Physiology: Cell, Muscle, and Cardiovascular Systems

Vocabulary practice flashcards covering cell electrophysiology, skeletal and smooth muscle mechanics, cardiac conduction, hemodynamics, vascular regulation, and systemic blood pressure control.

Phospholipid Bilayer

The structural basis of the cell membrane, composed of molecules with hydrophilic heads oriented outward toward the extracellular fluid and intracellular lumen, and hydrophobic tails oriented inward.

Simple Diffusion

The passive movement of substances across the cell membrane from a region of higher concentration to a region of lower concentration; applies to gases ($\text{O}_2$, $\text{CO}_2$), water, lipid-soluble vitamins (A, D, K, and E), and steroid hormones.

Facilitated Diffusion

The movement of water-soluble substances (such as $\text{Na}^+$, $\text{Ca}^{2+}$, $\text{K}^+$, and glucose) down their concentration gradient via a specialized membrane transporter or channel protein.

Ligand-Gated Ion Channel

A gated membrane channel protein that opens or closes in response to the binding of a specific chemical messenger (such as a neurotransmitter).

Voltage-Gated Ion Channel

An ion channel that opens or closes in response to alterations in the electrical membrane potential.

$\text{Na}^+/\text{K}^+$ ATPase (Sodium-Potassium Pump)

An active transport membrane enzyme present in every human cell that consumes one molecule of $\text{ATP}$ to pump three $\text{Na}^+$ ions out of the cell and two $\text{K}^+$ ions into the cell, maintaining a negative resting membrane potential and high intracellular $[\text{K}^+]$.

Excitable Cell

A cell type (predominantly muscle cells and neuronal cells) that expresses voltage-gated $\text{Na}^+$ and voltage-gated $\text{K}^+$ channels and is capable of generating and propagating an action potential when stimulated above threshold.

Action Potential Depolarization

The rapid upstroke phase of an action potential caused by the opening of voltage-gated $\text{Na}^+$ channels and the subsequent inward flow of $\text{Na}^+$.

Action Potential Repolarization

The phase during which voltage-gated $\text{Na}^+$ channels inactivate and voltage-gated $\text{K}^+$ channels open, restoring the membrane potential toward negative resting levels.

Endomysium

The delicate connective tissue layer that surrounds individual skeletal muscle fibers.

Perimysium

The connective tissue sheath surrounding a muscle fascicle, which contains a bundle of $10-100$ muscle fibers.

Epimysium

The outer dense connective tissue sheath that encircles the entire skeletal muscle belly.

Fascial Compartment

A section of an anatomical region enclosed by fascia that groups together muscles, nerves, and blood vessels.

Tendon

A fibrous connective tissue structure that attaches muscle to bone and transmits contractile force to generate bone movement.

Ligament

A fibrous connective tissue band that connects bone to bone, providing joint stabilization and structural support.

Myoblast

An embryonic mesodermal progenitor cell that fuses with others during development to form a multinucleated skeletal muscle fiber.

Myosatellite Cell

An unfused progenitor cell remaining within mature muscle tissue that persists into adulthood to assist in growth and repair.

Sarcolemma

The plasma membrane surrounding an individual skeletal muscle fiber, beneath which its multiple nuclei are located.

Sarcomere

The basic repeating contractile unit of a myofibril, delineated as the segment between two adjacent Z lines (Z discs).

A Band

The anisotropic, dark band of a sarcomere corresponding to the entire length of the thick filaments, which remains constant in length during contraction.

I Band

The isotropic, light band of a sarcomere consisting only of thin filaments; shortens during muscle contraction as thin filaments slide past thick filaments.

Titin

A large structural protein filament that stabilizes and anchors the thick (myosin) filaments to the Z disc within a sarcomere.

Tropomyosin

A filamentous protein of the thin filament that covers and blocks the myosin-binding sites on actin molecules in the resting muscle state.

Troponin Complex

A three-subunit protein complex on the thin filament consisting of Troponin-I (binds actin), Troponin-C (binds $\text{Ca}^{2+}$), and Troponin-T (binds tropomyosin).

