Explain renal, fluid and electrolyte physiology in O&G, including pregnancy adaptation and critical-care interpretation.

Renal, Fluid and Electrolyte Physiology in O&G

Clinical Overview

Start from one fact that organises everything else: the body is mostly water, and that water lives in compartments that the kidney guards. When obstetrics turns dangerous — pre-eclampsia, haemorrhage, sepsis, hyperemesis, eclampsia treatment — what is going wrong is almost always a disturbance of water, salt, acid or perfusion in those compartments. Renal and fluid physiology is therefore where "routine" obstetrics becomes critical care. It explains why pregnancy creatinine should be low, why estimated GFR equations mislead, why pre-eclampsia can produce oedema and intravascular vulnerability at the same time, why magnesium becomes dangerous in oliguria, why hyperemesis needs thiamine and potassium thinking, why sepsis causes capillary leak, and why excess crystalloid can worsen pulmonary oedema without fixing oxygen delivery.

The kidney is not just a urine-making organ. It is the system that regulates effective circulating volume, osmolality, sodium, potassium, acid-base balance, waste excretion, blood pressure, erythropoietin and vitamin D activation. Pregnancy pushes the kidney into a high-flow, high-filtration state and then asks clinicians to interpret numbers using a new baseline.

The safest Primary model is:

systemic vasodilatation -> renal vasodilatation -> increased renal plasma flow and GFR -> lower creatinine/urea -> altered sodium-water-osmolality set points -> less reserve when pre-eclampsia, sepsis, haemorrhage or CKD is added

This chapter builds that model in order: first the compartments and the forces that move water between them; then how the kidney filters and reabsorbs; then how pregnancy resets each control loop; then the electrolytes and acid-base systems; and finally the high-stakes clinical syndromes — hyperemesis, oliguria, pre-eclampsia, sepsis and fluid prescribing — that fall out of the physiology.

Core Knowledge

Body Water and Its Compartments

Begin with the simplest picture. In a non-pregnant adult, roughly 60% of body weight is water. That total body water divides into two great pools. About two-thirds is intracellular fluid (ICF) — the water inside cells — and about one-third is extracellular fluid (ECF). The ECF itself splits again: most of it is interstitial fluid bathing the cells, and a smaller fraction is plasma, the water inside the blood vessels. For a 70 kg adult that is approximately 42 L of total water: about 28 L intracellular, about 10.5 L interstitial and only about 3.5 L of plasma.

That last number matters more than its size suggests. The intravascular plasma is the smallest compartment, yet it is the one the heart pumps and the organs perfuse. A loss that looks modest in litres can be catastrophic in the plasma compartment, which is exactly why obstetric haemorrhage kills quickly.

The compartments differ not only in volume but in composition, and the difference is the key to electrolyte interpretation:

Two practical lessons follow immediately. First, sodium is the principal extracellular cation and potassium the principal intracellular cation, so a plasma sodium tells you mostly about water, and a plasma potassium can hide a huge total-body deficit because most potassium is out of sight inside cells. Second, plasma carries protein that the interstitial fluid largely lacks, and that protein is what holds water inside vessels. Lose protein or leak it into the interstitium and water follows — the basis of oedema.

What Moves Water Between Compartments

Water does not move because of habit; it moves because of pressure and particle gradients. Across the capillary wall, four Starling forces decide the direction of flow:

At the arteriolar end the hydrostatic push exceeds the oncotic pull and fluid leaves the vessel; at the venous end hydrostatic pressure has fallen and oncotic pressure draws most of that fluid back. A thin gel layer on the inner vessel surface, the glycocalyx, refines this selectivity and is increasingly recognised as a real determinant of leak. Oedema appears when this balance tips outward — when capillary hydrostatic pressure rises (venous obstruction by the gravid uterus pressing on the inferior vena cava in late pregnancy), when oncotic pressure falls (low albumin), or when the barrier leaks (endothelial injury in pre-eclampsia and sepsis). Holding these mechanisms in mind explains why the same word "oedema" can mean a benign late-pregnancy ankle swelling or a life-threatening capillary-leak state.

Osmosis is the partner concept. Osmolality is the concentration of dissolved particles per kilogram of water; it sets the gradient that drives water across cell membranes. Sodium with its accompanying anions, plus glucose and urea, are the main contributors, giving a normal plasma osmolality of about 290 mosmol/kg. Because sodium dominates, plasma sodium is largely a water-balance signal, not a sodium-store signal — a point we return to repeatedly.

What the Kidney Is Trying to Keep Stable

With the compartments and forces in place, the kidney's job becomes intuitive: it protects the internal environment while intake, losses and disease vary. It does this by filtering plasma, reclaiming what is useful, secreting what must be removed, and adjusting water and electrolytes under hormonal control.

Every fluid prescription should answer three questions:

1. What compartment is deficient or overloaded?
2. What solute problem accompanies the water problem?
3. Will this fluid improve oxygen delivery, or only make the chart look treated?

Filtration: Glomerular Physiology

Glomerular filtration is driven by pressure across a capillary filter. Blood enters through the afferent arteriole, passes through glomerular capillaries and leaves through the efferent arteriole. Water and small solutes enter Bowman's space; cells and most proteins stay in blood. The glomerular basement membrane acts as both a size filter (holes too small for cells and large proteins) and a charge filter (it normally resists negatively charged proteins like albumin). Damage either property — as glomerular disease and pre-eclamptic endotheliosis do — and protein leaks into the urine.

GFR is the filtered volume per minute. Serum creatinine is a rough marker because creatinine is produced by muscle at a fairly steady rate and cleared mainly by filtration. It is not a perfect marker — it lags behind acute changes, depends on muscle mass and is not linearly related to GFR — but in pregnancy it remains far more useful than eGFR equations. A useful Primary refinement: because creatinine roughly doubles for every halving of GFR, watching the trend in creatinine (and even its reciprocal) over time detects deteriorating function earlier than waiting for one value to cross a reference limit.

Tubular Handling: Reabsorption and Secretion

Filtering plasma is only the first step; the filtrate would empty the body of salt and water in hours if the tubule did not reclaim it. The tubule turns filtrate into urine segment by segment.