201. A graph plots heart rate on the vertical axis against time on the horizontal axis. The rate is initially stable near \(72\,\text{beats min}^{-1}\). When the spontaneous impulse frequency of the SAN is experimentally increased while the remaining conduction pathway stays intact, the heart rate rises and stabilises at a higher value. The pattern supports the conclusion that:
ⓐ. SAN impulse frequency normally helps determine the cardiac rhythm
ⓑ. atrioventricular valves independently set the number of beats
ⓒ. Purkinje fibres prevent impulses from leaving the right atrium
ⓓ. pericardial fluid converts electrical signals into blood cells
Correct Answer: SAN impulse frequency normally helps determine the cardiac rhythm
Explanation: The graph changes only after the spontaneous firing frequency of the SAN is increased, while the rest of the conduction system remains functional. A corresponding rise in heart rate indicates that the frequency of impulses generated by the pacemaker influences the frequency of completed cardiac cycles. The new plateau shows that the heart follows the altered rhythm rather than producing an unrelated transient response. Valves govern one-way blood movement through pressure-dependent opening and closure, not the number of impulses initiated. Purkinje fibres distribute excitation through ventricular muscle and do not normally block atrial output. The evidence links a controlled change in SAN activity with a predictable change in heart rate, supporting its pacemaker function. The higher plateau after faster SAN discharge shows a new stable rhythm rather than a brief response unrelated to pacemaker frequency.
202. The SAN continues to generate \(72\) action potentials per minute, but some impulses fail to pass through the conducting system to the ventricles. Which observation is most consistent with this condition?
ⓐ. ventricular contractions must exceed \(72\,\text{min}^{-1}\)
ⓑ. the ventricular rate may fall below SAN firing frequency
ⓒ. every failed impulse must permanently close the mitral valve
ⓓ. the SAN immediately moves to the left atrium
Correct Answer: the ventricular rate may fall below SAN firing frequency
Explanation: The SAN can continue producing \(72\) impulses per minute, so pacemaker initiation remains normal. If some impulses fail to travel through the AVN, AV bundle, bundle branches or Purkinje network, not every atrial excitation will produce a coordinated ventricular activation. The number of ventricular contractions may be lower than the SAN impulse frequency. This distinction separates impulse production from successful propagation and mechanical response. A conduction failure does not move the SAN anatomically or force a particular valve into permanent closure. The ventricular rate cannot be assumed to exceed the initiating frequency under the stated condition. The observation would reveal that a normal pacemaker rate alone is insufficient when transmission through the cardiac conduction pathway is incomplete. The atrial rate can remain tied to SAN discharge while failed conduction reduces the number of ventricular depolarisations, so the two measured frequencies need not remain equal.
203. Consider the following statements about joint diastole.
I. All four chambers of the heart are relaxed.
II. The atrioventricular valves are open.
III. The semilunar valves are closed.
IV. Blood entering the atria cannot pass into the ventricles.
ⓐ. I and IV only
ⓑ. II, III and IV only
ⓒ. I, II and III only
ⓓ. I, II, III and IV
Correct Answer: I, II and III only
Explanation: Joint diastole is the phase in which both atria and both ventricles are relaxed. Venous blood continues entering the atria, and the pressure relation across the atrioventricular openings allows the tricuspid and mitral valves to remain open. Blood can move through the atria into the ventricles without requiring atrial contraction for the entire filling process. The pulmonary and aortic semilunar valves remain closed, preventing blood already present in the pulmonary artery and aorta from returning to the relaxed ventricles. The fourth statement is inconsistent with this valve arrangement, since passive ventricular filling is a major event of joint diastole. Relaxation of all chambers does not mean that every valve is open; each valve responds to the pressure difference across it.
