Nursebob's MICU/CCU Survival Guide
Cardiology in Critical Care
Cardiovascular Anatomy and Physiology
by nursebob
© October 06
I.
The Structure of the Heart.
A. The pericardium is a fibroserous sac which surrounds the heart.
1. This is composed of two layers and anchors it to the surrounding structures.
2. The other layer adheres to the surface of the heart.
3. These two sacs are separated that a space called the pericardial cavity.
a. This space contains a serious lubricating fluid that lubricates the heart as it beats.
B. The heart is made up of three layers of distinctively different tissue.
1. The epicardium
a. The outer layer of the heart.
b. It is equivalent to this visceral of the pericardium.
2. The myocardium
a. The muscular portion of the heart.
b. It is the pump part of the heart and is also is the thickest.
c. Within the myocardium you have capillary muscles.
- Arise from the myocardial surface of the ventricles and attach the chordea tendineae
- Which prevents regurgitation of the valves.
3. The endocardium.
a. The inner membrane lining the surface of the heart
b. Lines the chambers of the heart.
c. Contains elastic fibers, smooth muscle fibers, blood vessels and nerves.
II.
The Chambers of the Heart.
A. The heart may be considered as a four-chambered pump.
1. Two atria
a. The atria are thin wall, low-pressure, receiving chambers.
b. They act as reservoirs for the blood for their respective ventricles.
c. The right atrium receives systemic venous blood from the superior and inferior vena cava, and coronary sinus.
d. The left atrium receives oxygenated blood returning to the heart from the lungs via way of the four pulmonary veins.
e. Seventy percent of the blood flows passively from the atria to the ventricles during early ventricular diastole.
f. When the atria contract forcefully, or atrial kick, another 10 to 20 percent of blood is supplied for ventricular output.
2. Two ventricles. The interventricular septum separates the left and right ventricle as well as the left and right atrium.
a. The ventricles are the major pumps of the heart.
b. The right ventricle (RV) contracts and propels deoxygenated blood into the pulmonary circulation via the pulmonary artery (PA). - This is a low-pressure system.
c.
The left ventricle (
- Ejects blood into the systemic circulation via the aorta during ventricular systole.
- It is a high-pressure system.
3. Atrial ventricular valves are located between the atria and the ventricles.
a. Allow for unidirectional blood flow from their respective atria to the respective ventricle during diastole.
b. Prevent retro grade flow during ventricular systole.
c. Tricuspid valve.
- Located on the right side of the heart.
- Separates the right atrium from the right ventricle.
d. Mitral valve
- Located on the left side of the heart
- Separates the left atrium from the left ventricle.
- With ventricular diastole, the ventricles and capillary muscles relax and the valve leaflets open.
- First heart sound (S1)
- Valves closing produces a sound that constitute the first heart sound (S1)
- Consisting of the mitral and tricuspid component (M1, T1), M1 is an initial and the major component of S1.
e. Semilunar valves
- Located between the ventricles and vessel
- Allowing unidirectional blood down from the outflow tract
- Prevent retrograde blood flow during ventricular diastole.
f. Pulmonic valves.
-Located between the right ventricle and pulmonary artery.
g. Aortic valve.
-Located between the left ventricle and the aorta.
h. Sequence of events.
- Valve openings occur when the respective ventricle contracts.
- When the pressure is greater than in the artery the valve will open.
-After ventricular systole, pressure in the artery exceeds pressure in the respective ventricle.
- This and retrograde blood flow now causes the valve to close.
i. Second heart sound. S2
- Valve closure produces a sound that constitutes the second heart sound (S2)
- Consisting of an aortic component (A2, P2).
- A2 is the initial and major component of S2.
III. Systemic
Vasculature
A. Function of the vascular system of the heart.
1. Supply tissues with blood nutrients and hormones and to remove metabolic waste.
B. This system consists of arteries, arterioles, the capillary system, and the venous system.
1. Arteries
a. Strong, compliant, elastic walled vessels.
b. Carry blood away from the heart and distribute it to capillary beds throughout the body.
c. High-pressure circuit.
