Pericardium
The pericardium is like a kind of bag that surrounds the lungs. The part of this "bag" that touches the lungs is called the visceral pericardium whereas the part of the "bag" that contacts the body wall is the parietal pericardium, as mentioned in a previous post. The parietal pericardium, to my understanding, is also reinforced by a fibrous layer that prevents overfilling of the heart. The space between these two layers is the pericardial cavity which contains some fluid. The pericardium does not completely cover the heart as there are still spaces for the blood vessels to enter and leave.
The parietal pericardium, as well as the fibrous layer, are supplied by the phrenic nerve which originates from the neck (specifically C3, C4 and C5). The visceral pericardium receives parasympathetic input from the vagus nerve (cranial nerve X) as well as sympathetic input from cardiac nerves from the cervical sympathetic trunk These latter nerves actually originally arise from T1-5, which has an important implication, as sensory nerves accompany the sympathetic nerves: referred pain from the heart may be across the T1-T5 dermatomes. This is why people get pain down their arms when they're having a heart attack.
Coronary circulation
The heart is mainly supplied by two main arteries (the left and right coronary arteries) which arise out of the aorta. In fact, they pretty much arise out of the wall of the aorta near the valve cusps. The significance of this is that blood tends to flow to the heart not when the valve cusps are open (since they'd be blocking the way), but when the valve cusps are closed and the blood is flowing backwards- i.e. during diastole.
The right coronary artery pretty much just runs around the coronary sulcus, though it also gives off a small artery called the marginal artery. The left coronary artery has two main branches: the circumflex artery, which runs around the coronary sulcus and runs into the right coronary artery, and the interventricular artery, which runs between the two ventricles and meets the two coronary arteries pretty much at right angles. This meeting up of arteries is important because it allows for anastomoses, or alternative routes that the blood can travel down if there are blockages.
The veins of the heart pretty much just run along with the arteries. The great cardiac vein follows the anterior interventricular artery, the small cardiac vein follows the right marginal artery and the middle cardiac vein follows the posterior interventricular artery. There is also an oblique cardiac vein which drains the left atrium. Most veins drain into the coronary sinus, which drains into the right atrium. The exceptions to this are the anterior cardiac veins, which drain the right ventricle. These drain directly into the right atrium.
Blood vessels of the heart tend to have a fair amount of variation. It's quite possible that this is why some people are more predisposed to heart attacks than others, but I'm not 100% certain, so don't quote me on that.
Chambers
As you should know by now, the adult heart has four chambers: the left and right atria and the left and right ventricles. Let's have a look at them one by one...
The Right Atrium
The right atrium has a rough-walled part, based off the primitive atria, and a smooth-walled part, based off the sinus venosus (for more information on the primitive heart and its bits and pieces, please see my post about the development of the heart). The rough walled part has some muscle called the pectinate muscles, or musculi pectinati, and has a flappy bit (it's not really flappy, but not really sure how to describe it) called the auricle. The smooth-walled part receives the veins, and at its posterior is the foramen ovale which is usually functionally closed (aside from in the cases of people who have "a hole in their heart"). Between the two is the crista terminalis. The tricuspid valve lies between the right atrium and right ventricle.
The Right Ventricle
The right ventricle is mainly rough-walled, except its rough part is called the "trabeculae carnae," which apparently roughly translates to "meaty ridges." There is, however, a small smooth part near the pulmonary valve derived from the bulbus cordis. The ventricles also contain chordae tendinae, which are little strings that join to the cusps and look kinda like parachute strings. These are held in place by papillary muscles. The chordae tendinae and muscles combined help to prevent the valve flaps from swinging open in the other direction.
The Left Atrium
The left atrium is mainly smooth-walled, except for the auricle. Part of the reason why it's so smooth-walled is probably because it has a whopping four veins that feed into it (the four pulmonary veins). The bicuspid (a.k.a. mitral) valve separates the left atrium from the left ventricle.
The Left Ventricle
The muscles of the left ventricle are roughly three times as thick as those of the right ventricle, as the left ventricle is responsible for pumping blood all over the body (whereas the right ventricle only pumps the blood to the lungs). Once again, the left ventricle has chordae tendinae and papillary muscles to stop the valve from swinging backwards.
Valves
All of the valves of the heart lie in a single plane, along with the coronary sulcus, or atrioventricular groove. They lie in a sheet of tissue that separates the atria from the ventricles and serves as an electrical insulator. This is important so that the atria and ventricles do not contract simultaneously- the ventricles need some time to fill following atrial contraction. (I'll go back to talking about conduction in a bit.)
As for the valves themselves, there isn't much that I need to say. I've already spoken about the tricuspid and mitral valves, and their chordae tendinae and papillary muscles. The aortic and pulmonary valves are a bit different structurally. They are called semilunar valves because of their shape. They each have three cusps made out of connective tissue and endothelial folds. Backflow of blood fills the cusps and seals them shut against each other.
Conduction
Some of the cardiac muscle cells are specialised to be able to transmit electrical signals, resulting in contraction. This is essentially due to leaky channels allowing them to create their own action potentials. (I may or may not go into this in more detail in a later post. It's in the physiology textbook, but we didn't cover it in the lectures.)