204. Use the cardiac arrangement described below. Both atria and both ventricles are relaxed. Blood entering through the veins passes through the upper chambers into the lower chambers, while blood in the pulmonary artery and aorta does not return to the heart. The valve pattern is:
ⓐ. all four valves closed throughout the phase
ⓑ. atrioventricular valves closed and semilunar valves open
ⓒ. all four valves open simultaneously
ⓓ. atrioventricular valves open and semilunar valves closed
Correct Answer: atrioventricular valves open and semilunar valves closed
Explanation: Movement of venous blood through the atria into the ventricles requires open atrioventricular valves. On the right side, the tricuspid valve permits flow from the right atrium to the right ventricle; on the left, the mitral valve permits flow from the left atrium to the left ventricle. At the same time, the semilunar valves at the pulmonary-artery and aortic openings remain closed, preventing arterial blood from moving backward into the ventricles. This combination is characteristic of joint diastole and passive ventricular filling. The chambers are relaxed, but valve states are not determined merely by relaxation. They reflect pressure differences across each opening, allowing forward filling while blocking reverse flow from the arteries. Joint diastole requires open atrioventricular valves for passive filling and closed semilunar valves to prevent arterial blood from returning to the ventricles.
205. A heart preparation is observed during a phase in which all chambers are relaxed. When atrial contraction is temporarily suppressed, blood still moves from the veins through the atria into the ventricles, although the final additional filling is lost. The observation supports the inference that:
ⓐ. ventricular filling occurs only during ventricular contraction
ⓑ. semilunar valves normally permit venous filling of the ventricles
ⓒ. atrial systole produces the entire ventricular filling volume
ⓓ. much ventricular filling occurs passively during joint diastole
Correct Answer: much ventricular filling occurs passively during joint diastole
Explanation: The experiment separates passive filling from the later contribution of atrial contraction. Blood continues to enter the relaxed ventricles even when atrial systole is suppressed, showing that a substantial portion of filling does not require active atrial pumping. During joint diastole, open atrioventricular valves provide a low-resistance route from the veins through the atria into the ventricles. Atrial systole normally adds approximately \(30\%\) more filling, so removing it reduces the final ventricular volume without eliminating the earlier passive component. Semilunar valves are not involved in venous filling; they remain closed at ventricular arterial exits. The observation demonstrates that ventricular filling is a staged process, beginning passively and being completed by atrial contraction. Joint diastole supports passive filling, atrial systole adds the final contribution, and ventricular systole raises pressure for valve closure and ejection.
206. Assertion: A cardiac cycle begins with all four chambers in a relaxed state.
Reason: The pulmonary and aortic semilunar valves are closed during joint diastole.
ⓐ. Assertion and Reason are true, and Reason correctly explains Assertion
ⓑ. Assertion and Reason are true, but Reason does not correctly explain Assertion
ⓒ. Assertion is true and Reason is false, so Reason cannot explain Assertion
ⓓ. Assertion is false and Reason is true, so Reason cannot explain Assertion
Correct Answer: Assertion and Reason are true, but Reason does not correctly explain Assertion
Explanation: The assertion is true in the standard description of the cardiac cycle: it begins with joint diastole, during which both atria and both ventricles are relaxed. The reason is also true. Semilunar valves remain closed during this phase and prevent blood in the pulmonary artery and aorta from returning to the ventricles. However, semilunar-valve closure does not explain why all four chambers are relaxed. It describes a pressure-dependent valve state occurring during the phase rather than the cause of chamber relaxation. Joint diastole also involves open atrioventricular valves and passive movement of blood into the ventricles. The two statements belong to the same phase, but they express different features without a direct explanatory relation.
207. In a simplified ventricular-filling model, passive filling during joint diastole contributes \(70\,\text{mL}\). Atrial systole then adds an amount equal to \(30\%\) of this passive volume. The final volume after atrial systole is:
ⓐ. \(91\,\text{mL}\)
ⓑ. \(79\,\text{mL}\)
ⓒ. \(100\,\text{mL}\)
ⓓ. \(121\,\text{mL}\)
Correct Answer: \(91\,\text{mL}\)
Explanation: The known passive-filling volume is \(70\,\text{mL}\). Under the stated simplified condition, atrial systole adds \(30\%\) of that value:
\[
\text{Additional filling}=70\,\text{mL}\times\frac{30}{100}=21\,\text{mL}
\]
The final volume is:
\[
70\,\text{mL}+21\,\text{mL}=91\,\text{mL}
\]
The calculation illustrates the biological role of atrial systole as an additional filling step rather than the sole source of ventricular blood. Most filling has already occurred passively during joint diastole through open atrioventricular valves. The stated values form a simplified proportional model and should not be confused with the normal stroke-volume value, which refers to blood ejected by each ventricle during one beat rather than the complete filled volume. The result also distinguishes ventricular filling from stroke volume: filling describes blood entering before systole, whereas stroke volume is the amount ejected during the beat. The atrial addition is \(70\times0.30=21\,\text{mL}\), and total filling becomes \(70+21=91\,\text{mL}\). The percentage is applied to passive filling, not to the final volume.
