Cardiovascular System and The Heart Help (page 2)
The heart is a hollow, four-chambered muscular organ that is specialized for pumping blood through the vessels of the body. It is located in the mediastinum where it is surrounded by a tough fibrous membrane called the pericardium. The parietal pericardium is a loose sac composed of an outer fibrous layer that protects the heart and an inner serous layer that secretes pericardial fluid. The visceral pericardium is a serous membrane that makes up the outer layer of the wall of the heart (the epicardium). The space between the parietal pericardium and the visceral pericardium is the pericardial cavity. Pericardial fluid is found within this cavity and functions to lubricates the surface of the heart.
The heart is composed of three layers from superficial to deep:
- Epicardium: Serous membrane of connective tissues covered with epithelium
- Outer coating that lubricates
- Myocardium: Cardiac muscle tissue and connective tissues Contractile layer, thickest layer
- Endocardium: Epithelial membrane and connective tissues Strengthened protective inner lining of heart
The heart is a four-chambered double pump (Figure 15-1). It consists of upper right and left atria that pulse together, and lower right and left ventricles that also contract together. The atria are separated by the thin, muscular interatrial septum, while the ventricles are separated by the thick, muscular interventricular septum. Atrioventricular valves (AV valves) are located between the upper and lower chambers of the heart, and the semilunar valves are at the bases of the two large vessels (the pulmonary trunk and aorta) that leave the heart.
Each cusp of the atrioventricular valves is held in position by strong tendinous cords, the chordae tendineae, which are secured to the ventricular wall by cone-shaped papillary muscles. All the valves in the heart prevent the backflow of blood to the previous chamber upon contraction of the muscular walls of the heart.
Blood Flow through the Heart
Deoxygenated blood returning from the body fills the right atrium while oxygenated blood returning from the lungs fills the left atrium. Blood flows from the atria into the ventricles, the atria contract, emptying the remaining blood into the ventricles. The ventricles then contract, forcing blood from the right ventricle into the pulmonary trunk and from the left ventricle into the aorta.
The pulmonary circuit refers to the circulation of blood from the heart to the lungs and then back to the heart. The structures that are part of the pulmonary circuit are the right ventricle, pulmonary trunk and pulmonary arteries, the capillary network in the lungs, the pulmonary veins returning blood to the heart, and the left atrium, which receives this oxygenated blood. The systemic circuit refers to the circulation of blood to and from all of the other body tissues. The components of this circuit are the left ventricle, the arteries, capillaries, and veins going to all the body tissues, and the right atrium, which receives deoxygenated blood as it returns from all the body tissues.
In a fetus, the lungs are nonfunctional and oxygen and nutrients are obtained from the placenta (Figure 15-2). An umbilical cord connects the fetus to the placenta. The umbilical cord consists of an umbilical vein that transports oxygenated blood toward the heart and two umbilical arteries that return deoxygenated blood to the placenta. Three fetal structures reroute blood in the fetus:
The umbilical vein forms the round ligament of the liver and the umbilical arteries become the lateral umbilical ligaments in an adult.
Blood supply to the myocardium is from the right and left coronary arteries, which exit the ascending aorta just beyond the aortic semilunar valve. The left coronary artery gives rise to the anterior interventricular and circumflex arteries, and the right coronary artery gives rise to the posterior interventricular and marginal arteries. The great cardiac vein and middle cardiac vein return blood to the coronary sinus, which empties into the right atrium.
Conduction System and Innervation
The conduction system consists of nodal tissues (specialized cardiac muscle fibers) that initiate the conduction of depolarization waves through the myocardium. The pacemaker of the heart is the sinoatrial node (SA node) located in the posterior wall of the right atrium. It depolarizes spontaneously at the rate of 70 to 80 times per minute, causing the atria to contract. Impulses from the SA node pass to the atrioventricular node (AV node) in the interatrial septum, the atrioventricular bundle (AV bundle) in the interventricular septum, and finally to the conduction myofibers (Purkinje fibers) within the ventricular walls. Stimulation of the conduction myofibers causes the ventricle to contract simultaneously. The SA and AV nodes are innervated by sympathetic and parasympathetic nerve fibers. Sympathetic impulses accelerate heart action; parasympathetic impulses through the vagus (CN X) decelerate heart action. These impulses are regulated by the cardiac centers in the hypothalamus and medulla oblongata.
The cardiac cycle consists of a phase of relaxation, called diastole, followed by a phase of contraction, referred to as systole. Major events of the cycle, starting in mid-diastole, are as follows:
- Late diastole. The atria and ventricles are relaxed, the AV valves are open, and the semilunar valves are closed. Blood flows passively from the atria into the ventricles.
- Atrial diastole. The atria contract and pump the additional blood into the ventricles.
- Ventricular systole. At the beginning of ventricular contraction, the AV valves close, causing the first heart sound, "lub." When pressure in the right ventricle exceeds the diastolic pressure in the pulmonary artery (10 mmHg) and the left ventricular pressure exceeds diastolic pressure in the aorta (80 mmHg) the semilunar valves open and ventricular ejection begins. Under normal resting conditions, the pressure reaches 25 mmHg on the right side and 120 mmHg on the left side. The stroke volume, volume of blood ejected from either ventricle, is 70 to 90 mL.
- Early diastole. As the ventricles begin to relax, the pressure drops rapidly. The semilunar valves close, preventing backflow into the ventricles from the arteries and causing the second heart sound, "dub." The AV valves open and blood begins to flow from the atria into the venticles.
Cardiac output, the volume of blood pumped by the left ventricle in 1 minute, may be calculated as:
Cardiac output (C.O.) = stroke volume (S.V.) × heart rate (H.R.)
C.O. is increased by:
- sympathetic stimulation of the heart.
- increased end-diastolic volume (Starling's law of the heart).
- various forms of anemia that result in decreased total peripheral resistance.
C.O. is decreased by decreased venous return.
Because the body is a good conductor of electricity, potential differences generated by the depolarization and repolarization of the myocardium can be detected on the surface of the body and recorded as an electrocardiogram (ECG or EKG) (Figure 15-3). The P wave indicates depolarization of the atria. The QRS complex is the record of ventricular depolarization; the T wave, of ventricular repolarization. The short flat segment between S and T represents the refractory state of the ventricular myocardium; that between P and Q a nonconductive phase of the AV node, during which atrial systole can be completed.
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