1. The Circulatory System

2. The circulatory system can be divided into the system loop and the pulmonary loop which together form a complete circuit. All of the blood pumped by the left heart passes through the systemic loop, returning to the right heart as systemic venous blood which is enriched in CO2 and which is partially depleted of O2. The right heart pumps the blood into the pulmonary loop where some CO2 is lost and O2 is gained by gas exchange with the atmosphere. This blood returns to the left heart for another trip through the circuit.

6. Photograph of a tricuspid valve during open heart surgery

8. Phase 1: Atria - relaxed, ventricles relaxed, filling, pulmonary and AV valves open, aortic valves closed
Phase 2: Atria contracting, ventricles relaxed, filling, pulmonary and AV valves open, aortic valves closed
Phase 3: Atria relaxed, Ventricles- isovolumetric contraction, pulmonary and AV valves closed, aortic valves closed (first heart sound)
Phase 4: Atria relaxed, Ventricles - contraction, ejection, pulmonary and AV valves closed, aortic valves open
Phase 5: Atria relaxed, Ventricles - isovolumetric relaxation, pulmonary and AV valves closed, aortic valves closed

9. Work of the Heart
Plot of ventricular pressure versus ventricular volume will give work of the heart
1-2 Filling phase
2-3 Isometric contraction
3-4 Ejection phase
4-1 Isometric relaxation
Work/cycle = enclosed area
Power:
LH = 1.1 W, RH = 0.2 W 4 3
234 - systole
412 diastole
Pressure 1 2
Volume

11. Structure of arterial walls - tunica intima is composed of basement membrane, single thickness endothelial cells reinforced with connective tissue. The tunica media is composed of elastic tissue, and circular smooth muscle and the tunica externa of longitudinal smooth muscle

12. Structure of veins similar

13. Arteries versus Veins
Tunica intima in veins forms one way valves that favour flow of blood to the heart
Thinner smooth muscle in veins
Veins more compliant (less collagen more elastin)
Accept large volume of blood with small pressure increase
Low compliance of arteries maintains blood flow
Compliance of arteries - blood ejected from heart is entering aorta more rapidly than able to flow away in rest of vasculature. Aorta is stretched by additional volume, but because of the low compliance of the vessel, it is stretched and a large fraction of the energy of the heart is retained - during diastole the recoil properties of the vessel thus maintain a high mean blood pressure throughout the arterial system and constant blood flow in the capillaries
Note that a compliance that is too high is not desirable either. Collagen content increases with age meaning that a 70 year old’s arteries are about 50% less compliant than ours. Therefore the person must expend higher energy to eject the same amount of blood. As well, the mean blood pressure in an older person is higher (123/76 for a 20 year old man, 145/82 for a 70 year old man)
More than half of blood is on venous side at any time. Yet venous pressure is only a few mm Hg

14. Flow to Various Organs Large flow to active organs
Distribution controlled mainly by changes in diameter (smooth muscle)