3. Organisms Exchange Substances - Control Of Heart Rate

Question 1

What is the autonomic nervous system?

Answer 1

It’s the nervous system that controls the involuntary or subconscious activities of internal muscles and glands.

Question 2

What are the two divisions of the autonomic nervous system?

Answer 2

• The sympathetic nervous system
• The parasympathetic nervous system

Question 3

What is the difference between the sympathetic and parasympathetic nervous systems?

Answer 3

The sympathetic nervous system stimulates effectors so speeds up activity, used more in emergencies as it raises heart rate and prepares us for activity and fight/flight response. The parasympathetic nervous system inhibits effectors and slows down any activity, controlling activities under resting condition and concerned with conserving energy and replenishing the body’s reserves.

Question 4

The cardiac muscle is myogenic, what does this mean?

Answer 4

The contraction is initiated from within the muscle itself, rather than by nervous impulses from outside (neurogenic).

Question 5

Where is the sinoatrial node (SAN) found?

Answer 5

Within the wall of the right atrium.

Question 6

What is the function of the SAN?

Answer 6

It is from here that the initial stimulus for contraction originates, the SAN determines the beat of the heart.

Question 7

Describe the sequence of events that controls the basic heart rate:

Answer 7

• A wave of electrical excitation spreads out from the sinoatrial node across both atria, causing them to contract.
• A layer of non-conductive tissue prevents the wave crossing to the ventricles.
• The wave of excitation enters a second group of cells called the atrioventricular node (AVN), which lies between the atria.
• The AVN, after a short delay, conveys a wave of electrical excitations between the ventricles along a series of specialised muscle fibres called the Purkyne fibres
• Which collectively make up a structure called the bundle of His
• The bundle of His conducts the wave through the atrioventricular septum to the base of the ventricles, where the bundle branches into smaller fibres of Purkyne tissue
• The wave of excitation is released form the Purkyne tissue, causing the ventricles to contract at the same time, from the base of the heart upwards.

Question 8

What is the layer of non-conductive tissue called?

Answer 8

The atrioventricular septum.

Question 9

What region of the brain controls changes to heart rate?

Answer 9

The medulla oblongata.

Question 10

Which nervous system increases heart rate and which one decreases heart rate?

Answer 10

• The sympathetic nervous system increases heart rate
• The parasympathetic nervous system decreases heart rate

Question 11

What are chemoreceptors?

Answer 11

They are receptors that are sensitive to changes in the pH of the blood that result from changes in carbon dioxide concentration.

Question 12

Where are chemoreceptors found?

Answer 12

In the wall of the carotid arteries and aorta.

Question 13

Describe the sequence of control of heart rate due to the chemoreceptors:

Answer 13

• Increased muscular/metabolic activity produces more carbon dioxide from tissues due to increased respiration
• In solution, the carbon dioxide dissolves to form carbon acid
• Increased H+ ions in blood lower the pH
• Chemoreceptors detect this change and increase the frequency of nervous impulses to the centre in the medulla oblongata that increases heart rate
• This centre increases the frequency of impulses via the sympathetic nervous system to the SAN
• This increases rate of production of electrical waves by the SAN therefore increases heart rate
• Increased blood flow allows more carbon dioxide to be removed from lungs so carbon dioxide concentration of blood returns to normal
• Therefore pH of blood rises to normal, so chemoreceptors reduce frequency of nerve impulses to the medulla oblongata
• Medulla oblongata reduced frequency of impulses to SAN, which leads to a reduction of heart rate