Cardiovascular Disorders ( ANATOMY AND PHYSIOLOGY )
Heart and heart wall layers
- The heart is located in the left side of the mediastinum.
- The heart consists of three layers.
- The epicardium is the outermost layer of the heart.
- The myocardium is the middle layer and is the actual contracting muscle of the heart.
- The endocardium is the innermost layer and lines the inner chambers and heart valves.
- Encases and protects the heart from trauma and infection
- Has two layers
- The parietal pericardium is the tough, fibrous outer membrane that attaches anteriorly to the lower half of the sternum, posteriorly to the thoracic vertebrae, and inferiorly to the diaphragm.
- The visceral pericardium is the thin, inner layer that closely adheres to the heart.
- The pericardial space is between the parietal and visceral layers; it holds 5 to 20 mL of pericardial fluid, lubricates the pericardial surfaces, and cushions the heart.
There are four heart chambers
- The right atrium receives deoxygenated blood from the body via the superior and inferior vena cava.
- The right ventricle receives blood from the right atrium and pumps it to the lungs via the pulmonary artery.
- The left atrium receives oxygenated blood from the lungs via four pulmonary veins.
- The left ventricle is the largest and most muscular chamber; it receives oxygenated blood from the lungs via the left atrium and pumps blood into the systemic circulation via the aorta.
There are four valves in the heart.
- There are two atrioventricular valves, the tricuspid and the mitral, which lie between the atria and ventricles.
- The tricuspid valve is located on the right side of the heart.
- The bicuspid (mitral) valve is located on the left side of the heart.
- The atrioventricular valves close at the beginning of ventricular contraction and prevent blood from flowing back into the atria from the ventricles; these valves open when the ventricle relaxes.
- There are two semilunar valves, the pulmonic and the aortic.
- The pulmonic semilunar valve lies between the right ventricle and the pulmonary artery.
- The aortic semilunar valve lies between the left ventricle and the aorta.
- The semilunar valves prevent blood from flowing back into the ventricles during relaxation; they open during ventricular contraction and close when the ventricles begin to relax.
Sinoatrial (SA) node
- The main pacemaker that initiates each heartbeat
- It is located at the junction of the superior vena cava and the right atrium.
- The sinoatrial node generates electrical impulses at 60 to 100 times per minute and is controlled by the sympathetic and parasympathetic nervous systems.
Atrioventricular (AV) node
- Located in the lower aspect of the atrial septum
- Receives electrical impulses from the sinoatrial node
- If the sinoatrial node fails, the atrioventricular node can initiate and sustain a heart rate of 40 to 60 beats/min.
The bundle of His
- A continuation of the AV node; located at the interventricular septum
- It branches into the right bundle branch, which extends down the right side of the interventricular septum, and the left bundle branch, which extends into the left ventricle.
- The right and left bundle branches terminate into Purkinje fibers. 785
- Purkinje fibers are a diffuse network of conducting strands located beneath the ventricular endocardium.
- These fibers spread the wave of depolarization through the ventricles.
- Purkinje fibers can act as the pacemaker with a rate between 20 and 40 beats/min when higher
- pacemakers (such as the sinoatrial nerve) fail.
- The right coronary artery supplies the right atrium and ventricle, the inferior portion of the left ventricle, the posterior septal wall, and the sinoatrial and atrioventricular nodes.
- The left main coronary artery consists of two major branches, the left anterior descending and the circumflex arteries.
- The left anterior descending artery supplies blood to the anterior wall of the left ventricle, the anterior ventricular septum, and the apex of the left ventricle.
- The circumflex artery supplies blood to the left atrium and the lateral and posterior surfaces of the left ventricle. The coronary arteries supply the capillaries of the myocardium with blood. If blockage occurs in these arteries, the client is at risk for myocardial infarction.
- The first heart sound (S1) is heard as the atrioventricular valves close and is heard loudest at the apex of the heart.
- The second heart sound (S2) is heard when the semilunar valves close and is heard loudest at the base of the heart.
- A third heart sound (S3) may be heard if ventricular wall compliance is decreased and structures in the ventricular wall vibrate; this can occur in conditions such as congestive heart failure or valvular regurgitation. However, a third heart sound may be normal in individuals younger than 30 years.
- A fourth heart sound (S4) may be heard on atrial systole if resistance to ventricular filling is present; this is an abnormal finding, and the causes include cardiac hypertrophy, disease, or injury to the ventricular wall.
- The faster the heart rate, the less time the heart has for filling, and the cardiac output decreases.
- An increase in heart rate increases oxygen consumption.
- The normal sinus heart rate is 60 to 100 beats/min.
- Sinus tachycardia is a rate more than 100 beats/min.
- Sinus bradycardia is a rate less than 60 beats/min.
Autonomic nervous system
- Stimulation of sympathetic nerve fibers releases the neurotransmitter norepinephrine, producing an increased heart rate, increased conduction speed through the atrioventricular node, increased atrial and ventricular contractility, and peripheral vasoconstriction. Stimulation occurs when a decrease in pressure is detected.
- Stimulation of the parasympathetic nerve fibers releases the neurotransmitter acetylcholine, which decreases the heart rate and lessens atrial and ventricular contractility and conductivity. Stimulation occurs when an increase in pressure is detected.
Blood pressure (BP) control
- Baroreceptors, also called pressoreceptors, are located in the walls of the aortic arch and carotid sinuses.
- Baroreceptors are specialized nerve endings affected by changes in the arterial BP.
- Increases in arterial pressure stimulate baroreceptors, and the heart rate and arterial pressure decrease.
- Decreases in arterial pressure reduce stimulation of the baroreceptors and vasoconstriction occurs, as does an increase in heart rate.
- Stretch receptors, located in the vena cava and the right atrium, respond to pressure changes that affect circulatory blood volume.
- When the BP decreases as a result of hypovolemia, a sympathetic response occurs, causing an increased heart rate and blood vessel constriction; when the BP increases as a result of hypervolemia, an opposite effect occurs.
- Antidiuretic hormone (vasopressin) influences BP indirectly by regulating vascular volume.
- Increases in blood volume result in decreased antidiuretic hormone release, increasing diuresis, decreasing blood volume, and thus decreasing BP.
- Decreases in blood volume result in increased antidiuretic hormone release; this promotes an increase in blood volume and therefore BP.
- Renin, a potent vasoconstrictor, causes the BP to increase.
- Renin converts angiotensinogen to angiotensin I; angiotensin I is then converted to angiotensin II in the lungs.
- Angiotensin II stimulates the release of aldosterone, which promotes water and sodium retention by the kidneys; this action increases blood volume and BP.
The vascular system
- Arteries are vessels through which the blood passes away from the heart to various parts of the body; they convey highly oxygenated blood from the left side of heart to the tissues.
- Arterioles control the blood flowinto the capillaries.
- Capillaries allow the exchange of fluid and nutrients between the blood and the interstitial spaces.
- Venules receive blood from the capillary bed and move blood into the veins.
- Veins transport deoxygenated blood from the tissues back to the right heart and then to the lungs for oxygenation.
- Valves help return blood to the heart against the force of gravity.
- The lymphatics drain the tissues and return the tissue fluid to the blood.