Cardiac Physiology and Blood Components

Thirty question-and-answer flashcards reviewing cardiac action potentials, autonomic modulation of heart rate, and fundamental blood components.

Why is it crucial that the myocardium never engages in tetanus?

Because sustained contraction would prevent relaxation, stopping ventricular filling and systemic blood pumping.

What is the plateau phase of a myocardial contractile cell action potential?

An extended depolarized period caused by Ca²⁺ influx that delays full repolarization.

Which ion influx is responsible for the initial depolarization of myocardial contractile cells?

Sodium (Na⁺) influx.

Which ion efflux drives the final repolarization of myocardial contractile cells?

Potassium (K⁺) efflux.

Which ion’s slow influx produces the plateau phase in myocardial contractile cells?

Calcium (Ca²⁺).

How does the plateau phase prevent tetanus in the heart?

It makes electrical activity end with the mechanical twitch, so the cell cannot be re-excited during contraction.

What do myocardial autorhythmic cells (MACs) do?

They spontaneously depolarize and repolarize, establishing the heart’s rhythm.

What is the pacemaker potential?

The slow depolarization in MACs that brings membrane voltage to threshold between action potentials.

Which specialized channels create the pacemaker potential?

Funny current (If) channels.

Which two ions move through funny current channels?

Na⁺ enters and K⁺ exits.

Why does sodium entry dominate over potassium exit through funny current channels?

Na⁺ has a strong electrochemical gradient, while K⁺ has only a chemical gradient.

Reaching −40 mV in MACs opens which channels to cause the rapid upstroke?

Voltage-gated Ca²⁺ channels.

What neurotransmitter mediates parasympathetic influence on MACs?

Acetylcholine.

Binding of acetylcholine to muscarinic receptors causes what permeability changes?

Increased K⁺ permeability and decreased Ca²⁺ permeability.

Approximately how often do MACs fire under resting parasympathetic (vagal) tone?

About 60–80 times per minute.

At what intrinsic rate do MACs fire in the absence of autonomic input?

Roughly 80–100 times per minute.

Which neurotransmitters provide sympathetic stimulation to MACs?

Norepinephrine and epinephrine.

Sympathetic stimulation via β1 receptors changes permeability of which ions?

It increases permeability to Na⁺ and Ca²⁺.

What effect does sympathetic stimulation have on heart rate?

It accelerates it to greater than 100 beats per minute.

Does the parasympathetic nervous system directly affect myocardial contractile cell force?

No, it has no direct influence on MCCs.

How does sympathetic activity affect myocardial contractile cells?

NE/Epi binding to β1 receptors increases ventricular contractile force.

What is hematocrit?

The percentage of blood volume made up of red blood cells, about 45%.

What is the primary function of erythrocytes?

Transport O₂ to tissues and carry CO₂ to the lungs.

Approximately what percentage of blood is plasma and what is its main component?

About 55%; it is mostly water with dissolved ions, nutrients, gases, and wastes.

What makes up the buffy coat in centrifuged blood?

Leukocytes (white blood cells) and platelets, <1% of blood volume.

Why are erythrocytes not considered true cells?

They lack a nucleus and organelles, functioning mainly as hemoglobin carriers.

What are platelets and what is their role?

Cell fragments that contribute to hemostasis (blood clotting).

Name two major dissolved gases and two common ions found in plasma.

Gases: O₂ and CO₂; ions: Na⁺ and Cl⁻.

Why are red blood cells described as “bags of hemoglobin”?

Their cytoplasm is largely hemoglobin, with minimal other cell structures.

In blood terminology, what are “formed elements”?

Erythrocytes, leukocytes, and platelets collectively.