Structure and function of heart Questions in English

(B) The $Conus$ $arteriosus$ is a conical pouch formed from the upper and left angle of the right ventricle in the hearts of certain vertebrates (like amphibians and fishes).
It is considered a part of the heart because it is composed of cardiac muscle tissue and functions as a contractile chamber that helps in pumping blood from the ventricle into the ventral aorta.

(D) The heart of a frog is myogenic,meaning the heartbeat originates from within the heart muscle itself,specifically from the pacemaker cells (sinus venosus).
Because it is myogenic,the heart can continue to beat even after being removed from the body,provided it is supplied with appropriate nutrients and kept in a moist environment to prevent desiccation.
If kept dry,the cells will quickly lose their ionic balance and die,causing the heart to stop beating.
Therefore,the correct observation is that it will continue to beat for a long time if kept moist.

(C) The heart wall consists of three layers: the outer epicardium,the middle myocardium,and the inner endocardium. The $Myocardium$ is the thickest layer of the heart wall because it is composed of specialized cardiac muscle tissue responsible for the contraction and pumping action of the heart. Therefore,the $Myocardium$ is primarily responsible for the thickness of the heart wall.

(C) Autorhythmicity is the ability of cardiac muscles to generate their own electrical impulses,which trigger contraction without external nerve stimulation. This allows the heart to beat rhythmically and continuously throughout an individual's life. Therefore,the correct option is $C$.

(A) The heart is myogenic,meaning the heartbeat originates from specialized cardiac muscle cells rather than from the central nervous system.
Even when the nerve supply to the heart is completely severed or obliterated,the heart continues to beat because the sinoatrial node ($SA$ node),which acts as the natural pacemaker,is composed of modified cardiac muscle fibers that possess the inherent property of self-excitation.
Therefore,the heart's rhythm is maintained by its own internal conduction system.

(B) The correct answer is $B$. Sodium ions $(Na^+)$ play a crucial role in the generation and conduction of action potentials in cardiac muscle cells. They are essential for initiating and maintaining the contraction of the heart muscle.

(C) The systemic heart refers to the left side of the heart in higher vertebrates (mammals and birds).
It consists of the left atrium (auricle) and the left ventricle.
This part of the heart receives oxygenated blood from the lungs and pumps it to the entire body through the systemic circulation.

(C) The nerve-like modified muscle tissue located in the wall of the right atrium (auricle) is the Sino-atrial $(S.A.)$ node.
It is responsible for initiating the rhythmic contraction of the heart,which is why it is commonly referred to as the 'Pacemaker' of the heart.

(C) The first heart sound,known as '$Lubb$',is produced by the closure of the atrioventricular $(AV)$ valves (tricuspid and bicuspid valves).
This event occurs at the beginning of ventricular systole when the ventricles start to contract,causing the pressure to rise and forcing the $AV$ valves to close to prevent backflow of blood into the atria.
The second heart sound,'$Dub$',is associated with the closure of the semilunar valves at the end of ventricular systole.

(C) The correct answer is $C$. Calcium ions $(Ca^{++})$ play a crucial role in the contraction of cardiac muscles. An excess of $Ca^{++}$ ions leads to prolonged and forceful contraction of the cardiac muscles,which eventually causes the heart to stop in a state of systolic arrest (spasm).

(A) The bicuspid valve,also known as the mitral valve,is a valve consisting of two membranous flaps or cusps. It is situated between the left atrium (auricle) and the left ventricle of the heart in mammals. Its primary function is to prevent the backflow of blood from the left ventricle into the left atrium during ventricular contraction.

(B) The auriculo-ventricular node $(AVN)$,also known as the node of $Tawara$,was discovered by the Japanese cardiologist $Sunao \ Tawara$ in $1906$.
$1$. $His$ discovered the Bundle of $His$.
$2$. $Keith$ and $Flack$ discovered the sino-atrial node $(SAN)$.
$3$. $William \ Harvey$ is known for describing the systemic circulation of blood.

(B) The heart of a healthy adult human typically beats at a rate of $70-80$ times per minute at rest.
This rhythmic contraction and relaxation of the heart muscle is known as the heartbeat.
The average heart rate is often cited as $72$ beats per minute,which falls within the $70-80$ range.