Motor Unit

A functional neuromuscular entity consisting of a single somatic motor neuron and all the skeletal muscle fibers it innervates.

End-Plate Potential (EPP)

A local, graded depolarization of the muscle motor end plate produced when acetylcholine binds to nicotinic acetylcholine receptors ($\text{nAChR}$), causing an influx of $\text{Na}^+$.

Transverse (T) Tubule

An invagination of the muscle sarcolemma that projects deep into the interior of the muscle fiber to rapidly conduct action potentials toward the sarcoplasmic reticulum.

Dihydropyridine (DHP) Receptor

A voltage-sensitive receptor located within the T-tubule membrane that undergoes a conformational shift upon membrane depolarization to trigger the opening of adjacent sarcoplasmic reticulum calcium channels.

Ryanodine Receptor (RyR)

A calcium-release channel in the sarcoplasmic reticulum membrane mechanically coupled to the DHP receptor that permits rapid efflux of $\text{Ca}^{2+}$ into the sarcoplasm.

SERCA

Sarcoendoplasmic reticulum calcium ATPase; an active transport pump that sequesters $\text{Ca}^{2+}$ back into the sarcoplasmic reticulum lumen to enable muscle relaxation.

Calsequestrin (CSQ)

A specialized calcium-binding protein located within the sarcoplasmic reticulum lumen that allows high quantities of $\text{Ca}^{2+}$ to be stored at a low free concentration.

Intercalated Disc

A specialized junctional complex linking adjacent cardiomyocytes, containing gap junctions for low-resistance ion diffusion and desmosomes to mechanically anchor intermediate filaments.

Functional Syncytium

A tissue arrangement in which cells are interconnected electrically via gap junctions so that an action potential spreads rapidly across the entire chamber, prompting synchronous contraction.

Plateau Phase (Phase 2)

A prolonged period ($\sim 0.2\,\text{s}$) of sustained depolarization in the ventricular cardiomyocyte action potential caused by the slow inward movement of $\text{Ca}^{2+}$ balancing outward $\text{K}^+$ current.

Sinoatrial (SA) Node

A specialized cluster of cardiac cells situated in the right atrium that spontaneously produces rhythmical action potentials, functioning as the natural pacemaker of the heart.

P Wave

The deflection on an electrocardiogram resulting from the electrical potentials generated during atrial depolarization prior to atrial contraction.

QRS Complex

The electrocardiographic wave representing rapid ventricular depolarization prior to ventricular systole; also conceals atrial repolarization.

T Wave

The deflection on an electrocardiogram produced by electrical potentials generated during ventricular repolarization.

First-Degree AV Block

A conduction disorder characterized on an ECG by abnormally slow conduction through the AV node, defined as a PR interval $> 0.2\,\text{s}$ without any dropped beats.

Tachycardia

An abnormally rapid cardiac rhythm characterized by a resting heart rate exceeding $100\,\text{BPM}$.

STEMI

ST-Elevation Myocardial Infarction; a type of heart attack characterized by an elevation of the ST segment on an ECG, indicative of transmural myocardial injury and contractile impairment.

Cardiac Output (CO)

The volume of blood pumped by each ventricle per minute, calculated as the mathematical product of heart rate ($\text{HR}$) and stroke volume ($\text{SV}$): $\text{CO} = \text{HR} \times \text{SV}$.

Frank-Starling Mechanism

The intrinsic physiological principle stating that the greater the myocardium is stretched by venous return during filling, the greater the force of contraction and the larger the stroke volume pumped into the aorta.

Systemic Circulation

The component of the cardiovascular circuit that delivers oxygenated blood from the left ventricle through arteries to body tissues and returns deoxygenated blood via veins to the right atrium.

Pulmonary Circulation

The vascular loop that carries deoxygenated blood from the right ventricle to the lungs for gas exchange and delivers re-oxygenated blood to the left atrium.

