Tubular Reabsorption Flashcards
All plasma constituents except the plasma proteins are indiscriminately filtered together through the
glomerular capillaries
The filtered fluid contains
nutrients, electrolytes, and other substances that the body cannot afford to lose in the urine
The essential materials that are filtered are
returned to the blood by tubular reabsorption
Tubular reabsorption is
tremendous, highly selective, and variable.
This means that Only a small percentage, if any, of filtered plasma constituents that are useful to the body are present in the urine
Of the 125 mL of fluid filtered per minute, typically 124 mL/min are reabsorbed
A large percentage of filtered waste products are present in the urine - These wastes are generally are not reabsorbed
Characteristics of the tubule wall
The tubule wall is one cell thick and is close to a surrounding peritubular capillary
Tubular reabsorption involves transepithelial transport
- Leave the tubular fluid by crossing the luminal membrane of the tubular cell.
- Pass through the cytosol from one side of the tubular cell to the other.
- Cross the basolateral membrane of the tubular cell to enter the interstitial fluid.
- Diffuse through the interstitial fluid.
- Penetrate the capillary wall to enter the plasma.
2 types of tubular reabsorption
- active
- passive
The two types of tubular reabsorption depend on
whether local energy expenditure is needed for reabsorbing a particular substance
In passive reabsorption
- the substance’s net movement occurs down electrochemical or osmotic gradients
In active reabsorption
- net movement of the substance occurs against an electrochemical gradient
______ of the total energy spent by the kidneys is used for Na+ transport
80%
Na+ is reabsorbed throughout
most of the tubule
99.5% of the Na+ that is filtered is
normally reabsorbed
on average the reabsorption of Na+ is as follows:
67% is reabsorbed in the proximal tubule
25% in the loop of Henle
8% in the distal and collecting tubules.
Sodium is reabsorbed throughout the tubule with the exception of the
descending limb of Henle’s loop
Na+ reabsorption involves the
energy-dependent Na+–K+ ATPase carrier located in the tubular cell’s basolateral membrane
Na+ Reabsorption
Sodium reabsorption in the proximal tubule plays a pivotal role in reabsorbing glucose, amino acids, H2O, Cl-, and urea.
Sodium reabsorption in the ascending limb of the loop of Henle, along with Cl- reabsorption, plays a critical role in the kidneys’ ability to produce urine of varying concentrations and volumes.
Sodium reabsorption in the distal and collecting tubules is variable and subject to hormonal control.
Aldosterone
stimulates Na+ reabsorption
In the proximal tubule and loop of Henle
a constant percentage of the filtered Na+ is reabsorbed regardless of the Na+ load
In the distal and collecting tubules,
the reabsorption of a small percentage of the filtered Na+ is subject to hormonal control
Sodium and its accompanying anion Cl- account for more than 90% of the
ECF’s osmotic activity
The Na+ load is subject to regulation;
Cl- passively follows along
The most important hormonal system involved in regulating Na+ is
the renin–angiotensin– aldosterone system (RAAS).
secrete renin
The granular cells
The granular cells secrete renin into the blood in response to
a fall in NaCl, ECF volume, and arterial blood pressure.
The following three inputs to the granular cells increase renin secretion:
The granular cells themselves function as intrarenal baroreceptors. When the granular cells detect a fall in blood pressure, they secrete more renin.
The macula densa cells of the juxtaglomerular apparatus are sensitive to the NaCl moving past them. In response to a fall in NaCl, the macula densa cells trigger increased renin secretion.
The granular cells are innervated by the sympathetic nervous system. When blood pressure falls below normal, the baroreceptor reflex increases sympathetic activity. Increased sympathetic activity stimulates the granular cells to secrete more renin.
Functions of the Renin–Angiotensin–Aldosterone System
Aldosterone increases Na+ reabsorption by the principal cells of the distal and collecting tubules.
