renal system Flashcards
Which metabolic waste products are excreted by the kidneys?
urea, uric acid, creatine, end products of hemoglobin
What are the main functions of the kidneys?
Regulation of water, ion balance, acid-base balance
Removal of metabolic waste products
Removal of foreign chemicals
Gluconeogenesis (synthesis of glucose from amino acids)
Production of hormones/enzymes (erythropoietin, renin, 1,25-dihydroxyvitamin D)
Flow of urine?
kidneys > ureters > bladder > urethra
Which blood vessles are at the hilum?
renal artery (perfusion)
renal vein (draining)
The ureter is formed from the ________, which are funnel-shaped structures that drain urine into the
__________, from which the urine enters the ureter
calyces
renal pelvis
Which two areas is are the kidneys divided into?
outer renal cortex
inner renal medulla
The connection between the tip of the medulla and the calyx is called the _________.
papilla
Each nephron is composed of:
an initial filtering component called the renal corpuscle
+ a tubule that extends from the renal corpuscle
Where does the fluid remaining at the end of each nephron go?
Combines in the collecting ducts + exits the kidneys as urine
Where does the glomerulus get its blood from and where does the blood exit?
An afferent arteriole
The efferent arteriole
Where does the glomerulus protrude into?
Bowmans capsule (glomerulus + bowmans capsule = renal corpuscle)
As blood flows through the glomerulus, what % of the plasma filters into Bowman’s capsule?
20%
Name the layers of the filtration barrier that separate the blood in the glomerulus from the fluid in
Bowman’s space:
single-celled capillary endothelium
basal lamina
single-celled epithelial (podocytes) lining of Bowman’s capsule
Each tubule in the cortex is surrounded by…
peritubular capillaries
What are the two types of nephrons in the kidneys?
Juxtamedullary nephrons
cortical nephrons
What is the juxtaglomerular apparatus?
Combination of the macula densa (patch of cells in ascending limb) and juxtaglomerular (JG) cells (secretory cells in afferent arteriole)
During the filtrates passage through the tubules, the filtrate’s composition is altered by…
movements of substances from the tubules to the peritubular capillaries, and vice versa
Tubular reabsorption:
movement is from tubular lumen to peritubular capillary plasma,
Tubular secretion:
from peritubular plasma to tubular lumen
Calculation of amount excreted of any substance:
Amount filtered + amt secreted - amt reabsorbed
Important solutes like ________ are completely reabsorbed, whereas
most ________ are secreted and not reabsorbed
glucose
toxins
What are the 4 basic renal processes?
glomerular filtration, tubular reabsorption, tubular secretion, metabolism by the tubular cells
Name the reasons for the exclusion of most plasma proteins from the glomerular filtrate?
- renal corpuscles restrict the movement of such high molecular-
weight substances. - filtration pathways in the corpuscular membranes are negatively charged, so they oppose the movement of these plasma proteins, most of which are also negatively charged
What are starling forces?
- the hydrostatic pressure difference across the capillary wall that favors filtration
- ## the protein conc difference across the wall = osmotic force that opposes filtration
What increases hydrostatic pressure in the glomerular capillaries?
efferent arteriolar constriction
(the efferent arteriole lies
beyond the glomerulus = efferent arteriolar constriction tends to “dam back” the blood in the glomerular capillaries)
What decreases hydrostatic pressure in the glomerular capillaries?
Constriction of the afferent arterioles
(due to a greater loss of pressure between arteries and capillaries)
How does constriction or dilation of both sets of arterioles influence Pgc?
leave PGC unchanged because of the opposing effects
What decreases the surface area of the glomerular capillaries?
Contraction of mesangial cells
= decrease in GFR
Whenever the quantity of a substance excreted in the urine is less than the filtered load, tubular ____________ must have occurred.
reabsorption
Tubular reabsorption occurs by which processes?
- diffusion, often across the tight junctions connecting the tubular epithelial cells
- mediated transport, which requires the participation of transport
proteins in the plasma membranes of tubular cells
What is tubular secretion?
moves substances from peritubular capillaries into the tubular lumen.