208. Atrial musculature fails to contract, but venous return, ventricular relaxation and atrioventricular-valve opening remain normal. The most likely immediate effect is:
ⓐ. complete absence of blood in both ventricles
ⓑ. opening of the semilunar valves throughout joint diastole
ⓒ. passive filling continues without the atrial contribution
ⓓ. reversal of blood flow from the ventricles into the arteries
Correct Answer: passive filling continues without the atrial contribution
Explanation: Open atrioventricular valves and relaxed ventricles still permit blood to move passively from the veins through the atria into the ventricles. Atrial contraction is not responsible for all ventricular filling. Its normal role is to provide an additional contribution of approximately \(30\%\) after the passive phase. Loss of atrial systole would reduce the final degree of filling while leaving the earlier passive movement possible. Semilunar valves remain closed until ventricular pressure rises sufficiently during ventricular systole, so atrial failure does not keep them open during joint diastole. The affected step is the active completion of filling, not the existence of the entire inflow pathway. This separates atrial assistance from the passive pressure-driven component of ventricular filling.
209. A graph plots ventricular volume on the vertical axis against time on the horizontal axis. Volume rises gradually during joint diastole and then shows a smaller final rise immediately after atrial electrical excitation. The final rise most directly represents:
ⓐ. arterial backflow after semilunar-valve closure
ⓑ. ventricular ejection through open semilunar valves
ⓒ. passive filling caused by ventricular systole
ⓓ. additional filling produced by atrial systole
Correct Answer: additional filling produced by atrial systole
Explanation: The vertical axis records ventricular volume, and the initial gradual increase occurs while the chambers are relaxed and the atrioventricular valves are open. That portion represents passive filling during joint diastole. The later smaller rise follows atrial electrical excitation, which is followed by atrial contraction. Atrial systole pushes additional blood into the ventricles and contributes approximately \(30\%\) more filling in the normal filling pattern. Ventricular ejection would cause ventricular volume to fall rather than rise. Arterial backflow is normally prevented by semilunar-valve closure, and ventricular systole does not drive passive filling. The timing and direction of the final graph change identify the active atrial contribution that completes ventricular filling before ventricular contraction begins. Because the final upward step follows atrial excitation and precedes ventricular ejection, it represents active completion of filling rather than an outflow event.
210. Arrange the events leading from pacemaker activity to completion of ventricular filling.
P. Atrial musculature contracts.
Q. The sinoatrial node generates an impulse.
R. Ventricular filling receives an additional contribution.
S. Excitation spreads through both atria.
ⓐ. S → Q → R → P
ⓑ. Q → S → P → R
ⓒ. Q → P → S → R
ⓓ. P → R → Q → S
Correct Answer: Q → S → P → R
Explanation: The sequence begins when the sinoatrial node generates an action potential. Excitation then spreads through the atrial musculature, electrically activating both atria. Mechanical atrial contraction follows this electrical event and pushes additional blood through the open atrioventricular valves into the ventricles. The result is the final atrial contribution to ventricular filling. This produces the order Q → S → P → R. Contraction cannot normally precede the electrical spread that activates the atrial muscle, and the added ventricular filling cannot occur before atrial systole. The sequence also demonstrates that electrical and mechanical events are related but not identical: electrical excitation occurs first, while pressure generation and blood movement follow. The complete pathway remains unambiguous when the specified starting event and the biological endpoint are kept in view. The dependence runs from impulse initiation to atrial excitation, then contraction and added filling; reversing any pair would place a mechanical event before its electrical trigger.
