Fluid and Electrolyte Balance

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Questions and Answers

What happens when there is decreased release of antidiuretic hormone (ADH)?

Urine becomes concentrated and body fluid volume increases

Dilute urine is produced and body fluid volume decreases (correct)

ECF solute concentration decreases dramatically

Blood pressure rises significantly

Which of the following factors can trigger the release of ADH?

Increased blood volume

Increased osmotic pressure in the ECF

Low sodium concentrations

Severe blood loss (correct)

What is the primary role of sodium in fluid and electrolyte balance?

To aid in the excretion of acids and bases

To control osmotic pressure and water distribution in ECF (correct)

To increase metabolism of other electrolytes

To provide electrical excitability in muscles only

What is a consequence of changes in sodium ion (Na+) levels?

It significantly impacts ECF and interstitial fluid (IF) volumes Signup and view all the answers

Electrolyte balance primarily concerns which of the following?

Salt balance and its effects on fluid movements Signup and view all the answers

What is the main mechanism driving water intake in the body?

Thirst mechanism governed by the hypothalamic thirst center Signup and view all the answers

Which of the following correctly describes osmolality?

The concentration of solute particles in a solution per kilogram of water Signup and view all the answers

What triggers the release of Antidiuretic Hormone (ADH) in the body?

Increase in extracellular fluid (ECF) osmolality Signup and view all the answers

Which of the following factors can activate the hypothalamic osmoreceptors?

Increased plasma osmolality of 1–2% Signup and view all the answers

What percentage of daily water output is typically lost through urine?

60% Signup and view all the answers

How does the drinking of water affect the thirst center?

It provides inhibitory feedback to shut off the thirst center Signup and view all the answers

What is considered a significant obligatory water loss that contributes to the body's water balance?

Insensible water loss from lungs or skin Signup and view all the answers

Which condition leads to the inhibition of ADH secretion?

Decrease in ECF osmolality Signup and view all the answers

What hormone plays the biggest role in the regulation of sodium by the kidneys?

Aldosterone Signup and view all the answers

How does atrial natriuretic peptide (ANP) affect blood pressure and volume?

Decreases blood pressure and volume Signup and view all the answers

Which mechanism is the main trigger for aldosterone release?

Renin-angiotensin-aldosterone mechanism Signup and view all the answers

What effect do female sex hormones, particularly estrogens, have on sodium balance?

They increase sodium chloride reabsorption Signup and view all the answers

What is the role of glucocorticoids in sodium regulation?

Increase Na+ reabsorption and promote edema Signup and view all the answers

What response occurs in the kidneys when cardiovascular baroreceptors detect increased blood volume and pressure?

Dilation of afferent arterioles Signup and view all the answers

What is the effect of aldosterone on Na+ levels over time?

Requires hours to days to exert effects Signup and view all the answers

Which hormone inhibits the production of both renin and aldosterone?

Atrial natriuretic peptide (ANP) Signup and view all the answers

Study Notes

Fluid, Electrolyte, and Acid-Base Balance

Understanding fluid, electrolyte, and acid-base balance is essential for correctly interpreting patient test results.
Infants have a higher water content (73% or more) compared to adults (males ~60%, females ~50%).
Adipose tissue is the least hydrated tissue type.
Total body water in adults averages ~40 liters.
Water content decreases to ~45% in old age.

Body Water Content

Two main fluid compartments exist: intracellular fluid (ICF) and extracellular fluid (ECF).
ICF: fluid inside cells, composing 2/3 of total body fluid.
ECF: fluid outside cells, composed of plasma (3 L) and interstitial fluid (IF) (12L).
IF includes lymph, cerebrospinal fluid (CSF), eye humors, synovial fluid, serous fluid, and gastrointestinal secretions.

Composition of Body Fluids

Water is the universal solvent.
Solutes are substances dissolved in water.
Solutes are classified as electrolytes and nonelectrolytes.
Nonelectrolytes do not dissociate in water, are mostly organic molecules, and examples include glucose, lipids, creatinine, and urea.
Electrolytes dissociate into ions in water, including inorganic salts, acids, bases, and some proteins.
Electrolytes have greater osmotic power than nonelectrolytes, leading to a greater ability to cause fluid shifts.
Examples of electrolytes include NaCl (2 particles), MgCl2 (3 particles), and glucose (1 particle).

