Transcript Reabsorption and Secretion

Reabsorption and Secretion
Learning Objectives
• Understand how fluid flow from the tubular lumen to the
peritubular capillaries.
• Know how the reabsorption of H2O, Cl-, Ca2+, Mg2+, glucose,
and amino acids are coupled to the active transport of Na+.
• Understand the function of the proximal tubule, loop of
Henle, distal tubule, collecting tubule, and medullary
collecting ducts.
• Know the renal mechanism for pressure diuresis and
natriuesis.
• Know how the kidney forms dilute or concentrated urine.
• Know how a hyperosmotic renal medulla is established.
• Know how antidiuretic hormone, aldosterone and
angiotensin II affect renal funciton.
• Know how the kidneys help regulate the body’s acid-base
balance.
Review of Urine Formation
Filtration and Reabsorption of Some
Compounds
Tubular Reabsorption
Solute Pathways
Transport Mechanisms
Transport back into Blood
Na+-K+ ATPase
Secondary Active Transport
H2O, Cl-, and Urea
• Generally, these follow Na+ through electrical
forces and osmosis.
• Urea is less permeable than H2O and Cl- and
thus, is not reabsorbed to the same level.
Proximal Tubule
Proximal Tubule
Secretion in the Proximal Tubule
• Bile salts, oxalate, urate and catecholamines are
secreted into the proximal tubule. Many of the
substances secreted are metabolic end-products.
• Many drugs are secreted, including penicillin.
• Para-aminohippuric acid (PAH) is rapidly secreted
through the same transporter used to secrete
penicillin. PAH is a derivative of para-aminobenzoic
acid (PABA) – used by bacteria to make folic acid.
• In WW II, PAH was administered with penicilin to
decrease Penicillin’s secretion. Probenecid was also
used for this purpose.
Loop of Henle
Distal Tubule and Cortical Collecting
Tubule
Early Distal Tubule
Late Distal Tubule and Cortical
Collecting Tubule
Medullary Collecting Duct
Flow of Fluid into the Peritubular
Capillaries
Flow of Fluid into the Peritubular
Capillaries
Top panal: Normal
Bottom panal: Increased peritubular
capillary hydrostatic pressure or
decreasing osmotic pressure.
Pressure Diuresis and Natriuresis
Forming a Dilute Urine
•Key to this antidiuretic
hormone (ADH) or
vasopressin, which
increases the permeability
of the distal tubules and
collecting ducts to H2O.
•When there is excess H2O
in the body, ADH levels are
low so that little H2O is
reabsorbed in the distal
tubule and collecting
ducts. However, Na+, K+,
and Cl- are reabsorbed.
Forming a Concentrated Urine
• H2O is reabsorbed in the distal tubule and
collecting ducts.
• 2 Keys:
- High level of ADH, to increase H2O reabsorption in
the distal tubules and collecting ducts.
- A high osmolarity of the medullary interstitial fluid.
• What process makes the medullary interstitial
fluid hyperosmotic?
Countercurrent Mechanism
Countercurrent Mechanism
Countercurrent Mechanism
Countercurrent Mechanism
Countercurrent Mechanism
Actually, the Na+ will make the medullary interstitial fluid ~ 600 mOsm/L
Urea, in the presence of ADH, contributes to achieve 1,200 mOsm/L.
Urea
•When there is a H2O deficit
and ADH is high, urea
becomes concentrated in
the distal tubule and cortical
collecting tubule when H2O
is reabsorbed.
•This high [urea] reaches the
medullary collecting duct.
There, ADH increases
permeability to urea and
activates urea transporters.
•Thus, urea diffuses out into
the interstitial fluid and
increases the osmolarity.
The high osmolarity of the medullary
interstitium increase H2O absorption
when H2O permeability is high (ADH).
This creates a concentrated urine.
Urea Recirculation
Forming Dilute and Concentrated Urine
Hormones Controlling Renal Function
• ADH or Vasopressin
- Increases H2O permeability of the distal tubul, cortical
collecting tubule, and medullary collecting tubule. Also,
- Increases the permeability of urea in the medullary
collecting tubule.
• Aldosterone – stimulates the Na+-K+-ATPase in
the cortical collecting tubule.
• Angiotensin II
- Stimulates aldosterone secretion.
- Constricts efferent arterioles.
- Stimulates Na+-K+-ATPase in the proximal tubule, loops of
Henle, distal tubules and collecting tubules.
Regulation of pH
• 3 Primary systems for regulating the pH of the
body:
1. Acid-base buffers of the body fluids.
2. Removal of CO2 via respiration.
3. Renal excretion of acid or alkaline urine.
• Today, we will briefly cover the bicarbonate
buffer and renal mechanisms.
Bicarbonate Buffer
Renal Control of
Acid-Base Balance
•The balance is achieved by
regulating the secretion of H+.
•Bicarbonate must react with H+
before it can be reabsorbed.
•If H+ is low, the kidneys do not
reabsorb as much bicarbonate,
increasing the amount of
bicarbonate excreted.
•If H+ is high, the kidneys
reabsorb nearly all the
bicarbonate. The excess H+ in the
tubular lumen combines with
phosphate and ammonia and is
excreted as salts.