(A) The valves in the heart are composed of thin,strong flaps of connective tissue known as cusps or leaflets.
These structures are essentially folds of the endocardium (the inner lining of the heart) reinforced with connective tissue.
Therefore,they are membranous in nature,allowing them to open and close efficiently to ensure unidirectional blood flow.

(D) The heart of a mammal is called $Myogenic$ because the contraction is initiated by a specialized patch of modified heart muscle fibers known as the $Sinu-auricular$ node or $S.A.$ node.
Unlike $Neurogenic$ hearts,where the heartbeat is initiated by nerve impulses from a ganglion,$Myogenic$ hearts generate their own electrical impulses within the cardiac muscle tissue itself.
This property allows the heart to continue beating even when removed from the body,provided it has an adequate supply of oxygen and nutrients.
Therefore,the correct option is $(d)$.

(D) Blood flows from the post caval vein (inferior vena cava) into the right atrium due to a pressure gradient. During the diastole of the heart,the right atrium relaxes,creating a lower pressure compared to the venous system. This pressure difference allows blood to flow passively from the higher pressure area (veins) to the lower pressure area (atrium).

(A) The human heart is myogenic,meaning the heartbeat is initiated by specialized cardiac muscle cells. The $Sino-atrial$ $node$ $(SAN)$,also known as the pacemaker,is located in the right atrium. It generates action potentials that initiate the heartbeat,making it the primary pacemaker of the heart.

(A) The heart is situated in the thoracic cavity in the mediastinum between the two lungs.
It is tilted towards the left,with the apex of the heart pointing towards the left side.
Since the left ventricle is the largest and most muscular chamber of the heart and is positioned towards the left,the heartbeat is most clearly heard on the left side of the chest.

(A) The $\text{Pulmonary vein}$ is unique because it is the only vein in the human body that carries oxygenated blood from the lungs to the left atrium of the heart.
In contrast, the $\text{Pulmonary artery}$ carries deoxygenated blood from the heart to the lungs.
The $\text{Renal vein}$ carries deoxygenated blood from the kidneys, and the $\text{Hepatic portal vein}$ carries nutrient-rich but deoxygenated blood from the digestive tract to the liver.

(A) $Purkinje$ fibres are specialized cardiac muscle fibres that are part of the heart's conduction system.
They are located in the subendocardial layer of the ventricular walls.
These fibres are responsible for conducting electrical impulses rapidly through the ventricles,ensuring coordinated contraction of the heart muscle.

(C) In the heart of amphibians and some reptiles,the $Truncus \ arteriosus$ is a large vessel that arises from the ventricle. The base of this vessel contains valves that prevent the backflow of blood. Specifically,the $Truncus \ arteriosus$ is guarded by spiral valves or tricuspid valves at its origin to regulate blood flow into the systemic and pulmonary arches.

(A) The atrio-ventricular valves (tricuspid and bicuspid valves) are attached to the papillary muscles of the heart wall by tough,fibrous strands of connective tissue known as $Chordae$ $tendineae$ or $Tendinous$ $cords$.
These structures prevent the valves from prolapsing or turning inside out into the atria during ventricular systole (contraction).

(A) The heart is enclosed in a double-walled membranous bag called the pericardium,which encloses the pericardial fluid.
$(a)$ The pericardium and the pericardial fluid protect the heart from friction,mechanical shocks,and injuries.
$(b)$ They keep the surface of the heart moist.
$(c)$ They allow the heart to move freely during contraction and relaxation.

(C) In mammals,including rabbits,the post caval vein (also known as the inferior vena cava) carries deoxygenated blood from the posterior parts of the body to the right auricle (atrium) of the heart.
This opening is guarded by a fold of the endocardium known as the $Eustachian$ valve.
The function of this valve is to prevent the backflow of blood from the right auricle into the post caval vein during atrial contraction.

(A) In mammals,the circulatory system follows a double circulation pattern.
$1$. The pulmonary circulation involves the transport of deoxygenated blood from the right ventricle to the lungs via the pulmonary artery.
$2$. Oxygenated blood is then returned from the lungs to the left auricle (atrium) via the pulmonary veins.
$3$. Therefore,the statement that the left auricle receives oxygenated blood from the lungs is correct.

