Human Physiology: Energy Homeostasis, Thermoregulation, and Cardiovascular System

Comprehensive vocabulary flashcards covering the energetic, metabolic, thermoregulatory, and cardiovascular principles presented in Human Physiology Weeks 7 and 8.

Exergonic Reaction

A chemical reaction that releases free energy, resulting in products that have less free energy than the reactants.

Endergonic Reaction

A chemical reaction that requires an input of free energy, resulting in products that have more free energy than the reactants.

Activation Energy

The initial energy input or "push" required to start a chemical reaction.

Reaction Coupling

The cellular process of using free energy released by an energy-releasing (exergonic) reaction to drive an energy-requiring (endergonic) reaction, commonly using ATP or electron carriers.

Energy Homeostasis

The balance between energy input from food and energy output through resting metabolism, physical activity, and the thermic effect of food, supporting body weight stability over time.

Daily Energy Expenditure (DEE)

The total energy expended by the body over a 24-hour period, comprising basal/resting metabolic rate ($\sim 60\text{--}75\%$), physical activity ($\sim 15\text{--}30\%$), and the thermic effect of feeding ($\sim 10\%$).

Resting Metabolic Rate (RMR)

The energy required to sustain essential bodily functions at rest, including ventilation, heart pumping, nervous signaling, and renal filtration.

Thermic Effect of Feeding (TEF)

The energy expended to digest, absorb, interconvert, transport, take up, and store nutrients from food (highest for protein at up to $30\%$, followed by carbohydrates at $10\text{--}15\%$, and fats at $2\text{--}3\%$).

Glycogenesis

The biochemical pathway that converts glucose into glycogen for storage in the liver and skeletal muscle.

Glycogenolysis

The breakdown of stored glycogen into glucose or glucose intermediates to release energy.

Gluconeogenesis

The metabolic production of glucose from non-carbohydrate precursors, such as amino acids and glycerol.

Lipolysis

The breakdown of stored triglycerides into free fatty acids and glycerol for tissue metabolism.

$\beta$-Oxidation

The mitochondrial catabolic pathway that breaks down fatty acids into two-carbon units to generate acetyl-CoA for cellular energy production.

Orexigenic Hormones

Hormones that act on hypothalamic circuits to stimulate hunger and increase food intake.

Anorexigenic Hormones

Hormones that act on hypothalamic circuits to promote fullness, induce satiety, and suppress appetite.

Ghrelin

A 28-amino acid peptide hormone secreted primarily by glands in the stomach that acts on the hypothalamus as a potent orexigen to stimulate hunger.

Leptin

A 165-amino acid peptide hormone secreted by adipocytes into the blood in proportion to fat stores that suppresses appetite by acting on receptors in the hypothalamus and brainstem.

Leptin Resistance

A state of reduced central neuronal responsiveness to circulating leptin, preventing normal satiety signaling despite elevated fat stores.

Homeothermy

The physiological capability and process of maintaining a relatively stable internal body core temperature despite fluctuations in ambient environmental temperature.

Endothermy

The generation of metabolic heat internally to maintain body temperature above ambient environmental temperature.

Thermoneutral Zone

The ambient environmental temperature range (approximately $27.8\text{--}30\,^{\circ}\text{C}$ in humans) where normal basal metabolic heat production maintains core body temperature without requiring extra heat production or active cooling.

Evaporation (Thermoregulation)

Heat dissipation that occurs when liquid sweat absorbs thermal energy ($\sim 540\,\text{cal/g}$ of water) and transitions into water vapor.

Conduction (Thermoregulation)

The transfer of thermal energy between two physical bodies in direct physical contact along a temperature gradient.

Convection (Thermoregulation)

Heat transfer facilitated by the movement of a fluid or gas (such as air currents or water) across the surface of the body.

Radiation (Thermoregulation)

The exchange of thermal energy between objects via electromagnetic infrared waves without requiring direct physical contact.

Shivering Thermogenesis

The generation of metabolic heat via rapid, involuntary skeletal muscle contractions regulated by the hypothalamic thermoregulatory center.

