Heart Regulation and Physiology

The
involuntary nature of cardiac muscles is the major quality that makes them
contract and relax continuously without pausing or without experiencing any fatigue
like the rest of the muscles of our body. The parasympathetic nervous system
has effects antagonistic to those of the sympathetic nervous system. The parasympathetic
stimulation oh cardiac muscles, however, cannot change the involuntary nature
of the cardiac musculature.

Angiotensinogen produced by the liver is broken into angiotensin I by renin which us an enzyme produced by the kidney. This angiotensin I is the precursor of angiotensin II which often plays a role in the constriction of glomerular arterioles, increase blood flow and blood pressure. 

Sympathetic neural signals, adrenal medullary hormones (like epinephrine), and thyroxine all increase heart rate. Parasympathetic signals and acetylcholine decrease heart rate, while the vagus nerve carries parasympathetic signals.

A refractory period is a period of time during which an organ or cell is incapable of repeating a particular action. 

The tenth cranial nerve (X) is the vagus nerve. It is the longest cranial nerve. (Its name means "wanderer"). It supplies most of the organs in the thoracic and abdominal cavities. This nerve also contains motor fibers to the larynx (voice box) and pharynx, and to glands that produce digestive juices and other secretions. The vagus nerve is responsible for such varied tasks as heart rate, gastrointestinal peristalsis, sweating, and quite a few muscle movements in the mouth, including speech. Parasympathetic innervation of the heart is partially controlled by the vagus nerve and is shared by the thoracic ganglia. To be specific, vagal and spinal ganglionic nerves mediate the lowering of the heart rate. 

Heart beat is initiated by the nodal cells of the SA node. It starts the heart beat. The electrical impulse is then carried by the nerves to the AV node and the Purkinje fibres. Thus the nerves help to spread the impulse throughout the heart. Hence the rate and rhythm of heart beat is controlled by the impulses travelling through the nerves.

The cardiac activity is controlled by the sympathetic nervous system through the epinephrine. It fastens the heartbeat and the cardiac output so that more concentration of oxygen can be supplied to the body parts.

The calcium ions are responsible for muscle contraction by generating the action potential in the muscle cells. More influx of calcium ions may contract the heart muscle to a prolonged state of contraction, which will lead to abnormal increase in the heart rate. 

Sympathetic nerves release norepinephrine and stimulate the adrenal medulla to release epinephrine, both of which accelerate the heart rate. Acetylcholine is a parasympathetic neurotransmitter that slows the heart rate.

The mammalian heart is myogenic, meaning the heartbeat originates from specialized muscle cells (the SA node) rather than from external nerve impulses.

The vasomotor center is a portion of the medulla oblongata, that together with the cardiovascular & respiratory centers regulates blood pressure too.

Cardiac output is the product of heart rate and stroke volume. During exercise, both increase significantly to meet oxygen demand, with the increase in stroke volume being a primary contributor to the rise in cardiac output.

Statement (i) is correct because nodal tissue generates action potentials. Statement (ii) is correct because the SA node is located in the upper right corner of the right atrium. Statement (iv) is correct as the AV bundle continues from the AV node. Statement (iii) is incorrect because the AV node is located in the lower left corner of the right atrium near the AV septum, not in the ventricle. Statement (v) is incorrect because Purkinje fibers emerge from the bundle of His, not directly as the sole origin from the AV node without the bundle.

• The fish heart contains both oxygenate and deoxygenated blood as it has one atrium and one ventricle.
• Closure of A-V valves produces the first heart sound.
• The vascular connection between the digestive tract and the liver is called the hepatic portal system.
• Purkinje fibres are nerve fibres present in the heart wall.

• The medulla oblongata of the brain controls cardiac activities.
• The action of sympathetic nerve fibres increases the heart rate, while that of the parasympathetic nerve fibres decrease it.
• Sensory fibres extend from the receptors present in the walls of the aortic arch, carotid sinuses and vena cava to the cardiovascular centre in the medulla oblongata.

The fibrous connective tissue skeleton separates the atria from the ventricling, preventing direct electrical propagation between them. This electrical discontinuity is bridged by the atrioventricular bundle, commonly known as the bundle of His, which transmits the action potential from the atria down to the ventricles.

Cardiac activity could be moderated by the autonomous neural system. The action of sympathetic nerve fibres increases the heart rate and the stroke volume, while that of the parasympathetic nerve fibres decrease these.

Thus, the correct answer is 'The sympathetic system stimulates heart rate and stroke volume.'

A permanent record of the electric events occurring during a cardiac cycle made on a graph paper in a wave form called electrocardiogram. The electric events include depolarization and repolarization of the auricles and ventricles bringing about their contraction and relaxation. The instrument used to record hearts electric event is named electrocardiograph.