controlling sodium

What is Effective circulating volume?

portion of ECF that is effectively perfusing the tissues

What is plasma [Na +] an indication of?

It is NOT an indication of the quantity of sodium in the body but is an indication of body water content

How does the pathway that controls blood pressure lead to a change in sodium concentration?

How do we lose sodium and what can we control?

urine + faeces

The only output that can be regulated is alteration in sodium excretion by the kidneys

Where are baroreceptors found in Low-Pressure Cardiopulmonary Circuit?

Atrial myocytes

Cardiac atria

Right ventricle

Large pulmonary vessels

Where are baroreceptors found in High-Pressure Cardiopulmonary Circuit?

Aortic arch

Carotid sinus

Afferent arterioles of the kidneys (juxtaglomerular apparatus)

What do low and high-pressure zones baroreceptors detect?

low: Changes in effective circulating volume
high: Changes in arterial pressure

What is more important: maintaining ECF or osmolality?

Defence of ECV usually has priority over defence of osmolality

What factors simulate the RAAS system?

1. Decreased renal perfusion pressure

2. Increased sympathetic nerve activity (β-adrenergic )

3. Decreased delivery of NaCl to the macula densa (NaCl sensor)

How does the RAAS system release ANG II?

juxtaglomeular cells sense low bp → release renin

liver makes angiotensinogen 

renin convets ANG to ANG I → reaches lungs → lungs release ACE → converts ANG I to ANG II

How does the RAAS system lead to decreased natriuresis and diuresis?

stimulates zona glomerulosa cells (adrenal cortex) → release aldosterone → DCT, thick ascending limb, collecting duct → increases Na+ reabsorption
• Stimulate supraopric nucleus of hypothalamus → release ADH → v2 receptors on collecting duct
• Stimulate hypothalamic thirst centres
• Act on PCT → reabsorption of Na+ and H20
• Stimulates vasoconstriction in blood vessels → increase TPR

How does ANP Increased natriuresis and diuresis

increase in bp is felt in the atria → release ANP

→ increased GFR → more Na+ and water excretion (less reabsorption)

→ inhibit renin secretion → no ANG II → decresed ADH + aldosterone

→ arteriole vasodialation → decreased TPR

How do renal sympathetic nerves decrease natriuresis and diuresis?

decreases GFR

increases renin secretion

increases Na+ reabsorption along the nephron

How does the nephron maintain Constant Na + Delivery to the Distal Tubule in Euvolemia

Autoregulation of the GFR: keeps the filtered amount of Na + constant

Glomerulotubular balance: if GFR increases → amount of Na + reabsorbed by PCT increases proportionately

Load dependence of Na+ reabsorption by the loop of Henle: loop of Henle to can increase its reabsorptive rate in response to increased delivery of Na+ 

How do we regulate the Distal Tubule and Collecting Duct Na + Reabsorption in Euvolemia?

Aldosterone is the primary regulator of Na + reabsorption by the distal nephron 

High aldosterone levels = increased Na + reabsorption (decreased Na + excretion)

Low aldosterone levels = decreased Na + reabsorption (increased Na + excretion)

How does the nephron Control Na+ Excretion with Volume Expansion (hypervolemia)?

1. less sympathetic activity → GFR increases → increased filtration of Na+

2. Reabsorption of Na + decreases in proximal tubule and loop of Henle

3. more Na+ in the DCT → macula densa → less renin + constriction of afferent arteriole → less water reabsorbed → more dilute urine 

How does the nephron Control Na+ Excretion with Volume contraction (hypovolemia)?

1. more sympathetic activity → GFR decreases→ decreased filtration of Na+

2. Reabsorption of Na + increases in proximal tubule and loop of Henle

3. less Na+ in the DCT → macula densa → more renin + dialation of afferent arteriole → more water reabsorbed → more concentrated urine 

How do we treat extracellular volume expansion?

Treat the underlying cause

Discontinue medications that promote sodium retention (e.g., NSAIDs)

Decrease dietary sodium

diuretics

How do we treat extracellular fluid volume contraction?

Mild: can be corrected orally
Severe: intravenous infusions

usually give normal saline (isotonic)

define oedema

Accumulation of excess fluid within the interstitial space

What causes oedema?

high capillary hydrostatic pressure

increased capillary permeability

reduced plasma oncotic pressure

lymphatic obstruction

renal retention of dietary sodium and water, → expanding the ECF volume

How can the kidney lead to oedema formation when there is myocardial dysfunction?

low bp + CO → low ECV (not ECF) → kidneys retain more Na+ and H20 → increased ECF + blood volume → increase venous hydrostatic pressure  → fluid moves into interstitium → oedema

How can we treat oedema?

decrease dietary NaCl intake
Inhibit the kidneys’ ability to retain NaCl: diuretics

Where is Na+ reabsorption adjusted in the nephron in euvolemia?

distal tubule and collecting duct