Kidney 3

*****Summary of transport across proximal tubule of Nephron***** Big point to know about regulation here. Reabsorption and/or secretion of 1. Na+ 2. glucose 3. amino acids 4. phosphate 5. H20 6. urea 7. K+ 8. H+ 9. Organic ions

1. 67% of filtered Na+ actively reabsorbed; not subject to control; Cl- follows passively 2. All filtered glucose and amino acids reabsorbed by secondary active transport (via Na+); not subject to control 3. Same as 2. 4. Variable amounts of filtered phosphate and other electrolytes reabsorbed; subject to control 5. 65% of filtered H2O osmotically reabsorbed; not subject to control 6. 50% of filtered urea passively reabsorbed; not subject to control 7. Almost all filtered K+ reabsorbed; not subject to control 8. Variable H+ secretion depending on acid base status of body 9. Organic ion secretion; not subject to control. ***Basically, the ways in which things are reabsorbed and secreted remain pretty constant in proximal tubule - it's in the distal and collecting tubules where you see things regulating reabsorption or secretion

*What are the 3 most important substances secreted by the tubules and at what points are they secreted in the loop of Henle?

1. H+ = secreted in proximal, distal, and collecting tubules 2. K+ = secreted only in distal and collecting tubules under control of aldosterone 3. Organic ions - secreted only in the proximal tubule

In the kidney tubules - where does the majority of Na+ reabsorption occur -

1. Proximal tubule - 67% 2. Ascending limb of the loop of Henle - 25% 3. Distal and collecting tubules - 8%

Role of Na+ reabsorption in 1. Proximal tubule 2. Ascending limb of the loop of Henle 3. Distal and collecting tubules

1. Proximal tubule - Plays role in reabsorbing glucose, amino acids, H2O, Cl-, and urea 2. Ascending limb of Loop of Henle - Plays critical role in kidneys' ability to produce urine of varying concentrations 3. Distal and Collecting tubules - Variable and subject to hormonal control; plays role in regulating ECF volume

*****Summary of transport across distal tubule and collecting duct of Nephron***** Big point to know here 1. Na+ 2. H2O 3. H+ 4. K+

1. Variable Na+ reabsorption, which is controlled by aldosterone; Cl- follows passively 2. Variable H2O reabsorption, controlled by vasopressin/ADH 3. Variable H+ secretion, depending on acid base status of body 4. Variable K+ secretion, controlled by aldosterone In the proximal tubule of nephron, you don't see variable reabsorption and secretion, things are mainly constant. But here in the distal tubule is where all the hormones and chemicals act to modulate what will be the final product.

***See slides 21-24 of kidney 4 to learn diagrams Explain the fate of the filtered plasma and give examples 1. For a substance filtered and not reabsorbed or secreted 2. For a substance filtered, not secreted, and completely reabsorbed 3. For a substance filtered, not secreted, and partially reabsorbed 4. For a substance filtered and secreted but not reabsorbed

1. inulin - all of the filtered plasma is cleared of the substance 2. glucose - none of the filtered plasma is cleared of the substance 3. urea - only of portion of the filtered plasma is cleared of the substance 4. Hydrogen ion - all of the filtered plasma is cleared of the substance, and the peritubular plasma from which the substance is secreted is also cleared.

1. Glucose and amino acids are reabsorbed by x-dependent, secondary active transport 2. How are electrolytes other than Na+ reabsorbed? 3. How does the reabsorption of H20 following Na+ reabsorption affect urea transport? 4. Generally, Generally, x products are not reabsorbed

1. sodium 2. Electrolytes other than Na+ that are reabsorbed by the tubules have their own independently functioning carrier systems within the proximal tubule 3. The reabsorption of water in the proximal tubule increases the concentration of urea in the tubule, as water is lost from the tubule. This produces a concentration gradient for urea from the tubule into the interstitial fluid. 4. Generally, unwanted waste products are not reabsorbed

*Big Na+ reabsorption review 1. Unlike most of the filtered solutes, Na is reabsorbed throughout most of the tubule, but 2. Of the filtered Na+, x% is normally reabsorbed? 3. Of the Na+ reabsorbed, how much on average is absorbed in proximal tubule, loop of Henle, distal tubule and collecting tubules 4. Of the total energy spent by the kidney, x% is used for Na+ transport, indicating its importance in their function.

1. to varying extents in different regions. 2. Of the filtered Na+ 99.5% is normally reabsorbed 3. Of the Na+ reabsorbed, on average, 67% is reabsorbed in the proximal tubule 25% in the loop of Henle 8% in the distal tubule and collecting tubules 4. Of the total energy spent by the kidney, 80% is used for Na transport, indicating its importance in their function.

Of total energy spent by kidneys, x% is used for Na+ transport

80%

What's essential for Na+ reabsorption into interstitial fluid?

An active Na+ - K+ ATPase pump in basolateral membrane is essential for Na+ reabsorption

How does vasopressin increase the reabsorption of H2O? Diagram where and how it is has its effects. How is this mechanism of water uptake different from that of proximal tubule?

