renal Flashcards
functions of kidneys
regulate ECF water (and thus long-term arterial pressure), ECF concentrations of many inorganic ions, and (together with the lungs) regulate ECF acid-base balance
remove metabolic wastes from ECF (urea from protein catabolism, uric acid from nucleic acids, creatinine from muscle creatine, end products of hemoglobin breakdown
removes some foreign chemicals (e.g. drugs, pesticides) from ECF
gluconeogenesis
produces hormones (renin, erythropoietin, and 1,25-dihydroxyvitamin D)
gluconeogenesis (occurs in kidneys but primarily in liver)
during fasting, synthesis of glucose from other molecules such as amino acids and release it into the blood
nephron (functional unit kidney) is comprised of:
renal corpuscle - the filtering component
(renal) tubule - extends out from the corpuscle and is surrounding by peritubular capillaries
renal corpuscle consists of
a tuft of capillaries called the glomerulus or GLOMERULAR CAPILLARIES
a balloon-like hollow Bowman’s capsule (BC) into which the capillaries protrude the fluid-filled space within which is called Bowman’s space
fluid from the capillaries filters into Bowman’s space across
the thin membranes of the capillaries and of Bowman’s capsule
kidney has an outer cortex and a deeper medulla; all renal corpuscles are
within the cortex
juxtamedullary nephrons have corpuscles
near the cortex/medulla junction
cortical nephrons have more superficial corpuscles nearer
the kidney surface
renal tubule: its lumen
is a continuation of the space in Bowman’s capsule
proximal tubule
first section of the tubule; it drains Bowman’s space (has a convoluted portion and a straight portion)
loop of henle
next section of the tubule (after proximal tubule); it has a descending limb followed by an ascending limb
distal convoluted tubule
next section of the tubule (after loop of henle)
collecting duct
final section formed by union of many distal tubules
composed of a cortical collecting duct followed by a medullary collecting duct
contains the fluid that will be transported to the bladder and eventually eliminated
the tubules of most (not all) nephrons
loop down into the medulla, sometimes deeply
afferent arteriole
carries blood (from a branch of the renal artery) into a tuft of glomerular capillaries
the glomerular capillaries recombine to form another arteriole, the ______ arteriole, through which
efferent
blood leaves the capillaries
the efferent arteriole divides into another set of capillaries, the _______, which
peritubular capillaries
branch to form a network surrounding the tubule
kidney function overview:
the low-molecular weight substances (_________) in glomerular capillary plasma are filtered ________, which is called ________
water, glucose, urea, ions
across the thin membranes of the capillaries and of Bowman’s capsule into Bowman’s space
glomerular filtration
glomerular filtration forms the glomerular filtrate (also called ultrafiltrate) which contains the same substances in _____ concentrations as their plasma concentrations
the same
what is not included in the filtrate and why?
cells or proteins
because they’re too large to cross
as the filtrate moves from BC into and through the tubule, its composition is altered by two general processes
- tubular reabsorption
2. tubular secretion
tubular reabsorption and secretion occur because
the tubule is very close to the peritubular capillary network, and substances can be transferred in either direction between peritubular capillary blood and the lumen of the tubule
filtration, tubular reabsorption, and tubular secretion combine to allow the kidneys to excrete
all, some, or non of a given substance in the blood
total excretion
substance that is filterable, cannot be reabsorbed, and undergoes 100 percent secretion
partial excretion
substance that is filterable, undergoes partial reabsorption, which only allows partial secretion
no excretion
substance that is filterable, undergoes 100 percent reabsorption, and cannot be secreted
not all substances undergo all 3 processes (filtration, reabsorption, and secretion); the rates at which the 3 processes proceed are often subject to
physiological control, and thus allow control of the concentrations of substances in body
where the ‘inner’ walls of BC abut the glomerular capillary walls, the BC cells are called ______ and contain many extensions (foot processes). spaces between foot processes are called _______
podocytes
filtration slits or “slit pores”
pathway of substance during filtration
a substance moves across the single layer of (or between the cells of) epithelial cells (called endothelium) forming the capillary walls –> across a layer of noncellular materials (basement membrane) –> and between foot processes into Bowman’s space
these filtration membranes serve only as a filtration barrier and have no energy-requiring function
the process of filtration involves using pressure to force a filtrate of plasma through the glomerular pores into Bowman’s space; except for being essentially protein-free, the filtrate is thus almost identical to plasma and contains:
water, glucose, urea, sodium, chloride, potassium, etc in their plasma concentrations
the force driving filtration is mostly
glomerular capillary fluid (blood) pressure, which is high (60 mmHg) relative to pressure in other capillaries. filtration is driven mostly by a fluid pressure gradient not a diffusion (concentration) gradient
filtration is opposed by two relatively small forces
- fluid pressure in Bowman’s space
2. osmotic force caused by proteins in plasma that are not present in Bowman’s space
the net pressure favoring filtration is
10 mmHg (or a bit more)
the glomerular capillaries are much more permeable to fluid than most other body capillaries, resulting in massive filtration of about ______ per day
180 L/day or 45 gallons
massive filtration by kidneys allows kidneys to regulate
some components of the internal environment very precisely
of the 900 liters of plasma entering the kidneys per day 180 liters are filtered into Bowman’s space and the rest passes into the
efferent arterioles and then into the peritubular capillaries
glomerular filtration rate (GFR)
the volume of fluid filtered into the combined Bowman’s spaces per unit time
total amount of a given substance filtered per unit time (the filtered load of the substance) =
GFR times the plasma concentration of the substance
if less of a substance is excreted than is filtered per unit time, _______ has occurred
tubular reabsorption
if more of a substance is excreted than is filtered per unit time, _____ has occurred
tubular secretion
GFR is not a fixed value but is subject to physiological control, achieved mainly by
neural and hormonal inputs to the afferent and efferent arterioles
impact of constriction of the afferent arteriole on net glomerular filtration pressure
decreases glomerular capillary blood pressure by increasing the pressure drop between arteries and capillaries
impact of constriction of the efferent arteriole on net glomerular filtration pressure
increases glomerular capillary blood pressure by causing blood to dam up in the capillaries
percent reabsorption of water
99
percent reabsorption of sodium
99.5
percent reabsorption of glucose
100
percent reabsorption of urea
44
filtered loads are huge, and filtered amounts of some substances would quickly
deplete body stores if reabsorption ceased
e.g. total plasma water would be urinated in 30 minute
rapid excretion of wastes (e.g. urea) can occur because quantities excreted are
sizable fractions of the filtered amounts
useful plasma component (e.g. organic nutrients, ions, water) undergo relatively complete reabsorption, so excreted amounts are
small fractions of the filtered amounts
the reabsorption process for some filtered substances (e.g. most organic nutrients) is not
physiologically regulated; these substances are typically completely reabsorbed at all times
reabsorption rates for other substances (water and many inorganic ions) are regulated
according to the body’s needs
can reabsorption regulate plasma glucose levels (minimize changes from a set point): assume ideal level is 80 mg glu/100 ml plasma
if plasma concentration is 100mg/ml (or 60 mg/ml) and complete reabsorption occurs, ____ glucose is lost from the body, and the plasma level remains at 100 (or 60)
thus, the kidney _____ set and regulate the plasma glucose concentration
no
does not
can reabsorption regulate plasma water levels?