When the clearance of any substance is greater than the GFR, that substance must undergo _________________
tubular secretion
When the clearance of a filterable substance is less than the GFR, that substance must undergo some _______________
reabsorption
Name the 2 sources of body water gain:
- water produced from the oxidation of organic nutrients
-water ingested in liquids and food
Which 4 sites lose water to the external environment:
skin, respiratory airways, gastrointestinal tract, and urinary tract
(+ menstrual flow for women)
Which external sites experience insensible water loss
evaporation via the skin and the lining of the respiratory passageways
the countercurrent multiplier establishes a ________ medullary interstitium that the ___________ help to preserve
hypertonic
vasa recta
Why is the peak osmolarity in the loop of Henle is lower in the absence of vasopressin?
vasopressin stimulates urea reabsorption in the medullary
collecting ducts.
In the absence of this effect of
vasopressin, urea concentration in the medulla decreases.
urea is responsible for ~ half of the solute in the medulla = the max osmolarity at the bottom of the loop
of Henle is decreased.
the amount of Na+ in the body determines the ____________ volume, the _________ volume component of
which helps determine cardiovascular pressures, which initiate the responses that control Na+ _________
extracellular fluid
plasma
excretion
The kidney is surrounded by a protective capsule made of ______________
connective tissue
Kidneys process ________ portion of blood
plasma
Each renal corpuscle contains ____________
glomerular capillaries
The proximal tubule is comprised of…
the proximal convoluted tubule and the proximal straight tubule
Order of tubule segments of kidneys
Proximal tubule > loop of henle > distal convoluted tubule > cortical collecting ducts > medullary collecting ducts
Where are all the renal corpuscles located?
The cortex
What lie close to juxtamedullary nephrons?
vasa recta
What happens to creatine levels when you only have one kidney before kidney donation?
Same amount of creatinine is produced into the blood, GFR is lower so less creatinine is excreted = creatinine levels in blood are higher
Is there reabsorption of Na+ in the descending limb of the loop of henle?
No
_________ Pgc favors fluid movement out of the glomerular capillaries and into bowmans space. Fluid in bowmans space exerts hydrostatic pressure (Pbs) that _______ filtration
Higher
opposes
Name another opposing force to glomerular filtration
osmotic force from presence of protein in glomerular capillary plasma favors movement of fluid by osmosis from bowmans into glomerular capillaries
What is GFR
The vol of fluid filtered from the glomeruli into bowmans space
Which factors determine GFR
net filtration pressure, permeability of corpuscular membranes, S.A available for filtration
How is the GFR value altered?
by neuronal + hormonal input to the afferent and efferent arterioles = changes in net filtration pressure
Useful plasma substances have practially complete ________. Waste products _________ is relatively incomplete. Organic nutrients are completely __________
reabsorption
reabsorption
reabsorbed
How does movement of substances from interstitial fluid into peritubular capillaries occur?
via bulk flow + diffusion
urea conc in bowmans space is equal to that in ___________ plasma + ___________ surrounding the tubule
peritubular capillary
interstitial fluid
What does urea reabsorption depend upon?
water reabsorption
How does urea reabsorption occur?
Filtered fluid flows through proximal tubule = water reabsorption occurs (removal of water) = increase conc of urea in tubular fluid (higher than conc in previously mentioned areas) = urea diffuses down conc gradient: tubular lumen > peritubular capillary
Process of Transcellular epithelial transport
Substances must first cross the apical membrane (separates tubular lumen from cell interior) > diffuse through cytosol of cell > basolateral membrane
In tubular reabsorption cotransported substance moves ‘______’ into a cell via a secondary active cotransporter as Na+ moves _________ into the cell via this same cotransporter
uphill
downhill
Why is there a transport maximum Tm for substances in tubular reabsorption
Due to binding sites becoming saturated
All filtered glucose is __________
reabsorbed
What occurs when the filtered load of glucose exceeds the glucose transport maximum for a sig number of nephrons
glucose starts to appear in urine
Which mechanisms enable tubular secretion? + name the most important substances secreted
diffusion or transcellular mediated transport
H+ and K+
(usually coupled to Na+ reabsorption)
In active secretion of a substance which are the transport pathways?