(C) The first heart sound,'$Lubb$',is a low-pitched,long-duration sound produced primarily by the closure of the atrioventricular valves (tricuspid and bicuspid valves) at the beginning of ventricular systole.
The second heart sound,'$Dup$',is a high-pitched,short-duration sound produced by the closure of the semilunar valves (aortic and pulmonary valves) at the beginning of ventricular diastole.
Therefore,the combination of these two sounds is due to the sequential closing of the atrioventricular valves followed by the semilunar valves.

(D) The posterior vena cava,also known as the inferior vena cava,is a large vein that carries deoxygenated blood from the lower body regions to the heart.
It originates at the posterior end of the abdomen by the union of the two common iliac veins.
Therefore,it begins at the hind end of the abdomen.

(D) The pacemaker of the heart is the Sinoatrial node ($SA$ node).
It is a specialized patch of tissue located in the right upper corner of the right atrium.
It is situated in the wall of the right atrium near the opening of the superior vena cava.
Therefore, the correct option is $D$.

(D) In the mammalian heart,the right auricle (right atrium) receives deoxygenated blood from the body.
This blood is collected by two large veins: the superior vena cava (precaval vein) which drains blood from the upper parts of the body,and the inferior vena cava (postcaval vein) which drains blood from the lower parts of the body.
Therefore,the right auricle receives blood from both pre- and post-cavals.

(C) Cardiac output is defined as the volume of blood pumped by each ventricle per minute. Since the heart pumps blood into the systemic circulation,it represents the amount of blood leaving the heart per unit time. The average value is approximately $5000 \ mL$ or $5 \ L$ in a healthy adult.

(B) The efficiency of cardiac contraction is defined as the ratio of work output to the total chemical energy expenditure.
The maximum efficiency of a healthy human heart is approximately $20-25\%$.
In cases of heart failure,this efficiency can decrease significantly,potentially falling as low as $5-10\%$.

(D) The heart wall is composed of three layers: the endocardium (inner), myocardium (middle, cardiac muscle), and epicardium (outer). The fibrous pericardium is a protective sac surrounding the heart. The term $Tunica \text{ } intima$ is specifically used to describe the innermost layer of blood vessels, not the heart wall. Therefore, $Tunica \text{ } intima$ is the correct answer.

(C) The correct answer is $C$.
Calcium ions $(Ca^{2+})$ play a critical role in the contraction of cardiac muscle.
They bind to troponin,which causes a conformational change in the tropomyosin complex,exposing the active sites on actin filaments for myosin binding.
An excess of calcium ions in the extracellular fluid can lead to a state of prolonged contraction known as calcium rigor,where the heart muscle remains in a contracted state.

(D) The first heart sound,known as $LUBB$,is produced by the sudden closure of the atrioventricular $(A.V.)$ valves (tricuspid and bicuspid valves).
This closure occurs at the beginning of ventricular systole due to a sharp rise in intraventricular pressure,which prevents the backflow of blood into the atria.

(A) The apex beat is the pulsation felt on the chest wall over the apex of the heart.
It occurs during the early part of ventricular systole.
The first heart sound $(S_1)$,often described as 'lub',is produced by the closure of the atrioventricular $(AV)$ valves (mitral and tricuspid valves) at the onset of ventricular systole.
Since the apex beat coincides with the beginning of ventricular contraction,it is synchronous with the first heart sound.

(B) Starling's law (also known as the Frank-Starling law of the heart) states that the stroke volume of the heart increases in response to an increase in the volume of blood filling the heart (the end-diastolic volume) when all other factors remain constant.
Essentially,within physiological limits,the greater the initial length of the cardiac muscle fibers (due to increased stretching by blood volume),the stronger will be the force of contraction.
Therefore,this law is directly related to the force of the heart beat.