Comparison of Extracellular and Intracellular Fluids

Each fluid compartment has a specific pattern of electrolytes.
ECF electrolyte contents are similar, except for higher protein and lower chloride contents in plasma.
Major cation in ECF is Na+, major anion is Cl-.
ICF contains more soluble proteins than plasma.
Major cation in ICF is K+, major anion is HPO42-.

Fluid Movement Among Compartments

Osmotic and hydrostatic pressures regulate continuous fluid exchange and mixing.
Water freely moves along osmotic gradients.
All body fluid osmolality is normally equal.
Changes in solute concentration in any compartment cause net water flow.
Increased ECF osmolality → water leaves cells.
Decreased ECF osmolality → water enters cells.

Exchange of Gases, Nutrients, Water, and Wastes

Nutrients, oxygen, water, and waste products are exchanged between the lungs, gastrointestinal tract, and kidneys with the blood plasma and interstitial fluid, ultimately reaching intracellular fluid in tissues.

Water Balance and ECF Osmolality

Water intake should equal water output (~2500 mL/day).
Water intake comes from ingested foods and beverages, as well as metabolic water produced by cellular metabolism.
Water output includes urine (60%), insensible water loss from skin and lungs, perspiration and feces
Osmolality is the concentration of a solution, expressed as the total number of solute particles per kilogram.
Normal osmolality is maintained around 280-300 mOsm.
Increased osmolality → thirst and ADH (antidiuretic hormone) release.
Decreased osmolality → ADH inhibition.

Regulation of Water Intake

Thirst is the driving force for water intake, controlled by the hypothalamic thirst center osmoreceptors activated by increased plasma osmolality (1-2%), dry mouth or decreased blood volume/pressure.
Drinking water inhibits the thirst center.
Inhibitory signals include relief of dry mouth and activation of stomach/intestinal stretch receptors.

Regulation of Water Output

Obligatory water losses must be considered, as they are necessary for life, including insensible water loss from lungs/skin or excretion of wastes in urine or sweat and feces.
Urine volume and solute concentration depend on fluid intake, diet, and water loss via other avenues.

Influence of Antidiuretic Hormone (ADH)

Water reabsorption in collecting ducts is proportional to ADH release.
Decreased ADH leads to dilute urine and reduced body fluid volume.
Increased ADH leads to concentrated urine due to water reabsorption, increasing body fluid volume.
Hypothalamic osmoreceptors sense ECF solute concentration and regulate ADH release.
Other factors triggering ADH release include large changes in blood volume or pressure, decreased blood pressure, intense sweating, vomiting, diarrhea, severe blood loss, and prolonged fever.

Electrolyte Balance

Electrolyte balance usually refers to salt balance, although electrolytes also include acids, bases, and some proteins.
Salts regulate fluid movement, providing minerals for excitability, secretory activity and membrane permeability.
Salts are acquired via ingestion and metabolism, and lost via perspiration, feces, urine, and vomiting.

Central Role of Sodium in Fluid and Electrolyte Balance

Sodium is the most abundant cation in ECF.
Sodium salts in ECF contribute to the 280 mOsm of total 300 mOsm ECF solute concentration, exerting significant osmotic pressure.
Sodium controls ECF volume and water distribution as water follows salt.
Changes in sodium levels affect plasma volume, blood pressure, and ECF/IF volumes.

Regulation of Sodium Balance

No known receptors directly monitor Na+ levels in body fluids.
Sodium-water balance is linked to blood pressure and volume control mechanisms.
Changes in blood pressure/volume trigger neural and hormonal controls to regulate Na+ content.
Aldosterone plays a major role in regulating sodium, but acts slowly (hours to days).
The renin-angiotensin-aldosterone mechanism is the main trigger for aldosterone release.

Regulation of Sodium Influence of Atrial Natriuretic Peptide (ANP)

Released by atrial cells in response to increased blood pressure, ANP decreases blood pressure and volume by inhibiting ADH, renin, and aldosterone production.
ANP increases the excretion of Na+ and water, promotes vasodilation.

