Heart

• Cardiac muscles control the regular beating of the heart.
• Cardiac muscle is 'myogenic' - it can contract and relax without any external stimulation.
• Heart block is a failure of conduction between atria and ventricles, the most common site is AVN and it may be either complete or incomplete block.
• The electrical activity in the heart creates the pattern of contractions which coordinates the regular heartbeat.
• The process starts in the Sino-atrial nodes (SAN), which is in the wall of the right atrium.
• The SAN is like a pacemaker - it sets the rhythm of the heartbeat by sending out regular waves of electrical activity (waves of depolarization) to the atrial wall.
• This causes the right and left atria to contract at the same time.
• A band of non-conducting collagen tissue prevents the waves of electrical activity from being passed directly from the atria to the ventricles.
• Instead, these waves of electrical activity are transferred from the SAN to the atrioventricular node (AVN).
• The AVN is responsible for passing the waves of electrical activity on to the bundle of His.
• The bundle of His is a group of muscle fibers in the septum responsible for conducting the waves of electrical activity to the finer muscles fibers in the right and left ventricle walls, called Purkyne fibers.
• The Purkyne fibers carry the waves of electrical activity into the muscular walls of the right and left ventricles, causing them to contract simultaneously, from the bottom up.
• Between atria and ventricles, there is a band of non-conducting muscle tissue.
• Electricity is forced to pass through the pathway described before (SAN → AVN → Bundle of His → Purkinje fibers → Apex then upwards).
• This leads to a delay of about 0.1 second between atrial systole and ventricular systole.
• Atrial systole is completed before ventricular systole starts which ensures proper ventricular filling.
• The direction of electric wave is from the apex (downwards) to the rest of the ventricles (upward), so blood flows towards great vessels (Aorta and Pulmonary artery).
• Stroke Volume is the volume of blood pumped by the heart in 1 beat.
• Heart Rate is the number of heart beats per minute.
• Sympathetic over stimulation is achieved through increased adrenaline secretion.
• Athletes have a higher cardiac output and higher systolic volume with a relatively lower heart rate than other people due to larger and stronger cardiac muscles that pump more blood per beat, reducing the need for more beats.
• The advantages of higher efficiency and more time for ventricular filling increase stroke volume.
• During exercise, athletes have a higher cardiac output and higher systolic volume with a relatively lower heart rate than other people due to larger and stronger cardiac muscles that pump more blood per beat, reducing the need for more beats.
• During rest, athletes have a lower heart rate and lower resting pulse.
• A doctor can check someone’s heart function using an electrocardiograph, a machine that records the electrical activity of the heart.
• The heart muscle depolarizes (loses electrical charge) when it contracts, and repolarizes (regains charge) when it relaxes.
• An electrocardiograph records these changes in electrical charge using electrodes placed on the chest.
• The trace produced by an electrocardiograph is called an electrocardiogram, or ECG.
• A normal ECG looks like this: The P wave is caused by contraction (depolarization) of the atria, the main peak of the heartbeat, together with the dips at either side, is called the QRS complex - it’s caused by contraction (depolarization) of the ventricles, the T waves is due to relaxation (repolarization) of the ventricles, and the height of the wave indicates how much electrical charge is passing through the heart - a bigger wave means more electrical charge, so (for the P and R waves) a bigger wave means a stronger contraction.
• SAN initiates the heart rate by starting the waves of excitation / depolarization, and it responds to nervous & hormonal stimuli so it also determines the heart rate.
• Interpretation of ECG results: It may reflect normal rhythm: The Heart is normally beating and functioning, arrhythmia: irregularity in heart rhythm due to defect in the conduction system of the heart caused by drugs such as caffeine, nicotine or alcohol, tachycardia: heart rate higher than 100 beats/min, bradycardia: heart rate less than 60 beats/min, ventricular fibrillation: loss of synchronization in ventricular muscle contraction.
• Cardiac Output is the volume of blood pumped by the heart in 1 minute.
• Cardiac Output (dm3.min-1) = Stroke Volume (dm3) x Heart Rate (beats.min-1).
• Adjusting the Cardiac Output (Homeostasis by negative feedback) is achieved by the cardiovascular control center which is found in the medulla oblongata (brain stem).
• This center is connected to the autonomic nervous system (involuntary) which has two arms: Sympathetic nervous system and Parasympathetic nervous system.
• The Sympathetic nervous system carries excitatory impulses causing more stimulation of the SAN which sends more signals for the heart muscles to contract thus increasing the heart rate and cardiac output.
• The parasympathetic nervous system carries inhibitory impulses to the SAN, opposite of the sympathetic nervous system.
• The parasympathetic nervous system mainly uses the neurotransmitter acetylcholine.
• The Cardiovascular control center receives impulses from baroreceptors, which are stretch receptors that detect the degree of stretch in the walls of arteries and in the cardiac muscle itself.
• When the blood pressure increases above normal, baroreceptors send impulses to the CVC control center which responds by sending impulses via the parasympathetic nervous system causing inhibition of the SAN and slowing of the heart rate.