The sinoatrial node, located in the right atrium, has some of the leakiest channels. Therefore, it has the fastest pace, and tends to set the pace for the rest of the heart. Hence, the sinoatrial node is often thought of as the "pacemaker" for the heart. Electrical impulses from here follow an interatrial pathway to stimulate the left atrium as well, as well as an internodal pathway that stimulates the atrioventricular node. The atrioventricular node is the only place in the atrioventricular groove that electrical current can pass through. Impulses from the atrioventricular node then pass through the bundles of His, which are between the two ventricles. From there, they travel through the Purkinje fibres to the ventricular cells that they stimulate.
Surface Anatomy
The heart is in the middle of the mediastinum, right behind the sternum. This is why CPR works: pressing down on the sternum squeezes the heart, which is directly behind it.
Because of the folding of the heart during development, the atrioventricular groove (and all of the valves that lie within it) are actually on an oblique (diagonal) plane. Sound from the closing of the valves travels downstream from the flow of blood. Hence you can listen to the valves in the following places:
- Pulmonary valve: Left side, 2nd intercostal space
- Aortic valve: Right side, 2nd intercostal space
- Mitral valve: Left side, 5th intercostal space
- Tricuspid valve: Right side, 5th intercostal space
- 6th costal cartilage, 1cm to right of sternum
- 3rd costal cartilage, 1cm to right of sternum
- 2nd intercostal space, 1cm to left of sternum
- 5th intercostal space, in the midclavicular line (i.e. in line with the midpoint of your clavicles)
The mediastinum, which is essentially just the area between the lungs, has four parts named after their relation to the heart. These are the anterior mediastinum, the posterior mediastinum, the superior mediastinum and the middle mediastinum (which is basically just the heart and pericardium).
Now for a (hopefully quick) overview of what's in each region! (I'm not going to include the middle mediastinum, because I just told you that it pretty much just contains the heart and pericardium). Also I'm only going to go into minimal detail, because there are shitloads of slides and I'm fairly sure this gets touched on again in a later lecture.
Anterior Mediastinum
The anterior mediastinum doesn't contain very much- only some fat, some lymph nodes and the thymus gland. The thymus gland is important in the immune system. It's larger in kids because they're sick all the time, but gets smaller as people get older and build up immunity to more things.
Superior Mediastinum
The superior mediastinum is above the pericardium, which incidentally ends at the level of the sternal angle. (I've mentioned it before- it's the bit where the manubrium meets the body of the sternum, usually at around T4-5.) The superior mediastinum can be broken down into three "layers," which I think is kinda misleading because the vagus nerves wrap around the arteries, but whatever.
The first layer of the superior mediastinum are the veins. Here we have the jugular veins coming down on either side of the neck, as well as the subclavian veins coming from the arms (subclavian = under the clavicle). These join into the brachiocephalic veins, which drain into the superior vena cava.
The second layer of the superior mediastinum are the nerves. Here you've got paired phrenic nerves from the neck, as well as paired vagus nerves (cranial nerve X). The vagus nerves also give off branches that loop around stuff. The left vagus gives off a branch that loops around the aorta, known as the left recurrent laryngeal nerve. The right vagus gives off a branch that loops around the right subclavian artery, known as the right recurrent laryngeal nerve. There are also cardiac branches from the cervical sympathetic trunk.
The third and final layer of the superior mediastinum are the arteries. The arch of the aorta lies in the superior mediastinum and gives off three main branches. The first branch is the right brachiocephalic artery (brachio = arm, cephalic = head). This branches off into the right subclavian artery (which runs alongside the right subclavian vein) and the right common carotid artery (which runs alongside the right jugular vein). The second branch is the left common carotid artery. The third and final branch is the left subclavian artery.
There are a few other structures in the superior mediastinum aside from those above. These include the trachea and oesophagus. (As for the bronchi- bifurcation of the trachea into two bronchi occurs at the sternal angle. Turns out that knowing that sternal angle is good for something...)
Posterior Mediastinum
The posterior mediastinum (which also has parts that are inferior to the heart, due to the curvature of the diaphragm) is mainly stuff like the descending oesophagus and the descending aorta with all of its branches. Also in here is the thoracic duct, which as far as I know is pretty much the main duct of the lymph system, and the azygos vein system, which connects the veins in the lower half of the body with the upper half. I'm going to give a quick note on each of these things (except for the oesophagus, because screw that).
The general pattern behind descending aorta branches (in the thorax) is reasonably simple. Paired body wall branches come off the sides of the aorta- these are the intercostal arteries. Visceral branches, which include the bronchial and oesophageal branches, come off the front.
There isn't much that I need to say about the thoracic duct, except that it starts off on the right and crosses to the left when it's level with the sternal angle. (Yup, it's that angle again...) The thoracic duct eventually drains into the subclavian veins.
The azygos system of veins is a way for the blood to get from the lower half of the body to the upper half, or vice versa, without having to pass through the heart. "Azygos" apparently means that the vein doesn't have a pair. The azygos vein is on the right side, right up against the oesophagus. It drains into the superior vena cava. There is also a hemiazygos on the left side which drains into the azygos.
Aaaaaand FINALLY this post is over! I swear this lecture was way too long... and to add insult to injury, there were extra slides at the end :P
No comments:
Post a Comment