The Cardiac Cycle Flashcards
The cardiac cycle
Has 3 stages:
- Atrial systole; contraction of the atria
- Ventricular systole; contraction of the ventricles
- Diastole; Atria and ventricles relax
Pressure changes
Atrial pressure:
1) During passive atrial filling there is little change in pressure as atrium can distend and blood simply passes through to the left ventricle (SLV CLOSED; AVV OPEN)
2) During atrial contraction the contents of the left atrium are emptied so there is a small spike in pressure as atrial contraction relatively weak (SLV CLOSED; AVV OPEN)
3) During ventricular contraction the pressure in the left atrium only gradually increases as it fills with blood (SLV OPEN; AVV CLOSED)
4) Pressure drops in left atrium as atrioventricular valve opens and atrial filling commences again (SLV CLOSED; AVV OPEN)
Ventricular pressure:
1) During passive atrial filling there is little change in pressure as left ventricle can distend and blood simply passes into it (SLV CLOSED; AVV OPEN)
2) During atrial contraction there is a small increase in pressure as blood is rapidly ejected into the left ventricle (SLV CLOSED; AVV OPEN)
3) During ventricular contraction there is a large spike in pressure as the thick muscle walls of the ventricle contract (SLV OPEN; AVV CLOSED)
4) Pressure in the left ventricle falls below the pressure in the left atrium as the ventricle empties and relaxes. Pressure in the left ventricle falls below that of the left atrium, so it is passively filled with blood (SLV CLOSED; AVV OPEN)
Myogenic
The heart beat is initiated from within the cardiac muscle itself and not due to stimulation from the nervous system or hormonal system
Sino Atrial Node (SAN)
- Located in the wall of the right atrium
- A natural pacemaker
-Initiates waves of excitation
Atrioventricular Node (AVN)
- Located in the wall of the heart between the atria and ventricles
- Composed of specialised cells which can conduct the waves of excitation from the atria to the ventricles
-Responsible for the delay between atrial systole and ventricular systole
Bundles of His
- Located in the ventricular septum of the heart
- Composed of specialised cardiac muscle cells arranged as fibres
-The fibres can conduct waves of excitation from the AVN to the apex of the heart
Purkinje fibres
- Located in the walls of the ventricles
- Composed of specialised cardiac muscle cells arranged as fibres
- Branch from the Bundles of His upwards
- Cause simultaneous contraction of the ventricles from the apex upwards
Events involved in coordinating the cardiac cycle
10
1) SAN initiates a wave of excitation that spreads across both atria
2) Both atria contract simultaneously
3) A thin band of connective tissue in the walls between the atria and ventricles acts as an electrical insulator, preventing the wave of excitation passing directly to the ventricles
4) This causes delay of approx. 0.1s between the contraction of the atria and contraction of ventricles
5) This allows the atria to empty their blood before the ventricles contract
6) The wave of excitation passes down the Bundles of His in the septum from the AVN to the apex of the heart
7) The wave of excitation passes from the apex up the Purkinje fibre in the walls of the ventricles
8) This causes the ventricles to contract simultaneously from the apex upwards
9) This ensures most of their blood is emptied into the aorta and pulmonary artery
10) A small residual volume of blood is left in the ventricles
Electrocardiogram (ECG)
- The electrical activity that spreads through the heart during the cardiac cycle can be detected by using electrodes placed on the subjects skin.
- The electrical signals can be shown on a cathode ray oscilloscope
- The record produced is called an electrocardiogram.
Analysis of ECG
P Wave:
- Shows depolarisation/contraction of the atria during atrial systole
- Shows the voltage change generated by the SAN, associated with the the wave of excitation that sweeps over the atrial walls, causing them to contract.
- The atria have less muscle than the ventricles so the P-waves are small
PR Interval:
-Represents the time for the wave of excitation to spread from the atria to the ventricles via the AVN
QRS wave:
-Shows the spread of depolarisation/contraction through the ventricles resulting in ventricular systole
T wave:
-Shows the repolarisation/relaxation of the ventricles during ventricular diastole
Pressure changes in blood vessels
- The higher the blood pressure, the faster the flow
- The further away from the heart the lower the blood pressure
- Friction and branching reduce blood pressure
- Pressure highest at Aorta and largest arteries, rising and falling with ventricular contraction
- Pressure is lowest in the veins, but due to their diameter blood travels
Plasma
Contains solutes such as:
- Food molecules e.g. Glucose, Amino acids and Mineral ions
- Waste products e.g. urea and CO2 as Hydrogen carbonate ions
- Hormones
- Plasma proteins e.g. antibodies and blood clotting proteins
Plasma also distributes heat
Red Blood Cells/Erythrocytes
Function:
To transport oxygen from the lungs to the respiring tissues
Structure:
-Biconcave discs with no nucleus
Oxygen disassociation curves
Haemoglobin can change its affinity for oxygen under different conditions
High affinity:
- Organisms that live in environments with little oxygen have haemoglobin with higher affinity for oxygen
- Oxygen disassociation curves will shift to the left
- E.g. Llama and lugworm
Low affinity:
- Organisms with high metabolic rates need to unload oxygen readily to meet respiratory demands
- Oxygen disassociation curves will shift to the right
- E.g. Small mammals
Haemoglobin
Haemoglobin combines with and then transports oxygen
