The Long-Term Control of Blood Pressure

Question 1

WHY IS Na+ SO IMPORTANT

Answer 1

• Na+ (with its associated anions Cl-
and HCO -) is the main osmotic constituent of the ECF
• Where Na+ moves, H2O must follow
• The body maintains ECF osmolality ~290 mOsm within narrow limits
• Thus, whole-body Na+ content – which the kidneys control – is the major determinant of ECF volume

Question 2

Oral Na+ intake

Answer 2

= Renal Na+ output + Extrarenal Na+ output

Question 3

extra renal Na+ output

Answer 3

Except for large fluid losses:
• GI tract (vomiting or diarrhoea)
• Skin (excessive sweating, extensive burns
these result in extra renal Na+ loss and therefore the kidneys would response by reducing Na+ secretion

Question 4

if there is excessive Na+ intake

Answer 4

the kidneys excrete surplus Na+

Question 5

what does renal excretion of Na+ depend on

Answer 5

AMOUNT of Na+ in the body (not concentration of Na+ in the ECF)

Question 6

Total body Na+

Answer 6

ECF volume x [Na+]ECF (with osmolality ~constant)

Question 7

what acts as a signal for Na+ homeostasis

Answer 7

the volume of ECF

Question 8

For ECF volume expansion to stimulate Na+ excretion

Answer 8

• This must occur in ECF compartments with volume sensors • Blood-filled compartments

Question 9

Critical parameter for regulating Na+ excretion is the

Answer 9

effective circulating volume
• “functional blood volume”
• Reflects extent of tissue perfusion in specific regions (detected as fullness/pressure in their blood vessels)

Question 10

EFFECTIVE CIRCULATING VOLUME

Answer 10

This can be distorted in disease
• Changes in effective circulating volume no longer parallel ECF volume
• Congestive heart failure, nephrotic syndrome, liver cirrhosis
• Total ECF volume is grossly expanded (oedema/ascites)
• But the ECV is low, therefore increasing Na+ retention

Question 11

volume expansion

Answer 11

VOLUME EXPANSION AND CONTRACTION
EXPANSION
• When Na+ intake persists in the face of impaired Na+ excretion
• Body retains isosmotic fluid
• Expansion of plasma fluid volume and of the interstitial fluid compartment
• When severe, interstitial volume increase so severe that subepidermal tissues swell (e.g. ankles) - pitting oedema

Question 12

volume contraction

Answer 12

• Excessive loss of Na+ into the urine.
• Dramatic shrinkage of the ECF volume, e.g. hypovolaemic shock
• Prolonged use of diuretics
• Osmotic diuresis in poorly controlled diabetes mellitus
• Adrenal insufficiency
• Recovery phase after AKI / urinary obstruction