Nonshivering Thermogenesis

Heat production occurring primarily in brown adipose tissue where uncoupling proteins dissipate the mitochondrial proton gradient as heat rather than synthesizing ATP.

Pyrogens

Fever-inducing substances, derived from infectious agents or immune cells, that act on the hypothalamus to elevate the thermoregulatory temperature set point.

Fever

A regulated physiological elevation of core body temperature caused by a pyrogen-mediated upward reset of the hypothalamic temperature set point.

Hyperthermia

An unregulated increase in core body temperature above normal levels occurring without a change in the hypothalamic set point, typically due to excessive environmental heat or impaired heat dissipation.

External Cardiac Anatomy and Great Vessels

The muscular four-chambered organ and connected vessels, including the superior and inferior venae cavae, ascending aorta, aortic arch, pulmonary trunk, pulmonary arteries, and pulmonary veins.

Pulmonary Circulation

The vascular circuit that moves deoxygenated blood from the right ventricle through pulmonary arteries to the lungs for gas exchange, returning oxygenated blood via pulmonary veins to the left atrium.

Systemic Circulation

The vascular circuit that carries oxygenated blood from the left ventricle through the aorta and systemic arteries to body tissues, returning deoxygenated blood through systemic veins and venae cavae to the right atrium.

Myocardium

The thick middle layer of the heart wall composed of cardiac muscle cells responsible for mechanical contraction.

Endocardium

The innermost layer of the heart wall, consisting of an endothelial lining that covers heart chambers and valves.

Epicardium

The outermost serous layer of the heart wall, also designated as the visceral pericardium.

Chordae Tendineae

Fibrous cords extending from papillary muscles to atrioventricular valve cusps that prevent valve eversion and prolapse into the atria during ventricular contraction.

Isovolumic Ventricular Contraction

Phase 3 of the cardiac cycle where ventricles contract, increasing pressure and snapping the AV valves shut ($S_1$) while semilunar valves remain closed, resulting in contraction without a change in ventricular volume.

Isovolumic Ventricular Relaxation

Phase 5 of the cardiac cycle where ventricles relax, intraventricular pressure drops, and semilunar valves close ($S_2$) while AV valves remain closed, resulting in relaxation without a change in ventricular volume.

First Heart Sound ($S_1$)

The "lub" sound caused by the vibrations associated with the closure of the atrioventricular (tricuspid and bicuspid) valves at the onset of ventricular systole.

Second Heart Sound ($S_2$)

The "dub" sound caused by the vibrations associated with the closure of the semilunar (aortic and pulmonary) valves at the beginning of ventricular diastole.

Autorhythmic Cells

Specialized cardiac pacemaker cells that lack organized sarcomeres and spontaneously generate rhythmic action potentials without external neural stimulation.

Funny Current ($I_f$)

A hyperpolarization-activated inward cation current carried largely by $Na^+$ through cyclic nucleotide-gated channels that drives spontaneous pacemaker depolarization in cardiac autorhythmic cells.

Intercalated Discs

Microscopic structural junctions between adjacent cardiomyocytes composed of desmosomes for mechanical linkage and gap junctions for electrical coupling.

Desmosomes

Cellular anchoring junctions within intercalated discs that physically fasten adjacent cardiomyocytes together and transfer mechanical tensile force during contraction.

Gap Junctions

Protein channels within intercalated discs that allow direct passage of ions and electrical currents between cardiac cells, enabling the myocardium to function as a syncytium.

Cardiac Conduction System

The specialized autorhythmic pathway that coordinates electrical depolarization through the heart: SA node $\rightarrow$ internodal pathways $\rightarrow$ AV node $\rightarrow$ AV bundle $\rightarrow$ bundle branches $\rightarrow$ Purkinje fibers.

Sinoatrial (SA) Node

The primary physiological pacemaker of the heart, located in the right atrium, that generates action potentials at the fastest intrinsic rate to set normal heart rhythm.

Atrioventricular (AV) Node

The specialized nodal tissue at the junction of the atria and ventricles that delays the electrical impulse before conducting it into the ventricles, ensuring complete atrial emptying.

