I. FUNCTIONS A. Pump blood. B. Determine direction of blood flow. II. STRUCTURE A. Layers: 1. Endocardium = inner layer of thick elastic connective tissue under
endothelial lining. 2. Myocardium = middle layer of cardiac muscle. 3. Epicardium = outside layer, surrounded by visceral pericardium.
B. Chambers: varies among vertebrate classes. (See Kent Fig. 13-9)
C. Blood supply: coronary arteries and veins.
III. DEVELOPMENTA. First organ system to become functional during development.
B. Functional significance: differentiation and growth of rest of body depends on internal transport.
C. Relationship between ontogeny and phylogeny more apparent in the development of the heart and circulatory system than in any other organ system.
A. Gill-breathing fishes: single circuit (see handout).
1. Advantages:
a. All blood is oxygenated.
b. No mixing of oxygenated and deoxygenated blood.
c. No oxygenated blood fails to enter capillary beds.
2. Disadvantages:
a. Two capillary networks in series reduces pressure in circuit.
b. Evolutionary significance: this is the major selective pressure for the increase in size and efficiency of the early vertebrate heart.
B. Dipnoi (lungfishes): intermediate between single and double circuits (see handout).
1. Blood flow pattern:
a. Heart receives oxygenated blood from lungs (via pulmonary veins) in addition to deoxygenated blood from the body.
b. The tendency of oxygenated blood from lungs to mix with deoxygenated blood from the body is the most important factor influencing subsequent evolution of the vertebrate heart.
2. First subdivision of atria and ventricles by incomplete septa:
a. Deoxygenated blood into right atrium via sinus venosus.
b. Oxygenated blood into left atrium via pulmonary veins.
c. Interventricular septum and division of conus arteriosus by spiral fold keep mixing of blood to a minimum.
d. Oxygenated blood from the left side of the heart is preferentially shunted to aortic arches 3 & 4, which supply body.
e. Deoxygenated blood from right shunted to arches 5 & 6 which lead to gills and lungs.
A. Loss of gills by adults and addition of cutaneous respiration results in partial mixing of blood, but eliminates a capillary network.
B. Typically completely divided atria, undivided ventricles.
1. Deoxygenated blood from body and oxygenated blood from skin enter right atrium; oxygenated blood from lungs enters left atrium.
2. Little mixing in ventricles due to trabeculae, further prevented by division of conus arteriosus by spiral fold.
3. Deoxygenated blood shunted to lungs and skin, oxygenated blood directed into systemic arches.
A. Lungs usually sole source of oxygen (turtles may use cloaca).
B. Sinus venosus and conus arteriosus reduced or divided.
C. Atria completely divided, ventricles incompletely divided (completely divided in crocodilians).
D. Mixing of blood kept to a minimum by complex structure of ventricle and contraction and dilation of pulmonary vessels.
A. Double circuit: complete division of atria and ventricles.
1. Low pressure pulmonary circuit using right side.
2. High pressure systemic circuit using left side.
3. No mixing of oxygenated and deoxygenated blood.
B. Sinus venosus incorporated into the right atrium of mammals, and represented by sino-atrial node; vestigial in birds.
C. Conus arteriosus divided into two trunks: pulmonary and systemic.