Anatomy and physiology of the cardiovascular system
The heart, positioned in the thoracic cavity between the lungs, is protected by the pericardium, a double-layered sac of tough connective tissue that protects and fixes the heart in its place. Between the sac’s layers, the pericardial fluid protects the heart from its continual motions by providing lubrication. The outer wall of the heart is made of three layers. The outer layer of the heart is the epicardium, also called visceral pericardium, the middle layer is the myocardium, which is a thick layer made of muscle, and the inner layer is the endocardium.
The heart is made up from four chambers, two atria and two ventricles. The interatrial septum, the internal partition that divides the heart longitudinally, separates the atria and the interventricular septum separates the ventricles. The right ventricle forms most of the anterior surface of the heart. The inferoposterior aspect of the heart is dominated by the left ventricle which forms the heart apex. Two grooves visible on the heart surface indicate the boundaries of its four chambers and carry the blood vessels supplying the myocardium. The coronary sulcus or atrioventricular groove encircles the junction of the atria and ventricles like a crown. The anterior interventricular sulcus, cradling the anterior interventricular artery, marks the anterior position of the septum separating the right and left ventricles. It continues as the posterior interventricular sulcus, which provides a similar landmark on the heart’s posteroinferior surface.
Atria and ventricles: Internally the right atrium has two basic parts: a smooth-walled posterior segment and an anterior portion in which the walls are ridged by bundles of muscle tissue called pectinate muscles. The crista terminalis, a C-shaped ridge, separates the posterior and anterior regions of the right atrium. The left atrium internally is mostly smooth and undistinguished. The fossa ovalis, a shallow depression situated in the interatrial septum, marks the place where the foramen ovale existed in the fetal heart. Most of the volume of the heart is made of the two ventricles. The internal walls of the ventricles have irregular ridges of muscles called trabeculae carneae. Here we can also find the conelike papillary muscles projecting into the ventricular cavity which play a role in valve action. The atria are the receiving chambers and the ventricles are the discharging chambers. The superior vena cava and the inferior vena cava deliver deoxygenated blood from the body to the right atrium and via the right ventricle blood is pumped into the lungs where carbon dioxide is exchanged for oxygen. The pulmonary veins deliver oxygenated blood to the left atrium, from which it flows to the left ventricle where is pumped out to the body via the aorta. Blood is forced through a heart valve to get from and atrium into a ventricle or from a ventricle into an artery. Heart valves prevent blood from moving backwards by acting like one way doors. High fluid pressure forces the valve open and when the fluid pressure declines the valve closes.
Cardiac blood supply: Blood supply to the tissue of the heart is provided through major arteries that lie within the coronary sulcus, a structure running oblique around the heart; and the interventricular sulci.
These major arteries include the right and left coronary arteries which project from the aorta. Branches originating from these two major arteries such as the anterior and posterior interventricular arteries, the right and left marginal arteries and the circumflex artery provide blood supply to the heart wall. Blood drainage for the organ, is achieved through the great cardiac vein, which drains the left side of the heart, and the small cardiac vein with drains the right portion. These veins empty into the coronary sinus, a large venous cavity, which in turn empties into the right atrium. Blood flow through the coronary blood vessels is not continuous. Vasculature of the heart wall is compressed when the cardiac muscle contracts, restricting blood flow – and resuming when the cardiac muscle relaxes.
Regulation of heart activity: Intrinsic and extrinsic regulatory mechanisms control cardiac output to maintain homeostasis according to changing physiological requirements. Intrinsic regulation of the heart does not depend on either neural or hormonal regulation, but is a result of the normal functional characteristics of the heart. Extrinsic regulation depends on neural control which includes sympathetic and parasympathetic reflexes and hormonal processes, derived from epinephrine and norepinephrine secreted from the adrenal medulla.
Intrinsic regulation: As venous return to the heart increases, the heart wall is stretched, an activity called preload. According to a principle called Starling’s law of the heart, t increased preload causes the cardiac muscle fibers of the heart to contract with greater force and produce a greater stroke volume, thus greater cardiac output. A decrease in preload will cause a concomitant decrease in cardiac output. Contracting ventricles, in order to push blood into the aorta, need to produce sufficient pressure to counterbalance aortic pressure. This is called afterload. The pumping effectiveness of the heart is greatly affected by relatively small changes in magnitude of preload, but it is relatively insensitive to large changes in the afterload.
