L6 Water And Electrolyte Balance

Homeostasis Lecture 6: Regulation of Water & Electrolyte Balance

MBBS PhaseI ModuleVII( 2008-2009)

Learning Objectives: 1.Define the term homeostasis & explain its importance to living organisms. 2.Describe the mechanisms for regulation of water & electrolyte balance. 3.Explain how chemical buffers regulate blood pH. 4.Explain how the lungs & red blood cells regulate blood pH. 5.Explain how the kidney plays a key role in regulation of body pH. 6.Describe the Anion gap & explain its use in study of acid-base disorders.

Definition Homeostasis refers to the maintenance of physical and chemical characteristics of extracellular fluid which represents the internal environment of the body. The organ systems involved in homeostasis are: • Circulatory system • Respiratory System • Gastrointestinal System • Musculoskeletal System • Renal System • Neuroendocrine System

Water Balance Total body water : 60% of total body weight (ECF- 33% & ICF – 67%) Sources of Water: 1. Exogenous 2. Endogenous Water output : 1. Urine (1-2 L/day; 500ml mini. Excretory vol). 2. Skin (450ml/day) 3. Lungs (400ml/day) 4. Feces (150 ml/day)

Electrolyte Balance 1. Sodium – Major extracellular cation. 2. Potassium – Major intracellular cation. General Functions of Electrolytes • Osmolality of body fluids • Maintaining hydration of cells • pH of body fluids • Membrane potential & electrical neutrality • Participation in metabolic events

Regulation of Water & Electrolyte Balance

Three mechanisms are involved: 4. Anti-diuretic hormone (ADH) 5. Renin-Angiotensin-Aldosterone System 6. Thirst Center

1. Anti-diuretic hormone Its secretion is regulated by: a) Osmoreceptors (in hypothalamus) b) Baroreceptors (in right atrium & carotid sinus) a.Osmoreceptors These respond to changes in ECF osmolality of about 2%. (ECF osmolality is determined by sodium)

b. Baroreceptors Respond to Intravascular Volume (IVV) changes of about 10% 1) ↓ IVV →

↑ ADH release ↓ Restores IVV ← ↑ renal H2O reabsorption

  2) ↑ IVV → ↓ ADH release ↓ Restores IVV ← ↓ Renal H2O reabsorption

2) Renin – Angiotensin -Aldosterone System:

3) Thirst Centre :   (↑ECF Na+ ↑ ECF osmolality → Thirst ↑ oral fluid intake ECF osmolality

Restoration of

Clinical Presentations of Water & Electrolyte Imbalance

1.  2.  3.  4.  5.  6.

Dehydration Overhydration Hypernatremia Hyponatremia Hyperkalemia Hypokalemia

Investigations used to study Water & Electrolyte Inbalance are :   Serum Na+, K+, Urea, Creatinine, Osmolality Urine Na+, K+, urea, Creatinine, Osmolality   N.V. → serum Na+ 135 - 145 mmol/L K+ 3.5 – 5.0 mmol/L Urea 10 – 35 mg/dl Creatinine – 0.6 – 1.2 mg/dl  Serum Osmolality 285 ± 10 mmol/Kg  Urine Osmolality (depends on the state of hydration) 50 - 1200 mmol/Kg

Laboratory Investigation of Hypernatremia

Laboratory Investigation of Hyponatremia

Laboratory Investigation of Dehydration

CASE STUDY 1

 

A 10 -year-old boy was referred to a pediatrician due to recent onset of marked swelling of his ankles. On examination he had pitting oedema of both legs. He was normotensive . The following investigations were performed:   Test (Serum)   Creatinine Urea Sodium Potassium Total protein Albumin

Result 0.9 32 137 4.0 4.4 1.8

Urine Total protein 17.6   Discuss the results and give your diagnosis.

Unit

Reference Range

mg/dL mg/dL mmol/L mmol/L g/dL g/dL

0.2 10 135 3.5

-

g/24 hours

< 0.15

1.2 40 145 4.8 6.0 - 8.0 3.2 - 4.5