RAAS promotes salt retention and a resulting H2O retention and rise in arterial BP
This system alleviates the factors that triggered the initial release of renin
Angiotensin II is a potent constrictor of the systemic arterioles - directly increasing BP
Stimulates thirst and stimulates vasopressin – results in plasma volume expansion and elevation of arterial pressure.
By varying the amount of renin and aldosterone secreted in accordance with the salt determined fluid load in the body, the kidneys can finely adjust the amount of salt conserved or eliminated.
inhibit Na+ reabsorption
The natriuretic peptides
These peptides produce natriuresis, or excretion of large amounts of sodium in the urine
A Na+ losing, blood pressure–lowering system that involves
the hormones atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP).
The heart, in addition to its pump action, produces
ANP and BNP
The main action of ANP and BNP is to
directly inhibit Na+ reabsorption in the distal parts of the nephron, thus increasing Na+ excretion and accompanying osmotic H2O excretion in the urine.
natriuretic peptides inhibit:
- renin secretion by the kidneys
- act on the adrenal cortex to inhibit aldosterone secretion.
- they inhibit the secretion and actions of vasopressin, the H2O-conserving hormone.
ANP and BNP also promote
natriuresis and accompanying diuresis by increasing the GFR
ANP and BNP directly
lower blood pressure by decreasing cardiac output and reducing total peripheral resistance
Large quantities of nutritionally important organic molecules are completely reabsorbed into the blood.
Glucose and amino acids are reabsorbed by
secondary active transport - the sodium and glucose cotransporter (SGLT)
All actively reabsorbed substances bind with plasma membrane carriers that transfer them across the membrane against a concentration gradient
An upper limit exists on how much of a particular substance can be actively transported from the tubular fluid in a given period
This maximum reabsorption rate is designated as the tubular maximum, or Tm
The normal plasma concentration of glucose is
100 mg of glucose for every 100 mL of plasma
With 125 mL of plasma normally being filtered each minute,125 mg of glucose pass into Bowman’s capsule with this filtrate every minute
Filtered load of a substance = plasma concentration X GFR of the substance
Filtered load of glucose = 100 mg/100 mL X 125 mL/min = 125 mg/min
The Tm for glucose averages 375 mg/min – the glucose carrier mechanism is capable of actively reabsorbing up to 375 mg of glucose per minute before it reaches its maximum transport capacity.
Renal Threshold for Glucose = 300 mg/100 mL
Phosphate is
actively reabsorbed and regulated
The kidneys do directly contribute to the regulation of many electrolytes
e.g. phosphate and calcium
Excess ingested phosphate is
quickly spilled into the urine, restoring the plasma concentration to normal
The reabsorption of phosphate and calcium is also subject to
hormonal control
The amount of Cl2 reabsorbed is determined by
the rate of active Na+ reabsorption instead of being directly controlled by the kidneys.
How is water absorbed?
Water is passively reabsorbed throughout the length of the tubule as H2O osmotically follows actively reabsorbed Na+
Where does water absorption occur?
80% is obligatorily reabsorbed in the early parts of the nephron. 20% is reabsorbed in the distal portions of the tubule
H2O passes primarily through aquaporins (AQPs)
– In the proximal tubule AQP-1, are always open.
- Whereas, the AQP-2 channels in the distal parts of the nephron are regulated by the hormone vasopressin.
Return of filtered H2O to the plasma is enhanced by the fact that
the plasma-colloid osmotic pressure is greater in the peritubular capillaries than elsewhere
Passive reabsorption of urea is indirectly linked to
active Na+ reabsorption
A concentration gradient is created for urea to
passively diffuse from the tubular lumen into the peritubular capillary plasma
Only about _____ of the filtered urea is passively reabsorbed
50%
Urea molecules
- the smallest of the waste products
- are the only wastes passively reabsorbed
The other wastes cannot be passively reabsorbed because they
cannot permeate the tubular wall.
What happens to other waste that cannot be passively reabsorbed?
These waste products generally remain in the tubules and are excreted in the urine in highly concentrated form
