ICU learning pathway | Regulation of Electrolyte and Acid-Base Balance

Regulation of Electrolyte and Acid-Base Balance
Session overview
Description
This session expands the role of antidiuretic hormone, and discusses other hormonal systems involved in regulating sodium absorption. The concept of 'effective circulating volume', which is central to these systems, is defined.

By the end of this session you should be able to:

• Describe the concept of effective circulating volume (ECV)
• Relate the renin-angiotensin-aldosterone system (RAAS) to ECV
• Describe the actions of renin, angiotensin and aldosterone
• Outline the other factors involved in regulation of sodium reabsorption

Before commencing this session you should complete:

• Renal Morphology, Blood Supply and Glomerular Filtration (001-0654)
• Tubular Transport and the Proximal Tubule (001-0655)
• Loop of Henle, Distal Tubule and Collecting Tubule (001-0656)

Antidiuretic hormone (ADH) is released in response to rises in plasma osmolality. Sodium salts are the main osmotically active solutes of the extracellular fluid (ECF). It follows that if the extracellular sodium concentration changes, then ECF volume also changes and that regulation of ECF volume is brought about by regulation of body sodium content (Fig 1).

ADH is also released in response to changes in the effective circulating volume (ECV), which is defined in this session.

Sodium excretion can be altered by changes in either filtration or reabsorption. Large changes in ECF volume can change glomerular filtration rate (GFR).

The focus of this session is on exactly how sodium reabsorption is modified by changes in ECF volume.