Chapter 20: Integrative Physiology II – Fluid and Electrolyte Balance

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20.1 Fluid and Electrolyte Homeostasis

Overview of Fluid and Electrolyte Regulation

Fluid and electrolyte homeostasis is essential for maintaining the internal environment of the body. The body uses several mechanisms and organ systems to regulate the excretion and retention of ions and water, ensuring cellular function and overall health.

Excretion Routes: Kidneys (primary), feces, sweat, lungs (water and bicarbonate via CO2).
Behavioral Mechanisms: Thirst and salt craving help regulate intake.
ECF Osmolarity: Changes affect cell volume and function.
Integrated Systems: Rapid responses via respiratory and cardiovascular systems (neural control); slower responses via kidneys (endocrine/neuroendocrine control).
beta cells- kidneys

20.2 Water Balance

Regulation of Water Intake and Loss

Water balance is achieved when daily intake equals daily loss. The kidneys play a central role in conserving water, but cannot restore lost volume.

Intake: Ingestion, metabolism, intravenous fluids.
Loss: Urine, feces, insensible loss (skin, exhalation), pathological (diarrhea, vomiting, excessive sweating).
Kidney Function: Conserve water by adjusting urine concentration.
I > O (congestion)
O < I (dehydration)
I = O (healthy)

Example: Average adult intake and output is about 2.5 L/day.

3 ways the body can increase Blood Pressure for endocrine sys

ADH/ Vasopressin
Aldosterone
Renin (RAAS renin angiotensin aldosterone system) most potent

The Renal Medulla Creates Concentrated Urine

Mechanisms of Urine Concentration

The osmolarity of urine reflects the amount of water excreted. The renal medulla's interstitial osmolarity allows the kidneys to produce concentrated urine.

Low Osmolarity: High water content in urine.
High Osmolarity: Low water content in urine.
Diuresis: Removal of excess water.
Loop of Henle: Descending limb loses water by osmosis; ascending limb actively transports Na+ out, impermeable to water.
Distal Nephron: Water permeability regulated by hormones (e.g., vasopressin).
Collecting Duct: Can reabsorb additional solute and water.
Descending loop of Henle is permeable to water

Ascending limb of loop of Henle water is impermeable , actively Na+ transported out

Vasopressin Controls Water Reabsorption

Role of Vasopressin (ADH)

Vasopressin (antidiuretic hormone, ADH) regulates water reabsorption in the kidneys by controlling the permeability of the collecting duct epithelium.

Mechanism: Stimulates insertion of aquaporins into the apical membrane, increasing water reabsorption.
Regulation: Blood volume and osmolarity activate osmoreceptors; secretion follows a circadian pattern (less urine at night).

Example: Increased plasma osmolarity leads to increased vasopressin secretion.

The Loop of Henle Is a Countercurrent Multiplier

Countercurrent Exchange and Multiplication

The loop of Henle and vasa recta form a countercurrent multiplier system, essential for concentrating urine and maintaining medullary osmolarity.

Loop of Henle: Ascending limb actively transports solutes into the medulla, increasing ECF osmolarity.
Vasa Recta: Removes water, preserving medullary osmolarity.
Urea: Contributes to interstitial osmolarity.

20.3 Sodium Balance and ECF Volume

Regulation of Sodium and Extracellular Fluid Volume

Sodium balance is crucial for ECF volume and osmolarity. Aldosterone, a steroid hormone from the adrenal cortex, controls sodium reabsorption.

Extra NaCl: Raises osmolarity, triggers vasopressin release and thirst.
Aldosterone: Increases Na+ reabsorption and K+ secretion in distal tubules and collecting ducts.
Regulation: Stimulated by low blood pressure, high plasma K+, and angiotensin II.

The Renin-Angiotensin Pathway

Hormonal Control of Blood Pressure and Sodium

The renin-angiotensin system (RAS) is activated by decreased blood pressure, leading to the production of angiotensin II, which has multiple effects on fluid and electrolyte balance.

Renin: Secreted by juxtaglomerular cells; converts angiotensinogen to angiotensin I.
ACE: Converts angiotensin I to angiotensin II.
Angiotensin II Effects:
1. Stimulates aldosterone production.
2. Increases vasopressin secretion.
3. Stimulates thirst.
4. Potent vasoconstrictor.
5. Increases Na+ reabsorption in proximal tubule.
Pharmacology: ACE inhibitors, ARBs, direct renin inhibitors are used to treat hypertension.

