2.2 Systemic and Pulmonary Circulation

Blood Flow Through Circuits

The cardiovascular system moves blood through two distinct loops, both powered by the heart. The systemic circuit delivers oxygen and nutrients to body tissues, while the pulmonary circuit sends blood to the lungs to pick up fresh oxygen and dump carbon dioxide. These two circuits run in series, meaning all blood must pass through both loops in sequence.

Systemic and Pulmonary Circuits

• The systemic circuit carries oxygenated blood from the left ventricle to body tissues, then returns deoxygenated blood to the right atrium.
• The pulmonary circuit carries deoxygenated blood from the right ventricle to the lungs, then returns oxygenated blood to the left atrium.
• Blood flows in one direction through the heart:
  • The right side receives deoxygenated blood from the body and pumps it to the lungs.
  • The left side receives oxygenated blood from the lungs and pumps it out to the body.

A helpful way to trace the path: right atrium → right ventricle → pulmonary arteries → lungs → pulmonary veins → left atrium → left ventricle → aorta → body → venae cavae → right atrium. That's one complete loop.

Major Arteries and Veins

Notice that artery/vein naming is based on direction of flow relative to the heart, not oxygen content. The pulmonary arteries carry deoxygenated blood (the only arteries in the body that do), and the pulmonary veins carry oxygenated blood.

• Aorta: the largest artery; carries oxygenated blood from the left ventricle into the systemic circuit.
• Pulmonary trunk/arteries: carry deoxygenated blood from the right ventricle to the lungs.
• Superior and inferior vena cava: the two large veins that return deoxygenated blood from the upper and lower body, respectively, to the right atrium.
• Pulmonary veins (four total): return oxygenated blood from the lungs to the left atrium.

Systemic vs. Pulmonary Circulation

Functions and Characteristics

The systemic circuit has a big job: deliver oxygenated blood and nutrients to every tissue in the body while picking up metabolic waste (like $CO_2$). The pulmonary circuit has a more focused role: gas exchange. Blood releases $CO_2$ and loads up on $O_2$ at the alveolar capillaries in the lungs.

A key difference is pressure. Systemic circulation operates at much higher pressures (mean arterial pressure around 93 mmHg) because it must push blood through a vast network of vessels reaching every organ. Pulmonary circulation runs at roughly one-fifth that pressure (mean around 15 mmHg) because the lungs are right next to the heart and offer low vascular resistance. That low pressure also protects the delicate alveolar capillaries from damage and allows efficient gas diffusion.

Circuit Size and Extent

The systemic circuit is far larger. It branches out to supply skeletal muscles, the brain, digestive organs, kidneys, skin, and every other tissue. The pulmonary circuit, by contrast, only spans the short distance between the heart and the lungs.

To put it in perspective: if you lined up all the systemic blood vessels end to end, they'd stretch roughly 60,000 miles. The pulmonary vasculature totals only about 1,500 miles. This size difference is the main reason the left ventricle must work harder and has thicker walls.

The Heart's Pumping Action

Cardiac Cycle and Contraction

The heart functions as a double pump: the right side and left side pump simultaneously, but to different circuits. Each pump cycle has two phases:

1. Diastole (relaxation): The heart chambers relax and fill with blood. The atria fill from the veins, and blood passively flows through the open AV valves into the ventricles.
2. Atrial systole: The atria contract, pushing the remaining blood into the ventricles (this "atrial kick" contributes about 20% of ventricular filling).
3. Ventricular systole (contraction): The ventricles contract forcefully. The AV valves snap shut (producing the first heart sound, S1), and blood is ejected through the semilunar valves into the aorta and pulmonary trunk. When the ventricles relax, the semilunar valves close (producing S2).

The contraction of the myocardium (heart muscle) generates the pressure that propels blood. The left ventricle generates significantly more pressure than the right because it feeds the high-resistance systemic circuit.

Conduction System and Pacemaker

The heart's electrical conduction system coordinates each beat in a precise sequence:

1. The sinoatrial (SA) node, in the upper wall of the right atrium, fires an electrical impulse. It sets the pace (typically 60-100 beats per minute at rest), which is why it's called the natural pacemaker.
2. The impulse spreads across both atria, causing atrial contraction.
3. The impulse reaches the atrioventricular (AV) node, located at the junction of the atria and ventricles. The AV node delays the signal by about 0.1 seconds, giving the atria time to finish contracting before the ventricles fire.
4. The impulse then travels down the bundle of His (in the interventricular septum) and splits into the right and left bundle branches.
5. The Purkinje fibers distribute the impulse rapidly throughout the ventricular myocardium, triggering a coordinated ventricular contraction from the apex upward.

This bottom-to-top contraction pattern in the ventricles is important because it pushes blood upward toward the outflow tracts (the aorta and pulmonary trunk).

Left vs. Right Heart Structure and Function

Blood Flow and Oxygenation

The two sides of the heart handle different blood:

• Right atrium receives deoxygenated blood from the body via the superior and inferior vena cava.
• Right ventricle pumps that blood into the pulmonary trunk/arteries toward the lungs.
• Left atrium receives freshly oxygenated blood from the lungs via the four pulmonary veins.
• Left ventricle pumps oxygenated blood into the aorta for systemic distribution.

Both sides pump the same volume of blood per beat (stroke volume). If they didn't, blood would pool on one side of the circuit.

Structural Differences

The structural differences between the left and right sides directly reflect their workloads:

• The left ventricle has a thick muscular wall (roughly 10-13 mm) because it generates the high pressures needed for systemic circulation. In cross-section, it appears circular or conical.
• The right ventricle has a thinner wall (roughly 3-5 mm) because pulmonary resistance is much lower. In cross-section, it wraps around the left ventricle in a crescent shape, almost like a bellows.
• The interventricular septum and interatrial septum separate the left and right sides, preventing oxygenated and deoxygenated blood from mixing. This complete separation is essential for maintaining the efficiency of the double-circuit system.

A common exam question: Why is the left ventricular wall thicker than the right? The answer ties directly back to pressure. The left ventricle pumps against systemic resistance (high), while the right ventricle pumps against pulmonary resistance (low). Thicker muscle generates more force.