Artlabeling Activity Blood Flow Through the Heart of a Newborn

19.3 Cardiac Cycle

Learning Objectives

By the end of this section, you volition be able to:

• Describe the human relationship between blood pressure and claret flow
• Summarize and explain the connection betwixt the various events of the cardiac bicycle
• Compare atrial and ventricular systole and diastole
• Chronicle middle sounds detected by auscultation to activity of centre'southward valves

The flow of timethat begins with contraction of the atria and ends with ventricular relaxation is known as the cardiac cycle (Figure 19.three.one). The flow of contraction that the middle undergoes while it pumps claret into circulation is called systole. The period of relaxation that occurs every bit the chambers fill with blood is called diastole. Both the atria and ventricles undergo systole and diastole, and it is essential that these components be carefully regulated and coordinated to ensure blood is pumped efficiently to the trunk.

Figure xix.three.one – Overview of the Cardiac Cycle: The cardiac cycle begins with atrial systole and progresses to ventricular systole, atrial diastole, and ventricular diastole, when the cycle begins once again. Correlations to the ECG are highlighted.

Pressures and Menses

Fluids, whether gases or liquids, are materials that flow according to pressure gradients—that is, they motion from regions that are higher in pressure to regions that are lower in pressure. Accordingly, when the middle chambers are relaxed (diastole), blood volition flow into the atria from the veins, which are higher in force per unit area. As claret flows into the atria, the pressure will rising, so the blood volition initially move passively from the atria into the ventricles. When the activity potential triggers the muscles in the atria to contract (atrial systole), the force per unit area within the atria rises farther, pumping claret into the ventricles. During ventricular systole, pressure rises in the ventricles, pumping blood into the pulmonary trunk from the right ventricle and into the aorta from the left ventricle. Again, as you consider this menstruum and relate it to the conduction pathway, the elegance of the arrangement should become credible.

Phases of the Cardiac Cycle

At the first of the cardiac cycle, both the atria and ventricles are relaxed (diastole). Blood is flowing into the correct atrium from the superior and inferior venae cavae and the coronary sinus. Blood flows into the left atrium from the four pulmonary veins. The two atrioventricular valves, the tricuspid and mitral valves, are both open, so blood flows unimpeded from the atria and into the ventricles. Approximately 70–fourscore pct of ventricular filling occurs by this method. The ii semilunar valves, the pulmonary and aortic valves, are airtight, preventing backflow of blood into the right and left ventricles from the pulmonary trunk on the right and the aorta on the left.

Atrial Systole and Diastole

Contraction of the atria follows depolarization, represented by the P wave of the ECG. As the atrial muscles contract from the superior portion of the atria toward the atrioventricular septum, pressure rises within the atria and claret is pumped into the ventricles through the open up atrioventricular (tricuspid, and mitral or bicuspid) valves. At the start of atrial systole, the ventricles are normally filled with approximately lxx–80 percent of their chapters due to inflow during diastole. Atrial contraction, also referred to as the "atrial kick," contributes the remaining 20–30 per centum of filling (see Figure 19.3.i). Atrial systole lasts approximately 100 ms and ends prior to ventricular systole, every bit the atrial muscle returns to diastole.

Ventricular Systole

Ventricular systole (see Effigy 19.3.1) follows the depolarization of the ventricles and is represented past the QRS complex in the ECG. It may be conveniently divided into two phases, lasting a total of 270 ms. At the end of atrial systole and just prior to atrial contraction, the ventricles contain approximately 130 mL blood in a resting developed in a continuing position. This volume is known equally the end diastolic volume (EDV) or preload.

Initially, as the muscles in the ventricle contract, the pressure of the blood within the sleeping room rises, only it is not yet high plenty to open the semilunar (pulmonary and aortic) valves and be ejected from the heart. However, blood pressure quickly rises above that of the atria that are at present relaxed and in diastole. This increase in pressure causes claret to catamenia back toward the atria, endmost the tricuspid and mitral valves. Since blood is non being ejected from the ventricles at this early stage, the volume of blood within the chamber remains constant. Consequently, this initial phase of ventricular systole is known equally isovolumic wrinkle, also chosen isovolumetric contraction (run across Figure nineteen.3.1).