- Elastic fibers located within arterial wall stretch during systole and recoil during diastole.
2. Arterioles
a. Maintain blood pressure.
b. Contain smooth muscle and are innervated by the autonomic nervous system.
- This stimulation causes constriction of the vessels.
- Decreased adrenergic discharge dilates vessels, best controlling the lead distribution to various capillary beds.
c. They comprise the major vessels controlling vascular resistance.
- They may give rise directly to either capillaries to or metarterioles (precapillaries).
3. Capillary system
a. Allows for exchange of oxygen, carbon dioxide and solutes between the blood and tissues
b. Permits fluid volume transfer between plasma and interstitial cells.
c. The capillaries do not have smooth muscle
- Control of their diameter is depending on changes in precapillary and post capillary resistance.
C. Venous system
1. Stores approximately 65 percent of the total volume of blood in the circulatory system.
2. A low-pressure system
a. Conducts blood back to the heart.
- The skeletal muscles act as a venous pump.
- Veins surrounded by skeletal muscles contract thus compressing the veins and moving blood toward the heart.
- Valves in the veins prevent retrograde blood flow.
b. The superior vena cava returns blood from the body area above the diaphragm.
c. The inferior vena cava returns blood from the body below the diaphragm.
d. The coronary sinus drains blood from the heart.
IV.
Systemic and Coronary Circulation.
A. Pulmonary circulation begins with the right heart.
1. It is here that the oxygenated blood from the venous system enters the right atrium through two large veins.
a. The superior and inferior vena cava.
2. Blood is transported to the lungs via the pulmonary artery and its branches.
3. Oxygen rich blood returns to the left atrium through cerebral pulmonary veins.
4. With systemic circulation blood is pumped out of the left ventricle through the aorta and major branches to supply all of the body tissues.
B. Coronary circulation
1 The heart with its own network of vessels.
2. The left and right coronary arteries originate at the base of the aorta and branch out to encircled the myocardium.
V.
Conduction System of the Heart.
A. The sinoatrial node or SA node
1. Commonly called the pacemaker of the heart because it possesses the fastest inherent rate of automaticity.
2. This is the part of the heart that initiates the electrical impulse.
3. It is a specialized area located near the superior vena cava and right atrial junction.
4. Normally it generates impulses at 60 to 100 times per minute.
B. These impulses travel across the atria via the internodal pathways to the atrial ventricular node or AV node.
1. These intranodal atrial pathways conduct the impulse from the SA node to the right atrial musculature to the atrial ventricular node.
C. Specialized areas, Bockman’s bundle, conducts impulses from the SA node to the left atrium.
1. Located on the floor of the interatrial septum is the atrioventricular (AV) node.
2. The small junctional fibers of the AV node slow the velocity of the impulse from the atria before it goes to the ventricles.
3. This allows time for both ventricles to fill prior to ventricular systole.
4. It then passes through the bundle of his at the AV junction.
D. Continues down the intraventricular septum through right and left bundle branches and out to the Punkinje fibers and ventricular wall.
1. These fibers transmits the impulse into the subendocardial layers of both ventricles.
2. Provides for depolarization from the endocardium to the epicardium,
3. Followed by ventricular contraction and ejection of blood out of the ventricles.
E. The bundle of his arises from the AV node and conducts impulses to the bundle branch system.
1. This system is composed of the left bundle branch and the right bundle branch.
a. The right bundle branch transmits the impulse down the right side of the intraventricular septum to the right ventricular myocardium.
b. The left bundle branch
- Separates into the left posterior fascicle
-Transmits the impulse over the posterior and inferior endocardial surface of the left ventricle, and the left anterior fascicle. - which transmits the impulse to the anterior and superior endocardial surface of the ventricle.