211. Four stages of a cardiac cycle are described.
| Stage | Chamber activity | Atrioventricular valves | Semilunar valves |
|---|
| P | All chambers relaxed | Open | Closed |
| Q | Atria contracting | Open | Closed |
| R | Ventricles beginning to contract | Closed | Closed |
| S | Ventricles ejecting blood | Closed | Open |
The stage immediately after ventricular pressure first exceeds atrial pressure but before it exceeds arterial pressure is:
ⓐ. Stage P
ⓑ. Stage Q
ⓒ. Stage R
ⓓ. Stage S
Correct Answer: Stage R
Explanation: When ventricular contraction begins, ventricular pressure rises. As soon as it exceeds atrial pressure, the tricuspid and mitral valves close to prevent backward movement into the atria. At this early point, ventricular pressure has not yet exceeded pressure in the pulmonary artery and aorta, so the semilunar valves remain closed. Stage R contains exactly this valve combination together with beginning ventricular contraction. Stage S occurs later, after ventricular pressure becomes high enough to open the semilunar valves and initiate ejection. Stages P and Q belong to filling phases with open atrioventricular valves. The two pressure thresholds separate atrioventricular closure from the later opening of the semilunar valves. Stage R is the brief early-systolic interval in which atrioventricular closure has occurred but arterial pressure still keeps both semilunar valves shut. Only after ventricular pressure rises further does Stage S begin with semilunar opening and ejection.
212. Ventricular electrical excitation has reached the myocardium. Ventricular pressure has just become greater than atrial pressure but remains lower than pressure in the pulmonary artery and aorta. At this moment:
ⓐ. atrioventricular valves close, while semilunar valves remain closed
ⓑ. atrioventricular valves remain open, while semilunar valves open
ⓒ. both valve groups open and blood moves in both directions
ⓓ. semilunar valves close after ventricular ejection is completed
Correct Answer: atrioventricular valves close, while semilunar valves remain closed
Explanation: Ventricular depolarisation is followed by ventricular contraction and a rise in ventricular pressure. The first relevant threshold is atrial pressure. Once ventricular pressure becomes greater, blood tends to move toward the atria, pushing the tricuspid and mitral valves shut. The ventricular pressure is still below arterial pressure, so it cannot yet open the pulmonary and aortic semilunar valves. A brief stage exists in which both valve groups are closed and ejection has not begun. Further pressure development is required before the semilunar valves open. This pressure-based reasoning distinguishes the onset of ventricular systole from the later ejection phase and shows why valve changes occur in a definite order.
213. During an attempted ventricular systole, ventricular pressure fails to rise above atrial pressure. The valves themselves are structurally normal. The most likely consequence is:
ⓐ. semilunar valves open as ventricular pressure exceeds arterial pressure
ⓑ. arterial pressure falls below ventricular pressure despite weak systole
ⓒ. atrial systole ceases because ventricular pressure remains low
ⓓ. atrioventricular valves remain open because ventricular pressure stays low
Correct Answer: atrioventricular valves remain open because ventricular pressure stays low
Explanation: Normal closure of the tricuspid and mitral valves depends on ventricular pressure becoming greater than atrial pressure. The pressure reversal pushes the valve cusps into the closed position and prevents backward flow during ventricular contraction. Under the stated condition, this decisive gradient never develops, even though the valves are anatomically intact. They would not undergo normal pressure-driven closure, and effective one-way systolic pumping would be compromised. Semilunar opening requires ventricular pressure to exceed arterial pressure, which is even less likely when ventricular pressure cannot surpass atrial pressure. The defect lies in force generation and pressure development rather than in valve structure. This case shows that a normal valve can function abnormally when the pressure conditions required for its movement are absent. Without the normal atrium-to-ventricle pressure reversal, the atrioventricular cusps are not driven shut, so backward flow can accompany the failed systolic pressure rise.
