Chapter 1 Introduction

“Renal physiology has now passed into a quantitative phase where unsupported speculation and empirical description are no longer warranted.”

— Homer Smith, 1951

One of the great attractions of renal physiology is that is amenable to simple quantitative analysis. Long before we learnt of their molecular basis, the fundamental processes of the kidney were understood by Homer Smith and others in terms of quantitative models.

In learning these models, the student of nephrology comes to understand the physical basis of the underlying physiology. For example, the fractional excretion (2.2) of a substance is a concept that considers glomerular filtration and tubular reabsorption as independent, sequential processes: as they are in reality. Moreover these equations can be used to diagnose electrolyte and acid-base disorders and to guide their clinical management.

However, the equations can be difficult to remember, as can the assumptions that underlie their validity. As a result they are often not applied – or worse, misapplied. Therefore this book aims to provide a quick-reference guide to clinicians wishing to use simple calculations to help diagnose and treat disorders in electrolyte and acid-base homeostasis.

1.1 Work in progress

At present this is a somewhat rough-and-ready collection of short notes. It is very much a work in progress. However - in the spirit of #FOAM - I will keep it in the public domain as I edit and update it. Any comments are very gratefully received (@renalrob on twitter).

1.2 What this book is - and what it is not

This book was initially written as a personal, quick-reference aide-memoire. It is written for those moments when one cannot quite remember how to interpret a TTKG or whether you are supposed to add-on or subtract to correct the anion gap in hypoalbuminaemia. It assumes a certain amount of pre-existing knowledge of basic electrolyte, acid-base and renal physiology.

This book does not aim to act as a guide to clinical management (for the reasons stated in the disclaimer). Neither does this book aim to provide a comprehensive discussion of renal or fluid-electrolyte physiology; there are plenty of excellent books and articles that already do this.

1.3 How to use this book

This book was written so that it should be easy to read on a smartphone. However, some of the tables and equation panels will only render correctly when the phone is held in landscape orientation.

Equations are in blue panels, along with any associated notes or assumptions.

Clinical checklists or algorithms are in yellow panels.

A guide to notation and glossary of abbrevations is given in appendix 1.