F. Depolarization.
1. Results in myocardial contraction.
2. The ion exchange reverses and the cell returns to its resting state of electronegativity
3. The cell is repolarized and the cardiac muscle relaxes.
4. This is the basis for EKG.
VI. Cardiovascular
Anatomy and Physiology
A. Coronary arteries
1. Branch off at the base of the aorta supplying blood to the conduction system and to the myocardium of the heart.
2. The right coronary arteries
a. Supply blood to the SA node in 55 percent of the hearts, to the AV node in 90 percent of the hearts.
b. It also supplies blood to the right atrium and right ventricular heart muscle as well as to the inferior posterior wall of the left ventricle
c. In 80 percent of the hearts; the right coronary artery provides a branch to the posterior descendent artery.
3. The left main coronary artery branches into the left anterior descending (LAD) artery.
a. Supplies blood to the anterior part of the intraventricular septum
b. Supplies blood to the anterior wall of the left ventricle, right bundle branch, the anterior superior division of the left bundle branch.
4. The left main coronary artery branches into the circumflex (CF)
a. Supplies blood to the anterior ventricular node in 10 percent of the hearts.
b. Supplies blood to SA node in 45 percent of the hearts, the lateral posterior surface of left ventricle by the obtuse marginal branch.
VII.
Control of Peripheral Blood Flow.
A. Autoregulation.
1. Control of peripheral blood flow by local mechanisms.
2. The ability to the tissues to control their own blood flow.
B. Vasodilatory Theory.
1. As metabolism increases, oxygen usage is also increased.
2. With this increased oxygen use there is an increase in a production of vasodilator substances, which increases blood supply.
C. The oxygen demand theory.
1. Oxygen is required to maintain vascular contraction.
2. When oxygen level decreases, dilatation of the blood vessels occurs. This increases blood flow.
D. Autonomic regulation of the vessels.
1. Adrenergic sympathetic nervous system regulation secretes norepinephrine at the nerve endings, which produces vasoconstriction. 2. Arterioles help to regulate blood flow and arterial pressure.
3. Veins help to regulate the amount of blood flow.
E. Vasoconstriction causes the increase in venous return to the heart.
1. The parasympathetic nervous system secretes acetycholine at the nerve ending producing vasodilation.
2. Stretch receptors or baroreceptors are located at the aortic arch, carotid sinus, vena cava, pulmonary arteries, and atria.
a. They are activated by elevated blood pressure usually greater than 60 millimeters hg. or blood volume.
b. They respond to the stretch of arterial walls.
c. Activation results in inhibitions of sympathetic action.
d. The vagal reflects will always dominate.
e. A decrease in blood pressure results in decreased vital tone and sympathetic nervous system becomes dominant.
3. The vasomotor center
a. Located in the Medulla of the brain.
b. Called the cardioaccelerator center or cardiac center.
c. This may act as a vasoconstriction
- Stimulation causes secretion of norepinephrine
- Increases heart rate, stroke volume, cardiac output, and ultimately blood pressure.
d. At times, it may act as a vasodepressor
- When it is stimulated by inhibition of the vasoconstriction or area producing vasodilatation.
- Decreases heart right, and blood pressure. It works with the stretch receptors and chemoreceptors.
VIII. Electrophysiology.
A. A Resting Membrane Potential (RMP)
1. Occurs when the sodium ion concentration is greater outside the cell than inside.
2. The potassium ion concentration is greater inside the cell than outside and unbound calcium ions are greater outside the cell.
3. The membrane potential for myocardial muscle fibers is 80 to 90 microvolts.
B. The stimulation of the cell results from chemical, electrical or mechanical stimulation.
1. This stimulation reduces the resting membrane potential to a less than negative or depolarization charge.
2. The temporary inability of the depolarized cell to respond to other stimuli is called the refractory period.
a. During a absolute refractory period, another stimulus will not produce a stimulation.
b. A relative refractory period, is one where a stimulus that is stronger than normal can produce the polarization.
c. A supernormal refractory period, is one where a very weak stimulus can produce the polarization.
d. This is the principle behind pacemakers.
REFERENCES:
Priscilla Lemone/Karen M. Burke, Medical Surgical Nursing, 1996, Chapter 28
White, Kathrine, Fast Facts for Adult Critical Care, 1998,
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