P Wave

The deflection wave on an electrocardiogram (ECG) representing electrical depolarization of the atria, which precedes atrial mechanical contraction.

QRS Complex

The ECG waveform representing progressive electrical depolarization of both ventricles, which precedes ventricular mechanical systole (and obscures atrial repolarization).

T Wave

The ECG deflection wave representing electrical repolarization and recovery of the ventricular myocardium.

End-Diastolic Volume (EDV)

The total volume of blood contained within a cardiac ventricle immediately before the onset of ventricular systole.

End-Systolic Volume (ESV)

The residual volume of blood remaining in a ventricle at the conclusion of ventricular systole and ejection.

Stroke Volume (SV)

The volume of blood pumped out of a ventricle with each heartbeat, calculated as $SV = EDV - ESV$ (normally averaging approximately $70\,\text{mL}$ at rest).

Cardiac Output (CO)

The total volume of blood pumped by a ventricle per minute, calculated as $CO = SV \times HR$ (averaging approximately $5\,\text{L/min}$ at rest).

Preload

The degree of mechanical stretch experienced by ventricular myocardial fibers immediately prior to contraction, directly determined by end-diastolic volume (EDV).

Afterload

The combined arterial pressure and vascular resistance that a contracting ventricle must overcome to open the semilunar valves and eject blood.

Frank–Starling Law of the Heart

The physiological relationship stating that an increase in venous return raises end-diastolic volume (EDV), stretching myocardial fibers and resulting in an increased stroke volume.

Skeletal Muscle Pump

The mechanism by which contracting skeletal muscles compress adjacent deep veins, driving venous blood through one-way valves toward the heart to increase venous return.

Respiratory Pump

The mechanism by which pressure changes in the thoracic and abdominal cavities during inhalation assist venous blood return toward the right atrium.

Tunica Media

The middle layer of a blood vessel wall composed of smooth muscle cells and elastic tissue that regulates vessel diameter via vasoconstriction and vasodilation.

Laminar Flow

The smooth, silent, streamlined concentric pattern of blood movement through an unconstricted, healthy vessel with highest velocities in the center.

Precapillary Sphincters

Smooth muscle rings situated at the entrance of individual capillary channels that constrict or relax to regulate local microvascular perfusion.

Fenestrations

Pores or openings within capillary endothelial cells that enhance microvascular wall permeability to water and dissolved solutes without permitting red blood cell transit.

Pulse Pressure (PP)

The mathematical difference between systolic and diastolic arterial blood pressure ($PP = SBP - DBP$).

Mean Arterial Pressure (MAP)

The average perfusion pressure driving blood through systemic beds across a complete cardiac cycle, estimated at rest as $MAP \approx DBP + \frac{1}{3}(SBP - DBP)$.

Peripheral Resistance

The opposition to blood flow encountered across systemic blood vessels, inversely proportional to the fourth power of the vessel radius ($R \propto \frac{1}{r^4}$).

Alpha-1 ($\alpha_1$) Adrenergic Receptors

Adrenergic receptors located on vascular smooth muscle cells whose activation by norepinephrine triggers smooth muscle contraction and vasoconstriction.

Beta-2 ($\beta_2$) Adrenergic Receptors

Adrenergic receptors located on vascular smooth muscle cells (especially in skeletal muscle) whose activation by epinephrine triggers smooth muscle relaxation and vasodilation.

Baroreceptors

Mechanosensitive stretch receptors located in the carotid sinus and aortic arch that monitor changes in arterial blood pressure and transmit signals to the brainstem.

Cardiovascular Control Center (CVCC)

The integrating regulatory center situated within the medulla oblongata that coordinates autonomic efferent outputs to modulate heart rate, myocardial contractility, and vascular diameter.

Hydrostatic Pressure (Capillary)

The physical fluid pressure exerted by blood against the capillary endothelial wall that promotes outward fluid filtration into the interstitial space.

Colloid Osmotic Pressure

The inward osmotic pull exerted primarily by non-diffusible plasma proteins (such as albumin) that favors fluid absorption back into the capillary lumen.

