Hormonal Control, Fluid Balance, and Acid-Base Homeostasis

Comprehensive vocabulary flashcards covering renal endocrine functions, fluid intake and output pathways, the RAAS cascade, ADH signaling, electrolyte dynamics, and respiratory/renal mechanisms of acid-base balance.

Nephron

The functional unit of the kidney that processes blood plasma into urine through filtration, reabsorption, and secretion.

Erythropoietin

A glycoprotein hormone secreted by healthy kidneys that stimulates red blood cell production in the bone marrow.

Body Fluid

The total water within the human body along with all the chemicals, electrolytes, and solutes dissolved inside it.

Intracellular Fluid (ICF)

The fluid compartment contained entirely inside cell membranes, constituting roughly two-thirds of total body water and containing high concentrations of potassium and phosphate.

Extracellular Fluid (ECF)

All body fluids found outside cell membranes, making up roughly one-third of total body water and subdividing into interstitial tissue fluid and circulating blood plasma.

Metabolic Water

Approximately $200\,\text{mL}$ of water produced daily as an oxidative byproduct of cellular respiration during nutrient catabolism.

Fluid Ingestion

The primary avenue of water intake, contributing approximately $1{,}500\,\text{mL}$ per day through liquid drinking and $700\text{--}800\,\text{mL}$ through moist foods.

Insensible Water Loss

Unavoidable, continuous water loss occurring via cutaneous evaporation and exhaled respiratory vapor, combining with sweat to account for approximately $500\,\text{mL}$ daily under resting conditions.

Osmoreceptors

Sensory neurons located in the hypothalamus that detect increases in blood plasma osmotic pressure and stimulate thirst and hormone secretion.

Thirst Reflex

A homeostatic physiological mechanism triggered by elevated plasma solute concentration and mucosal dryness that produces the conscious desire to drink water.

Dehydration

A condition occurring whenever fluid output exceeds fluid intake, causing fluid compartment depletion and resulting in the excretion of low-volume, dark amber-colored urine.

Antidiuretic Hormone (ADH)

A neurohormone synthesized in the hypothalamus and released from the posterior pituitary that promotes water reabsorption in renal collecting ducts to conserve systemic blood volume.

Aquaporins

Transmembrane water channel proteins that insert into collecting duct epithelial cell apical membranes under ADH stimulation, dramatically elevating tubule water permeability.

Diabetes Insipidus

A clinical condition marked by deficient ADH synthesis or renal tubule unresponsiveness to ADH, causing the excretion of massive daily volumes ($15\text{--}20\,\text{L}$) of dilute urine.

Hypotonic Overhydration Response

The physiological shutdown of ADH secretion triggered when excess pure water intake decreases plasma osmotic pressure, leading to the excretion of abundant dilute urine.

Juxtaglomerular Cells

Specialized vascular smooth muscle cells in the afferent arterioles of the renal corpuscle that monitor renal perfusion pressure and release renin when pressure declines.

Renin

An enzymatic hormone secreted by juxtaglomerular cells into the bloodstream that cleaves liver-derived angiotensinogen into angiotensin I.

Angiotensinogen

A circulating inactive precursor plasma protein synthesized constitutively by the liver that serves as the substrate for enzymatic cleavage by renin.

Angiotensin-Converting Enzyme (ACE)

An endothelial enzyme located primarily within pulmonary capillary beds that converts circulating angiotensin I into active angiotensin II.

Angiotensin II

A potent peptide hormone that triggers systemic arterial vasoconstriction and stimulates the adrenal cortex to synthesize and secrete aldosterone.

Aldosterone

A steroid hormone secreted by the adrenal cortex that acts on the distal convoluted tubules and collecting ducts to enhance $Na^+$ reabsorption and promote $K^+$ excretion.

Atrial Natriuretic Peptide (ANP)

A cardiac hormone released by atrial myocytes in response to high venous return and stretching; it inhibits renin and aldosterone release, promoting renal sodium and water loss to reduce arterial blood pressure.

Electrolytes

Inorganic chemical substances that dissociate into electrically conductive charged ions when dissolved in biological water.

Cations

Positively charged electrolyte ions present in body fluid compartments, including sodium ($Na^+$), potassium ($K^+$), calcium ($Ca^{2+}$), and magnesium ($Mg^{2+}$).

Anions

Negatively charged electrolyte ions present in body fluids, including chloride ($Cl^-$), bicarbonate ($HCO_3^-$), and hydrogen phosphate ($HPO_4^{2-}$).

Sodium ($Na^+$)

The principal extracellular cation, comprising greater than $90\%$ of all ECF cations and exerting primary control over extracellular osmotic balance, fluid volume, and action potential propagation.

Potassium ($K^+$)

The predominant intracellular cation, crucial for establishing cellular resting membrane potential, maintaining cardiac rhythmicity, and enabling neuromuscular signaling.

Calcium ($Ca^{2+}$)

A vital extracellular and intracellular cation required for bone and tooth matrix hardness, blood coagulation cascade activation, and initiating skeletal and cardiac muscle contraction.

Chloride ($Cl^-$)

The most abundant extracellular anion; it passively follows sodium to ensure electroneutrality and provides the chloride ion for gastric hydrochloric acid ($HCl$) synthesis.

Principle of Osmotic Volume Follow

The physical rule where water cannot be actively pumped but migrates passively by osmosis toward higher solute concentrations, making sodium retention the primary driver of circulatory volume expansion.

Normal Blood pH Range

The narrow, strictly defended arterial blood pH window between $7.35$ and $7.45$ that maintains proper protein conformation and enzyme function.

Chemical Buffer Systems

The body's initial line of acid-base defense that acts within fractions of a second to tie up or release free hydrogen ions ($H^+$).

Bicarbonate Buffer System

The primary extracellular chemical buffering pair composed of weak carbonic acid ($H_2CO_3$) and basic bicarbonate salt ($HCO_3^-$).

Carbonic Anhydrase

An enzyme that rapidly and reversibly catalyzes the hydration of carbon dioxide into carbonic acid ($CO_2 + H_2O \rightleftharpoons H_2CO_3$).

Respiratory pH Regulation

The intermediate homeostatic control system that alters ventilation rate and depth within minutes to adjust the rate of volatile $CO_2$ elimination.

Renal pH Regulation

The ultimate, permanent acid-base regulatory mechanism occurring over hours to days in which tubular cells secrete excess $H^+$ into urine while reabsorbing and generating $HCO_3^-$ into the blood.

Acidosis

A pathophysiological state wherein arterial blood pH drops below $7.35$, leading to central nervous system depression, lethargy, stupor, and potential coma.

Alkalosis

A pathophysiological state wherein arterial blood pH rises above $7.45$, leading to central and peripheral nervous system hyperexcitability, muscle spasms, convulsions, and tetany.

Respiratory Acidosis

An acid-base disturbance resulting from pulmonary hypoventilation, airway obstruction, or emphysema that retains excess $CO_2$ in the blood, driving pH below $7.35$.

Metabolic Alkalosis

An acid-base disturbance characterized by elevated blood pH caused by excessive loss of gastric hydrochloric acid during persistent vomiting or overconsumption of alkaline antacids.