In the second phase of ventricular systole, the ventricular ejection phase, the contraction of the ventricular muscle has raised the force per unit area within the ventricle to the bespeak that information technology is greater than the pressures in the pulmonary trunk and the aorta. Claret is pumped from the middle, pushing open the pulmonary and aortic semilunar valves. Pressure level generated by the left ventricle will be appreciably greater than the pressure generated by the correct ventricle, since the existing pressure in the aorta will be and then much higher. Nevertheless, both ventricles pump the same amount of blood. This quantity is referred to as stroke book. Stroke volume will normally exist in the range of 70–80 mL. Since ventricular systole began with an EDV of approximately 130 mL of blood, this ways that there is still l–lx mL of blood remaining in the ventricle following contraction. This volume of claret is known every bit the stop systolic volume (ESV).

Ventricular Diastole

Ventricular relaxation, or diastole, follows repolarization of the ventricles and is represented by the T wave of the ECG. It besides is divided into two distinct phases and lasts approximately 430 ms.

During the early on phase of ventricular diastole, every bit the ventricular muscle relaxes, pressure on the remaining blood inside the ventricle begins to autumn. When pressure within the ventricles drops beneath pressure in both the pulmonary trunk and aorta, blood flows back toward the center, producing the dicrotic notch (pocket-size dip) seen in blood pressure tracings. The semilunar valves close to prevent backflow into the center. Since the atrioventricular valves remain closed at this point, at that place is no change in the volume of blood in the ventricle, and so the early phase of ventricular diastole is called the isovolumic ventricular relaxation stage, also called isovolumetric ventricular relaxation phase (see Figure xix.3.one).

In the second phase of ventricular diastole, called late ventricular diastole, as the ventricular muscle relaxes, pressure on the blood inside the ventricles drops even further. Somewhen, it drops below the pressure level in the atria. When this occurs, blood flows from the atria into the ventricles, pushing open the tricuspid and mitral valves. As pressure drops within the ventricles, blood flows from the major veins into the relaxed atria and from there into the ventricles. Both chambers are in diastole, the atrioventricular valves are open up, and the semilunar valves remain closed (see Figure 19.iii.1). The cardiac cycle is complete.

Figure 19.3.2 illustrates the human relationship between the cardiac wheel and the ECG.

Effigy 19.iii.2 – Relationship between the Cardiac Cycle and ECG: Initially, both the atria and ventricles are relaxed (diastole). The P moving ridge represents depolarization of the atria and is followed by atrial wrinkle (systole). Atrial systole extends until the QRS complex, at which point, the atria relax. The QRS complex represents depolarization of the ventricles and is followed by ventricular contraction. The T wave represents the repolarization of the ventricles and marks the beginning of ventricular relaxation.

Centre Sounds

One of the simplest, still effective, diagnostic techniques practical to appraise the state of a patient's eye is auscultation using a stethoscope.

In a normal, healthy heart, in that location are only two audible heart sounds: S_one and South₂. S_one is the sound created by the closing of the atrioventricular valves during ventricular contraction and is normally described equally a "lub," or offset heart audio. The second eye sound, S₂, is the sound of the closing of the semilunar valves during ventricular diastole and is described as a "dub" (Figure 19.3.three). In both cases, every bit the valves close, the openings inside the atrioventricular septum guarded by the valves will become reduced, and blood flow through the opening will become more turbulent until the valves are fully closed. In that location is a third heart sound, S_iii, but it is rarely heard in healthy individuals. It may be the sound of blood flowing into the atria, or blood sloshing dorsum and along in the ventricle, or even tensing of the chordae tendineae. S₃ may be heard in youth, some athletes, and pregnant women. If the sound is heard later in life, it may indicate congestive heart failure, warranting farther tests. Some cardiologists refer to the collective S₁, S_ii, and Southward₃ sounds as the "Kentucky gallop," because they mimic those produced past a galloping horse. The fourth heart sound, Southward₄, results from the wrinkle of the atria pushing blood into a stiff or hypertrophic ventricle, indicating failure of the left ventricle. Southward₄ occurs prior to S₁ and the collective sounds S₄, S₁, and South₂ are referred to by some cardiologists equally the "Tennessee gallop," because of their similarity to the sound produced by a galloping horse with a different gait. A few individuals may have both S₃ and S₄, and this combined audio is referred to as S₇.

Figure 19.3.three – Heart Sounds and the Cardiac Cycle: In this analogy, the x-centrality reflects time with a recording of the centre sounds. The y-axis represents pressure.

The term murmur is used to describe an unusual sound coming from the heart that is caused by the turbulent period of blood, ordinarily due to valve problesms. For example an incompetent valve does not shut completely leading to a "swish" audio as the claret flows backwards through the valve. A high pitch sound results as claret moves through a stiff (stenotic) valve.  Murmurs are graded on a scale of 1 to 6, with 1 being the near common, the virtually difficult audio to detect, and the to the lowest degree serious. The most severe is a vi. Phonocardiograms or auscultograms can be used to record both normal and aberrant sounds using specialized electronic stethoscopes.