the kidney does not always reabsorb all (or even 99 percent) of the total amount of water filtered; instead, the water reabsorption rate is
constantly subject to physiological control
increase water intake –> ____ reabsorption –> ____ excretion –> restoration of plasma water level to normal
decreased
increased
tight junction exist between the _____ borders of the tubular epithelial cells
luminal
transcellular path (into and out of the tubule lumen) is through the plasma membranes forming the luminal borders and thus through cells; occurs by
diffusion or active transport
paracellular path (into and out of the tubule lumen) is through the tight junctions and thus between cells; occurs by
diffusion
a substance need not be actively transported across both the _____ and ______ membranes to be actively transported (actively reabsorbed)
luminal
basolateral
substances move from the renal interstitial fluid into the _____ by
peritubular capillary plasma
a combination of diffusion and bulk flow
substances are reabsorbed by _____ and/or ______ transport mechanisms
diffusion
carrier-mediated
carrier-mediated transport mechanism
glucose is reabsorbed by secondary active transport (sodium cotransport) and thus uses energy and can occur against the glucose concentration gradient
steps in glucose carrier mediated transport
- Na/K pump pumps Na out of cell and into rental interstitial fluid (creating Na concentration gradient favoring Na into the cell)
- Na/glucose cotransporter moves Na with its concentration gradient (into the cell from tubule lumen) and takes glucose with it
- GLUT transporter transports glucose out of cell and into interstitial fluid
when considering glucose reabsorption, because a carrier protein is involved, a very high glucose concentration in blood (and thus in interstitial fluid) can exceed the capacity (called the transport maximum) of the carrier, in which case glucose will appear
in the urine (diabetes mellitus)
urea is reabsorbed by
diffusion
since urea is filtered at the renal corpuscle, its concentration in the first portion of the tubule is equal to its concentration in the interstitial fluid around the tubule and in the peritubular capillary plasma.
as the filtrate flows along the proximal tubule, water reabsorption occurs, thus ______ the tubular concentration of urea
urea then diffuses passively down its concentration gradient from ____ to ____
increasing
tubule to peritubular capillary
most ____ and ____ in urine enters the tubules by secretion; thus renal regulation of these is accomplished primarily by controlling the rates of their
K+ and H+
tubular secretion
some organic ions (e.g. creatinine) and foreign substances (e.g. penicillin) are
secreted
secretion is by ______ or ______ , and the secretion of many substances is coupled to
diffusion or carrier-mediated transport
sodium reabsorption
secretion from the renal interstitial fluid into the filtrate within the tubule draws substances from the peritubular capillary plasma, is a way to increase the kidney’s ability to excrete substances at a higher rate than if the kidneys depended solely on the
filtered load
clearance
describes the rate at which substances are removed (cleared) from the plasma by the kidneys
renal clearance of a substance is the
volume of plasma completely cleared of a substance per unit time by the kidneys (units are volume of plasma/unit time)
Cs =
Us*V / Ps
example of clearance:
40 mg or a substance S shows up in each liter of urine (urine concentration of S = 40 mg/L)
during an hour, 0.5 L of urine is produced (this half liter of urine contains 20 mg of S– the kidney are getting rid of 20 mg of S per hour)
Cs =
(40 * 0.5) / 5
urinary excretion rate of substance S =
Us * V
Clearance rate of inulin =
GFR
creatinine is an endogenous filtered substance that undergoes 0 reabsorption and almost 0 secretion (similar to inulin), so clearance rate of creatinine =
GFR
low GFR is hallmark of kidney disease
hypothetical situation:
have a patient in which GFR decreased by half: only half as much creatinine will be filtered and thus excreted (but change is transient). creatinine will start accumulating – its plasma concentration rises. this will cause the filtered load eventually to increase because there is so much creatinine in the blood. excretion eventually returns to normal and a balance is re-established between creatinine production and excretion, but this only occurs when the plasma creatinine concentration is about _____ what it was before
double
a blood test for creatinine levels is commonly used as an indicator of _____
GFR
high creatinine is an indicator of a _____ GFR, which is one hallmark of kidney disease
low
