Human Physiology Test 1

What is the goal of the heart?

To pump blood and maintain circulation

What is the resting membrane potential of myocardial cells?

-90 mV

What is unique about the resting potential of nodal cells?

They do not have a steady resting potential; undergo slow depolarization called pacemaker potential

What ions are key for action potentials in both myocardial and nodal cells?

Na⁺, K⁺, and Ca²⁺

Which ion is key to reaching threshold in myocardial cells?

Na⁺

Which ion is key to reaching threshold in nodal cells?

Ca²⁺

What does the SA node exhibit?

Autorhythmicity (automatic rhythm generation)

How is excitation-contraction coupling in cardiac muscle similar to skeletal muscle?

Involves action potential triggering Ca²⁺ release for contraction

What is a key difference in excitation-contraction coupling between cardiac and skeletal muscle?

No summation or tetanic contraction in cardiac due to long absolute refractory period (250 ms in cardiac vs 2-4 ms in skeletal)

What are the two major phases of the cardiac cycle?

Systole (contraction) and diastole (relaxation)

For a heart rate of 72 bpm, how long does a cardiac cycle last?

Approximately 0.8 seconds (0.3 sec systole, 0.5 sec diastole)

What is stroke volume (SV)?

End-diastolic volume (EDV) minus end-systolic volume (ESV)

What are typical resting values for EDV, ESV, and SV in an adult?

EDV = 135 mL, ESV = 65 mL, SV = 70 mL

What is cardiac output (CO)?

Volume of blood pumped by each ventricle per minute; CO = HR × SV

What is a typical resting CO for an average adult?

5.5 L/min (72 bpm × 0.07 L/beat)

What can CO reach during exercise?

Up to 35 L/min

How do parasympathetic neurons affect heart rate?

Decrease heart rate

How do sympathetic neurons affect heart rate?

Increase heart rate

What is the intrinsic heart rate without neural input?

100 bpm

Why is resting heart rate typically 72 bpm?

Due to dominant parasympathetic tone

What other factors can influence heart rate?

Epinephrine, body temperature, plasma electrolytes, hormones

What is the Frank-Starling mechanism?

The ventricle contracts more forcefully when filled to a greater degree during diastole (more preload = stronger contraction)

What is the role of the vascular system?

Regulates blood pressure and distributes blood to tissues

What is the inner lining of all blood vessels?

Single layer of epithelial cells (endothelium)

What are characteristics of arteries?

Thick walls of elastic and muscular tissue with large radius

What determines arterial blood pressure?

Volume of blood within arteries and their compliance (stretchability)

What is systolic pressure (SP)?

Maximum arterial pressure during peak ventricular ejection

What is diastolic pressure (DP)?

Minimum arterial pressure just before ventricular ejection begins

What is pulse pressure?

Difference between SP and DP (e.g., 120/80 mmHg = 40 mmHg pulse pressure)

What factors influence pulse pressure?

Stroke volume, speed of ejection, arterial compliance (stiffer arteries increase pressure)

How is systemic arterial pressure measured?

Using a sphygmomanometer; listens for Korotkoff sounds

What role do arterioles play?

Determine relative blood flow to organs via resistance changes

How is blood flow through arterioles determined?

Pressure gradient divided by resistance (resistance key due to variable radius)

What happens when arteriolar smooth muscle contracts or relaxes?

Contraction = vasoconstriction (increases resistance, decreases flow); relaxation = vasodilation (decreases resistance, increases flow)

What percentage of total blood is in capillaries?

5%

What is the primary function of capillaries?

Exchange of nutrients, metabolic end products, and cell secretions

How close are body cells to capillaries?

No more than 0.1 mm; efficient for diffusion

What is the estimated total length of capillaries in an adult?

40,000 km

What is the typical length and diameter of a capillary?

1 mm long, 8 µm diameter (just wide enough for an erythrocyte)

How does flow rate change in capillaries?

Decreases significantly (slow flow for exchange)

What drives blood return in venules and veins?

Pressure difference (10-15 mmHg in veins vs 0 mmHg in right atrium)

What percentage of total blood volume is in systemic venules and veins?

Approximately 60%

How does venous smooth muscle contraction affect flow?

Increases pressure and forward flow (decreases diameter but increases velocity)

How does arteriolar constriction affect flow?

Increases resistance, reduces forward flow

How does venous constriction affect flow?

Increases forward flow

What aids venous return besides pressure?

Skeletal muscle pump (muscle contractions squeeze veins)

What prevents backflow in veins?

One-way valves

What sections should be read for the quiz?

Pages 384 (Heart Sounds) and 407-409 (Lymphatic System)

What is the primary goal of the heart?

Contract as a unit, move deoxygenated blood from right ventricle to lungs, move oxygenated blood from left ventricle to body, and contract in a coordinated fashion

What is the resting membrane potential of myocardial cells?

-90 mV

What is unique about nodal cells' membrane potential?

No steady resting potential; undergoes slow depolarization known as pacemaker potential

What ions are key for action potentials in both myocardial and nodal cells?

Na⁺, K⁺, and Ca²⁺

Which ion primarily reaches threshold in myocardial cells?

Na⁺

Which ion primarily reaches threshold in nodal cells?

Ca²⁺

What property does the SA node exhibit?

Autorhythmicity (spontaneous rhythm generation)

Why does the SA node control heart rate over the AV node?

SA node reaches threshold more quickly than AV node

What happens if AV node conduction fails?

Bundle of His and Purkinje fibers can exhibit autorhythmic properties as backup pacemakers (ectopic pacemakers at slower rates, e.g., Purkinje 25–40 bpm)

What is Phase 0 of the myocardial action potential?

