Cardiac cycle and heart sound - Online Biology Notes

Cardiac cycle

The sequence of events related to the flow of blood or blood pressure that occurs from the beginning of one heartbeat to the beginning of another can be referred to as cardiac cycle.
Average heart beat per minute= 75 beats
Then, cardiac cycle= 60secs/75 beats= 0.8 sec/beat.
The frequency of the cardiac cycle is explained by the heart rate:
Each cardiac cycle has four major events:
- Atrial Systole = 0.1 sec
- Atrial Diastole = 0.7 sec
  - Total = 0.8 sec
- Ventricular Systole = 0.3 sec
- Ventricular Diastole = 0.5 sec
  - Total = 0.8 sec

Atrial systole:

It is the contraction of heart muscle (myocardium) of left and right atria.
Generally, both atria contract at the same time.
As the atria contract, the blood pressure in each atrium increase forcing additional blood into the ventricles.
The additional flow of blood is also known as atrial kick.

Atrial diastole:

It is the phase when the atria are in relaxing mode.

Detection of atrial systole:

Electrical systole of the atria begins with the onset of the P wave on the Electrocardiogram.
The wave of bipolarization or depolarization that stimulates both atria, to contract at the same time is due to sinoatrial node (SA node) which is located on the upper wall of the right atrium.

Ventricular systole:

Ventricular systole events contains:
- Isomeric contraction (0.05sec)
- Maximum ejection (0.11sec)
- Reduced ejection (0.14sec)
First sound of heart produced by closure of AV valve.
For certain period, semi lunar valve also stays closed.
The period when both AV and semilunar valve stay closed is known as isovolumetric contraction as there is no change in volume.
It occurs for about 0.05sec.
Then semilunar valve opens and blood flows from ventricle to aorta and pulmonary artery through left and right ventricle respectively which is known as maximum ejection (0.11 sec).
Then reduced ejection occurs (0.14sec).
After which sharp closure of semilunar valve occurs producing second sound of heart.

Ventricular diastole:

Ventricular diastole events:
- Proto-diastole (0.04sec)
- Isometric relaxation (0.08sec)
- 1^st rapid filling (0.113sec)
- Diastasis (0.107sec)
- Last rapid filling (0.1sec)
For certain period before diastole, semilunar valve close, the AV are open and the whole heart is relaxed.
This period is known as prodiastole and it lasts for about 0.04 sec.
During isovolumetric ventricular relaxation (0.08sec), both AV and SLV closed, pressure decreases, no blood enters the ventricles, the ventricles stop contracting and begin to relax.
3^rd sound of heart is produced by 1^st rapid filling (0.113).
Diastasis is the intermediate period when the pressure lowers.
4^th sound of heart is produced by last rapid filling (0.1sec).

Detection of ventricular systole or heart sound:

The closure of the mitral and tricuspid valve at the beginning of ventricular systole cause the first part of the Lubb-dubb sound made by the heart as it beats.
Formally, this sound is termed as the first heart tone or S1.
This first heart tone is created by the vibration of leaves of the valves during the closure of mitral and triscuspid valve and is actually a two component sounds H1, T1.
The second part pf the lubb-dubb (the second heart tone or S2) is caused by the closure of the aortic and pulmonary valves at the end of ventricular systole.
As the left ventricle empties, its pressure falls below pressure in the aorta, and the aortic valve closes.
The second heart sound is also of two components A2 and P2 i.e. aortic valve closes prior to the pulmonary valve and they are audibly separated from each other in the second heart sound.
The splitting of S2 is only audible during inhalation.
But, some cardiac conduction abnormalities such as left bundle branch block (LBBB) permit the P2 sound to be heard before the A2 sound during respiration.
With LBBB, inhalation brings A2 and P2 together where they cannot be audibly distinguished.

Cardiac output:

The cardiac output is the amount of blood ejected from each ventricles every minute.
The amount expelled by each contraction of each ventricle is the stroke volume.
Cardiac output is expressed in litres per minute and is calculated by:
- Cardiac output = stroke volume X heart rate
In a healthy adult at rest, the stroke volume is approximately 70ml and if the heart rate is 72 per minute, the cardiac output is 5 litre/min.
This can be greatly increased to meet the demands of exercise to around 25litre/minute and in athletes up to 35 litre/min.
This enhancement during exercise is called the cardiac reserve.

What is Blood pressure?