Natriuretic Peptides Promote Na+ and Water Excretion

Hormonal Regulation of Volume Reduction

Natriuretic peptides (ANP, BNP) are hormones that promote sodium and water excretion, reducing blood volume and pressure.

ANP: Produced in atrial myocardial cells.
BNP: Produced in ventricular myocardial cells and certain brain neurons.
Actions: Dilate afferent arterioles (increase GFR), decrease Na+ reabsorption, suppress RAS.
Natriuresis - loss of Na+

20.4 Potassium Balance

Importance of Potassium Regulation

Potassium balance is tightly regulated to maintain normal cellular function, especially in excitable tissues like muscle and heart.

Hypokalemia: Muscle weakness, respiratory and cardiac failure.
Hyperkalemia: Cardiac arrhythmias.
Disturbances: Can result from kidney disease, diarrhea, diuretics.
Aldosterone: Plays a key role in K+ regulation.

20.5 Behavioral Mechanisms in Salt and Water Balance

Behavioral Responses to Fluid and Electrolyte Changes

Behavioral mechanisms complement physiological responses to maintain fluid and electrolyte balance.

Drinking: Replaces fluid loss.
Salt Appetite: Triggered by low plasma Na+ levels.
Avoidance Behaviors: Prevent dehydration (e.g., desert animals avoid heat).

20.6 Integrated Control of Volume and Osmolarity

Homeostatic Responses to Dehydration and Volume Changes

Volume and osmolarity can change independently, requiring integrated homeostatic responses involving multiple systems.

Dehydration: Decreases blood volume/pressure, increases osmolarity.
Compensation: Cardiovascular responses, ANG II, vasopressin, thirst.
Kidneys: Assist in blood pressure homeostasis.
Endocrine Disorders: Diabetes insipidus (low ADH), SIADH (high ADH).

Volume	Osmolarity Decrease	No Change	Osmolarity Increase
Increase	Drinking large amount of water	Ingestion of isotonic saline	Ingestion of hypertonic saline
No Change	Replacement of sweat loss with plain water	Normal volume and osmolarity	Eating salt without drinking water
Decrease	Incomplete compensation for dehydration	Hemorrhage	Dehydration (e.g., sweat loss or diarrhea)

20.7 Acid-Base Balance

Regulation of Plasma pH

Acid-base balance is vital for normal cellular and enzymatic function. The body maintains plasma pH within a narrow range (7.38–7.42).

Protein Denaturation: pH changes can alter protein structure and function.
Abnormal pH Effects: Acidosis (CNS depression), alkalosis (CNS hyperexcitability, muscle tetanus).
Associated with K+ Disturbances: pH and potassium balance are interrelated.

Sources of Acids and Bases

Acid Input: Organic acids (diet, metabolism), CO2 production, ketoacidosis.
Base Input: Few dietary/metabolic sources.

pH Homeostasis: Buffers, Lungs, and Kidneys

Buffers: Proteins, phosphate ions, HCO3- moderate pH changes by binding/releasing H+.
Henderson-Hasselbalch Equation: Describes the relationship between pH, bicarbonate, and CO2:
Ventilation: Compensates for 75% of pH disturbances (hypo/hyperventilation).
Kidneys: Use ammonia and phosphate buffers to excrete acid and reabsorb bicarbonate.

Types of Acid-Base Disturbances

Respiratory Acidosis: Hypoventilation increases CO2, decreases pH.
Metabolic Acidosis: Excess acid production or loss of bicarbonate decreases pH.
Respiratory Alkalosis: Hyperventilation decreases CO2, increases pH.
Metabolic Alkalosis: Loss of acid (vomiting) or excess bicarbonate increases pH.

Disturbance	Plasma pH	Plasma HCO3-	PCO2
Respiratory Acidosis	Low	High	High
Metabolic Acidosis	Low	Low	Low
Respiratory Alkalosis	High	Low	Low
Metabolic Alkalosis	High	High	High

Summary Table: Responses Triggered by Changes in Volume, Blood Pressure, and Osmolarity

Change	Organ/System	Response
Low blood pressure	Kidneys	Renin release, RAS activation
High osmolarity	Hypothalamus	Vasopressin secretion, thirst
High blood volume	Heart	ANP/BNP release
Low plasma K+	Adrenal cortex	Decrease aldosterone
High plasma K+	Adrenal cortex	Increase aldosterone

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