either from the blood side (interstitial fluid) into the tubule cell (across the basolateral membrane) OR out of the cell into the lumen (across the apical membrane)
What do cells of renal tubules do during fasting?
synthesize glucose + add it to blood
+ can catabolize certain organic substances (e.g peptides)
Tubular reabsorption/secretion is under which physiological control?
regulation of activity or conc of membrane channels and transporter proteins
Regulation done via hormones and paracrine or autocrine factors
Why must GFR be very large?
to excrete waste products adequately
What is the role of the proximal tubule?
reabsorb water + nonwaste solutes
a major site of solute secretion (except K+)
General substances of henle’s loop reabsorption?
reabsorbs relatively large amounts of the major ions + water
Renal clearance
the vol of plasma from which that substance is completely removed by the kidneys
Calculation for Clearance of S(ubstance)
Mass of S excreted per unit time / plasma conc of S
Mass of S excreted per unit time = (Urine conc of S) x (urine vol per unit time)
Clearance of glucose normally ___ since its fully __________ (value would only be positive when Tm for glucose is ___________, which would suggest renal disease or v high blood glucose)
0
reabsorbed
exceeded
What is used to approximate GFR?
Creatinine clearance
Creatinine is filtered at the _______________ but does not undergo ____________ (undergoes small amount of __________)
renal corpuscle
reabsorption
secretion
As creatine production is constant, what does an increase in creatinine conc in the blood indicate?
indicator of disease in GFR
Na+ and water have low-molecular weights + circulate in the plasma in the free form, what does this enable?
they filter freely from glomerular capillaries into bowmans space
% Na+ and water undergo reabsorption
> 99%
Where does the bulk of Na+ and water reabsorption occur?
in the proximal tubule
Major hormonal control of reabsorption is exerted on which areas?
the distal convoluted tubules and collecting ducts
Na+ and water reabsorption mechanisms:
- Na+ reabsorption is an active process occurring in all tubular segments except the descending limb of the loop of Henle
- Water reabsorption is by osmosis (passive) and is dependent upon Na+ reabsorption
Primary Active Na+ Reabsorption is done by…
+ what does this enzyme do?
Na+/K+ -ATPase which pumps in the basolateral membrane of the cells
Keeps intracellular conc of Na+ low so Na+ moves out of tubular lumen + into tubular epithelial cells
Mechanism of the downhill Na+ movement across the apical membrane in proximal tubule?
Apical entry step of Na+ occurs by cotransport w/ variety of organic molecules e.g glucose or countertransport w/ H+
H+ moves out of cell into the lumen as Na+ moves into the cell
Na+ reabsorption here drives the reabsorption of the cotransported substances and the secretion of H+
Mechanism of the downhill Na+ movement across the apical membrane in ascending limb of loop of Henle?
reabsorb NaCl but not water
Via Na-K-2Cl cotransporter
K+ absorbed through that cotransporter from the tubular lumen is then recycled back to the tubular lumen through an apical potassium channel (to ensure enough K+ to maintain conc grad necessary for the function of the cotransporter)
Smaller amount of K+ is absorbed into the interstitial fluid by basolateral potassium pumps
Chloride is absorbed into the interstitial fluid via a basolateral chloride channel
What does the Na-K-2Cl cotransporter depend upon for Na+ movement across the apical membrane in ascending limb of loop of Henle?
the Na+ conc gradient generated by the basolateral Na+/K+ -ATPase pump) (NKCC)
Mechanism of the downhill Na+ movement across the apical membrane in the collecting ducts?
Apical entry step for Na+ occurs primarily by diffusion through Na+ channels
basolateral membrane step is ________ in all Na+ reabsorbing segments
the same
Steps of Coupling of water Reabsorption to Na+ Reabsorption process?
