Human circulatory system - class-XII

Purkinje system is mainly present in the ventricle myocardium. They spread impulse in ventricles generated by the AV node.

Systole is defined as the ventricular contraction of heart which occurs between the first and second heart sounds of the cardiac cycle. There is pumping of blood into the aorta and pulmonary trunk and lasts for 0.3-0.4 second. During systole arterial blood pressure is maximum. 

The cardiac cycle refers to a complete heartbeat from its generation to the beginning of the next beat. The frequency of the cardiac cycle or the heart rate is expressed as heart beats per minute.

During each cardiac cycle two prominent sounds are produced, which can be easily heard through a stethoscope. The first heart sound (lubb) is associated with the closure of the tricuspid and bicuspid valves, whereas the second heart sound (dupp / dubb) is associated with the closure of the semilunar valves. These sounds are of clinical diagnostic significance.

The cardiac cycle is regulated by specialized areas in the heart wall, that forms the conduction system of the heart. Two of these areas are tissue mass, called as nodes; the third is a group of fibers called as the atrioventricular bundle. 

'Bundle of His' is a network of muscle fibres, found in the walls of ventricles. They originate from AV node and further give rise to Purkinje fibres. This helps in maintaining the rhythmic contraction of the ventricles. 

The cardiac impulse is initiated in the sino-atrial node also known as natural pacemaker. It then travels to the atrio-ventricular node. From there through the Purkinje fibres to the ventricles. The atrio-ventricular node regulates the rhythm of the heart rate mainly. The delay in conduction of impulse in this node changes the duration of contraction between the atria and the ventricles. Hence AV node is called as the pace-setter.

A normal heart rate is 60 to 80 beats per minute. However, in certain conditions, the heart beat is accelerated. The major system involved in acceleration of heartbeat is sympathetic nervous system. It releases hormones like adrenaline and noradrenaline during stress or excitement. Exercise also stimulates sympathetic nervous system to accelerate the heartbeat. Apart from heartbeat, sympathetic nervous system and noradrenaline constricts blood vessels, goose bumps, sweating, increased blood pressure, dilation of pupil.

A) Bundle of His is a muscular tissue.

Heart has two ventricles and two atria, left and right respectively. Deoxygenated blood enters the left atrium through superior & inferior vena cava, when the atria relaxes, blood enters into it, whereas impulse from SA node causes contraction of atria which leads to the flow of blood into ventricles.

The cardiac output, which is the volume of blood pumped by each ventricle per minute, is approximately 5 liters in an average adult at rest.

The pulmonary trunk is a significant human blood vessel from the right ventricle. It branches into the pulmonary arteries of the right and left that lead to the lungs.

The aorta, including the dorsal aorta, originates from the left ventricle, which pumps oxygenated blood to the rest of the body.

The bundle of His is a collection of heart muscle cells, specialized for electrical conduction, that transmits the electrical impulses from the AV node (located between the atria and the ventricles) to the Purkinje fibers, which provide electrical conduction to the ventricles.

The Purkinje fibres are located in the inner ventricular walls of the heart, just beneath the endocardium in a space, called as the subendocardium. These fibres consist of specialized cardiomyocytes, that are able to conduct cardiac action potentials.

• The SA node is the heart's natural pacemaker. 
• The SA node is a muscular tissue where electrical impulses are generated for the rhythmic contractions of the heart.
  
  Hence, option D is correct.

Pulmonary circulation carries blood to and from the lungs. They include the pulmonary artery (aorta) and its branches to the capillaries in the lungs, as well as the veins, that drain those capillaries. The pulmonary arteries carry blood low in oxygen from the right ventricle, while the pulmonary veins carry blood high in oxygen from the lungs into the left atrium. This circuit functions to eliminate carbon dioxide from the blood and replenish its supply of oxygen.

Myogenic contraction refers to a contraction initiated by the myocyte cell itself.

The sinoatrial node (SAN) can generate the maximum number of action potentials, i.e., 70 - 75 beats/min, and is responsible for initiating and maintaining the rhythmic contractile activity of the heart. Therefore, it is called as the pacemaker of the heart.

The Purkinje fibers are specialised conducting fibres present in the inner ventricular walls of the heart. They help in the contraction of the ventricles and thus maintain the rhythmicity of the heart. 

The bundle of His is a group of muscle cells that transmits the electrical impulses from AV node to the ventricle wall via Purkinje fibres. 

Arteries carry blood oxygenated blood away from the heart. While the veins carry deoxygenated blood towards the heart.

Arteries carry oxygenated blood away from the heart.  Pulmonary artery carries the de-oxygenated blood from the right ventricle to the lungs for oxygenation.

The bundle of His is the upper end of the electrical path,which through the Purkinje fibres allows the electrical signal to activate the ventricles and thus to pump the blood.

Under the normal conditions heart rate has an inverse relation with the size of the animal.

The vagus nerve is part of the parasympathetic nervous system, which slows the heart rate by releasing acetylcholine at the SA node.

A neurogenic heart is one where the heartbeat is initiated by nerve impulses from a ganglion, which is characteristic of many lower invertebrates. Humans have a myogenic heart, where the impulse originates in the heart muscle itself.

