Module 5 Communication, homeostasis & energy Block 2C – 5.5 Animal responses 5.5.9 Controlling heart rate

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Starter – Explain this diagram SAN AVN Cardiac muscle = myogenic – initiates its own beat at regular intervals. Atrial has a higher myogenic rate than the ventricular muscle. The SAN sets a regular heart rate, but this can be increased or slowed when necessary.

Learning Outcomes To explain the role of carbon dioxide, chemoreceptors and baroreceptors in controlling heart rate (Grade C) To explain the role of hormones in controlling heart rate (Grade C)

Monitoring blood pressure Electrocardiograms (ECG’s) There are 3 ways of monitoring the heart we are going to look at: Checking pulse Monitoring blood pressure Electrocardiograms (ECG’s)

Pulse Find your pulse, count it for 15 seconds. What is your pulse rate per minute? What do you think a ‘normal’ pulse should be? - A typical pulse should be 60-80 bpm.

Heart rate The heart rate needs to be modified to meet the needs of tissues by: Raising or lowering the heart rate. Number of beats per minute. Altering the force of the contractions of the ventricular walls Altering the stroke volume (volume of blood pumped per minute) Heart rate = cardiac output ÷ stroke volume

Heart rate Heart rate is controlled by the autonomic nervous system and hormones Nervous inputs – Cardiovascular centre in the brain’s medulla receives information about the concentration of carbon dioxide in the blood. Affecting the frequency of contractions. Hormonal inputs – In stressful situations adrenaline is released into the blood.

Chemoreceptors and baroreceptors (pressure receptors) in the aorta and carotid sinus inform the brain about changing blood pressure and blood pH.

The control of the heartbeat during increased muscular activity Look at the diagram. Can you work out how the information from the aorta and carotid sinus lead to a change in heart rate?

Why Increase Heart Rate? The blood supply to the lungs is increased, so CO2 is taken to the lungs for removal more quickly. The blood supply to the tissues is increased, so O2 supply for respiration is increased.

Create a flow chart to explain how the heart rate is controlled – use worksheet Include what causes the heart rate to increase or decrease. Use the pages 101-102 - help

During exercise After exercise Blood CO2 levels increase due to more respiration. Blood pH drops. Increase in Blood pressure Blood CO2 levels decreases due to reduced respiration. Blood pH rises. Detected by chemoreceptors (in aorta & carotid sinus) Stretch receptors in carotid sinus (carotid artery) Detected by chemoreceptors (in aorta & carotid sinus) Decrease in nerve impulses to cardiac centre in medulla oblongata in brain. Increase in nerve impulses to cardiac centre in medulla oblongata in brain. APs sent down the vagus nerve (parasympathetic nerve) release of acetylcholine at the SAN AP’s sent down the along sympathetic nerve (accelerans nerve) release of noradrenaline at the SAN Heart rate increases Heart rate decreases

Pacemakers If the mechanism controlling the heart rate fails then an artificial pacemaker must be fitted. A pacemaker is a device that delivers an electrical impulse to the heart muscle. Modern pacemakers are only about 4cm long. They may be implanted under the skin and fat on the chest (or sometimes within the chest activity) and are capable of responding to the activity of the patient Some pacemakers deliver impulses to the ventricle walls. This deals with conditions where the AVN that normally relays the impulse from atria to ventricles, via the Purkyne fibres, is not functioning but the SAN may be functioning.

Past paper question

Plenary - Exam questions Name the region of the heart responsible for maintaining the resting heart rate at 60-80 beats per minute (1 mark) A) Which 2 types of receptors provide feedback to the cardiovascular centre in the brain? (2 marks) B) Where are they located? (2 marks) Sinoatrial node (SAN) Chemoreceptors (1) Stretch receptors (1) Chemoreceptor – carotid artery/aorta/brain Stretch receptor – muscles/carotid sinus