Tunica Media

The middle layer of a blood vessel wall, composed predominantly of circularly arranged smooth muscle cells and elastic tissue.

Vein Compliance

The capacity of veins to distend and store large volumes of blood with small increases in pressure, allowing systemic veins and venules to hold approximately $60\%$ to $64\%$ of the total blood volume.

Calmodulin

A cytosolic calcium-binding regulatory protein in smooth muscle cells that forms a $\text{Ca}^{2+}$-calmodulin complex to activate myosin light chain kinase (MLCK).

Myosin Light Chain Kinase (MLCK)

An enzyme in vascular smooth muscle activated by the calcium-calmodulin complex that phosphorylates the regulatory light chain on myosin heads to initiate cross-bridge cycling.

Myogenic Autoregulation

The local intrinsic ability of vascular smooth muscle (especially in arterioles) to contract in response to increased intravascular stretch/pressure and relax in response to reduced stretch, maintaining stable organ blood flow.

Post-Occlusion Reactive Hyperemia (PORH)

A transient surge in blood flow to a vascular bed following the removal of a temporary arterial occlusion, driven by the local accumulation of vasodilator metabolites.

Endothelial Nitric Oxide Synthase (eNOS)

An enzyme in endothelial cells that converts $\text{O}_2$ and L-arginine into nitric oxide ($\text{NO}$) and L-citrulline in response to luminal shear stress.

Transcytosis

A transport pathway across capillary endothelium whereby large, lipid-insoluble substances are endocytosed into vesicles on the luminal side, moved through the cytoplasm, and exocytosed onto the opposite side.

Blood-Brain Barrier (BBB)

A highly selective vascular permeability barrier composed of capillary endothelial cells with tight junctions, astrocytic end-feet, and pericytes, lacking fenestrae to protect brain tissue.

Pulse Pressure

The numeric difference between the systolic blood pressure ($\text{SP}$) and the diastolic blood pressure ($\text{DP}$): $\text{Pulse Pressure} = \text{SP} - \text{DP}$.

Mean Arterial Pressure (MAP)

The average arterial pressure across a complete cardiac cycle, calculated by the formula $\text{MAP} = \text{DP} + \frac{1}{3}(\text{SP} - \text{DP})$.

Hypertension Stage 1

A clinical blood pressure classification defined by the American Heart Association as a systolic reading of $130-139\,\text{mmHg}$ or a diastolic reading of $80-89\,\text{mmHg}$.

Total Peripheral Resistance (TPR)

The cumulative frictional resistance against blood flow offered by systemic vessels, determined under Poiseuille's law by vessel length, blood viscosity, and most importantly, vessel internal radius.

Baroreceptor Reflex

A homeostatic negative feedback loop mediated by stretch receptors in the carotid sinus and aortic arch that adjusts sympathetic and parasympathetic output to regulate blood pressure.

Orthostatic Hypotension

A sudden drop in systemic blood pressure occurring when an individual transitions from lying or sitting to standing, caused by gravitational pooling of blood in lower extremities reducing venous return and cardiac output.

Renin

An enzymatic hormone synthesized and released by the kidneys into circulation that cleaves circulating liver-derived angiotensinogen into angiotensin I.

Angiotensin-Converting Enzyme (ACE)

An endothelial-bound enzyme concentrated in the pulmonary circulation that hydrolyzes angiotensin I to form the bioactive peptide angiotensin II.

Angiotensin II

A potent vasoconstrictive peptide that raises blood pressure by constricting blood vessels, stimulating the posterior pituitary to secrete $\text{ADH}$, and inducing the adrenal cortex to secrete aldosterone.

Aldosterone

A steroid hormone secreted by the adrenal cortex that acts on the renal nephron to stimulate sodium reabsorption and water retention, expanding blood volume.

Antidiuretic Hormone (ADH / Vasopressin)

A peptide hormone synthesized in the hypothalamus and released from the posterior pituitary gland that promotes water reabsorption in the collecting tubules of the kidney and causes vasoconstriction.