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The Human Body:
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The Heart
The heart is the first organ to become functional , when an embryo is half mm long (6 weeks). This because the circulation feeds the whole body. Unlike a chick, human embryos have little yolk. So, circulation develops in order to interact with the maternal circulation. It’s about the size of one’s clenched fist and weighs about 250 grams. He heart beats about once every second. It is surrounded by the lunds and protected by the ribcage. It works as a powerful pump to send blood around the body depending on the body’s need. The heart is divided into 2 halves, the right and the left, separated by a wall known as the septum. Each half is composed of 2 chambers, the upper atria and the lower vernticle. The upper chambers fill up with blood at the same time. Their thin muscular walls contract and squeeze blood into the lower ventricles through valves. When the ventricles are filled with blood, they give a powerful squeeze and push blood out into the arteries. This co-ordinated movement is controlled y the actions of the SA node, AV node, Bundle of His and the Purkinge fibers shown below.
Diagram 1 the heart showing the SA and AV node with the Purkinge fibers
Within the body there are 2 types of circulation, the pulmonary and the systemic circulation. The pulmonary circulation refuels the blood with fresh oxygen. It is slow flowing so as to enable proper gaseous exchange. The systemic circulation delivers nutrients and picks up waste all round the body. So, to give it extra strength, the left side of the heart has extra muscular walls to deal with the increased pressure need to push the blood around. Every blood cell only goes to one part of the body and does not circulate around different organs.
The heart has a few valves to enable it to work. The main functions of the valves is to prevent the backflow of blood so that the blood can be propelled forward where it needs to be. The following is a view that shows that all the valves lie on the same plane. There is no fibrous skeleton, all of which are muscle mass( cardiac muscle). There is dense connective tissue (fibrous skeleton) for firm attachment for 4 heart valves. The closing of the valves produces the “lub-dub” sound.
Diagram 1.2 Heart showing valves
Unlike skeletal muscle, there are no motor nerves in the heart, but there are millions of small muscle cells. The Auto-rhythmic Action potentials originates from Nodes (specific autorhythmic cells) and affects contractile muscle cells (99% of cells in heart). There is a delay in the AV node( at the base of right atria, just above junction of A and V) , which is necessary because atria must contract before ventricles (as when vents contract, AV valves close). The other node is the Sino-atrial node (SA node) in the right atria wall near opening of superior vena cava. Below are the Bundle of His-Specialized tract originating from AV node and travels down interventricular septa. Following which are the Purkinje Fibers-Terminal fibres spreading through myocardium like tree branches.
Unlike skeletal muscles and nerves, cardiac auto-rhythmic cells do not have resting membrane potential. Instead, they have pacemaker ability:- membrane potential slowly depolarizes between APs until threshold is reached. There are no voltage-gated Na+ channels, Na+ channels are always open. The Cardiac action potentials of pacemaker cells is decreased in outward flux of K+, and slow unchanging inward leak of Na+. K+ channels slowly close at –ve potentials. Thus, the inside gradually becomes less negative, causing the transient voltage gated Ca2+ channels open and reaches threshold. Therefore, there is Ca2+ long-lasting opens on AP (not Na+ like nerves)
However, there maybe irregular pace-maker activity. In short, the fastest pace-maker wins. Going down the line.. SA node, AV node, Bundle, Purkinje, much slower than normal. If conduction of impulse is blocked between atria and vent (ie, the AV node) , atria still go normally (eg 70 bpm) but ventricles (using purkinje) will go at 30 bpm. This is called a complete Heart Block, usually patient loses consciousness.
The action potential in a cardiac contractile (muscle) cell is very different from the pacemaker cell.
The resting membrane potential is -90mV until excited. Then, there will be a rapid increase in Na+ permeability, but it rapidly returns to normal when 30mV. But, slow Ca2+ (L) channels are activated, thus Ca2+ enters slowly. At the same time, K+ perm drops, so rapid repolarization is prevented – PLATEAU
Repolarization – inactivation of L-type Ca2+ and activation of K+ channels.
Like all other muscles of the body, the heart requires nutrition and removal of waste products. The heart is supplied by its own special blood vessels, the coronary arteries and the cardiac veins that lie on its surface. The branches of these vessels penetrate inward to supply the cardiac muscle. The right and left coronary arteries supply the myocardium of the heart. They originate from dilations at the origin of the ascending aorta, opposite of the cusps of the aortic valve. The cardiac veins accompany the coronary arteries’ branches. They empty directly into the right atria. The largest of them is the coronary sinus which lies on the posterior surface of the heart in the groove between the atria and the ventricles.