Blood pressure is the force or pressure that the blood exerts on the wall of the blood vessels.
It is also known as systematic arterial blood pressure and it maintains the essential flow of blood into and out of the organs of the body.
Blood pressure = cardiac output X peripheral resistance
The blood pressure in a healthy person is 120/80mm Hg.
The blood pressure is measured by an ausculatory method with the help of an instrument called sphygmomanometer. It was invented by Karot kolff in 1905. It is the indirect method of measurement of blood pressure.
It is of two types:
- Systolic blood pressure
- Diastolic blood pressure

Systolic blood pressure:

When the left ventricle contracts and pushes blood into the aorta, the pressure produced within the arterial system is called the systolic blood pressure.
In a healthy resting adult man, it is about 120mm Hg.
It is maximum in arteries because arteries because arteries are always stretched.
It indicates the force with which the left ventricle pushes the blood in the aortic arch.
It is the higher limit of arterial blood pressure (or 16kpa).

Diastolic blood pressure:

When complete cardiac diastole occurs and the heart is resting following the ejection of blood, the pressure within the arteries is much lower and is called diastolic blood pressure.
At this time, ventricle is maximally relaxed.
In a healthy resting adult man, it is about 80mm Hg or 11 kpa.
It indicates the elasticity of blood vessels.
It is called the lower limit of arterial blood pressure.
Therefore, during each heartbeat, the arterial blood pressure rises to about 120mm Hg in systolic phase and falls to about 80mm Hg in diastolic phase.

Pulse pressure:

The difference between the systolic and diastolic pressure is called pulse pressure which is 40mm Hg.

The effects of the autonomic nervous system on heart and blood vessels:

	Sympathetic stimulation	Parasympathetic stimulation
Heart	Rate increases strength of contraction increases	Rate decreases strength of contraction decreases
Blood vessels	Most constrict but arteries supplying skeletal muscles and brain dilate.	Most blood vessels do not have a parasympathetic blood supply.

Elasticity of arterial walls:

There is a considerable amount of elastic tissue in the arterial walls especially in large arteries.
Therefore, when the left ventricle ejects blood into the already full aorta, the aorta expands to accommodate it and then recoils because of the elastic tissue in the wall.
This rushes the blood forwards into the systemic circulation.
This distension and recoil occur throughout the arterial system.
During cardiac diastole the elastic recoil of the arteries maintains the diastolic pressure.
If BP becomes too high, blood vessels can be damaged, causing clots or bleeding from sites of blood vessel rupture.
The blood flow through tissue beds may be inadequate if BP falls too low.
This is especially critical for such essential organs as the heart, brain or kidneys.

Blood pressure disorders:

Hypertension:

It is the condition in which a person has persistent high blood pressure.
In this condition, blood pressure is high 150/90mm Hg.
The main factors responsible for hypertension are:
- Tension
- Fear
- Exercise
- Obesity
- Anxiety
- Sorrow
- Other emotional stress
It can also be caused due to intake of cholesterol rich diet, smoking, nephritis etc.
Constriction of arteries, loss of elasticity and fatty deposition (cholesterol) inside arteries narrowing the lumen results in high blood pressure.

Hypotension:

Hypotension is a condition in which a person has persistent low blood pressure.
In this condition, blood pressure becomes low as 100/50mm Hg.
The main factors responsible for hypotension are the loss of blood by hemorrhage, failure of the pumping action of the heart.
It may cause a person senseless.

Artificial pacemakers:

It is an artificial instrument to keep the pace of heart.
It generates heartbeat somewhat at a normal rate.
At first Chardack implanted it in 1960.
A pacemaker is implanted when heart rate of a patient falls to about 30-40 percent has to certain reason.
The implanted pacemaker raises the heartbeat rate and maintains to a normal.
This device is being widely used and has become a boon in the history of medical science.
A pacemaker is made of a pulse generator and an electrode.
The pulse generator is a sealed box that contains lithium halide cells to generate power for more than ten years and an electric circuit to regulate the heart beat rate.
The electrode is a fine metallic string that is connected to the pulse generator.
Its special tip remains in contact with the interior of right ventricle.
The pacemaker is placed under the collar bone below the skin.
It is done by a simple operation.
A pacemaker can be replaced or removed easily.

Distribution of blood volume in body:

Total blood volume = 5 liter (approx.)
Systemic circulation= 64% in veins and venules,
13% in arteries,
7% in systemic arterioles and capillaries.
Heart = 7%
Pulmonary vessels = 9%
The whole course of circulation is shown as follows:
- Organs —-> venous capillaries —-> venules (deoxygenated blood) ——> veins ——-> pre and post cavals ——> right auricle —–> right ventricles —-> pulmonary aorta m -> lungs (for oxygenation) ——> pulmonary veins —–> left auricle (oxygenated blood) —–> left ventricle ——> systemic aorta —–> arteries —–> arterioles and arterial capillaries —–> organs