Blood pressure declines from arteries to veins due to vascular resistance

Steps:

• Blood exits the heart into arteries at high pressure, around 100 mmHg systolic.
• Pressure drops sharply in arterioles and capillaries due to increased resistance and total cross-sectional area.
• In veins, pressure remains low (near 0-15 mmHg) as blood flows back to the heart with minimal pulsation.
• The diagram's line for veins shows this steady, lowest pressure profile.

Why D is correct:

• Veins exhibit low pressure per the definition of venous return, where upstream resistance in capillaries dissipates most arterial pressure (Bernoulli's principle and Poiseuille's law confirm flow-driven drop).

Why the others are wrong:

• A: Depicts high, pulsatile arterial pressure leaving the heart.
• B: Shows moderately high pressure in smaller arteries post-initial drop.
• C: Represents capillary pressure, intermediate and declining but above venous levels.

Final answer: D