The Icebreaker: Contraction Alkalosis

The Icebreaker:

What is Contraction alkalosis??

The word 'contraction' in contraction alkalosis refers to fluid contraction or in other words, fluid loss.

So when the body loses fluid (typically seen with diuretics) it tries to get all that fluid back.

Q: How can the body do this? 

A: By increasing the serum sodium concentration. (increased sodium concentration --> increased osmolarity of serum --> increase in intravascular volume)

Q: How does the body do it?

A:1) The decreased blood volume is sensed as decreased blood pressure felt by the kidneys at the glomerulus ie. decreased renal perfusion pressure. There are modified smooth muscle cells in the afferent arteriole that detect this change called JG cells. They secrete Renin in response to the low pressures. 

2) By the diagram shown below Renin activates an axis that gives rise to Angiotensin II and aldosterone.

 The Renin-Angiotensin-Aldosterone axis 

3) Angiotensin II  causes some changes in the kidney including:

a) Increased Na exchange for H+ at the proximal tubule

b) Increased HCO3- reabsorption at the proximal tubule

4) Aldosterone exerts its effects on the Distal Tubule and Collecting Duct by:

c) Absorption of Na+  by principal cells. (This is by opening of certain channels called ENac, that selectively increase the permeability of Sodium here)

d) Increased distal tubule H+ secretion by alpha intercalated cells. (Predominantly by H+ ATPase)

e) Increased production of HCO3- in alpha intercalated cells

The distal tubule and collecting duct

So we're essentially seeing:

i) an increase in Na+ resorption that helps maintain inravascular fluid volume (points a + c)

ii) Increased secretion of H+ and reabsorption of HCO3- in the kidneys ( points c+d+e)

                 (Picture from a screenshot of a video 'Metabolic Alkalosis: ABG intepretation - Lesson 10' from Eric Strong, MD, Stanford university, You can checkout his Youtube channel:Strong Medicine)

Hence the pH rises above 7.45 causing an alkalosis as Serum HCO3- is conserved and H+ lost.

Hence the pH of the blood increases with loss of intravascular fluid causing contraction Alkalosis.

PS: If the renal perfusion pressures are increased the renin secretion would stop and so would this entire process: Hence the treatment involves Intravenous NaCl.

By Dr Ashwin Mahesh