The heart is myogenic-it has a special rhythmic control power to initiate the contraction of the cardiac muscles. The heart beat is generated at the SA node. The SA node generates action potentials that are transmitted in concentric waves to reach the AV node near the a-v septum. The 2 atria then contracts in unison forcing blood into the ventricles. The a-v valves are open. The action potentials from the SA node then stimulates the AV node. The Av node delays impulse for 0.15s, then generates APs which are propogated along the Purkyne fibres to the ventricular walls. This delay allows atria to force blood into the ventricles prior to ventricular contraction. Impulses from the AV node pass down the fibers, the Bundle of His. The bundle of His divides into right and left branches which spreads the APs through the walls at the base of the ventricles. Purkyne fibres carry impulses up through the walls of the ventricles. Impulses from the Purkyne fibres causes the ventricles to contract, forcing blood from ventricles to aorta and pulmonary arteries. The Tricuspid and Bicuspid valves are closed but the semi lunar valves are open.
Cardiac heart muscle contraction then occurs specifically as follow when the action potential arrives and travels down the T-tubule. A small entry of Ca+ from the extracellular fluid which triggers an increase in the release of Ca+ from the sacroplasmic reticulum, giving an overall rise in Ca+ concentrations. This causes the tropomyosin complex in the thin filaments of the muscle to be pulled aside and allows X-bridge sliding. Thin filaments slide inwards of thick filaments of the muscle causing contraction.
The average heart beats at a rate of 60-80 times a minute but a child’s is 120 times a minute. As blood leaves the heart, it travels at a speed of about 30cm per second. In 1 minute, 5-7 litres of blood are pumped around the body. In a day, the heart can move about 10,000 litres, pumping more than 200 million litres in a lifetime!
The heart rate is modified by the cardiac centres located in the medulla oblongata of the hind brain. The heart is supplied with motor nerves from both the sympathetic and parasympathetic divisions of the autonomic nervous system. The sympathetic fibers accelerate heart beat. On the other hand, the parasympathetic fibers lower the heart rate.
The sympathetic nerves originate in the cardio-acceleratory centre of the medulla oblongata, run parallel to the spinal cord and emerge from the thoracic region to the SA node. Sympathetic stimulation increases the activity of the heart- increased heart rate, vigour of cardiac contraction and rapidity of conduction of the cardiac impulse through the heart. Adrenaline and noradrenaline (epinephrine, nor-) on b1 receptors causes the effect. It is enabled by the decrease of K+ permeability by the increase in the rate of K+ channel inactivation. So, less K+ can exit, thus inside of membrane more positive, so it gets closer to the threshold. The AV node stimulation decreases and the AV node delay by the increased rate of conduction (increase slow inward current of Ca2+): cAMP formation. The increased spread of action potentials down specialized conduction pathway and the contractile strength of cardiac muscle causes the heart to beat more strongly (more blood pumped). This is due to the increased of the Ca2+ permeability, leading to elevated slow Ca2+ influx and more Ca2+ participation in muscle contraction. Thus, the SA node is stimulated to increase the rate of depolarization, reaching threshold faster, thus fire more rapidly
The parasympathetic fibers are branches of the vagus nerve. They originate in the cardio-inhibitory centre of the medulla oblongata and run on both sides of the trachea and lead toSA node, AV node and the bundle of His. Parasympathetic stimulation decreases all activities of the heart. It decreases the heart rate, force of cardiac contraction and conduction of impulses through the AV node, thus lengthening the delay period between atrial and ventricular contraction. The main hormone involved here is Acetylcholine, which triggers the vagus stimulation of M2 receptors. The SA node - driven by parasympathetic stimulation causes the increase in K+ perm by reducing closure of K+ channels, thus raising the time taken to get to threshold. Also, causes a hyperpolarization, so membrane potential starts further away from threshold. The Ach on AV node diminishes excitability, prolonging transmission to vents (also by increase K+ perm and hyperpolarization). So, Ach on atrial contractile cells shortens action potential due to reduction in Ca2+ slow inward current, thus plateau phase reduced and contraction weaker
Ach/PS innervation of ventricles is limited so PS NS has limited effect.
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