Regulation of Sodium Influence of Other Hormones

Female sex hormones (e.g., estrogen) increase NaCl reabsorption, leading to H₂O retention.
Progesterone decreases Na⁺ reabsorption and increases Na+ and H₂O loss.
Glucocorticoids increase Na⁺ reabsorption and promote edema.
Cardiovascular baroreceptors alert the brain to increases in blood volume and pressure, leading to sympathetic nervous system impulses to dilate afferent arterioles in the kidneys, increasing GFR, Na+ and water output, and reducing blood volume and pressure.

Regulation of Potassium Balance

Potassium affects resting membrane potential (RMP) in neurons and muscle cells.
Increased ECF potassium causes decreased RMP, followed by reduced excitability.
Decreased ECF potassium causes hyperpolarization and nonresponsiveness.
Disruption in potassium (hyper- or hypokalemia) can interfere with electrical conduction, leading to sudden death.
Potassium is part of the body's buffer system, where H+ shifts in and out of cells in the opposite direction of K+ to maintain cation balance.
ECF K+ levels rise with acidosis; ECF K+ levels fall with alkalosis.

Influence of Aldosterone on Potassium Balance

Aldosterone stimulates K+ secretion and Na+ reabsorption by principal cells.
Increased K+ in the adrenal cortex causes aldosterone release, which increases K+ secretion.

Acid-Base Balance

pH affects all functional proteins and biochemical reactions and is closely regulated by the body.
Normal pH of arterial blood is 7.4; venous blood and interstitial fluid ~7.35; ICF ~7.0.
Alkalosis/alkalemia: arterial pH >7.45
Acidosis/acidemia: arterial pH <7.35.
Small amounts of acidic substances enter bodies in food, but most H+ is a by-product of metabolism.
Examples of metabolic acids released from cellular functions include those from phosphorus-containing proteins, lactic acid from anaerobic respiration, fatty acids and ketone bodies from fat metabolism.

Chemical Buffer Systems

Chemical buffer systems are rapid, first-line of defense against pH changes, including the bicarbonate, phosphate, and protein buffer systems.
Bicarbonate buffer system: mixture of H₂CO₃ (carbonic acid, weak acid) and HCO3- (bicarbonate) salts.
Phosphate buffer system: effective in urine and ICF where phosphate concentrations are high , H₂PO₄⁻ and HPO₄²⁻
Protein buffer system: intracellular proteins are most powerful buffers; plasma proteins are also important; act as weak acids and weak bases depending on pH.

Respiratory Regulation of H+

Rising CO₂ (hypercapnia) in blood activates medullary chemoreceptors causing increased respiratory rate and depth.
Rising plasma H+ in blood (acidosis) activates peripheral chemoreceptors causing increased respiratory rate and depth.
Both increase removal of CO₂ from the blood and reduces H+ concentration.
Alkalosis depresses respiratory center, causing respiratory rate and depth to decrease and H+ concentration to increase.

Renal Regulation of Acid-Base Balance

Lungs eliminate volatile carbonic acid by eliminating CO₂.
Kidneys eliminate nonvolatile acids (phosphoric, uric, and lactic acids, and ketones) to prevent metabolic acidosis.
Kidneys regulate blood levels of alkaline substances and renew chemical buffers.

Conserving Filtered Bicarbonate Ions: Bicarbonate Reabsorption

To maintain alkaline reserve, kidneys replenish bicarbonate (reabsorb).
Tubule cells are impermeable to bicarbonate but permeable to CO₂.
Bicarbonate can enter the body by being converted to CO₂, then converted back into bicarbonate or leaves as CO₂.
The mechanism is coupled to H+ secretion: for every H+ secreted, a bicarbonate is reabsorbed.

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Description

This quiz covers the essential concepts of fluid, electrolyte, and acid-base balance in the human body. Understand the differences between intracellular and extracellular fluids, the composition of body fluids, and the significance of water content in various age groups. Prepare to test your knowledge on these fundamental physiological principles.