Overview
This lecture covers the structure and function of the human circulatory system, including types of circulatory systems, heart anatomy, blood vessel pathways, blood composition, and heart function.
Types of Circulatory Systems
- Insects and spiders have open circulatory systems with hemolymph, combining blood and interstitial fluid.
- Humans and most vertebrates have closed circulatory systems, keeping blood separate from interstitial fluid.
- Fish have two-chambered hearts and single-loop circulation from heart to gills to body.
- Amphibians and reptiles have three-chambered hearts with double-loop circulation but mixing of oxygenated and deoxygenated blood.
- Birds and mammals have four-chambered hearts and double-loop circulation, preventing blood mixing for greater efficiency.
Heart Structure and Blood Flow
- The right atrium receives deoxygenated blood from the superior and inferior vena cava.
- Blood flows from right atrium through the tricuspid (AV) valve to the right ventricle.
- The right ventricle pumps blood through the semi-lunar valve into the pulmonary artery to the lungs.
- Oxygenated blood returns from lungs to the left atrium, then passes through the left AV valve to the left ventricle.
- The left ventricle pumps blood into the aorta to supply the body.
Blood Vessels
- Blood flows from arteries → arterioles → capillaries → venules → veins.
- Arteries carry blood away from the heart under high pressure; veins return blood to the heart with valves to prevent backflow.
- Blood is always red; diagrams use blue to indicate deoxygenated blood, not its actual color.
Blood Composition
- Red blood cells transport oxygen.
- Plasma is the fluid portion of blood, carrying nutrients and carbon dioxide.
- White blood cells fight infection.
- Platelets help in blood clotting and scab formation.
Cardiac Cycle and Heart Function
- The cardiac cycle has two phases: diastole (relaxation/filling) and systole (contraction/pumping).
- The sinoatrial (SA) node initiates the electrical signal causing atria and then ventricles to contract in sequence.
- Ventricles have more muscle, especially on the left side, to pump blood throughout the whole body.
Heart Health
- Coronary arteries supply the heart muscle with oxygen and nutrients.
- Blockage in coronary arteries can cause a heart attack by cutting off oxygen to heart tissue.
Key Terms & Definitions
- Open Circulatory System — circulatory fluid (hemolymph) mixes with body fluids, found in insects.
- Closed Circulatory System — blood circulates within vessels, separate from interstitial fluid.
- Atria — upper heart chambers that receive blood.
- Ventricles — lower heart chambers that pump blood out.
- Arteries — vessels carrying blood away from the heart.
- Veins — vessels carrying blood back to the heart.
- Capillaries — tiny vessels where gas and nutrient exchange occurs.
- Plasma — liquid component of blood.
- Cardiac Cycle — sequence of heart filling (diastole) and pumping (systole).
- Sinoatrial (SA) Node — heart’s natural pacemaker.
Action Items / Next Steps
- Practice tracing the flow of blood through the heart from vena cava to aorta.
- Review diagrams of heart chambers and major blood vessels.