7) Human Transport

• The human cardiovascular system (CVS) consists of a heart, blood vessels, and blood
• Humans have a double circulation with a 4-chambered heart where the blood travels twice through the heart in one complete circuit
• There are two types of circulation:

  • Pulmonary circulation: right ventricle pumps blood to the lungs
  • Systemic circulation: left ventricle pumps blood to all body tissues
• Double circulation maintains high blood pressure to overcome pressure loss in the lung capillaries
• Humans have a closed circulatory system where blood is enclosed within blood vessels
• The heart has 4 chambers: 2 thin-walled atria and 2 thick-walled ventricles
• Atria contract to pump blood into the ventricles, while ventricles contract to pump blood through arteries to body tissues
• Atrioventricular valves prevent backflow of blood from the ventricles to the atria during ventricular contraction
• The left ventricle is 3 times thicker than the right ventricle to generate more pressure for systemic circulation
• The pulmonary artery and aorta have semilunar valves that open during ventricular contraction and close during ventricular relaxation
• The heart is made of cardiac muscles (Myocytes) supplied by numerous capillaries branching from the coronary arteries
• The sequence of events in one heartbeat:

  • Atrial Systole: atrial muscles contract to pump blood into the ventricles
  • Ventricular Systole: ventricular muscles contract to pump blood through arteries
  • Ventricular Diastole: atria and ventricles relax
• The cardiac muscle is myogenic and contracts spontaneously without nerve impulses
• The sinoatrial node (SAN) acts as a pacemaker, sending waves of excitation through the heart
• Electrocardiogram can detect and record waves of excitation in the heart muscle
• Arteries carry blood away from the heart at high pressure to the tissues
• Veins return blood to the heart with thinner walls and wider lumen for less resistance
• Capillaries facilitate rapid transfer of substances between cells and blood in capillary beds
• Blood pressure is the force exerted by flowing blood on the surface area of blood vessels
• Blood components include blood cells (RBCs, WBCs, platelets) and blood plasma with various substances
• Capillary walls have gaps allowing movement of plasma except proteins, facilitating exchange of substances
• At the arteriolar end of the capillary, there is a net loss of fluid into interstitial spaces, forming tissue fluid
• At the venous end of the capillary, there is a net movement of fluid into the capillary from the tissue fluid
• Tissue fluid is almost identical in composition to blood but without the plasma proteins, RBCs & platelets
• 90% of the fluid that leaks from capillaries eventually returns back
• The remaining 10% is collected and returned to the blood system by lymph vessels or lymphatics
• Lymphatics are blind-ended vessels with valves that allow tissue fluid to leak in but prevent backflow
• Valves are wide enough to allow large proteins (tissue proteins) to pass
• Lymphatics join up to form larger lymph vessels that transport lymph back to the subclavian veins
• Lymph is identical to tissue fluid but has a different name because it is in a different place
• Movement of lymph is caused by contraction of surrounding skeletal muscles and smooth muscles in lymphatics
• Lymph nodes are rich in WBCs that remove bacteria, unwanted substances, and secrete antibodies
• RBCs are the most common cell in blood
• Red Blood Cells (RBCs)
• RBCs have a lifespan of 120 days before their membranes become fragile and rupture
• RBCs contain millions of hemoglobin molecules responsible for the red color of RBCs
• RBCs have adaptations:

  • Biconcave disc shape increases surface area to volume ratio for faster O2 diffusion
  • No nucleus, mitochondria, or ER to maximize O2 carrying capacity
  • 7μm in diameter to squeeze through capillaries and reduce diffusion distance
  • Flexible due to specialized cytoskeleton to deform and pass through tight vessels
• Oxygen (O2) Transport
• O2 is transported around the body inside RBCs in combination with hemoglobin (Hb)
• Association & dissociation of Hb with O2 is affected by partial pressure of O2 & CO2, temperature, pH, and 2,3 bisphosphoglycerate (2,3 BPG)