214. A graph shows atrial, ventricular and arterial pressures during the onset of ventricular systole. The ventricular-pressure curve first crosses above the atrial-pressure curve and later crosses above the arterial-pressure curve. The two crossings correspond respectively to:
ⓐ. atrioventricular-valve closure followed by semilunar-valve opening
ⓑ. semilunar-valve closure followed by atrioventricular-valve opening
ⓒ. atrial contraction followed by joint diastole
ⓓ. semilunar-valve opening followed by atrioventricular-valve closure
Correct Answer: atrioventricular-valve closure followed by semilunar-valve opening
Explanation: The graph presents two successive pressure thresholds. At the first crossing, ventricular pressure becomes greater than atrial pressure. This reverses the gradient across the atrioventricular valves and closes the tricuspid and mitral valves. Ventricular pressure continues rising while all outlets are briefly closed. At the second crossing, ventricular pressure becomes greater than pressure in the pulmonary artery and aorta. The pulmonary and aortic semilunar valves then open, allowing ventricular ejection. The order of the crossings explains the order of valve events. Semilunar closure and atrioventricular reopening occur later during ventricular relaxation, when the ventricular-pressure curve falls below arterial and then atrial pressure. Ventricular ejection begins only after pressure exceeds arterial pressure, and semilunar closure follows when the gradient reverses during relaxation. The two crossings therefore mark sequential reversal of the atrioventricular and ventricular-arterial pressure gradients.
215. Both ventricles are contracting. The tricuspid and mitral valves are closed, while the pulmonary and aortic semilunar valves are open. Blood is moving into the pulmonary artery and aorta. The heart is in:
ⓐ. atrial systole with passive ventricular filling
ⓑ. the ventricular-ejection phase of ventricular systole
ⓒ. joint diastole with all chambers relaxed
ⓓ. ventricular diastole before atrioventricular-valve reopening
Correct Answer: the ventricular-ejection phase of ventricular systole
Explanation: The combination of contracting ventricles, closed atrioventricular valves and open semilunar valves identifies ventricular ejection. Ventricular pressure has risen above atrial pressure, closing the tricuspid and mitral valves, and has also exceeded pressure in the pulmonary artery and aorta, opening the semilunar valves. The right ventricle ejects blood into the pulmonary circuit, while the left ventricle ejects blood into the systemic circuit. Joint diastole and atrial systole have open atrioventricular valves and closed semilunar valves. During early ventricular relaxation, the semilunar valves close rather than remain open. The described valve states, chamber activity and blood routes converge on one specific part of ventricular systole.
216. In an experimental heart, ventricular pressure rises above arterial pressure, but both semilunar valves are mechanically prevented from opening. The atrioventricular valves close normally. The expected observation is:
ⓐ. normal ejection into both arteries despite closed outlets
ⓑ. passive ventricular filling during continued ventricular contraction
ⓒ. high ventricular pressure with severely impaired arterial ejection
ⓓ. immediate reopening of both atrioventricular valves
Correct Answer: high ventricular pressure with severely impaired arterial ejection
Explanation: The ventricles develop the pressure normally required for ejection, and the atrioventricular valves close as expected. However, the experimental treatment blocks the actual outlets into the pulmonary artery and aorta. Blood cannot be expelled normally even though the driving pressure is present. Ventricular pressure may remain high, but arterial ejection becomes severely impaired. Passive filling cannot proceed while the atrioventricular valves are closed, and those valves do not reopen until ventricular pressure later falls below atrial pressure. The experiment distinguishes the force that drives flow from the valve opening that provides a route. Adequate ventricular pressure is necessary for ejection, but effective forward movement also requires structurally mobile semilunar valves. Raised ventricular pressure alone cannot eject blood when both arterial exit valves are fixed shut, so pressure generation is preserved while forward flow is selectively lost.
217. Assertion: The pulmonary and aortic semilunar valves close during ventricular relaxation.
Reason: Ventricular pressure falls below pressure in the pulmonary artery and aorta.
ⓐ. Assertion and Reason are true, and Reason correctly explains Assertion
ⓑ. Assertion and Reason are true, but Reason does not correctly explain Assertion
ⓒ. Assertion is true and Reason is false, so Reason cannot explain Assertion
ⓓ. Assertion is false and Reason is true, so Reason cannot explain Assertion
Correct Answer: Assertion and Reason are true, and Reason correctly explains Assertion
Explanation: The assertion is true. As ventricular systole ends, ventricular musculature relaxes and pressure inside the ventricles falls. Arterial pressure then becomes greater than ventricular pressure, creating a tendency for blood to move backward from the pulmonary artery and aorta. This reversed pressure gradient fills the cusps of the semilunar valves and closes them. Their closure prevents arterial blood from re-entering the ventricles. The reason correctly states the pressure change that produces the valve movement and directly explains the assertion. Atrioventricular valves do not necessarily open at the exact same instant; ventricular pressure must fall further until atrial pressure exceeds it. The pressure sequence produces an orderly transition from ejection to relaxation and later filling.
