BIOL 1911 Chapter Notes - Chapter 42.2: Electrocardiography, Sinoatrial Node, Atrioventricular Node
29 Apr 2018
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42.2 – Coordinated cycles of heart contraction drive
double circulation in mammals
H ’ ( ) ?
Mammalian Circulation
• Overall organization of the mammalian cardiovascular system (beginning with pulmonary circulation)
1. Contraction of right ventricle pumps blood to lungs via Pulmonary arteries
2. Blood flowing through capillary bed in left and right lungs loads O2 and unloads CO2
3. Oxygen-rich blood returns from the lungs via the pulmonary veins to the left atrium of the heart
4. Oxygen- ’
• H’ en-rich blood to body tissues via the systemic circuit
5. Blood leaves the left ventricle via the aorta, which conveys blood to arteries leading throughout the body
a. Coronary arteries are the first branches leading from the aorta, and they supply blood to the heart
muscle itself
6. Branches lead to capillary beds in the head and arms
7. Aorta descends into abdomen and supplies oxygen-rich blood to arteries leading to capillary beds in the
abdominal organs and legs
a. In capillaries, there is a net diffusion of O2 from blood to the tissues and of CO2 into the blood
8. Capillaries rejoin and form venules, which convey blood to veins
9. Oxygen-poor blood from head, neck and forelimbs is channeled into a large vein, the superior vena cava
10. Inferior vena cava drains blood from trunk and hind limbs
11. Two venae cavae empty blood into right atrium, from which oxygen-poor blood flows into right ventricle
The Mammalian Heart: A Closer Look
• Located behind the sternum (breastbone)
• About the size of a clenched fist
• Consists mostly of cardiac muscle
• Two atria have relatively thin walls and serve as collection chambers for blood returning to the heart from lungs or other
body tissues
o Much of the blood that enters the atria flows into the ventricles while all four heart chambers are relaxed
o Remaining blood is transferred by contraction of atria before ventricles begin to contract
• The ventricles have thicker walls and contract much more forcefully
o The left ventricle especially contracts with greater force than the right ventricle because it pumps blood
throughout the body via the systemic circuit
o Left ventricle and right ventricle pump the same volume of blood during each contraction despite the
difference in strength of contraction
• Cardiac cycle – one complete sequence of pumping and filling
o Heart contracts and relaxes in a rhythmic cycle
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