Common Misconceptions About Circulatory System

The most common misconception about the circulatory system is that it is a closed system, where blood is pumped by the heart and then returns to the heart without any interaction with the outside environment.

Misconceptions

• Myth: The circulatory system is a closed system, with no exchange of substances between the blood and the outside environment.
• Fact: The circulatory system is actually an open system, with exchange of oxygen and carbon dioxide occurring in the lungs through the process of respiration, and exchange of nutrients and waste products occurring in the digestive system and kidneys, with ~25% of cardiac output going to the kidneys (Guyton and Hall, Textbook of Medical Physiology).
• Source of confusion: This myth persists due to oversimplification in introductory textbooks, which often depict the circulatory system as a simple loop with no external interactions.
• Myth: The heart is the only organ responsible for pumping blood throughout the body.
• Fact: Other organs, such as the skeletal muscles, also play a crucial role in propelling blood forward, with the muscular pump mechanism in the legs contributing significantly to venous return, particularly during exercise, as demonstrated by the work of William Harvey.
• Source of confusion: This myth may arise from the fact that the heart is often the primary focus of study in circulatory physiology, leading to neglect of the contributions of other organs and tissues.
• Myth: Arteries always carry oxygenated blood, while veins always carry deoxygenated blood.
• Fact: The pulmonary arteries carry deoxygenated blood from the heart to the lungs, while the pulmonary veins carry oxygenated blood from the lungs to the heart, with this unique arrangement allowing for efficient gas exchange, as described by Marcello Malpighi.
• Source of confusion: This myth may be due to oversimplification of the circulatory pathway, neglecting the special case of the pulmonary circulation.
• Myth: The circulatory system is solely responsible for delivering oxygen and nutrients to tissues.
• Fact: The circulatory system also plays a key role in regulating body temperature, with blood flow to the skin helping to dissipate heat, and ~10% of cardiac output going to the skin at rest (Berne and Levy, Cardiovascular Physiology).
• Source of confusion: This myth may arise from a narrow focus on the delivery of oxygen and nutrients, neglecting other important functions of the circulatory system.
• Myth: Blood pressure is constant throughout the body.
• Fact: Blood pressure varies significantly between different parts of the body, with pressures in the aorta and large arteries being much higher than those in the capillaries and veins, with a mean arterial pressure of ~100 mmHg (Guyton and Hall, Textbook of Medical Physiology).
• Source of confusion: This myth may be due to the use of a single, average value for blood pressure, rather than recognizing the complexity and variability of the circulatory system.
• Myth: The circulatory system is not affected by factors outside of the body.
• Fact: The circulatory system is influenced by a range of external factors, including gravity, temperature, and altitude, with changes in these factors affecting blood distribution and cardiac output, as demonstrated by the work of August Krogh.
• Source of confusion: This myth may arise from a failure to consider the interactions between the body and its environment.

Quick Reference

• Closed system → open system with exchange of oxygen, carbon dioxide, nutrients, and waste products
• Heart as sole pump → heart and skeletal muscles contribute to blood propulsion
• Arteries carry oxygenated blood → pulmonary arteries carry deoxygenated blood
• Circulatory system delivers only oxygen and nutrients → also regulates body temperature
• Constant blood pressure → varies between different parts of the body
• Circulatory system unaffected by external factors → influenced by gravity, temperature, and altitude