Blood flow in our body is like a complex highway system. The heart acts as a central hub, pumping blood through two main circuits: systemic and pulmonary. These circuits work together to deliver oxygen and nutrients to every cell in our body.

The systemic circuit sends oxygen-rich blood to our organs, while the pulmonary circuit refreshes blood with oxygen from our lungs. Understanding how these circuits work helps us grasp the incredible efficiency of our cardiovascular system in keeping us alive and healthy.

Blood Flow Through Circuits

Systemic and Pulmonary Circuits

• The systemic circuit carries oxygenated blood from the left ventricle of the heart to the body tissues and returns deoxygenated blood to the right atrium
• The pulmonary circuit carries deoxygenated blood from the right ventricle to the lungs for oxygenation and returns oxygenated blood to the left atrium
• Blood flows in a unidirectional manner through the heart
  • The right side receives deoxygenated blood from the systemic circuit and pumps it to the lungs
  • The left side receives oxygenated blood from the lungs and pumps it to the systemic circuit

Major Arteries and Veins

• The aorta is the main artery that carries oxygenated blood from the left ventricle to the systemic circuit
• The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs
• The vena cava (superior and inferior) are the main veins that return deoxygenated blood from the systemic circuit to the right atrium
• The pulmonary veins return oxygenated blood from the lungs to the left atrium

Systemic vs Pulmonary Circulation

Functions and Characteristics

• The systemic circulation delivers oxygenated blood and nutrients to the body tissues and removes waste products
• The pulmonary circulation is responsible for gas exchange between the blood and the lungs
• The systemic circulation operates at a higher pressure than the pulmonary circulation
  • This is due to the greater resistance in the systemic blood vessels and the need to deliver blood to all parts of the body
• The pulmonary circulation has a lower blood pressure and resistance compared to the systemic circulation, facilitating efficient gas exchange in the lungs

Circuit Size and Extent

• The systemic circulation is a much larger circuit compared to the pulmonary circulation, as it needs to supply blood to all body tissues
• The pulmonary circulation is a shorter circuit, as it only needs to transport blood between the heart and the lungs for gas exchange
  • Example: The total length of the systemic circulation in an adult human is approximately 60,000 miles (96,560 km), while the pulmonary circulation is only about 1,500 miles (2,414 km) long
• The systemic circulation reaches all organs and tissues in the body (skeletal muscles, digestive organs, brain, etc.)
• The pulmonary circulation is confined to the lungs and the blood vessels connecting them to the heart

The Heart's Pumping Action

Cardiac Cycle and Contraction

• The heart acts as a double pump
  • The right side pumps deoxygenated blood to the lungs
  • The left side pumps oxygenated blood to the body
• The contraction of the heart muscle (myocardium) generates the force necessary to propel blood through the circulatory system
• The cardiac cycle consists of systole (contraction) and diastole (relaxation) of the atria and ventricles, which coordinates the filling and emptying of the heart chambers
  • Example: During atrial systole, the atria contract and push blood into the ventricles; during ventricular systole, the ventricles contract and pump blood out of the heart

Conduction System and Pacemaker

• The sinoatrial (SA) node, located in the right atrium, acts as the heart's natural pacemaker, initiating the cardiac cycle and setting the heart rate
• The atrioventricular (AV) node, located between the atria and ventricles, delays the electrical impulse from the SA node, allowing the atria to contract before the ventricles
• The bundle of His and Purkinje fibers rapidly conduct the electrical impulse from the AV node to the ventricles, ensuring coordinated contraction of the ventricular myocardium
  • Example: The SA node generates an electrical impulse that spreads through the atria, causing them to contract; the impulse is then delayed at the AV node before being rapidly conducted through the bundle of His and Purkinje fibers to the ventricles, causing them to contract

Left vs Right Heart Structure and Function

Blood Flow and Oxygenation

• The right side of the heart receives deoxygenated blood from the systemic circuit and pumps it to the lungs
• The left side receives oxygenated blood from the lungs and pumps it to the systemic circuit
• The right atrium receives deoxygenated blood from the superior and inferior vena cava
• The left atrium receives oxygenated blood from the pulmonary veins
• The right ventricle pumps blood into the pulmonary artery, which carries deoxygenated blood to the lungs
• The left ventricle pumps blood into the aorta, which distributes oxygenated blood to the body

Structural Differences

• The left ventricle has a thicker muscular wall compared to the right ventricle
  • This is because it needs to generate higher pressure to pump blood through the systemic circuit
• The right ventricle has a thinner muscular wall and a more crescent-shaped cavity compared to the left ventricle
  • This is because it pumps blood against lower resistance in the pulmonary circuit
• The left side of the heart is separated from the right side by the interatrial and interventricular septa
  • This prevents the mixing of oxygenated and deoxygenated blood
• The left ventricle has a more conical shape, while the right ventricle appears triangular or crescent-shaped when viewed in cross-section