Introduction
The blood system, also known as the circulatory system, is crucial for the transport of nutrients, gases, hormones, and waste products throughout the body. This study note will cover the key topics in the IB Biology syllabus related to the blood system, including its history, the various types of blood vessels, the concept of double circulation, the cardiac cycle, and heart rate.
6.2.1 The Blood System: History
Historical Theories and Discoveries
Galen's Theories
- The ancient Greek philosopher and surgeon Galen proposed several theories regarding blood circulation:
- Blood is formed in the liver from ingested food.
- Blood is pumped back and forth between the liver and the right ventricle.
- Some blood moves into the left ventricle through invisible pores and mixes with air from the lungs.
- This mixture produces spirits, which are distributed to the body via the brain.
- Blood is consumed by tissues, necessitating continuous production.
Galen's theories lacked empirical evidence and were based largely on speculation and philosophical reasoning.
William Harvey's Contributions
- William Harvey, an English physician, overturned Galen's theories through empirical research and observations:
- Blood is pumped to the brain and body by the heart.
- Blood circulates through pulmonary and systemic circulation systems.
- Capillaries link arteries to veins.
- Blood flow is too fast for it to be consumed by tissues; instead, it recirculates.
Harvey's work laid the foundation for modern understanding of the circulatory system.
ExampleHarvey demonstrated his theories by showing that blood could not be consumed by tissues at the rate it flows, thus proving it must recirculate.
6.2.2 The Blood System: Vessels
Types of Blood Vessels
Arteries
- Function: Carry blood away from the heart.
- Structure: Thick muscular walls to withstand high pressure.
- Example: The aorta, the largest artery in the body.
Arterioles
- Function: Smaller branches of arteries that lead to capillaries.
- Structure: Thinner walls than arteries but still muscular, allowing regulation of blood flow.
Capillaries
- Function: Sites of nutrient, gas, and waste exchange between blood and tissues.
- Structure: Extremely thin walls (one cell thick) to facilitate diffusion.
Venules
- Function: Collect blood from capillaries and transport it to veins.
- Structure: Thinner walls than arterioles.
Veins
- Function: Carry blood back to the heart.
- Structure: Thinner walls than arteries, contain valves to prevent backflow.
- Example: The vena cava, the largest vein in the body.
Remember the sequence: arteries → arterioles → capillaries → venules → veins.
