A 40-year-old female volunteers for an invasive study to measure her cardiac function. She has no previous cardiovascular history and takes no medications. With the test subject at rest, the following data is collected using blood tests, intravascular probes, and a closed rebreathing circuit: Blood hemoglobin concentration 14 g/dL Arterial oxygen content 0.22 mL O2/mL Arterial oxygen saturation 98% Venous oxygen content 0.17 mL O2/mL Venous oxygen saturation 78% Oxygen consumption 250 mL/min The patient's pulse is 75/min, respiratory rate is 14/ min, and blood pressure is 125/70 mm Hg. What is the cardiac output of this volunteer?

Body surface area is required to calculate cardiac output.

Stroke volume is required to calculate cardiac output.

Cardiac muscle serves many necessary functions, leading to a specific structure that serves these functions. The structure highlighted is an important histology component of cardiac muscle. What would be the outcome if this structure diffusely failed to function?

Failure of propagation of the action potential from the conduction system

Failure of potassium channels to appropriately open to repolarize the cell

Ineffective excitation-contraction coupling due to insufficient calcium ions

Inappropriate formation of cardiac valve leaflets

A researcher measures action potential propagation velocity in various regions of the heart in a 42-year-old Caucasian female. Which of the following set of measurements corresponds to the velocities found in the atrial muscle, AV Node, Purkinje system, and ventricular muscle, respectively?

1.1 m/s, 0.05 m/s, 2.2 m/s, 0.3 m/s

0.05 m/s, 1.1 m/s, 2.2 m/s, 3.3 m/s

2.2 m/s, 0.3 m/s, 0.05 m/s, 1.1 m/s

0.3 m/s, 2.2 m/s, 0.05 m/s, 1.1 m/s

0.5 m/s, 1.1 m/s, 2.2 m/s, 3 m/s

A 65-year-old woman comes to the physician because of a 3-month history of intermittent palpitations and shortness of breath. Cardiopulmonary examination shows no other abnormalities. An ECG shows an absence of P waves, an oscillating baseline, and irregular RR intervals at a rate of approximately 95 beats per minute. The difference between atrial and ventricular rates in this patient is most likely due to which of the following?

Temporary inactivation of Na+ channels in the AV node

Prolonged influx through voltage-gated Ca2+ channels in the bundle of His

Transient activation of K+ current in Purkinje fibers

Inhibition of the Na+/K+-ATPase pump in ventricular cells

Limited speed of conduction through the left bundle branch

A 42-year-old Caucasian woman is enrolled in a randomized controlled trial to study cardiac function in the setting of several different drugs. She is started on verapamil and instructed to exercise at 50% of her VO2 max while several cardiac parameters are being measured. During this experiment, which of the following represents the relative conduction speed through the heart from fastest to slowest?

Purkinje fibers > atria > ventricles > AV node

Purkinje fibers > ventricles > atria > AV node

Atria > Purkinje fibers > ventricles > AV node

AV node > ventricles > atria > Purkinje fibers

Purkinje fibers > AV node > ventricles > atria

A 55-year-old man comes to the emergency department because of left-sided chest pain and difficulty breathing for the past 30 minutes. His pulse is 88/min. He is pale and anxious. Serum studies show increased cardiac enzymes. An ECG shows ST-elevations in leads I, aVL, and V5-V6. A percutaneous coronary intervention is performed. In order to localize the site of the lesion, the catheter must pass through which of the following structures?

Left coronary artery → left circumflex artery

Right coronary artery → posterior descending artery

Left coronary artery → left anterior descending artery

Right coronary artery → right marginal artery

Left coronary artery → posterior descending artery

Cardiovascular system overview — USMLE Lesson

Heart Anatomy - The Body's Pumping House

Posterior view of human heart with labeled structures

Chambers: Four hollow spaces that receive and pump blood.
- Atria (R/L): Superior, receiving chambers.
- Ventricles (R/L): Inferior, pumping chambers. The left ventricle wall is ~3x thicker than the right.
Valves: Ensure unidirectional blood flow.
- Atrioventricular: Tricuspid (R), Mitral (L).
- Semilunar: Pulmonary (R), Aortic (L).
- 📌 Mnemonic: "Try Pulling My Aorta"
Layers: From out to in: Pericardium → Myocardium (muscle) → Endocardium.

⭐ In ~85% of people, the Right Coronary Artery (RCA) gives rise to the Posterior Descending Artery (PDA), determining "right dominance."

Blood Flow Pathway - The Double-Loop Journey

Pulmonary Circuit (Low-Pressure): The right side of the heart pumps deoxygenated blood through the pulmonary arteries to the lungs for gas exchange. Oxygenated blood then returns to the left atrium.
Systemic Circuit (High-Pressure): The left side of the heart pumps this oxygenated blood via the aorta to the rest of the body. Veins and vena cavae return deoxygenated blood to the right atrium.

⭐ Physiological Shunt: A small volume of deoxygenated blood from the bronchial circulation (supplying lung tissue) and Thebesian veins (draining the heart muscle) empties directly into the left atrium. This is a normal finding.

Coronary Circulation - The Heart's Fuel Line

Anterior View of the Heart with Coronary Arteries

Arterial Supply: Originates from the root of the aorta; perfused during diastole.
Left Coronary Artery (LCA) Branches:
- Left Anterior Descending (LAD): Supplies anterior 2/3 of IV septum, apex, anterior LV wall. The "widow maker."
- Left Circumflex (LCX): Supplies lateral/posterior LV wall, anterolateral papillary muscle.
Right Coronary Artery (RCA): Supplies RA, RV, SA & AV nodes.

⭐ Coronary Dominance: Determined by which artery gives rise to the Posterior Descending Artery (PDA). Right dominance is most common (~85%). Occlusion of the RCA can thus cause AV nodal block.

Cardiac Conduction - The Heart's Electrical Beat

Sinoatrial (SA) node: The heart's primary pacemaker, firing at 60-100 bpm.
Atrioventricular (AV) node: Delays the signal to allow for ventricular filling; intrinsic rate of 40-60 bpm.
Bundle of His & Purkinje fibers: Transmit impulse to the ventricles; intrinsic rate of 20-40 bpm.

Heart electrical conduction system with ECG correlation

⭐ The AV node's refractory period is crucial; it protects the ventricles from dangerously high rates during atrial tachyarrhythmias like atrial fibrillation.

High‑Yield Points - ⚡ Biggest Takeaways

The right heart pumps deoxygenated blood to the lungs; the left heart pumps oxygenated blood to the body.

Systemic circulation is a high-pressure system, while pulmonary circulation is a low-pressure system.

The heart muscle is supplied by the right and left coronary arteries, the first branches of the aorta.

Electrical conduction pathway: SA node → AV node → Bundle of His → Purkinje fibers.

Key fetal shunts-foramen ovale, ductus arteriosus, ductus venosus-bypass the non-functional fetal lungs.

The pericardium encloses the heart, providing protection and preventing overfilling.

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