circulatory system

Circulatory system:

Heart:

The heart is located, tilted with it's apex to the left, in the center of the chest, between the lungs. It is slightly larger than a fist, weighing about a pound
The apex is formed by the tip of the stronger and larger left ventricle, projecting inferiorly and anteriorly. The left border is formed almost entirely by the left ventricle. The superior border, where the major blood vessels leave the heart, is formed by both the left and right atria. The anterior surface is also referred to as the sternocostal aspect, and the inferior, the diaphragmatic aspect or surface
The entire heart is cover by and held in place by the pericardium. The fibrous pericardium is made of a heave, fibrous tissue resting on the diaphragm with the open ends of the 'bad' connecting to the blood vessels leaving the heart. The serous pericardium is like a partially filled bag in which the heart is set, the inner surface, or visceral layer (epicardium) embraces the heart. The outer (parietal) layer comes in contact with the fibrous pericardium. In the sac that makes up the visceral and parietal peritoneum is pericardial fluid, in the cavity aptly named the pericardial cavity. Inflammation or infection of the pericardium is known as pericarditis. Pressure builds in the pericardial cavity, compressing the heart (cardiac tamponade)
The wall of the heart itself is made up of three layers (from superficial to deep):

epicardium-- thin layer of serous tissue and mesothelium. It is the epicardium, the visceral pericardium, but is often considered a part of the heart, proper
myocardium-- cardiac muscle, the main bulk of the heart tissue and responsible for the beating of the heart
endocardium-- thin layer of endolithium over another thin layer of connective tissue. Covers the valves and the tendons which operate them. The endocardium is continuous with the endolithial lining of the blood vessels

The interior of the heart is divided into four chambers. Between the two lower chambers, the ventricles, is the interventricular septum. The two smaller and more superior atria are separated by interatrial septum. One feature of this septum is an oval depression, the fossa ovalis, corresponding with the opening of the foramen ovale, an opening in the interatrial septum of the fetal heart. Each atrium has an appendage called auricle ('ear', specifically, in this case, a dog's ear), which lies on the surface of the heart, giving it more surface area. Externally, a groove, the coronary sulcus, separates the ventricles from the atria and housing the right coronary artery. The anterior interventricular sulcus and posterior interventricular sulcus separate the two ventricles and is the location of the left coronary artery. All three sulcus contain blood vessels and adipose deposits
The right atrium receives blood from the entire body, with the exception of the lungs. It does so through three major veins:

superior vena cava-- brings blood down from all areas of the body superior to the heart
inferior vena cava-- brings blood up from areas inferior to the heart
coronary sinus-- drains blood from from the vessels of the heart itself

The right atrium then then moves blood to the right ventricle. The right ventricle pumps blood through the pulmonary trunk, which splits into the right and left pulmonary arteries, carrying blood to the right and left lungs to pick up oxygen. Blood returns to the heart via pulmonary veins. The blood moves from the pulmonary veins to the left atrium, to the left ventricle, from which it is pumped to the rest of the body via the ascending aorta. From the aorta, blood travels through the coronary artery, aortic arch, and abdominal aorta. As the aorta arches, three major arteries come from it. The first is the brachiocephalic artery, second, the left common carotid artery, then the left subclavian artery

To keep blood from flowing backwards, the heart has some collagen structures; valves. Atrioventricular valves lie between the atria and ventricles. The AV valve between the right atrium and ventricle is a tricuspid, called so because it is made up of three flaps. These flaps are fibrous tissue covered with the endocardium. Tendon cords, chordae tendineae, connect the pointed ends of the roughly triangular flaps to the papillary muscles on the ventricle walls. The ridges and folds of the ventricles are known as the trabeculae carneae. Between the left ventricle and atrium is a bicuspid, or mitral, valve
The heart also contains two semilunar valves to keep blood from flowing back into the heart from the arteries. They are the pulmonary and aortic semilunar valves. Both consist of three half-moon (semi-lunar) shaped cusps. The free edges curve up into artery. Pressure back into the hollows, towards the heart, forcing them shut
The blood flow to the heart muscle itself is the coronary or cardiac circulation. Just under the auricle of the right atrium runs the right coronary artery, the anterior and small cardiac veins. On the left are the left coronary (branching into the circumflex branch and anterior interventricular branch) arteries and the great cardiac vein. On the posterior aspect, the middle branch of the veins, the middle cardiac vein, the posterior interventricular branch and one circumflex branch of the left coronary artery
Ischemia is the name for a reduced oxygen supply to the heart, weakening the cells but not killing them. Angina is ischemia of the myocardium
The sinoarterial or sinuatrial node (SA node), also called the pacemaker, is a compact mass of cells located in the right atrium, just below the opening of the superior vena cava. It sets the pace of the heart, spontaneously depolarizing and beginning action potentials. When the SA node depolarizes, it causes the artioventricular (AV) node, also located in the right atrium, is one of the last areas in the atria to be depolarized. From the AV node, a bundle of fibers, the atrioventricular (AV) bundle or bundle of His, distributes the action potential through the left and right bundles to the conduction myofibers (Purkinje fibers) in the ventricles, and the heart beats
In normal heartbeats, the two atria contract while the ventricles relax, then vis versa. Systole is contraction, diastole is relaxation. The phases of the cardiac cycle are as follows:

atrial systole (0.1 seconds)
ventricular filling (0.3 seconds)
ventricular systole
ventricular diastole (combined with ventricular systole, 0.4 seconds)

The first sound heart in a heart beat, the 'lubb', is the closing of the atrioventricular valves. The second is 'dupp', the closing of the semilunar valves

Each portion of the cardiac cycle creates a different impulse, read by an EKG. The first wave is the P wave, a small, upward wave, the atrial depolarization. The second wave is the QRS (each letter a different point), the ventricular depolarization. The low, long T wave follows, the ventricular repolarization. Enlarged P waves indicates mitral stenosis, the narrowing of the mitral valve and backing of blood into the left atrium and enlargement thereof. An enlarged Q wave may show a heart attack (myocardial infarction). Enlarged R waves indicate a enlarged ventricles. A flat T wave indicated that the heart is not receiving enough oxygen, also shown by a depressed S-T segment (the flat area between the QRS wave and T wave). Elevated S-T segments are another indicator of a heart attack

The aortic branches:

ascending aorta: right and left coronary arteries, supplying the heart
aortic arch: brachiocephalic (branching into the right common carotid and subclavian, supplying the right upper extremity and brain), left common carotid (left head and neck, brain), and left subclavian (upper left extremity)
thoracic aorta: intercostals (intercostal and chest muscles and pleurae), superior phrenics (posterior and superior diaphragm), bronchials (bronchi of the lungs), and esophageals (esophagus)
abdominal aorta: inferior phrenics (inferior diaphragm), celiac (dividing into the common hepatic [liver], left gastric [stomach and esophagus], and splenic [spleen, pancreas, and stomach]) superior mesenteric (small intestine, cecum, ascending and transverse colon), suprarenals (adrenal, or suprarenal, glands), renals (kidneys), gonadals ( branching into testicular or ovarian), inferior mesenteric (transverse, descending, and sigmoid colon, and rectum), and common iliacs (branching into the external iliacs or internal iliacs, also called hypogastrics, supplying the lower extremities, then the uterus, prostate, buttock muscles, and urinary bladder, respectively)

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