The pulmonary artery carries deoxygenated (impure) blood from the right ventricle of the heart to the lungs for oxygenation.

The pulmonary artery receives deoxygenated blood pumped out by the right ventricle to transport it to the lungs.

Pulmonary veins carry oxygenated blood from the lungs back to the left atrium of the heart. This is the exception to the rule that veins carry deoxygenated blood.

The Bundle of His (AV bundle) conducts the electrical impulse from the AV node to the Purkinje fibers, which then distribute the impulse to the ventricular muscle.

Oxygenated blood returns from the lungs to the left atrium of the heart via the pulmonary veins.

Stroke volume is the amount of blood pumped out of a ventricle with each individual heartbeat (systole).

The pulmonary veins carry oxygenated (pure) blood from the lungs to the left atrium (auricle) of the heart. Other options represent deoxygenated blood pathways or systemic venous return.

• Blood enters the heart through two large veins, the inferior and superior vena cava brought from different parts of the body.
• This oxygen-deficient blood from the body is emptied into the right atrium of the heart which is further pumped to the right ventricle. 
• As the right ventricle contracts, blood is pumped into the pulmonary artery and to the lungs where it is oxygenated.
• Thus, the pulmonary arteries carry deoxygenated blood from the right ventricles to the lungs. So, the correct answer is 'Deoxygenated blood to the lungs'.

The pulmonary artery is the artery that carries blood from the right ventricle of the heart to the lungs for oxygenation. It is involved in the pulmonary circulation of the cardiovascular system.

During $3$ to $4$ weeks of gestation, the heart usually starts beating in human foetus.

• Pulmonary artery and the Pulmonary vein are a set of blood vessels that are related to the lungs. 
  
• The Pulmonary artery carries deoxygenated blood from the right ventricle to the lungs for oxygenation, this oxygenated blood is carried back to the heart by the Pulmonary vein for systemic circulation i.e. for ​circulation to different parts of the body.
  

Pulmonary artery and the Pulmonary vein are a set of blood vessels that are related to the lungs. The Pulmonary artery carries deoxygenated blood from the right ventricle to the lungs for oxygenation, this oxygenated blood is carried back to the heart by the Pulmonary vein for systemic circulation i.e. for ​circulation to different parts of the body.

Heart sounds are noises generated by the closing and opening of the heart valves. In a healthy individual, there are two normal heart sounds called lub and dub. Lub is the first heart sound. It is associated with the closure of the tricuspid and bicuspid valves at the beginning of systole. The second heart sound dub is associated with the closure of the semilunar valves at the beginning of diastole.

Atria receive blood from all parts of the body which subsequently flows to the ventricles. Action potential generated at sinoatrial node passes from the atria to the ventricles. Both these events take place at the same time.

The heart beat originates at SA node and passes to the AV node which finally conducts it to the bundle of His. The AV node is placed between the atria and the ventricles of the heart, specifically in the posteroinferior region of the interatrial septum near the opening of the coronary sinus, which conducts the normal electrical impulse from the atria to the ventricles.

Sino-atrial node(SAN)  is known as a pacemaker and has the ability to generate action potentials without any external stimuli. 

The pulmonary artery is the vessel transporting de-oxygenated blood from the right ventricle to the lungs. Deoxygenated blood from the systemic circulation enters the right atrium from the superior and inferior venae cavae and passes to the right ventricle. From here it is pumped into the pulmonary circulation through the pulmonary artery, to the lungs where it receives oxygen and gives off carbon dioxide. 

• The only artery which carries deoxygenated blood is called the pulmonary artery. It carries deoxygenated blood from the right ventricle into the lungs for oxygenation. 
  
• Hepatic artery is a short blood vessel that supplies oxygenated blood to the liver.
• Aorta is the largest artery in the body. It carries oxygenated blood from left ventricle to the other parts of the body.
• Renal artery carries oxygenated blood to kidneys.

The natural (SA node) or artificial pacemaker has a function to send electrical impulses from atrium for contraction and pumping action into the ventricles. The impulses get transferred in the following route :

Pulmonary veins are responsible for carrying oxygenated blood from the lungs back to the left atrium of the heart. The pulmonary artery is responsible for transporting oxygen-depleted blood away from the heart and back toward the lungs. Carotid artery is present in the neck region and supply blood to the brain, neck, and face. The aorta is the artery that carries blood away from the heart to the rest of the body. So, the correct option is 'Pulmonary artery'.

Right atrium is just below the right atrium and receives deoxygenated blood from the right atrium and then pumps this deoxygenated blood to the lungs through the pulmonary artery.

Bundle of His is a part of heart, It is collection of heart muscles cells and specialized for electrical conduction .So option (B) Heart is correct answer.

Blood is considered a connective tissue due to two basic reasons: (1) Embryologically, it has the same origin (mesodermal) as other connective tissues (2) Blood connects the body systems together and transports the required oxygen, nutrients, hormones and other signaling molecules, and removes the wastes. 
So, the correct answer is option B.