Heart rate and its regulation

Physiology

Lec.(3) Dr. Rafah Sami
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Heart rate and its regulation
The normal heart rate in adults during rest ranges between 65-80/minute.in new born its higher(120-140)/minute .during old age it becomes slow
Regulation of heart rate is usually brought about by alterations in the pace maker activity. Nervous influences may operate directly through vagus or sympathetic nerves or reflex through Sino aortic or atrial reflexes. A significant increase in heart rate is called Tachycardia while a reduction is termed Bradycardia

Tachycardia is seen in:-

1- Muscular exercise 2- Emotional excitement
3- Fever 4- Sympathetic stimulation
5- Catecholamine release 6- Thyrotoxicosis

Bradycardia occurs in:-

1- Heart block 2- Vagal stimulation
3- Acetylcholine release 4- Myxoedema
5- Barorecepter stimulation

Sympathetic stimulation produces:-

1- Increase excitability and conductivity of myocardium
2- Increase in heart rate
3- Increase force of cardiac contraction


Parasympathetic innervation Stimulation of vagus produce:-

1- Reduced excitability and conductivity of myocardium

2- Fall in heart rate
3-Reduced force of contraction these effects are due to release of
Acetylcholine at vagal nerve endings

The heart ability to increase its output according to body needs mainly depends on 4 factors

The preload(ventricular filling)

After load(resistance to ejection of blood from the heart)
Cardiac contractility
Heart rate
Preload represents the work imposed on the heart before the contraction begins and also preload represents the amount of blood that the heart must pump with beach beat and is largely determined by the venous return to the heart.
After load-is the pressure or tension work of the heart ,it is the pressure that the heart must generate to move blood into the Aorta, it is called the after load because it is the work presented to the heart after the contraction has commenced, the systemic arterial pressure is the main source of after load work on the left heart and the pulmonary arterial pressure is the main source of after load work for right heart

Cardiac output:- is the quantity of blood pumped into the aorta each

minute by the heart. Its value is 5 liters
Stroke volume:- it is the amount of blood pumped per beat by either
ventricle. The normal value is 70 to80 ml .
Venous return:- is the quantity of blood flowing from the veins into the right atrium each minute. The venous return and the cardiac output must equal each other except for a few heartbeats at a time when blood is temporarily stored in or removed from the heart and lungs.


Role of the Frank-Starling Mechanism of the Heart:-
Peripheral factors are usually more important than the heart itself in controlling cardiac output is that the heart has a built-in mechanism that normally allows it to pump automatically whatever amount of blood that flows into the right atrium from the veins. This mechanism, called the Frank-Starling law of the heart, was . Basically, this law states that when increased quantities of blood flow into the heart, the increased blood stretches the walls of the heart chambers. As a result of the stretch, the cardiac muscle contracts with increased force, and this empties the extra blood that has entered from the systemic circulation. Therefore, the blood that flows into the heart is automatically pumped without delay into the aorta and flows again through the circulation. Another important factor, is that stretching the heart causes the heart to pump fasterat an increased heart rate. That is, stretch of the sinus node in the wall of the right atrium has a direct effect on the rhythmicity of the node itself to increase heart rate as much as 10 to 15 per cent. In addition, the stretched right atrium initiates a nervous reflex called the Bainbridge reflex, passing first to the vasomotor center of the brain and then back to the heart by way of the sympathetic nerves and vagi, also to increase the heart rate. .

Determination of cardiac output

The methods commonly used
1- Ficks principle
2- Indicator dilution method

Ficks principle:-

Cardiac output or blood flow is determined by knowing the O2 consumption per minute and the arteriovenous O2 difference

Cardiac output = O2consumption/minute 100

(A-V) O2

the oxygen consumption can be determined by closed circuit spirometry and the oxygen content of arterial blood, by drawing blood sample from a peripheral artery the collection of mixed venous blood from pulmonary artery involves cardiac catheterization

Indicator dilution technique:-

A known concentration of Evans blue(T1824)or cardio green is injected. Cardiac output can be determined by knowing the amount of dye injected. Mean concentration of the dye and the time taken for its appearance

F= I/CT 60

Where
( F)----- is cardiac output
(I)------ indicates the amount of dye injected
(C)------is the mean concentration
(T)------is the time in seconds


Regulating Heart Rate Reflexes :-
1-Bainbridge reflex:- Bainbridge in 1915, observed that increased venous return produces tachycardia and this effect is abolished after section of vagi. This reflex is due to the stimulation of the receptors present in the atrial wall and is not seen when the initial heart rate is high

2-sinoaortic reflex:- It has been demonstrated that a rise of blood pressure stimulates the '' baroceptors in the carotid sinus and the aortic arch and the heart rate is reduced reflexly. The nerves mediating these reflexes are the sinus nerve and the vagus. The canters are situated in the medulla and they are the cardio inhibitory and the vasomotor canters.
3-Right atrial Reflex:- Two types of receptors have been identified in the wall of the right atrium. Type A receptors are stimulated during atrial systole and B type show discharge during diastole. Stimulation of both these receptors results in tachycardia.
4-Coronary chemo reflex:- In experimental animals the injection of veratridine in the left coronary artery produces a reflex bradycardia and hypotension.
5-Reflex from lungs:- Distention of pulmonary vascular bed causes reflex bradycardia.
6-Reflex from higher canters:- Stimulation of higher canters by stress or emotional excitement cause a rise of heart rate due to the activation of sympathetic system.
7-Reflex from periphery :-The stimulation of pain fibres results in an increase of heart rate.

:-Blood flow

َQuantity of blood that passes a given point in the circulation in a given period ,blood flow is expressed in milliliters or liters/minute
velocity of blood flow is inversely proportional to vascular cross-sectional area because approximately the same volume of blood flows through each segment of the circulation each minute, vessels with a large cross-sectional area such as the capillaries, have a slower blood flow velocity pressure vary in the different parts of the circulation pressure in systemic capillaries vary from high as a32mmHg near the arteriolar end as low as 18mmHg near the venous end the capillary hydrostatic pressure is more than osmotic pressure(25mmHg) and this causes the filtration of fluid into tissue spaces while pressure is 18mmHg at venous end this facilities the reabsorption

methods for measuring blood flow:-

1-Electromagnetic flow meters
2-Ultrasonic Doppler flow meter

Laminar flow:-

The flow of blood in straight blood vessels, like the flow of liquids in narrow rigid tube, is normally laminar(streamline),within blood vessels, an infinitely thin layer of blood in contact with the wall of the vessel does not move, the next layer within the vessel has a low velocity, the next a higher velocity, and soforth,velocity being greatest in the center of the stream. Laminar flow occurs at velocities up to certain critical velocity. At above this velocity ,flow is turbulent ,stream flow is silent, but turbulent flow creates sounds










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