In the human body, the heart is found slightly to the left of the middle of the thorax, underneath the sternum. It is inside a sac known as the pericardium and is surrounded by the lungs. The average wight for an adults heart is 250-350g, but extremely diseased hearts can be up to 1000 g in mass. It consists of four chambers, the two upper atria and the two lower ventricles. A septum divides the right atrium and ventricle from the left atrium and ventricle, preventing blood from passing between them. Valves between the atria and ventricles, called atrioventricular...
by contracting the vascular smooth muscle in the vessel walls. It is regulated by vasoconstrictors, which are chemicals that cause vasoconstriction. These include paracrine factors like a chemicle called prostaglandins a number of hormones like adreneline. Blood vessels play a role in virtually every medical condition. Cancer, for example, cannot progress if the tumor does not cause formation of new blood vessels to supply the malignant cells" metabolic demand. Atherosclerosis, the formation of lumps of fat in the blood vessel wall, is the prime cause of cardiovascular disease, which is the main cause of death in the Western world.