Renal Physiology: Chpt 40- Renal Sodium and Potassium Regulation

Movement of a solute from the interstitial fluid across the basolateral plasma membrane of the cell, through the cytoplasm, and across the luminal (apical) plasma membrane of the cell is an example of what renal process?

Secretion

NaCL Intake = NaCl Loss

Has everything with homeostasis

Food & Beverage: Need to lose the same amount

• Kidneys will excrete

Intake: 8.50g

Urine: 8.00g (kidneys excreting b/c they change rate of renal absorption)

Output: 8.50g

Transport of Sodium Through Kidneys:

Maintain ECFV by regulating amount of NA+ in the urine (Na+ is freely filtered, reabsorbed, NOT secreted)

Na+ MOST IMPORTANT to ECF osmolality

What is Natriuresis?

Rate of urinary Na+ excretion by kidneys above usual levels (extra sodium in urine)

What is Antiatruresis?

Low rate of Na+ excretion (by the kidney)

How does the SNS correlate?

Acts to promote natriuresis and/or antinatriuresis

MABP is determined by:

Blood volume, Cardiac Output, SVR, Veins

What is Long-Term Control BP

Due to regulation of blood volume

• BV: Major determinant of MABP

• Influences venous pressure, venous return, EDV, SV, CO

What is Long-Term Control BP Controlled by?

CV & Kidney by regulation of BV

• Renin-angiotensin-aldosterone system (RAAS): responsible for regulated blood pressure in the kidneys

• Sodium and Water retention, thirst drive

What is Long-Term Control BP complex and integration of?

Multiple organ systems all working together to maintain homeostasis

Hormonal Regulation of Kidney Function: Renin-angiotension-aldosterone

Arginine vasopressin (ANTIdiuretic hormone), sympathetic nerves (all affect kidney function)

• Increased salt and water reabsorption to increase ECFV

• Decreased salt and water excretion to increase ECFV

• Systemic and renal arteriole constriction to increase MABP

Hormonal Regulation of Kidney Function: Arterial NATRIuretic peptide

Decreased salt and water reabsorption to decrease ECFV

Increased salt and water excretion to decrease ECFV

Systemic and renal arteriole dilation to decrease MABP

Renal Sodium Regulation

Na+ major EC solute, with associated anions, ~ 90% solute in ECF

• Mechanisms regulate kidney Na+- handling related to changes in ECFV

ECF compartment includes plasma volume changes and changes in BP

What type of Feedback is Renal Sodium Regulation?

Negative Feedback Mechanism

• Low plasma Na+ —> low plasma volume —> low BP —> decreased baroreceptors —> increase reflexes to decrease GFR & Increase Na+ reabsorption

Renal Na+ Reabsorption

Na+- most abundant cation in filtrate

Na+ reabsorption almost always active transport

Na+ NEVER secreted by nephron

Active pumping of Na+ via Na+/K+ ATPase

• Generates electrochemical gradient

• Couples to passive entrance of other substances (glucose and amino acids) via co-transporters

What kind of Transport is Renal Na+ Reabsorption

Secondary Active Transport

Na+ Reabsorption- Collecting Duct

Apical trnasoporters for Na+ and K+ are different ion channels

Na+ Reabsorption- Proximal Tubule

“X” represents organic molecules like glucose, amino acids that are cotransported with Na+

H+ is countertransported with Na+

• Hydrogen moving out (secretion)

Basic Renal Processes for Sodium and Water: Na+ Reabsorption

Active process occurring in all tubular segments except descending limb of loop of Henle

• They don’t have proteins

Basic Renal Processes for Sodium and Water: Water Reabsorption

By diffusion, dependent upon Na+ reabsorption

Water moves through aquaporin channels (water always follows after sodium)

• Highly expressed in proximal tubule (2/3 of water)

• Absent in collecting ducts unless antidiuretic hormone, ADH present

What is the Amount of NaCl: Filtered Load

1,500 g/d

What the the Amount of NaCl: Contained in ECF

140g

What us the Amount of NaCl: Excreted in Urine

6 g/d

What happens to the leftover NaCl after it is excreted?

The rest is reabsorbed through primary or secondary active transport

Renin-Angiotension System

1. Angiotensiongen (starting point) (453 aa)

   • Renin-Proteolytic ENZYME- rate limiting step (produced in kidneys)

2. Angiotension I (10 aa)

   • Angiotension converting ENZYME

3. Angiotension II (8 aa)

4. Receptor

   • Aldosterone, Vasoconstriction, Increase Sodium Reabsorption

Renin-Angiotension System: Aldosterone

Steroid hormone, produced by adrenal cortex

• Increases Na+ reabsorption in DT, CD

• Increases synthesis of K+ & Na+ (ENaC) channels, Na+/ K+ ATP pump in CD

JUXTAGLOMERULAR APPARATUS (JGA) =

Macula densa (tubule) + juxtanglomerular cells (vessels) —> secretes endocrine signal- RENIN

Kidney Anatomy: Juxtaglomerular Apparatus: Granular Cells

Part of vasculature

• Mechanoreceptors- stretch receptors sense stretch in afferent arteriole, innervated by SNS

• Enlarged smooth muscle cells with secretory granules, contain hormone renin (synthesize & store)

• part os renin-angiotension system (RAS), raises BP

Kidney Anatomy: Juxtaglomerular Apparatus: Macula Densa Cells

Part of tubule

• Chemoreceptors- respond to changes in NaCl content of filtrate

• Group of tall, closely-packed tubular epithelial cells adjacent to granular JG cells

• Cells work in tandem & critical regulators of BP

Control of Renin Secretion:

Pathways that increase RENIN secretion : Increase ANGIOTENSIN II levels

Decreased Plasma Volume

1. Increase SNS

2. Decrease BP

3. Decrease GFR

Control of Renin Secretion Order:

1. Decrease Plasma Volume

2. Increase Renin

3. Increase Ang II

4. Increase Aldosterone

5. Decrease Na+ & Water Excretion

Atrial Natriuretic peptide (ANP)

Synthesized, secreted- cardiac atria

• Stimulus: Increase Atrial distention and plasma volume

• Response: Increase GFR and Decrease Renal Na+ reabsorption

Increase Na+ & Water Excretion (too much salt & water in kidneys)

What does Aldosterone do to K+ Secretion and Excretion by the Kidneys?

increases

• Too high and drop to adrenal gland

• Urine needs to have more potassium to excrete the urine

Aldosterone- Increases Na+ Reabsorption and K+ secretion in Collecting Duct

Ingesting Increase K+ stimulates aldosterone from adrenal cortex;

Na+ reabsorption (channel) & K+ (channel) secretion; leads to greater K+ excretion in urine

Can you explain each step in the RAS cascade?

1. Liver secretes angiotensiogen

2. Renin release JG cells cleaves angiotensinogen - Angl

3. Angl cleaved by ACE- Angll

4. Angll actos on Hypothalamus- + thirst & ADH

5. Angll acts on adrenal gland- secretes aldosterone- Na+ excretion

6. Angll- Kidney

   • Vasoconstriction AA & EA

   • Increase Na+ reabsorption

7. Net effect = Decrease Na+ & H20

   • Excretion