Control of the Heart Rate



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Control of the Heart Rate  


The rate at which the heart beats and the volume of blood pumped at each beat (the stroke volume) can both be controlled. The product of these two is called the cardiac output – the amount of blood flowing in a given time:

Write the formula for calculating cardiac output:

Work out the missing values from the table



 

heart rate (beats/min)

stroke volume
(cm3/ beat)

cardiac output
(cm3/min)

 

at rest

75

75







at exercise

180




22 000







 As the table shows, the cardiac output can increase dramatically when the body exercises. There are several benefits from this:

Complete the labels on the heart diagram to show the structures involved in a heartbeat

But what makes the heart beat faster?

This is an involuntary process and is controlled by a region of the medulla called the cardiovascular centre. The cardiovascular centre receives inputs from various receptors around the body and sends output through two nerves to the sino-atrial node in the heart.








How does the cardiovascular centre control the heart? 


The cardiovascular centre can control both the heart rate and the stroke volume. Since the heart is myogenic, it does not need n_________ i_________to initiate each contraction. But the nerves from the cardiovascular centre can change the heart rate. There are two separate nerves from the cardiovascular centre to the sino-atrial node: the sympathetic nerve (_______________) to speed up the heart rate and the parasympathetic nerve (___________) to slow it down.

The cardiovascular centre can also change the stroke volume by controlling blood _________. Impulses are received from pressure receptors in the a__________ and c___________ bodies when the pressure of the blood needs altering. The increase the stroke volume when the cardiovascular centre sends nerve impulses to the arterioles to cause v_______________, which increases blood pressure so more blood fills the heart at diastole. Alternatively it can decrease the stroke volume by causing v_______________ and reducing the blood pressure.


How does the cardiovascular centre respond to exercise? 


When the muscles are active they respire more quickly and cause several changes to the blood, such as decreased oxygen concentration, increased carbon dioxide concentration, decreased pH in blood plasma (since the carbon dioxide dissolves to form carbonic acid) and increased temperature. All of these changes are detected by various receptor cells around the body, but the pH changes are the most sensitive and therefore the most important. The main chemoreceptors (receptor cells that can detect chemical changes) are found in:

  • The walls of the aorta (the aortic body), monitoring the blood as it leaves the heart

  • The walls of the carotid arteries (the carotid bodies), monitoring the blood to the head and brain

  • The medulla, monitoring the tissue fluid in the brain

The chemoreceptors send nerve impulses to the cardiovascular centre indicating that more respiration is taking place, and the cardiovascular centre responds by increasing the heart rate.

A similar job is performed by temperature receptors and stretch receptors in the muscles, which also detect increased muscle activity.



  1. A woman takes moderate exercise. Explain what causes her heart rate to increase why she exercises. (7)


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