Tubular Secretion Flashcards
Tubular Secretion involves
transepithelial transport
Provides a second route of entry into the tubules for selected substances
Tubular Secretion
The most important substances secreted by the tubules:
Hydrogen ion
Potassium ion
Organic anions and cations
Renal H+ secretion is extremely important in regulating
acid–base balance in the body.
H+ secreted into the tubular fluid is eliminated from the body in
the urine.
H+ can be secreted by the
proximal, distal, and collecting tubules
H+ secretion _________ as required
increases/decreases
K+ is selectively moved
in opposite directions in different parts of the tubule
Filtered K+ is almost completely reabsorbed in the
proximal tubule
most K+ in the urine is derived from
controlled K+ secretion in the distal parts of the nephron rather than from filtration
During K+ depletion,
K+ secretion in the distal parts of the nephron is reduced to a minimum
When plasma K+ levels are elevated,
K+ secretion is adjusted to reduce the plasma K+ concentration to normal
K+ secretion is varied in a controlled fashion to
maintain the desired plasma K+ concentration
Mechanism Of K+ Secretion
K+ secretion in the distal and collecting tubules is coupled to Na+ reabsorption
Control of K+ Secretion
Aldosterone stimulates K+ secretion by the principal tubular cells late in the nephron while simultaneously enhancing these cells’ reabsorption of Na+
Effect of H+ Secretion on K+ Secretion
The organic ion secretory systems serve three important functions:
Organic secretory pathways facilitate excretion of organic ions – e.g. certain blood-borne chemical messengers having served their purpose, must be rapidly removed from the blood so that their biological activity is not unduly prolonged
To be transported in blood, organic ions are extensively but not irreversibly bound to plasma proteins and therefore cannot be filtered through the glomeruli. Tubular secretion facilitates elimination of these nonfilterable organic ions in urine.
Most important, the systems play a key role in eliminating many foreign compounds from the body. Speeds up the removal of many foreign organic chemicals, including food additives, environmental pollutants, drugs, and other non-nutritive organic substances that have entered the body
Summary of Reabsorptive and Secretory
The final quantity of urine formed averages 1 mL/min
- of the 180 liters filtered per day, 1.5 liters of urine are excreted.
Urine contains high concentrations of
various waste products plus variable amounts of the substances regulated by the kidneys
A relatively small change in the quantity of filtrate reabsorbed can bring about a large change in the
volume of urine formed
The plasma clearance of any substance is defined as
the volume of plasma completely cleared of that substance by the kidneys per minute
a more useful measure than urine excretion
Plasma clearance
The plasma clearance rate varies for different substances, depending on
how the kidneys handle each substance.
Plasma clearance
At normal fluid balance and solute concentration, the body fluids are
isotonic at an osmolarity of 300 milliosmols per liter (mOsm/L)
Hypotonic vs Hypertonic
This gradient enables the kidneys to produce urine that ranges in concentration from 100 to 1200 mOsm/L, depending on the body’s state of hydration
Long-looped nephron establishes a vertical osmotic gradient in the
renal medulla
Long-looped nephron establishes a vertical osmotic gradient in the renal medulla. How?
via countercurrent multiplication
Uncontrolled osmotic reabsorption of filtered H2O occurs in the
proximal tubule secondary to active Na+ reabsorption
By the end of the proximal tubule, about _____ of the filtrate has been reabsorbed, but the remaining % in the tubular lumen still has the _________
65%
same osmolarity as the body fluids
An additional 15% of the filtered H2O is obligatorily reabsorbed from the
loop of Henle
Properties of the Descending Limb of a Long Henle’s Loop
- Is highly permeable to H2O (via abundant, always-open AQP-1 water channels)
- Does not actively extrude Na+
Properties of the Ascending Limb of a Long Henle’s Loop
- Actively transports NaCl out of the tubular lumen into the surrounding interstitial fluid
- Is always impermeable to H2O, so salt leaves the tubular fluid without H2O osmotically following along.
Mechanism of Countercurrent Multiplication
https://www.youtube.com/watch?v=hjQd9nWAxQk
https://www.youtube.com/watch?v=cYyJF_aSC6o
Benefits of Countercurrent Multiplication
- It establishes a vertical osmotic gradient in the medullary interstitial fluid. This gradient, in turn, is used by the collecting ducts to concentrate the tubular fluid so that a urine more concentrated than normal body fluids can be excreted.
