The Human Heart

                   The Human Heart

                     PKGhatak,MD

The Heart:

The heart is a muscular pump. It has four chambers. The top two, one on the right and the other on the left are called the Right and Left Atrium. They are thinned walled receiving chambers where venous blood returns. The right atrium receives oxygen poor blood from the entire body via two large veins called the Superior and Inferior Vena Cava. The left atrium receives oxygen rich blood from the lungs by four Pulmonary veins.

Both atria are filled simultaneously and direct blood flow to 2 lower chambers - called the Right and Left Ventricles. The ventricles have a thick muscular wall, thicker on the left than the right. The opening passages to ventricles, one in each ventricle, are guarded by one-way valves. These two valves remain open when blood is returning to the atria.

The blood exits the ventricles when the ventricles contract. The exit openings are also guarded by one-way valves but they remain closed all the time except when ventricles eject blood out. Both ventricles contract in unison. The blood from the right ventricle goes to the lungs by the Pulmonary arteries and blood from the left ventricle enters the Aorta and then travels the entire body.

Both atria contract simultaneously, a few milliseconds before ventricles contract and push blood into the nearly full (80%) ventricles and contribute 20% of the ventricular load.

The heart pumps out about 70 ml (2 ¼ oz) of blood each time it contacts, the output is called Stroke Volume. But each ventricle retains in its cavity about 30 to 40 ml of blood at the end ejection. That amount is called End Diastolic Volume. The amount of blood ejected out of each ventricle is expressed as a percentage of the total initial volume and is called Ejection Fraction (EF).

The time taken from one heartbeat to the next is called a Cardiac Cycle. When the heart is beating 72/ minute, it takes only 0.8 sec. to complete one cardiac cycle; of which 0.3 sec. is taken for the ventricular contraction and the remaining 0.5 sec. ventricles remain relaxed and continue to receive blood from the atria. It takes more time for blood to fill ventricles than to empty. So, when the heart beats faster, say, 150/min., there will be not enough time to fill the ventricles to their full capacities and as a result, the cardiac output will fall. The patient may experience dizziness and may fall on the ground unconscious.

How Heartbeats are Generated and Controlled:

There is a group of specialized cells situated at the point where the superior vena cava enters the right atrium, called the Sinus Node or Sinoatrial Node (SA Node). These cells are capable of generating an electrical current.  The Sinus node is known as the Cardiac pacemaker. The normal Sinus node generates 60 to 100 impulses a minute but is capable of generating much higher rates if needed. The impulse generated by the Sinus node spreads over the walls of both atria. The electrical impulse then reaches another node situated in the atrial septum near the right atrium and right ventricular junction. It is called Atrioventricular Node (AV Node). It takes 0.18 sec. for the cardiac impulse to reach the A V node. From the AV node, the impulse spreads all over both ventricular walls by way of a specialized conductive tissue called the Bundle of His and its branches.  It takes only 0.4 sec. for the impulse to reach all muscles of both ventricles. The AV node, the Bundle of His and its branches are also capable of generating the cardiac impulse but at a much lower rate when the pacemaker-sinus node fails to generate the cardiac impulse. 

Electrical impulse depolarizes muscles, and then the heart contracts immediately, first atria and then a few milliseconds later ventricles.

The ventricular contraction generates the heartbeat, and the First Heart Sound and the ejected blood from the left ventricle into the aorta causes a Pulse wave.

The heart is richly supplied by nerve fibers from the Autonomic Nervous System. The sympathetic nerve fibers come from the Celiac plexus and the Parasympathetic fibers reach the heart via the Vegas nerve. Sympathetic nerves cause rapid heart rate and stronger heart contractions. The parasympathetic nervous system slows the heart rate. In addition, a group of hormones, collectively called Catecholamine, produced by the Adrenal Medulla, increases the heart rate, blood pressure, and increases force contraction of the heart. A hormone from the thyroid gland - Thyroxine increases heart rate and Aldosterone from the Adrenal cortex and Renin from the kidneys influence heart activities by way of maintaining normal levels of sodium, potassium and blood volume.
All living tissues must receive blood consistently and in varying amounts based on demands so as to keep the body fit for any action. The system is very well regulated and an essential part of our fitness. The cardiac output must be tailored accordingly. Heart rate, stroke volume, blood pressure and blood volume must be maintained within the normal parameters. The heart is the center of that system.

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Edited 3/20/2024