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Human Circulatory System - II

Blood Pressure (BP)

Blood pressure is the pressure exerted by the blood on the wall of the blood vessels through which it flows. It is also known as arterial blood pressure. It is measured in mercury (Hg). It is of two types:

a. Systolic blood pressure

It is the pressure which the blood exerts on the wall of blood vessels at the end of systolic contraction of ventricles. In a healthy resting adult man, it is about 120 mm Hg. It is maximum in arteries because arteries are always stretched. It indicates the force with which the left ventricle pumps the blood in the aortic arch. It is called higher limit of arterial blood pressure.

b. Diastolic blood pressure

It is the pressure which the blood exerts on the wall of arteries when the ventricle is maximally relaxed. In a healthy resting adult man, it is about 80mm Hg. It indicates the elasticity  of the blood vessels. It is called lower limit of arterial blood pressure.

Therefore, during each heartbeat, the arterial blood pressure rises to about 120 mm Hg in systolic phase and falls to about 80 mm Hg in diastolic phase. The difference between the systolic and diastolic pressures is called pulse pressure which is 40 mm Hg.

The blood pressure in a healthy person is 120/80 mm Hg. The blood pressure is measured by an auscultatory method with the help of an instrument called Sphygmomanometer. It is invented by Karot Koff in 1905. It is the indirect method of measurement of blood pressure. BP is generally measured from brachial artery.


Factors affecting Blood Pressure

1. Exercise: During physical exercise BP slightly increases.

2. Emotions of excitements: During emotions and excitements, BP increases.

3. Meals: Soon after meals BP increases.

4. Sex: BP is slightly higher in men than in women.

5. Constriction of blood vessels: Constriction of arteries and capillaries increases the BP.

6. Body posture: BP remains marginally low in the person in lying position than in erect position.

7. Amount of concentration of Blood: When the amount of blood decreases, BP also decreases.


Problems related to Blood pressure


1. Hypertension

It is this condition in which a person has persistent high blood pressure. In this condition, blood pressure is high 150/90 mm Hg. The main factors responsible for hypertension are- tension, fear, exercise, obesity, anxiety, sorrow and other emotional stresses.

2. Hypo tension

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



Heart Sounds


During heart beat the valves have to open and close passively in order to prevent back flow of blood. The closure of these valves is associated with an audible "lub-dub-lub-dub" sound, which is termed as the heart sounds. Generally, heart sounds can be heard by placing the ear or using stethoscope on the chest.

Heart sounds give information about how the valves are working.

Pulse

Pulse is defined as a wave of distension felt in an artery wall due to contraction of the left ventricle. Arterial pulse can be felt by placing a fingertip on the artery near the wrist. This artery palpitates at a rate corresponding to that of the heart beat. Thus, pulse rate increases during exercise, fever, emotional and psychological excitements.




Heart rate

Pulse per minute is called heart rate. In normal person, the heart beats about 72 times a minute; this is termed as the heart rate. The heart rate determines the cardiac output. If the heart rate rises, cardiac output increases, and if falls, cardiac output also falls. The heart rate increases during exercise, fever, anger and fear.

Stroke volume

It is the volume of blood pumped out of the heart in each heart beat. It is about 70 to 80 ml. Stroke volume is determined by the volume of blood  in the ventricle before they contract.

Cardiac Cycle

The heart contracts rhythmically and ceaselessly throughout the life. The heart undergoes systole (contraction phase) and diastole (relaxation phase). The contractions and relaxation of different parts of heart takes place in a cyclic pattern. This is called cardiac cycle. Total duration of a heartbeat of cardiac cycle is 0.8 seconds.





Cardiac output

The cardiac output is determined by the stroke volume and the heart rate. The stroke volume and heart rate on the multiplication gives the volume of blood pumped out of each ventricle per minute. This is called the cardiac output.



Electrocardiogram (ECG)

You are probably familiar with this scene from a typical hospital television show: A patient is hooked up to a monitoring machine that shows voltage traces on a screen and makes the sound “... pip... pip... pip.....peeeeeeeeeeeeeeeeeeeeee” as the patient goes into cardiac arrest. This type of machine (electro-cardiograph) is used to obtain an electrocardiogram
(ECG). ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle. To obtain a standard ECG, a patient is connected to the machine with three electrical leads (one to each wrist and to the left ankle) that continuously monitor the heart activity. For a detailed evaluation of the heart’s function, multiple leads are attached to the chest region. Here, we will talk only about a standard ECG.

Figure: Diagrammatic presentation of a standard ECG


Each peak in the ECG is identified with a letter from P to T that corresponds to a specific
electrical activity of the heart. The P-wave represents the electrical excitation (or depolarization) of the atria, which leads to the contraction of both the atria. The QRS complex represents the depolarization of the ventricles, which initiates the ventricular contraction. The contraction starts shortly after Q and marks the beginning of the systole. The T-wave represents the return of the ventricles from excited to normal state (repolarization). The end of the T-wave marks the end of systole. Obviously, by counting the number of QRS complexes that occur in a given time period, one can determine the heart beat rate of an individual. Since the ECG obtained from different individuals have roughly the same shape for a given lead configuration, any deviation from this shape indicates a possible abnormality or disease. Hence, it is of a great clinical significance.

Distribution of Blood Volume

Out of total blood volume (5 liters), approximately 84% of the blood is in the systemic circulation (64% in veins and veinules and 13% in arteries and 7% in systemic arterioles and capillaries). Heart contains 7% and pulmonary vessels contain 9% of the blood.