Basically, blood borne vasopressin exits peritubular capillary - binds to its receptor on basolateral membrane of principal cell in distal or collecting tubule Vasopressin binding activates cAMP second messenger system in the cell - which causes the promotion of insertion of AQP-2 water channels into the apical membrane. Membrane usually impermeable isn't anymore and water now enters the cells, goes to other side - and binds with AQP-3,4 channels that are always there and gets reabsorbed.

Important in regulating acid-base balance Secreted in proximal, distal, and collecting tubules

H+

Secreted only in the distal and collecting tubules under control of aldosterone

K+

Diagram K+ secretion from the peritubular capillary to the lumen of convoluted tubule

K+ passive diffusion from the peritubular capillaries, uptake to the cell in distal or collecting tubule via Na+/K+ ATPase, passive diffusion out into lumen via K+ channel

How do substances in the early proximal tubule get reabsorbed into the tubular cell cytoplasm for filtration?

Most of the substances, Glu, AA, Lactic acid, HPO4 2-, are transported in with Na+ via a symport mechanism (active transport).

Accomplish more efficient elimination of foreign organic compounds from the body Secreted only in the proximal tubule

Organic ions

When is tubular reabsorption passive? Active?

Passive reabsorption No energy is required for the substance's net movement Occurs down electrochemical or osmotic gradients Active reabsorption Occurs if any one of the steps in transepithelial transport of a substance requires energy Movement occurs against electrochemical gradient

***H2O reabsorption in the various tubular segments of the nephron proximal, loop of henle, distal and collecting tubules

Proximal tubule - 65% H2O reabsorption - Passive; obligatory osmotic reabsorption because of active Na+ reabsorption. Loop of Henle - 15% H2O reabsorption - passive; obligatory osmotic reabsorption from the descending limb as the ascending limb extrudes NaCl into the interstitial fluid and reabsorbs NaCl Distal and Collecting tubules - Passive; not linked to solute reabsorption; variable quantities of "Free" H2O reabsorption subject to vasopressin control; driving force is the VERTICAL OSMOTIC GRADIENT in the medullary interstitial fluid established by the long loops of Henle; important in regulating ECF osmolarity

***Na+ reabsorption in the various parts of the nephron - proximal tubule - loop of Henle - Distal and Collecting Tubules

Proximal tubule - 67% Na+ reabsorption, active and uncontrolled - PIVOTAL ROLE in helping glucose, amino acids, Cl-, H2O, and urea reabsorption via symport. Loop of Henle - 25% reabsorption - active and uncontrolled. Na+(with Cl-) reabsorption from the ASCENDING limb helps establish the MEDULLARY INTERSTITIAL VERTICAL OSMOTIC GRADIENT, which is important in the kidney's ability to produce urine of varying concentrations and volumes depending on body needs Distal and collecting tubules - 8% of Na+ reabsorption - active; variable and subject to aldosterone control; important in the REGULATION OF ECF VOLUME and LONG TERM CONTROL OF BLOOD PRESSURE; linked to K+ secretion and H+ secretion.

Diagram reflex and voluntary control of urination

Reflex- 1. Bladder fills 2. Stretch receptors activate 3. Parasympathetic nerve activated while motor neuron to external sphincter inhibited 4. Bladder contracts 5. Internal urethral sphincter mechanically opens when bladder contracts and external urethral sphincter opens when motor neuron is inhibited. Voluntary - 1. Bladder fills 2. Stretch receptors activate and try to activate parasympathetic nerves and inhibit motor neuron to external sphincter. Voluntary neurons from cerebral cortex say no no and stimulate motor neuron to external sphincter 3. External urethral sphincter remains closed when motor neuron is stimulated 4. No urination

What follows reabsorbed sodium?

Water follows reabsorbed sodium by osmosis which has a main effect on blood volume and blood pressure

2 ways that NaCl is reabsorbed

can be absorbed via the paracellular pathway, or it can enter the cytoplasm via Cl- antiport with base and Na+ antiport with an H+. Na+ antiport with K+ out to IF, while Cl- passively goes out

Na+ is not reabsorbed in the x of the loop of Henle

descending limb

Kidney tubules can selectively add some substances to the substances that are already x

filtered

Metabolic acidosis

inability of the kidneys to adequately secrete H+ that is continually being added to the body fluids as a result of metabolic activity

At the end of proximal tubule, what happens in urea transport?

passive diffusion of urea down its concentration gradient. As H20 gets reabsorbed, this increases urea concentration in tubule, which then passively diffuses down its gradient.

Potassium retention

results from inadequate tubular secretion of K+

Uremic toxicity

retention of waste products

Involves the transfer of substances from tubular lumen into peritubular capillaries

tubular reabsorption

When does transepithelial transport happen in the kidney

tubular reabsorption

Transfer of substances from peritubular capillaries into the tubular lumen

tubular secretion