During auscultation, it is common practice for the clinician to enquire the patient to breathe deeply. This procedure not but allows for listening to airflow, simply it may also amplify heart murmurs. Inhalation increases blood flow into the correct side of the heart and may increment the amplitude of right-sided heart murmurs. Expiration partially restricts blood menstruum into the left side of the heart and may amplify left-sided heart murmurs. Figure 19.3.4 indicates proper placement of the bong of the stethoscope to facilitate auscultation.

Figure 19.3.iv – Stethoscope Placement for Auscultation: Proper placement of the bell of the stethoscope facilitates auscultation. At each of the 4 locations on the chest, a different valve can be heard.

Chapter Review

The cardiac cycle comprises a consummate relaxation and contraction of both the atria and ventricles, and lasts approximately 0.viii seconds. Beginning with all chambers in diastole, blood flows passively from the veins into the atria and past the atrioventricular valves into the ventricles. The atria begin to contract (atrial systole), following depolarization of the atria, and pump blood into the ventricles. The ventricles begin to contract (ventricular systole), raising pressure within the ventricles. When ventricular pressure level rises above the pressure in the atria, blood flows toward the atria, producing the first centre sound, South_one or lub. As pressure in the ventricles rises to a higher place two major arteries, blood pushes open the two semilunar valves and moves into the pulmonary body and aorta in the ventricular ejection phase. Following ventricular repolarization, the ventricles begin to relax (ventricular diastole), and pressure within the ventricles drops. As ventricular pressure drops, there is a tendency for blood to flow back into the atria from the major arteries, producing the dicrotic notch in the ECG and closing the two semilunar valves. The 2d centre sound, Due south₂ or dub, occurs when the semilunar valves close. When the pressure falls beneath that of the atria, blood moves from the atria into the ventricles, opening the atrioventricular valves and marking 1 consummate heart cycle. The valves preclude backflow of blood. Failure of the valves to operate properly produces turbulent claret flow within the heart; the resulting heart murmur can oftentimes be heard with a stethoscope.

Review Questions

Critical Thinking Questions

one. Describe one cardiac cycle, kickoff with both atria and ventricles relaxed.

Glossary

cardiac wheel

period of time between the onset of atrial contraction (atrial systole) and ventricular relaxation (ventricular diastole)

diastole

period of time when the heart musculus is relaxed and the chambers fill with blood

end diastolic volume (EDV)

(likewise, preload) the amount of blood in the ventricles at the end of atrial systole merely prior to ventricular contraction

cease systolic book (ESV)

amount of blood remaining in each ventricle following systole

heart sounds

sounds heard via auscultation with a stethoscope of the closing of the atrioventricular valves ("lub") and semilunar valves ("dub")

isovolumic wrinkle

(also, isovolumetric contraction) initial phase of ventricular contraction in which tension and pressure in the ventricle increase, only no blood is pumped or ejected from the heart

isovolumic ventricular relaxation phase

initial stage of the ventricular diastole when pressure in the ventricles drops below pressure in the two major arteries, the pulmonary trunk, and the aorta, and claret attempts to flow back into the ventricles, producing the dicrotic notch of the ECG and closing the two semilunar valves

murmur

unusual heart audio detected past auscultation; typically related to septal or valve defects

preload

(as well, cease diastolic volume) amount of blood in the ventricles at the end of atrial systole just prior to ventricular wrinkle

systole

menstruation of time when the middle muscle is contracting

ventricular ejection phase

2nd stage of ventricular systole during which claret is pumped from the ventricle

Solutions

Answers for Disquisitional Thinking Questions

1. The cardiac cycle comprises a complete relaxation and contraction of both the atria and ventricles, and lasts approximately 0.8 seconds. Beginning with all chambers in diastole, blood flows passively from the veins into the atria and past the atrioventricular valves into the ventricles. The atria begin to contract following depolarization of the atria and pump blood into the ventricles. The ventricles brainstorm to contract, raising pressure inside the ventricles. When ventricular pressure rises above the pressure in the two major arteries, claret pushes open up the two semilunar valves and moves into the pulmonary trunk and aorta in the ventricular ejection phase. Post-obit ventricular repolarization, the ventricles brainstorm to relax, and pressure inside the ventricles drops. When the pressure level falls below that of the atria, blood moves from the atria into the ventricles, opening the atrioventricular valves and marking one complete heart wheel.