Depolarization: rapid Na⁺ influx (initial rise), slow Ca²⁺ influx

What is Phase 1 of the myocardial action potential?

Early repolarization: voltage-gated Na⁺ channels close (peak of curve)

What is Phase 2 (plateau) of the myocardial action potential?

Plateau phase: rapid K⁺ efflux balanced by slow Ca²⁺ influx (from extracellular space and SR); Ca²⁺ causes prolonged plateau

What is Phase 3 of the myocardial action potential?

Rapid repolarization: Ca²⁺ channels close, rapid K⁺ efflux; ion positions reverse relative to sarcolemma

What is Phase 4 of the myocardial action potential?

Resting potential: maintained by leaky K⁺ channels; sarcolemma impermeable to Na⁺

What is an ectopic pacemaker?

A pacemaker outside the SA node that drives contraction (typically slower, e.g., Purkinje fibers at 25–40 bpm)

How does excitation-contraction coupling in cardiac muscle compare to skeletal muscle?

Similar mechanisms (AP triggers Ca²⁺ release), but cardiac has no summation/tetanic contraction due to long absolute refractory period

What is the absolute refractory period in cardiac muscle vs. skeletal muscle?

Cardiac: 250 ms (almost as long as contraction); Skeletal: 2–4 ms (contraction 20–100 ms)

Why is a long refractory period important in cardiac muscle?

Prevents tetanus/sustained contraction; allows relaxation for chamber filling and forward blood flow

What are the two major phases of the cardiac cycle?

Systole (contraction) and diastole (relaxation)

For a resting HR of 72 bpm, how long is one cardiac cycle?

~0.8 seconds (0.3 sec systole, 0.5 sec diastole)

What drives the mechanical events of the cardiac cycle?

Electrical events (action potentials)

What occurs during isovolumetric ventricular contraction?

Ventricles contract with all valves closed; volume constant; tension develops, pressure rises (isometric-like)

What causes the aortic and pulmonary valves to open?

Ventricular pressure exceeds pressure in aorta/pulmonary trunk

What is stroke volume (SV)?

Volume of blood ejected per beat; SV = EDV – ESV

What are typical resting values for EDV, ESV, and SV?

EDV = 135 mL, ESV = 65 mL, SV = 70 mL

What is cardiac output (CO)?

Volume of blood pumped by each ventricle per minute; CO = HR × SV (in L/min)

What is typical resting CO for an average adult?

~5.5 L/min (72 bpm × 0.07 L/beat); circulates all blood once per minute

What can CO reach during maximal exercise?

Up to 35 L/min

How do parasympathetic neurons affect heart rate?

Decrease HR (via SA node innervation)

How do sympathetic neurons affect heart rate?

Increase HR (via SA node innervation)

What is the intrinsic (no neural input) heart rate?

~100 bpm

Why is resting HR typically 70–75 bpm instead of 100 bpm?

Dominant parasympathetic tone (greater parasympathetic activity)

What other factors can influence heart rate?

Epinephrine, body temperature, plasma electrolyte concentrations, hormones

What is the Frank-Starling mechanism?

Intrinsic autoregulation: ventricle contracts more forcefully when more filled during diastole (greater preload = greater SV)

How does increased myocyte stretch contribute to Frank-Starling?

Increases proximity of contractile proteins → greater sensitivity to Ca²⁺ → increased contractility (no extra Ca²⁺ needed)

What are functional benefits of the Frank-Starling mechanism?

Matches right/left CO during respiratory cycle; smooths preload changes from exercise/posture; compensates for HR transients (e.g., after extrasystole); acts within one beat

What occurs during isovolumetric ventricular relaxation?

Ventricles relax; aortic/pulmonary valves close; AV valves closed; volume constant

When do AV valves open during diastole?

When ventricular pressure falls below atrial pressure → ventricular filling begins

What percentage of ventricular filling occurs before atrial contraction at rest?

~80% (most filling passive; atrial kick adds the rest at end of diastole)

What is end-systolic volume (ESV)?

Volume of blood remaining in ventricle after ejection

What is the myocardium?

The muscular layer of the heart responsible for contraction

What is the pericardium?

Protective fibrous sac surrounding the heart

What is the epicardium?

The layer covering the heart closely; fluid in the pericardial space lubricates movement

What are the AV valves?

Tricuspid (right side) and bicuspid/mitral (left side); control blood flow from atria to ventricles

What prevents AV valve prolapse into atria during ventricular contraction?

Papillary muscles and chordae tendineae

What are the semilunar valves?

Pulmonary and aortic valves; permit forward flow during ventricular contraction but prevent reverse flow during relaxation

How do AV valves open and close?

Passive process driven by pressure differences: open when atrial pressure > ventricular pressure; close when ventricular pressure > atrial pressure

What happens if papillary muscles or chordae tendineae are damaged?

Valve prolapse can occur (e.g., mitral valve prolapse)

How is cardiac muscle arranged?

In layers that encircle blood-filled chambers; contraction squeezes like a fist on a fluid-filled balloon

Do all heart cells contract with every beat?

Yes; every myocardial cell contracts once per beat (~once per second)

What percentage of cardiac cells form the conducting system?

~1%; specialized for spreading action potentials (not contraction-focused)

How is the heart supplied with blood/nutrients?

Coronary arteries (first branches off aorta) deliver blood to myocardium via arterioles, capillaries, venules, veins → coronary sinus → right atrium (blood in chambers does not nourish myocardium)