I. Function of Circulation
A. Transport/distribute essential substances
B. Remove wastes
C. Closed Circulation-includes pump, distributing and collecting tubes, exchange tubes (capillaries), ICF (cytoplasm) and ECF (interstitial fluid)
D. Open Circulation –includes pump, distributing tubes (arteries), collecting tubes (veins), ICF, ECF
II. Heart as a pump
A. Muscular pump with chambers and valves. Number of chambers vary, with birds and mammals having 2 atria and 2 ventricles; left ventricle has thickest muscular wall due to having the hardest job
B. Cardiac Muscle
1. Features –Striated with sarcomeres, small cells, one nucleus, extensive SR, T tubules, cells connected electrically at intercalated disks which have gap junctions
2. Gap junctions allow electrical current to flow from one cell to another resulting in a functional syncitium (a single coordinated unit)
3. Rhythmic systolic and diastolic contractions & relaxations are the result of action potentials
a. Pacemaker cells in sinoatrial node initiate action potentials. All cells are capable of spontaneous action potentials but the Sinoatrial node does it sthe fastest and thus sets the pace.
-unstable resting membrane potential that drifts upward gradually depolarizing due to slow influx of sodium (this is called the pacemaker potential) without corresponding outflow of potassium
-membrane shows spontaneous depolarization until threshold is reached and this causes fast calcium – sodium channels to open and mainly calcium and some sodium flow inward from the ECF causing depolarization to threshold, leading to an action potential
-for repolarization, potassium channels open and potassium leaves the cell and then potassium channels close and cycle can start again – every .8 seconds for 75 bpm. However, as repolarization begins, the calcium channels are very slow to close and thus a plateau appears before rapid repolarization.
b. Conduction pathway through heart: SA node to both atria, to AV node (slight delay to finish atrial contraction) AV bundle to Purkinje fibers of ventricular walls
4. Electrocardiogram (EKG or ECG) records electrical activity of heart
a. P wave – atrial depolarization
b. QRS complex – ventricular depolarization
c. T wave – ventricular repolarization
5. No tetany in cardiac muscle due to lengthy absolute refractory period.