A woman with coronary artery disease is starting to go for a walk. As she begins, her heart rate accelerates from a resting pulse of 60 bpm until it reaches a rate of 120 bpm, at which point she begins to feel a tightening in her chest. She stops walking to rest and the tightening resolves. This has been happening to her consistently for the last 6 months. Which of the following is a true statement?

Increasing the heart rate decreases the relative amount of time spent during diastole

This patient's chest pain is indicative of transmural ischemia

Perfusion of the myocardium takes place equally throughout the cardiac cycle

Increasing the heart rate increases the amount of time spent during each cardiac cycle

Perfusion of the myocardium takes place primarily during systole

Which of the following physiologic changes decreases pulmonary vascular resistance (PVR)?

Inhaling the entire vital capacity (VC)

Inhaling the inspiratory reserve volume (IRV)

Exhaling the entire vital capacity (VC)

Exhaling the expiratory reserve volume (ERV)

Breath holding maneuver at functional residual capacity (FRC)

A 44-year-old man is brought to the emergency department 45 minutes after being involved in a high-speed motor vehicle collision in which he was the restrained driver. On arrival, he has left hip and left leg pain. His pulse is 135/min, respirations are 28/min, and blood pressure is 90/40 mm Hg. Examination shows an open left tibial fracture with active bleeding. The left lower extremity appears shortened, flexed, and internally rotated. Femoral and pedal pulses are decreased bilaterally. Massive transfusion protocol is initiated. An x-ray of the pelvis shows an open pelvis fracture and an open left tibial mid-shaft fracture. A CT scan of the head shows no abnormalities. Laboratory studies show: Hemoglobin 10.2 g/dL Leukocyte count 10,000/mm3 Platelet count <250,000/mm3 Prothrombin time 12 sec Partial thromboplastin time 30 sec Serum Na+ 125 mEq/L K+ 4.5 mEq/L Cl- 98 mEq/L HCO3- 25 mEq/L Urea nitrogen 18 mg/dL Creatinine 1.2 mg/dL The patient is taken emergently to interventional radiology for exploratory angiography and arterial embolization. Which of the following is the most likely explanation for this patient's hyponatremia?

Physiologic ADH (vasopressin) secretion

Pathologic aldosterone secretion

Physiologic aldosterone secretion

Pathologic ADH (vasopressin) secretion

Regional blood flow distribution for USMLE Step 2 CK: High-Yield Physiology Lessons

Flow Fundamentals - The Body's Pipes

Blood flow (Q) follows a version of Ohm's Law: driven by pressure gradient (ΔP), opposed by resistance (R).
- $Q = ΔP / R$
Resistance is detailed by Poiseuille's Law: $R \propto (\eta \cdot L) / r^4$
- η: Viscosity (e.g., ↑ in polycythemia)
- L: Vessel length (largely constant)
- r: Vessel radius (primary site of physiologic control)
📌 Radius rules all: Halving the vessel radius increases resistance 16-fold and reduces flow by 16x.

⭐ The arterioles are the principal site of systemic vascular resistance (SVR), acting as the main "faucets" regulating blood flow into capillary beds.

Poiseuille's Law: Flow, Radius, Length, Pressure, Viscosity

Autoregulation - Keeping Flow Steady

Intrinsic ability of an organ to maintain constant blood flow despite changes in perfusion pressure (60-160 mmHg).
Mediated by altering arteriolar resistance: $Q = oldsymbol{\Delta} P / R$

Key Mechanisms:

Myogenic Mechanism:
- ↑ Pressure → stretches arteriolar wall → smooth muscle contraction → vasoconstriction → ↑ Resistance.
- Primary mechanism in kidney, brain.
Metabolic (Local) Mechanism:
- Blood flow is matched to metabolic activity.
- ↑ Metabolism → ↑ local vasodilators (CO₂, adenosine, K⁺, H⁺, lactate) → vasodilation → ↑ Blood Flow.

⭐ The heart is highly dependent on metabolic autoregulation. Adenosine is the most critical vasodilator linking coronary blood flow to myocardial oxygen consumption.

Organ Blood Flow Autoregulation

Organ-Specific Control - A VIP Tour

Heart: Flow is governed by local metabolic needs ($O_2$ demand).
- Key vasodilators: Adenosine, $NO$, $CO_2$.
- Most blood flow occurs during diastole due to systolic compression of vessels.
Brain: Tightly autoregulated; flow is constant.
- Primary controller: Local metabolites, especially arterial $PCO_2$.
Skeletal Muscle: Dual control.
- At rest: Sympathetic tone dominates (vasoconstriction).
- During exercise: Local metabolites (lactate, adenosine, $K^+$) override, causing massive vasodilation.
Kidney: Strong autoregulation (myogenic & tubuloglomerular feedback) to maintain constant GFR & RBF.
Lungs: Unique hypoxic vasoconstriction-directs blood away from poorly ventilated areas.
Skin: Sympathetic nervous system control is primary for temperature regulation.

⭐ The heart has the highest oxygen extraction (~75%) of any organ at rest. To increase its oxygen supply during exercise, it must primarily increase blood flow, as it cannot extract much more oxygen from the blood passing through.

High‑Yield Points - ⚡ Biggest Takeaways

The heart has the highest blood flow per gram at rest, but skeletal muscle receives the most during intense exercise.

Coronary blood flow is unique, peaking during ventricular diastole due to systolic compression of vessels.

Cerebral blood flow is tightly autoregulated, primarily responsive to changes in local PCO₂.

Pulmonary circulation is a low-pressure, low-resistance system that accommodates the entire cardiac output.

Renal blood flow is also highly autoregulated to maintain a stable GFR.

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