Fluid, Electrolyte, and Acid-Base Balance: ANP College Study Guide

Fluid and Electrolyte Balance

Overview of Fluid Compartments

The human body maintains a delicate balance of fluids and electrolytes, which is essential for normal physiological function. Body fluids are distributed into two main compartments: extracellular fluid (ECF) and intracellular fluid (ICF). ECF includes interstitial fluid, plasma, and other body fluids, while ICF refers to the cytosol within cells.

• ECF: Comprises plasma, interstitial fluid, and minor components such as lymph, cerebrospinal fluid, and synovial fluid.

• ICF: Makes up about two-thirds of total body water, primarily within the cytosol of cells.

Fluid and Electrolyte Balance

Fluid balance is achieved when water intake equals water loss. Electrolyte balance occurs when ion intake matches ion loss. Acid-base balance is maintained when hydrogen ion (H+) gain is offset by loss.

• Fluid balance: Maintains constant body water volume.

• Electrolyte balance: Regulates ions such as sodium, potassium, and calcium.

• Acid-base balance: Keeps pH within the narrow range of 7.35–7.45.

Major Electrolytes in Body Fluids

Electrolytes are ions that conduct electricity in solution. Their concentrations differ between ECF and ICF, which is crucial for cellular function.

Cation	Symbol	Extracellular	Intracellular
Sodium	Na+	135–145 mM	10 mM
Calcium	Ca2+	2–3 mM	0.1 μM
Potassium	K+	3.5–5 mM	140 mM
Protein	—	7 g/dL (7%)	—
Glucose	—	70–100 mg/dL	—
Osmolarity	—	280–300 mOsm/L	280–300 mOsm/L

Regulation of Fluids and Electrolytes

Homeostatic Mechanisms

Homeostatic mechanisms respond to changes in ECF, as there are no direct receptors for fluid or electrolyte balance. Water moves passively in response to osmotic gradients, and body content rises if intake exceeds outflow.

• Regulation: Monitors plasma volume and osmotic concentration.

• Water movement: Passive, driven by osmotic gradients.

Primary Regulatory Hormones

Three hormones play key roles in fluid and electrolyte regulation:

• Antidiuretic hormone (ADH): Stimulates water conservation and activates the thirst center.

• Aldosterone: Controls sodium absorption and potassium loss in the distal convoluted tubule (DCT).

• Natriuretic peptides (ANP and BNP): Reduce thirst and block ADH and aldosterone release.

Fluid Movement and Water Balance

Fluid moves freely within the ECF compartment. Water losses are balanced by gains from eating, drinking, and metabolic generation.

Source	Daily Input (ml)
Water content of food	1000
Water consumed as liquid	1200
Metabolic water produced during catabolism	300
Total	2500

Method of Elimination	Daily Output (ml)
Urination	1200
Evaporation at skin	750
Evaporation at lungs	400
Loss in feces	150
Total	2500

Fluid Shifts and Tonicity

Fluid Shifts Between ECF and ICF

Water moves between ECF and ICF based on tonicity. If ECF is hypertonic, water moves from ICF to ECF; if ECF is hypotonic, water moves into cells.

• Isotonic: No net gain or loss of water.

• Hypotonic: Net gain of water into cell (hemolysis).

• Hypertonic: Net water flow out of cell (crenation).

Electrolyte Balance

Sodium Balance

Sodium uptake is proportional to dietary intake, and losses occur through urine and perspiration. Homeostatic mechanisms correct large variations in sodium concentration.

• Low sodium: ADH and aldosterone secreted.

• High sodium: ANP secreted.

Potassium Balance

Potassium concentrations in ECF are low and less tightly regulated than sodium. Excretion increases as ECF concentrations rise, aldosterone is secreted, or pH rises. Retention occurs when pH falls, as K+ is exchanged for H+.

Acid-Base Balance

Importance of pH Control

The pH of ECF is tightly regulated between 7.35 and 7.45. Deviations can cause acidosis or alkalosis, affecting all body systems and potentially leading to coma, cardiac failure, or circulatory collapse.

• Acidemia: pH below 7.35 (acidosis).

• Alkalemia: pH above 7.45 (alkalosis).

Types of Acids in the Body

• Volatile acids: Can leave solution and enter the atmosphere (e.g., carbonic acid).

• Fixed acids: Do not leave solution (e.g., sulfuric and phosphoric acids).

• Organic acids: By-products of aerobic metabolism (e.g., lactic acid).

Key equation: $\mathrm{CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-}$

Buffer Systems

Buffer systems resist changes in pH. The three major systems are:

• Protein buffer system: Amino acids act as weak acids or bases.

• Hemoglobin buffer system: Buffers H+ in red blood cells.

• Carbonic acid-bicarbonate buffer system: Buffers changes caused by organic and fixed acids.

Regulation of Acid-Base Balance

Acid-base balance is maintained by buffer systems, respiration, and renal function. The lungs regulate pH through changes in respiratory rate, affecting PCO2. The kidneys regulate pH through renal compensation, adjusting H+ and HCO3- excretion.

Acid-Base Disorders

Respiratory Acid-Base Disorders

Respiratory disorders result from abnormal respiratory function, causing changes in CO2 levels in ECF.

Metabolic Acid-Base Disorders

Metabolic disorders are caused by generation of organic or fixed acids, or changes in bicarbonate ion concentration.

• Metabolic acidosis: Depletion of bicarbonate reserve, inability to excrete H+, or excessive acid production.

• Metabolic alkalosis: Elevated HCO3- concentrations, often due to repeated vomiting.

Detection of Acid-Base Disorders

Diagnostic blood tests measure blood pH, PCO2, and bicarbonate levels to distinguish between respiratory and metabolic disorders.

Summary Table: Key Fluid and Electrolyte Concepts

Concept	Definition	Example
Fluid Balance	Water intake equals water loss	Urine output matches water consumed
Electrolyte Balance	Ion intake equals ion loss	Sodium regulation by aldosterone
Acid-Base Balance	H+ gain equals H+ loss	Buffering by bicarbonate system

Additional info:

• Metabolic water is produced during catabolism of nutrients, contributing to daily water balance.

• Clinical conditions such as hyponatremia, hypernatremia, and dehydration are directly related to fluid and electrolyte imbalances.

• Buffer systems are essential for maintaining pH stability in the body, with protein, hemoglobin, and carbonic acid-bicarbonate systems playing major roles.