- Because the fluid is hypotonic as it enters the distal parts of the tubule, the kidneys can excrete a urine more dilute than normal body fluids
Vasopressin controls variable H2O reabsorption
After obligatory H2O reabsorption from the proximal tubule and loop of Henle, 20% of the filtered H2O remains in the lumen to enter the distal and collecting tubules for variable reabsorption under hormonal control.
20% X GFR (180 L/day) = 36 L/day to be reabsorbed to varying extents, depending on the body’s state of hydration
The fluid leaving the loop of Henle enters the distal tubule at 100 mOsm/L, so it is hypotonic to the surrounding isotonic interstitial fluid of the renal cortex
The distal tubule then empties into the collecting duct, which is bathed by progressively increasing concentrations (300 to 1200 mOsm/L) of the surrounding interstitial fluid as it descends through the medulla.
For H2O absorption to occur across a segment of the tubule, two criteria must be met:
- An osmotic gradient must exist across the tubule
- The tubular segment must be permeable to H2O
The distal and collecting tubules are impermeable to H2O except in the presence of
vasopressin (antidiuretic hormone)
Water Reabsorption
In the tubular segments permeable to H2O, solute reabsorption is always accompanied by comparable H2O reabsorption because of osmotic considerations.
Solute excretion is always accompanied by comparable H2O excretion because of osmotic considerations.
Osmotic Diuresis and Water Diuresis
A loss or gain of pure H2O that is not accompanied by comparable solute deficit or excess in the body (that is, “free” H2O) leads to changes in ECF osmolarity
Renal failure has a variety of causes, among the causes are the following:
- Infectious organisms, either blood-borne or gaining entrance to the urinary tract through the urethra
- Toxic agents
- Inappropriate immune responses
- Obstruction of urine flow by kidney stones, tumors, or an enlarged prostate gland
- An insufficient renal blood supply that leads to inadequate filtration pressure, which can occur secondary to circulatory disorders
Acute vs Chronic renal failure
Once urine has been formed by the kidneys, it is transmitted through the
ureters to the urinary bladder.
Urine does not flow through the ureters by gravitational pull alone.
Peristaltic (forward-pushing) contractions of the smooth muscle within the ureteral wall propel the urine forward from the kidneys to the bladder.
The ureters penetrate the wall of the bladder obliquely, coursing through the wall several centimetres before they open into the bladder cavity.
As the bladder fills, the ureteral ends within its wall are compressed closed
Role of the Bladder
The bladder can accommodate large fluctuations in urine volume.
as the bladder alternately fills and empties.
The epithelial lining can increase and decrease in surface area by the orderly process of membrane recycling
Bladder muscle can
stretch tremendously without building up bladder wall tension
In addition, the highly folded bladder wall
flattens out during filling to increase bladder storage capacity.
empties urine from the bladder
Contraction
The exit from the bladder is guarded by two sphincters:
Internal urethral sphincter
External urethral sphincter
internal urethral sphincter
is smooth muscle and, accordingly, under involuntary control.
When the bladder is relaxed, the anatomic arrangement of the internal urethral sphincter region closes the bladder outlet.
Farther down the passageway, the urethra is encircled by a layer of skeletal muscle, the external urethral sphincter.
This sphincter is reinforced by the entire pelvic diaphragm, a skeletal muscle sheet that forms the floor of the pelvis and helps support the pelvic organs
The motor neurons that supply the external sphincter and pelvic diaphragm fire continuously at a moderate rate unless they are inhibited
Normally, when the bladder is relaxed and filling, both the internal and the external urethral sphincters are closed to keep urine from dribbling out.
Furthermore, because the external sphincter and pelvic diaphragm are skeletal muscle and thus under voluntary control, the person can deliberately tighten them to prevent urination from occurring even when the bladder is contracting and the internal sphincter is open.
Micturition
urination
Micturition (urination) the process of bladder emptying, is governed by two mechanisms:
The micturition reflex
Voluntary control
Voluntary Control of Micturition
Bladder filling gives rise to the conscious urge to urinate
The perception of bladder fullness appears before the external sphincter reflexly relaxes, warning that micturition is imminent
If the time when the micturition reflex is initiated is inopportune for urination, you can voluntarily prevent bladder emptying by deliberately tightening your external sphincter and pelvic diaphragm
Urination cannot be delayed indefinitely
Micturition can also be deliberately initiated, even though the bladder is not distended, by voluntarily relaxing the external sphincter and pelvic diaphragm