1) Na+ is transported tubular lumen > interstitial fluid across the epithelial cells. Other solutes, e.g glucose, amino acids, and HCO3 also contribute to osmosis.
2) Removal of solutes from the tubular lumen decreases local osmolarity of the tubular fluid adjacent to the cell. + solute in the interstitial fluid just outside the cell increases the local osmolarity
3) difference in water conc between lumen + interstitial fluid = diffusion of water from lumen across the tubular cells’ plasma membranes and/or tight junctions > the interstitial fluid.
4) Water, Na+, and everything else dissolved in the interstitial fluid move together by bulk flow into peritubular capillaries
What is tubular epithelium water permeability dependent upon?
presence of aquaporins
In which section is the conc of aquaporins always high?
in membranes of epithelial cells of proximal tubules
Which portions can have their water permeability altered via vasopressin?
the last portions of the tubules, the cortical and medullary collecting ducts
ADH doesnt influence water reabsorption in parts of the tubule prior to the collecting ducts
Diabetes insipidus
failure to synthesize or release vasopressin or inability of kidneys to respond to vasopressin
High conc of vasopressin = _________ + ______________ vol of urine
small
concentrated
Process of vasopressin mechanism:
ADH > binds to receptors on basolateral membrane > production of cAMP increases > activates PKA which phosphorylates proteins that increase the rate of fusion of vesicles containing aquaporins with the apical membrane = increase in diffusion of water down its conc grad across apical membrane into the cell > diffuses through basolateral membrane > interstitial fluid > blood
Urininary conc is established as tubular fluid flows through the _____________________
medullary collecting ducts
is the interstitial fluid surrounding the ducts very hypoosmotic or hyperosmotic ?
hyperosmotic
What factors cause medullary interstitial fluid to become hyperosmotic:
the countercurrent anatomy of the loop of henle of juxtamedullary nephrons
Reabsorption of NaCl in the ascending limb of those loops of henle
Impermeability to water of those ascending limbs
Trapping of urea in the medulla
Hairpin loops of vasa recta to minimize washout of the hyperosmotic medulla
Fluid entering the ___________ has the same osmolarity as the plasma (300 mOsmol/L)
+ why is this?
descending limb
Because proximal tubule absorbs Na+ and water in the same proportions
The Countercurrent Multiplier System in the ascending limb:
Na+ and Cl- are reabsorbed into the medullary interstitial fluid via NKCC transporters (relatively impermeable to water) = interstitial fluid of medulla becomes hyperosmotic
The Countercurrent Multiplier System in the descending limb:
does not reabsorb NaCl, is very permeable to water = net diffusion of water out the limb into the interstitial fluid = osmolarities of descending limb and interstitial fluid become equal (both higher than ascending limb)
The countercurrent multiplier system concentrates the ________ loop fluid but then decreases the osmolarity in the __________ loop so that the fluid entering the distal convoluted tubule is actually ____________
descending
ascending
hypoosmotic
How does the distal convoluted tubule make the fluid even more dilute?
actively transports Na+ and Cl- out the tube (relatively impermeable to water)
Order of transport and osmolarity of fluid when there is a high conc of ADH
High conc of ADH = water reabsorption occurs by diffusion from hypoosmotic fluid into interstitial space
Isoosmotic tubular fluid now enters the medullary collecting ducts
This water then enters the medullary capillaries + carried out the kidneys
The final urine is hyperosmotic
Which structure of the vasa recta minimize excessive loss of solute from the interstitium by diffusion?
hairpin loops that run parallel to the loops of Henle + medullary collecting ducts
Which process maintains the steady-state countercurrent gradient set up by the loops of Henle?
both the salt and water being reabsorbed from the loops of Henle and collecting ducts are carried away in equivalent amounts by bulk flow
The Recycling of Urea Helps to…
The Recycling of Urea Helps to Establish a Hypertonic Medullary Interstitium
Summary of Vasopressin Control of Urine Volume and Osmolarity
The direct effect of ADH in the collecting ducts = increased osmolarity in the renal medullary interstitium = increased water reabsorption from the lumen in the descending loop of Henle + increase in tubular fluid osmolarity
Outline the process of urea recycling
In the thin descending/ascending limbs of the loop of Henle, urea that has accumulated in the medullary interstitium is secreted back into the tubular lumen by facilitated diffusion = virtually all of the urea that was originally filtered in the glomerulus is present in the fluid that enters the distal tubule.