218. At the beginning of ventricular diastole, arterial pressure is greater than ventricular pressure, while ventricular pressure is still greater than atrial pressure. The expected valve state is:
ⓐ. atrioventricular valves open and semilunar valves open
ⓑ. atrioventricular valves open and semilunar valves closed
ⓒ. atrioventricular valves closed and semilunar valves open
ⓓ. atrioventricular valves closed and semilunar valves closed
Correct Answer: atrioventricular valves closed and semilunar valves closed
Explanation: Arterial pressure exceeding ventricular pressure closes the pulmonary and aortic semilunar valves, preventing blood from flowing back into the relaxing ventricles. At the same time, ventricular pressure remains higher than atrial pressure, so the tricuspid and mitral valves have not yet reopened. Both valve groups are closed during this early part of ventricular relaxation. As ventricular pressure continues to fall, it eventually becomes lower than atrial pressure. Only then do the atrioventricular valves open and passive filling resumes. Ventricular pressure must be compared with two different neighbouring compartments. One comparison determines the semilunar state, while the other independently determines the atrioventricular state.
219. Arrange the following events in one cardiac cycle.
P. Atrial systole adds to ventricular filling.
Q. Semilunar valves close as ventricular pressure falls.
R. Joint diastole permits passive filling.
S. Ventricular pressure closes the atrioventricular valves.
T. Semilunar valves open and blood is ejected.
U. Atrioventricular valves reopen.
ⓐ. R → S → P → T → Q → U
ⓑ. R → P → S → T → Q → U
ⓒ. P → R → T → S → U → Q
ⓓ. R → P → T → Q → S → U
Correct Answer: R → P → S → T → Q → U
Explanation: The cycle begins with joint diastole, during which passive ventricular filling occurs through open atrioventricular valves. Atrial systole follows and supplies the additional filling contribution. Ventricular contraction then raises ventricular pressure above atrial pressure, closing the tricuspid and mitral valves. Further pressure increase opens the semilunar valves and permits ejection. When the ventricles relax, their pressure falls below arterial pressure and the semilunar valves close. A further decline below atrial pressure reopens the atrioventricular valves and restores passive filling. The order R → P → S → T → Q → U follows changing chamber activity and sequential pressure thresholds rather than a memorised list of valve names. Joint diastole precedes atrial systole, ventricular systole follows filling, and ventricular relaxation then restores the pressure conditions for the next filling phase. Valve movements follow the associated pressure reversals throughout this order.
220. Pressure and valve observations are recorded at four moments.
| Moment | Pressure relation | Atrioventricular valves | Semilunar valves |
|---|
| P | Ventricular pressure greater than arterial pressure | Closed | Open |
| Q | Ventricular pressure greater than atrial pressure but lower than arterial pressure | Closed | Closed |
| R | Atrial pressure greater than ventricular pressure; arterial pressure greater than ventricular pressure | Open | Closed |
| S | Ventricular pressure lower than atrial pressure | Closed | Open |
The moment representing restoration of passive ventricular filling is:
ⓐ. Moment P
ⓑ. Moment Q
ⓒ. Moment R
ⓓ. Moment S
Correct Answer: Moment R
Explanation: Passive ventricular filling resumes when atrial pressure becomes greater than ventricular pressure and opens the atrioventricular valves. At the same time, arterial pressure remains greater than ventricular pressure, keeping the semilunar valves closed and preventing arterial backflow. Moment R contains both required pressure relations and the matching valve states. Moment P represents ventricular ejection, while Moment Q represents the early contraction interval after atrioventricular closure but before semilunar opening. Moment S is internally inconsistent with normal pressure-controlled valve behaviour, since ventricular pressure below atrial pressure should favour atrioventricular opening rather than closure. The correct row is identified by integrating both valve groups with the direction of the relevant pressure gradients. Atrial contraction supplements rather than initiates all ventricular filling, since substantial passive flow occurs while the chambers are relaxed. Moment R instead marks passive filling: atrial pressure opens the atrioventricular valves while higher arterial pressure keeps the semilunar valves closed.
