created by the fluid ascending
ascending limb in the countercurrent mechanism
is removing Na and Cl ions from the tube into the interstitial fluid causing the fluid to become hypertonic while the fluid inside the tube is hypotonic
descending limb in the countercurrent mechanism
is permeable to water so water tends to leave the descending limb so the fluid in the tubule becomes hypertonic while the the interstitial fluid is hypotonic.
how the countercurrent mechanism
1. so when the hypertonic fluid from the descending limb move into the ascending limb NaCl is actively reabsorbed into the medullary interstitial fluid further raising the concentration of solutes making the fluid more hypertonic
2. As a result from the increase in concentration of solutes in the medullary interstitial fluid more water flows out of the descending limb increasing the hypertonicity of the fluid inside the limb
what is the solute concentration near the top of the ascending loop?
it is 4 times more then the solute concentration of plasma
vasa recta
another countercurrent mechanism by which food is brought into nephrons
- by product of amino acid catabolism in the liver
-enters the renal tubule by filtration and is reabsorbed and secreted by different portions of the renal tube
-it contributes to the reabsorption of water
uric acid
-its a product of the metabolism of certain nucleic acid bases
-active transport reabsorbs the filtered uric acid
-only 10% enter the urine
What happens when there is an increase in K ion concentration in renal tubule?
1. Adrenal cortex is signal
2. Aldosterone is secreted
3. Renal Tube increase absorption of Na and increase secretion of K ions
4. Na ions are conserved