Some of the original urea is reabsorbed from the distal tubule and cortical collecting duct.
~ half of the urea is reabsorbed from the medullary collecting duct
The remaining amount is secreted back into the loop of Henle.
Of the urea originally filtered, 15% remains in the collecting duct and is excreted in the urine.
ADH also increases the permeability of the inner medullary collecting ducts to urea.
the responses that regulate urinary Na+ excretion are initiated mainly by various ________________, such as _____________, and by _________ in the kidneys that monitor the filtered load of Na+.
cardiovascular baroreceptors
the carotid sinus
sensors
What is the link between blood pressure and total body sodium levels?
Low total-body sodium = low plasma vol = decrease in cardiovascular pressures.
Explain the mechanism of baroreceptors in regulating sodium levels
via baroreceptors, initiate reflexes that influence the renal arterioles and tubules so as to decrease GFR and increase Na+ reabsorption
When is an increase in GFR is usually elicited by neural + endocrine inputs?
when an increased total-body-sodium level increases plasma volume = increased renal Na+ loss that returns extracellular volume to normal
What is the main direct cause of the decreased GFR in regulation of total-body sodium?
a decreased net glomerular filtration pressure (which occurs as a consequence of a decreased arterial pressure in the kidneys + reflexes acting on the renal arterioles)
What is the major hormonal factor determining the rate of tubular Na+ reabsorption?
Aldosterone
Adrenal cortex produces aldosterone which stimulates Na+ reabsorption by which structures?
the distal convoluted tubule and the cortical collecting ducts
What does aldosterone induce which promotes Na+ reabsorption?
the synthesis of the ion channels and pumps shown in the cortical collecting duct
What is the role of Angiotensin II?
acts directly on the adrenal cortex to stimulate the secretion of aldosterone (it also stimulates the constriction of arterioles)
Renin-angiotensin-aldosterone system:
Renin splits a small polypeptide, angiotensin I, from a large plasma protein, angiotensinogen
Angiotensin I > ACE > angiotensin II
the chain of events in sodium depletion: increased renin secretion → increased plasma renin concentration → increased plasma angiotensin I concentration → increased plasma angiotensin II concentration → increased aldosterone release → increased plasma aldosterone concentration
inputs to the juxtaglomerular cells by which sodium depletion causes an increase in renin secretion:
the renal sympathetic nerves
intrarenal baroreceptors
the macula densa
Which innervation occurs for the juxtaglomerular cells?
renal sympathetic nerves
What is the result of an increase in the activity of renal sympathetic nerves?
stimulates renin secretion
How are renal sympathetic nerves activated?
via cardiovascular baroreceptors whenever a reduction in body sodium decreases cardiovascular pressures
What happens when blood pressure in the kidneys decreases (w/ juxtaglomerular cells)?
When blood pressure in the kidneys decreases = these cells are stretched less = secrete more renin
Which structure is strategically located near the ends of the ascending loops of Henle + senses the amount of Na+ in the tubular fluid flowing past it?