Lymphedema

Abnormal interstitial fluid accumulation and soft tissue swelling resulting from obstructed, absent, or damaged lymphatic drainage vessels.

Hematocrit

The percentage of total centrifuged blood volume occupied by packed erythrocytes (red blood cells), typically about $45\%$.

Erythrocytes

Anucleate, biconcave, flexible formed elements containing hemoglobin and carbonic anhydrase specialized for the transport of oxygen and carbon dioxide.

Hemoglobin

A tetrameric oxygen-binding globular protein within red blood cells composed of two $\alpha$ and two $\beta$ subunits, each carrying a heme-iron complex capable of binding one $O_2$ molecule ($4\,O_2$ molecules per hemoglobin).

Carbonic Anhydrase

An enzyme present within erythrocytes that rapidly catalyzes the reversible hydration of carbon dioxide into carbonic acid, dissociating into $H^+$ and bicarbonate ($HCO_3^-$).

Erythropoietin (EPO)

A peptide hormone secreted in response to cellular hypoxia that stimulates erythrocyte progenitor differentiation and production in the red bone marrow.

Megakaryocyte

A large bone marrow cell whose cytoplasmic fragmentation generates thousands of circulating platelets (thrombocytes).

Thrombin

An active serine protease in the coagulation cascade that converts soluble plasma fibrinogen into insoluble fibrin strands.

Fibrin

An insoluble filamentous protein polymer that aggregates into an interlaced mesh, stabilizing the initial platelet plug to form an organized hemostatic clot.

Plasmin

An active fibrinolytic enzyme derived from plasminogen that degrades fibrin meshwork to dissolve blood clots following vascular repair.

Phagocytosis

The cellular internalization and endosomal enzymatic digestion of foreign pathogens, cellular debris, or particulate matter by cells such as neutrophils and macrophages.

Agglutination

The immune-mediated cross-linking and clumping of foreign red blood cells or antigenic particles when bound by multivalent specific antibodies.

Type O Blood

Blood characterized by erythrocytes lacking both surface A and B antigens and plasma possessing both anti-A and anti-B antibodies; serves as the universal red cell donor.

Type AB Blood

Blood characterized by erythrocytes expressing both surface A and B antigens and plasma lacking anti-A and anti-B antibodies; serves as the universal red cell recipient.

Rh Factor (Rh D)

An erythrocyte surface protein antigen whose presence designates blood as Rh-positive and absence designates blood as Rh-negative.

Brachiocephalic Trunk

The first major branch of the aortic arch that subsequently divides into the right common carotid artery and right subclavian artery.

Celiac Trunk

An anterior branch of the abdominal aorta that divides into the left gastric, splenic, and common hepatic arteries to supply the upper abdominal viscera.

Hypertension

A chronic clinical disorder characterized by persistently elevated resting arterial blood pressure exceeding normal physiological reference ranges.

Cardiac Tamponade

A life-threatening complication of severe pericardial effusion where fluid accumulation in the pericardial sac elevates intrapericardial pressure, severely compressing the ventricles and restricting diastolic filling.

Valve Stenosis

Pathological narrowing of a heart valve opening that creates elevated resistance and restricts forward transvalvular blood flow.

Valve Incompetence (Regurgitation)

Failure of a heart valve leaflet mechanism to close completely, allowing retrograde or backward blood flow during cardiac contraction.

Coronary Artery Disease (CAD)

A condition characterized by atherosclerotic plaque buildup within the coronary arteries that restricts blood and oxygen delivery to the working myocardium.

Myocardial Infarction (MI)

The irreversible ischemic necrosis and death of a region of cardiac myocardium resulting from an abrupt, severe obstruction of coronary arterial blood flow.

Angina

Substernal chest pain or tightness provoked by transient myocardial ischemia without acute myocardial cell death, often triggered by exertion and relieved by nitroglycerin.

Sickle Cell Anemia

A genetic hemoglobin disorder characterized by abnormal hemoglobin polymerization under low oxygen tension, resulting in rigid, crescent-shaped erythrocytes that obstruct capillaries and undergo early hemolysis.