Fluid, Electrolyte, and Acid-Base Balance

Flashcards covering the fundamentals of fluid compartments, water balance regulation, hormonal mechanics such as the RAAS, electrolyte imbalances, and the physiological and chemical buffering systems used to maintain acid-base homeostasis.

Intracellular fluid (ICF)

The fluid contained within cells, also known as cytosol, which accounts for approximately $65\%$ of total body fluid.

Extracellular fluid (ECF)

The fluid outside of cells, representing about $35\%$ of total body fluid, comprising interstitial fluid, blood plasma, lymph, and transcellular fluids.

Preformed water

Water that enters the body through the ingestion of food and drink.

Metabolic water

Water produced internally as a by-product of aerobic respiration and dehydration synthesis reactions.

Insensible water loss

Output of water that is usually unnoticed, occurring via expired air and cutaneous transpiration.

Sensible water loss

Observable water output, primarily through feces and urine.

True dehydration

A condition where blood volume declines and sodium concentration rises, caused by factors like lack of drinking, profuse sweating, or diabetes insipidus.

Hypovolemia

An imbalance where proportionate amounts of water and sodium are lost without replacement, often due to hemorrhage, severe burns, or chronic vomiting.

Macula densa

A patch of cells located at the border of the nephron loop and distal convoluted tubule that sense $NaCl$ content to help regulate glomerular filtration rate.

Granular cells

Also known as juxtaglomerular cells, these are cells that secrete the enzyme renin when signaled by the macula densa or sympathetic reflexes.

Angiotensin II

A potent vasoconstrictor produced by the conversion of angiotensin I by $ACE$; it raises MAP, stimulates thirst, and promotes the secretion of aldosterone and $ADH$.

Aldosterone

The "salt-retaining hormone" produced by the adrenal cortex that promotes sodium reabsorption, water retention, and potassium secretion in the kidneys.

Antidiuretic hormone (ADH)

Also known as vasopressin, this hormone increases the number of aquaporins in the renal tubules to promote water reabsorption independently of sodium.

Natriuretic peptides

Hormones released by the heart in response to high blood pressure that inhibit renin, aldosterone, and $ADH$ to increase salt and water excretion.

Principal cells

The most abundant cells in the $DCT$ and collecting duct, possessing receptors for hormones and functioning in salt and water balance.

Intercalated cells

Cells in the distal convoluted tubule and collecting duct that reabsorb $K^+$ and secrete $H^+$ into the tubule for acid-base balance.

Fluid sequestration

A condition where total body water may be normal, but volume is lost from circulation due to accumulation in specific locations like the pleural cavity or interstitial spaces.

Edema

The abnormal accumulation of fluid in interstitial spaces, leading to tissue swelling.

Hyponatremia

A state where the sodium concentration in the $ECF$ is below $136\,mEq/L$, often caused by overhydration or inadequate salt intake.

Hypernatremia

A state where the sodium concentration in the $ECF$ is above $145\,mEq/L$, most commonly caused by dehydration.

Hyperkalemia

An excess of potassium in the blood that can lead to cardiac arrhythmias or arrest due to easier cell depolarization.

Hypokalemia

A deficiency of potassium in the blood that causes cells to hyperpolarize, making nerves and muscles harder to excite.

ACE2

An enzyme that inactivates angiotensin II and serves as the entry point for the SARS-CoV-2 spike protein; its down-regulation leads to inflammation and fibrosis.

Physiological buffer

A system, such as the respiratory or urinary system, that stabilizes pH by controlling the body's output of acids, bases, or $CO_2$.

Chemical buffer

A substance that resists pH changes by binding $H^+$ when concentrations rise or releasing it when concentrations fall; includes bicarbonate, phosphate, and protein systems.

Bicarbonate Buffer System

The most important buffering system in the Extracellular Fluid ($ECF$), consisting of a solution of carbonic acid and bicarbonate ions.

Acidosis

A clinical state where the blood pH falls below $7.35$.

Alkalosis

A clinical state where the blood pH rises above $7.45$.