+ its mechanism
macula densa
A decreased Na+ delivery = release of paracrine factors that diffuse from the macula densa to the nearby JG cells = activates them = release of renin
If salt intake is low, less Na+ is filtered and less appears at the macula densa
high salt intake = very low rate of release of renin
Role of ANP
Acts on several tubular segments to inhibit Na+ reabsorption
Acts on renal blood vessels to increase GFR (+ = increased Na+ excretion)
Whats happens if blood pressure is sig decreased?
glomerular filtration rate can decrease = decrease the tubular flow rate such that less Na+ is presented to the macula densa
This input also results in increased renin release at the same time that the sympathetic nerves and intrarenal baroreceptors are doing so
Cardiac atria cells synthesize and secrete…
ANP
An osmotic diuresis that is caused by an increase in Na+ excretion is called a…
natriuresis
What does ANP directly inhibit?
aldosterone secretion
When does ANP secretion increase?
when there is excess Na in the body
What is the stimulus for ANP secretion?
atrial distension (which occurs due to the expansion of plasma vol)
What does an increase in arterial pressure cause?
+ how does it do this?
An increase in arterial pressure = inhibits Na+ reabsorption, increases Na+ excretion (via pressure natriuresis)
It does this via: inhibiting the activity of the renin-angiotensin-aldosterone system + It acts locally on the renal tubules
What is the major determinant of for how much water is excreted?
The rate of water reabsorption
under conditions due predominantly to water gain or loss, the sensory receptors that initiate the reflexes controlling vasopressin secretion are…
osmoreceptors in the hypothalamus
These receptors are responsive to changes in osmolarity
What happens when the osmolarity of the body fluids increases because of water deprivation?
vasopressin secretion is reflexively increased via the osmoreceptors = water reabsorption by the collecting ducts increases, + a very small volume of highly concentrated urine is excreted
What happens when you drink a lot of water?
Absorption of the excess water from the GI tract = decreases the body fluid osmolarity = inhibition of vasopressin secretion via the hypothalamic osmoreceptors = water permeability of the collecting ducts decreases dramatically = water reabsorption of these segments is greatly reduced + a large volume of hypoosmotic urine is excreted
Link between extracellular vol and vasopressin?
Decreased extracellular volume also triggers an increase in vasopressin secretion
This reflex is initiated by several baroreceptors in the cardiovascular system
Under which circumstances do baroreceptors decrease their rate of firing?
when cardiovascular pressures decrease = increased vasopressin secretion
(increased cardiovascular pressures cause more firing by the baroreceptors)
Vasopressin causes widespread arteriolar _____________
constriction
Other stimuli to vasopressin secretion
ethanol inhibits vasopressin release
hypoxia alters vasopressin release via afferent input from peripheral arterial chemoreceptors to the hypothalamus via ascending pathways from the medulla oblongata to the hypothalamus
Nausea also stimulates vasopressin release
What can severe sweating lead to?
a decrease in plasma volume + increase in plasma osmolarity
GFR decreased and aldosterone increased; together decrease Na+ excretion
increases vasopressin, which decreases H2O excretion
Net result: Renal retention of Na+ and H2O acts to minimize hypovolemia and maintain plasma osmolarity.
What happens during potassium depletion?
there is no K+ secretion by the cortical collecting ducts
Mechanism by which cortical collecting ducts can secrete K+:
In the cortical collecting ducts K+ pumped into the cell across the basolateral membrane by
Na+/K+-ATPases diffuses into the tubular lumen through K+ channels in the apical membrane.
= secretion of K+ by the cortical collecting duct = reabsorption of Na+ by this tubular segment.
How does K+ secretion occur in the ascending limb of the loop of Henle?
K+ secretion into the tubular lumen does occur through K+ channels on the apical membrane.
However, this is basically a recycling process to maintain tubular K+ concentrations sufficient to drive the NKCC transporter
Factors influencing K+ secretion by the cortical collecting ducts:
High potassium diet = K+ plasma conc increases = directly drives enhanced basolateral uptake via the Na+/K+-ATPase pumps = enhanced K+ secretion
Aldosterone enhances K+ secretion by cortical collecting ducts. aldosterone-secreting cells of the adrenal cortex are sensitive to the K+ concentration of the extracellular fluid (an increased intake of K+ = increased extracellular K+ concentration = stimulates the adrenal cortex to produce aldosterone. The increased plasma aldosterone concentration increases K+ secretion = eliminates excess K+ from the body.
