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Extrinsic control of heart rate is the input from outside the heart that affects the heart rate. This is achieved by the effect of nervous stimulation or hormones on the SAN.

This method of control is via the cardiovascular centre of the medulla in the brain. Within the cardiovascular centre there are two areas, shown in the table 3, below.

You may also refer to Nervous Communication   click to make the link

Table 3 : Cardiovascular centre
First. Area of cardiovascular centre. cardio-acceleratory centre. nerve linking to S. A. N. in heart. sympathetic nerve (nerve of herring). effect of stimulation. increased cardiac output. Second. Area of cardiovascular centre. cardio-inhibitory centre. nerve linking to S. A. N. in heart. parasympathetic nerve (vagus depressor). effect of stimulation. decreased cardiac output.

The cardiovascular centre in the medulla receives input from receptor cells that are located in various positions around the body, and are stimulated by changes in certain factors.

Table 4
1. Factors are, pH. CO2 concentration. O2 concentration and Pressure. 2. Position of receptors (in the walls of ). aortic arch. carotid sinus. and vena cava.

Changes in the above factors in the blood cause stimulation of the appropriate section of the cardiovascular centre (in the medulla) and therefore an increase or a decrease in heart rate.

Table 5
Heart Rate increased by. 1. Increase in blood pressure in the Vena Cava. 2. Increase in blood CO2 concentration. 3. Decrease in blood O2 concentration. 4. Decrease in blood pH. 5. Increase in body temperature (core temperature). 
									
									Heart Rate decreased by. 1. Increase in blood pressure in Aorta and large arteries. 2. Decrease in blood CO2 concentration. 3. Increase in blood O2 concentration. 4. Increase in blood pH. 5. Decrease in body temperature (core temperature).
Table 6
1.Exercise Begins. 
									2. Increased CO2 production due to increased rate of respiration in muscle cells - this also causes a decrease in blood pH (makes it more acidic). 
									3. Increased CO2 and decreased p.H. are detected by the chemoreceptors in the wall of the aorta and the carotid sinus. 
									4. Impulses are sent from these chemoreceptors to the medulla, where they stimulate the cardio-acceleratory centre. 
									5. Impulses are sent from the cardio-acceleratory centre through the sympathetic nervous system to the S. A. N.
									6. Impulses cause S. A. N. to increase it's output.
									7. This increases heart rate.

Certain hormones have an effect on the SAN and therefore heart rate. The two main hormones are adrenaline and thyroxine.  Exam Advice

Adrenaline is released from the adrenal glands, which are situated on top of the kidneys. Thyroxine is released from the thyroid gland, which is situated in the neck. Adrenaline and thyroxine have many different effects within the body. The effects of these two hormones on heart rate are detailed in table 7, below.

Table 7 : The effects of adrenaline and thyroxine on heart rate
The effects of adrenaline and thyroxine on heart rate: Heart Rate increased by. 1. Increase in hormone adrenaline. 2. Decrease in hormone thyroxine. Heart Rate decreased by. 1. Decrease in adrenaline. 2. Increase in thyroxine.


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