Chapter 23 Circulation

I. Circulatory Systems

1. Circulatory systems facilitate exchange with all body tissues.

2. Two basic types of circulatory systems have evolved in animals:

(1)  Open circulatory system: most molluscs and all arthropods

(2)  Closed circulatory system

3. There are 3 kinds of vessels: arteries, veins, and capillaries. The cardiovascular system of a fish has two chambers: the atrium (plural form atria) and the ventricle. The large arteries branch into arterioles, while the capillaries converge into venules, which in turn converge into large veins.

4. Single circulation: blood passes through the heart of a fish only once in each circuit through the body.

5. Double circulation: systemic circuit and pulmonary circuit in three-chambered (2 atria and a ventricle) heart of amphibians and four-chambered (2 atria and 2 ventricles) heart of birds and mammals.

II. The Human Cardiovascular System and Heart

1. Blood leaves the heart through the pulmonary arteries which carry oxygen-poor blood to the lungs, and the aorta which starts oxygen-rich blood to the body tissues.

2. Blood flows into the heart through the pulmonary veins which bring oxygen-rich blood from the lungs, and two large veins (superior vena cava and inferior vena cava) carry oxygen-poor blood to the heart.

3. The heart contracts and relaxes rhythmically by sinoatrial (SA) and atrioventricular (AV) nodes  (pacemakers).

(1)  The cardiac cycle represents the heart contracts (systole) and relaxes (diastole) in a rhythmic sequence.

(2)  Cardiac output is the volume of blood that each ventricle pumps per minute.

(3)  Heart rate is the number of heart beats per minute. Cardiac output (5~7L/min) = stroke volume (~70 ml/beat) x heart rate (~72 beats/min)

(4)  The sinoatrial (SA) node is the pacemaker situated in the upper wall of the right atrium and generates electrical impulses. These signals spread rapidly by gap junctions between cardiac muscle cells through both atria, making them contract in unison.

(5)  The atrioventricular (AV) node is located between the right atrium and right ventricle. The AV node delays the signal about 0.1 second to empty the atria before the ventricles contract.

4. The electrical impulses in the heart can be detected on the skin by electrodes and recorder as an electrocardiogram (ECG or EKG). An EKG can provide data about heart health, such as the existence of arrhythmias (abnormal heart rhythms), heart rate, and fibrillations.