EMPA revision

Electrical Activity in the Heart 1

Regular heartbeat
1. Impulse arrives at the sinoatrial node (SAN) in the wall of the right atrium. The SAN sends out waves of electrical activity to the atrial walls, causing the left and right atria to contract at the same time
2. The waves are transferred to the atrioventricular node (AVN), as a band of non-conducting collagen prevents impulses from being transferred from atria to ventricles. The AVN transmits the wave of electrical activity to the bundle of His (with a delay to make sure atria empty before ventricles contract)
The bundle of His is a group of fibres which conducts the impulse to the finer muscle fibres of the left and right ventricles, called the Purkyne fibres.
The Purkyne fibres carry the waves into the ventricles, causing them to contract simultaneously from the bottom up
An electrocardiograph is a machine which records the electrical activity of the heart. The heart muscle depolarises when it contracts, and repolarises when it relaxes
P wave- contraction (depolarisation) of atria
QRS complex- contraction (depolarisation) of ventricles
T wave- relaxation (repolarisation) of ventricles

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During exercise, breathing rate and depth is increased to obtain more O2 and get rid of CO2, and heart rate is increased to deliver O2 to the muscles faster and remove extra CO2 produced by increased rate of respiration in muscle cells
The medulla has two areas called the inspiratory and expiratory centres, which control rate of breathing
1. The inspiratory centre in the medulla sends nerve impulses to the intercostal and diaphragm muscles to make them contract (also impulses to inhibit expiratory centre).
The diaphragm flattens/ ribs move up and out. Lung volume increases, pressure decreases bellow atmospheric pressure.
2. Air enters the lungs due to pressure difference between lungs and air.
3. As the lungs inflate, stretch receptors in the lungs are stimulated. This send impulses to the medulla to inhibit the insp. centre and stimulate the expiratory centre.
4. The lack of impulses from the exp. centre in the medulla cause the intercostal and diaphragm muscles to relax. Causes lungs to deflate, expelling air. Stretch receptors become inactive, insp. no longer inhibited.
Internal intercostal muscles contract during deeper exipration/exercise.
During exercise, CO2 in blood increases, decreasing the pH of blood. Chemoreceptors (in medulla, aortic bodies and carotid bodies) detect a decrease in pH and send impulses to the medulla to increase rate and depth of breathing.

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Medulla also controls heart rate
A decrease in blood pH is detected by chemoreceptors, which send impulses to the medulla, which send impulses to the SAN to increase heart rate
Increased bp causes a decrease in heart rate
1. Baroreceptors (pressure receptors) in the aorta wall and carotid sinuses detect rise in blood pressure
2. Nerve impulses are sent to the cardiovascular centre, which send impulses the SAN to slow the heart rate
If pressure is too low, baroreceptors send impulses to the cardiovascular centre which sends impulses to the SAN to speed up heart rate
Cardiac output- the volume of blood pumped by a ventricle per minute
Cardiac output (cm^3/min) = heart rate (beats per minute) x stroke volume (cm^3)

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Tidal volume- volume of air in each breath
Breathing rate- number of breaths taken per minute
Ventilation rate- volume of air breathed in and out per minute. Ventilation rate = tidal volume x breathing rate
A spirometer gives a reading of tidal volume and breathing rate. Has an O2 filled chamber with a moveable lid, and a tube containing soda lime to absorb CO2. A person breathes through a tube into the O2 chamber, moving the lid up (breathing out) and down (breathing in)
These movements are recorded by a pen attached to the lid, creating a spirometer trace
The spirometer can be used to measure the change in breathing rate/ tidal volume before, during and after exercise. The person is connected to a spirometer using a mask, and recordings are taken.
During exercise, the body needs to take in more O2 and remove more CO2, so breathing rate and tidal volume increase. During recovery, the breathing rate/tidal volume is still high (to remove built up lactate), but eventually returns to rest level
Training decreases breathing rate at all stages as the lung muscles are strengthened, so more air taken in with each breath, so less frequent breaths needed
Training increases tidal vol at all stages as more air taken in with each breath
During recovery, breathing rate/tidal volume decreased faster due to training as the muscles are strengthened so lungs can return to normal O2 and CO2 levels quicker

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