Conditions where net retention of CO2 occurs:
+ effect
hypoventilation or respiratory disease
= net gain of H+
Conditions where net loss of CO2 occurs:
vomitus or hyperventilation = net elimination of H+
effect of loss of alkaline GI secretions
their conc of HCO3 is usually higher than in plasma. Loss of these fluids, as in diarrhea, = a gain of H
Buffer
+ equation
+ effect w/ changes in H+ conc
Any substance that can reversibly bind H+
(The major extracellular buffer is the CO2/HCO3− system)
Buffer + H+ ⇋ HBuffer
When H+ concentration increases the reaction is forced to the right = more H+ is bound by buffer to form HBuffer
when H+ concentration decreases eq proceeds to the left = H+ is released from HBuffer
Factors causing net gain of H+:
net gain of H+ can occur with increased production of these nonvolatile acids, with hypoventilation or respiratory malfunction, or with the loss of alkaline gastrointestinal secretions = kidneys increase the elimination of H+ from the body
effect of increased arterial H+ conc:
increased arterial H+ concentration stimulates ventilation, = lowers arterial PCO2 = reduces H+ concentration.
effect of decreased arterial H+ conc:
a decreased plasma H+ concentration inhibits ventilation = increasing arterial PCO2 and the H+ concentration.
If the respiratory system is the actual cause of the H+ imbalance, then what is the homeostatic responder?
the kidneys
What occurs when the respiratory response is the only one in control of regulating H+ imbalances?
malfunctioning kidneys can create an imbalance of H+ by eliminating too little or too much H+ from the body
How do the kidneys eliminate or replenish H+ from the body?
by altering plasma HCO3− concentration.
excretion of HCO3− in the urine increases the plasma H+ concentration
When the plasma H+ ion concentration decreases (alkalosis) for whatever reason, the kidneys’ homeostatic response is to excrete large quantities of HCO3
Where can HCO3- be secreted?
in the collecting ducts
HCO3- undergoes sig reabsorption, where does this occur?
in the proximal tubule, ascending loop of Henle, and cortical collecting ducts
What does HCO3− reabsorption depend on?
+ process
the tubular secretion of H+, which combines in the lumen with filtered HCO3−.
H2CO3 immediately dissociates to yield H+ and HCO3−.
The HCO3− moves down its concentration gradient via facilitated diffusion across the basolateral membrane into interstitial fluid and then into the blood.+ H+ is secreted into the lumen
How is H+ secretion achieved?
this secretion is achieved by some combination of primary H+-ATPase pumps, primary H+/K+-ATPase pumps, and Na+/H+ countertransporters
What happens when all HCO3- has been reabsorbed?
the extra secreted H+ combines in the lumen with a filtered nonbicarbonate buffer, the most important of which is HPO42−.
The H+ is then excreted in the urine as part of H2PO4
Name another mechanism via which the Addition of New HCO3− to the Plasma occurs?
Tubular cells, mainly those of the proximal tubule, take up glutamine from both the glomerular filtrate and peritubular plasma and metabolize it. = NH4+ and HCO3− are formed inside the cells.
NH4+ is secreted via Na+/NH4+ countertransport into the lumen and excreted, while the HCO3− moves into the peritubular capillaries and constitutes new plasma HCO3−
When does respiratory acidosis occur?
when the respiratory system fails to eliminate CO2 as fast as it is produced (an increase in both arterial PCO2 and H+ concentration)
When does respiratory alkalosis occur?
when the respiratory system eliminates CO2 faster than it is produced (decrease in both arterial PCO2 and H+ concentration)
common causes of metabolic acidosis:
excessive production of lactic acid (during severe exercise or hypoxia) or of ketone bodies (in uncontrolled diabetes mellitus or fasting (or excessive loss of HCO3−, as in diarrhea)
increased H+ conc associated w/ metabolic acidosis reflexively stimulates…
ventilation and decreases arterial PCO2