(D) The cardiac impulse originates at the $SA$ (Sinoatrial) node,which acts as the natural pacemaker of the heart.
From the $SA$ node,the impulse spreads through the atrial muscles to the $AV$ (Atrioventricular) node.
From the $AV$ node,the impulse travels through the Bundle of His (Atrioventricular bundle).
Finally,it is transmitted to the Purkinje fibres,which distribute the impulse throughout the ventricular musculature,causing the heart muscles to contract.
Therefore,the correct sequence is: $SA$ node $\to$ $AV$ node $\to$ Bundle of His $\to$ Purkinje fibres $\to$ Heart muscles.

(A) Purkinje fibres are specialized cardiac muscle fibres that are part of the conduction system of the heart. They are located in the subendocardial surface of the ventricular walls. Their primary function is to rapidly conduct electrical impulses from the atrioventricular bundle to the ventricular myocardium,ensuring coordinated contraction of the ventricles.

(C) $Sinus \ venosus$ is the first chamber of the heart in fish,amphibians,and reptiles. In fish,it receives blood from the cuverian ducts and hepatic veins and leads into the single auricle.
In amphibians and reptiles,it receives blood from the three venae cavae and leads to the right auricle. Birds and mammals,including rabbits,do not possess a $sinus \ venosus$.

(D) The heart has a specialized conducting system. The $Sino-atrial$ $node$ $(SAN)$ is known as the pacemaker because it generates the maximum number of action potentials to initiate the heartbeat. The $Atrio-ventricular$ $node$ $(AVN)$ is known as the pace-setter because it delays the impulse of contraction before transmitting it to the ventricles,thereby regulating the time interval between the contraction of the atria and the ventricles.

(A) Among land animals,giraffes have the largest heart. Their heart can have a length up to $2 \text{ feet}$ and can weigh more than $11 \text{ kg}$.
Thus,the correct answer is 'Giraffe'.

(C) Cardiac output is defined as the volume of blood pumped by each ventricle per minute.
It is calculated by the formula: $\text{Cardiac Output} = \text{Stroke Volume} \times \text{Heart Rate}$.
Given that the average stroke volume is approximately $70 \text{ ml}$ and the average heart rate is $72 \text{ beats per minute}$.
$\text{Cardiac Output} = 70 \text{ ml/beat} \times 72 \text{ beats/min} = 5040 \text{ ml/min}$.
This is approximately $5.04 \text{ litres per minute}$,which is closest to $5.3 \text{ litres per minute}$ among the given options.

(B) The pacemaker,specifically the $Sinoatrial$ $(SA)$ node,is a specialized cluster of cells located in the wall of the right atrium of the heart.
It acts as the natural pacemaker because it has the ability to generate electrical impulses spontaneously.
These impulses spread through the heart muscle,causing it to contract and relax in a rhythmic manner,which directly regulates the heart beat.
Therefore,the pacemaker is responsible for initiating and controlling the rate of the heart beat.

(D) The mitral valve,also known as the bicuspid valve,is located between the left atrium (auricle) and the left ventricle in mammals.
It consists of two flaps that prevent the backflow of blood from the left ventricle into the left atrium during ventricular systole.

(C) The coronary sinus is a collection of veins joined together to form a large vessel that collects blood from the heart muscle (myocardium). It is located on the posterior part of the heart,specifically within the atrioventricular groove on the diaphragmatic surface. It opens into the right atrium.

(D) The wall of the heart consists of three distinct layers:
$1$. $Epicardium$: The outermost layer.
$2$. $Myocardium$: The middle, thick muscular layer.
$3$. $Endocardium$: The innermost layer lining the heart chambers.
Therefore, the correct answer is $All \text{ of the above}$.

(A) The heartbeat originates from the pacemaker,which is the $S.A.$ node (Sinoatrial node).
It is a specialized patch of cardiac muscle tissue located in the right atrium that generates electrical impulses to initiate the contraction of the heart.

(B) The sinu-auricular valve is a specialized structure found in the hearts of certain vertebrates,such as amphibians and reptiles.
It is located at the sinu-auricular aperture,which is the opening through which deoxygenated blood from the sinus venosus enters the right atrium (auricle).
This valve prevents the backflow of blood from the atrium into the sinus venosus during atrial contraction.
Therefore,the correct location is the edges of the sinu-auricular aperture.