Extrinsic regulation - parasympathetic control: Parasympathetic nerve fibers extend through the vagus nerves from the brainstem to the SA node, AV node, coronary vessels and atrial myocardium of the heart. Parasympathetic stimulation has an inhibitory effect on the heart, primarily by decreasing the heart rate, but little effect on stroke volume. Acetylcholine is critical neurotransmitter produced by the parasympathetic neurons. By binding to ligand-gated channels they cause cardiac plasma membranes to become more permeable to K+, which will cause membrane hyperpolarization. This will cause the heart rate to decrease because the hyperpolarized membrane takes longer to depolarize and cause an action potential.
Sympathetic control: Preganglionic neurons, originating in the thoracic region of the spinal cord, synapse with postganglionic neurons of the inferior cervical and upper thoracic sympathetic chain ganglia which innervate the heart at the SA and AV nodes, the coronary vessels and the atrial and ventricular myocardium, and cardiac nerves. Sympathetic stimulation increases both the heart rate and the force of muscular contraction. Norepinephrine is the postganglionic sympathetic neurotransmitter. It increases the frequency and intensity of depolarization of the cardiac muscle so the incidence of action potential firing and its amplitude is increased.
Hormonal control: The pumping effectiveness of heart can be significantly influenced from epinephrine and norepinephrine released from the adrenal medulla. Both of them have similar effect on cardiac muscle thus increasing the rate and force of heart contractions. There secretion is influenced by sympathetic stimulation of the adrenal medulla, which happens in response to emotional excitement, stressful conditions and increased physical activity. Both epinephrine and norepinephrine are transported to the heart through the blood and act on cardiac muscle cells by binding to β-adrenergic receptors and stimulate cAMP synthesis. Sympathetic stimulation on the heart is faster than epinephrine but the effects of epinephrine last longer time.
Anatomy and physiology of blood vessels: With the exception of microcapillaries, the walls of blood vessels have three distinct layers called tunics, which surround a central blood containing space, the lumen. The tunica intima is the innermost layer which contains the endothelium, a simple squamous epithelial cell layer that lines the lumen of all vessels. This endothelium is a continuation of the endocardial lining of the heart, composed of flat cells situated closely together, which form a slippery surface that diminishes friction as blood moves through the lumen. The tunica media is the middle layer, which is composed of smooth muscle cells and sheets of elastin. Generally the tunica media is the largest layer in arteries, which have the principal responsibility for maintaining blood pressure and continuous blood circulation. The tunica externa is the outermost layer of the blood vessels wall, composed of collagen fibers that protect and reinforce the vessel and anchor it to surrounding structures. The heart pumps blood into large, elastic arteries that branch to form many smaller vessels. Wall structure of large-diameter arteries, high in the branching system include a large amounts of elastic tissue and lesser quantities of smooth muscle, whereas smaller vessels lower in the system, contain smaller amounts of elastic tissue larger amounts of smooth muscle. Veins are structurally distinct from arteries. The walls of the veins are thinner and contain fewer smooth muscle cells and less elastic tissue. As the veins branch towards the heart, their diameter increases, their number decreases, with increase in wall thickness.
Arterial tissue structure: Arteries have a muscular wall reinforced with elastic tissue which has evolved to maintain blood flow, even when ventricles are not contracting. Blood is pushed in the arteries with each contraction of the ventricles and large quantities of blood, cause swelling. When the ventricles relax, the artery wall recoils and pushes the blood further forward inside the artery.
Arteriole structure: Constantly varying internal and external conditions force the body to modify the distribution of blood flow by adjustment of arteriole diameter. Signals from the nervous and endocrine systems cause the smooth muscle cells of the arterioles to relax, causing vasodilation, and contract (vasoconstriction).
Capillary and venule structure: Capillary walls are composed of a single layer of endothelial cells. These vessels are present at high density in most tissues, providing large surface areas available for gas and nutrient exchange. Blood from capillary beds enters venules which merge into veins.
Vein structure: Large diameter vessels with flexible walls may stretch significantly under pressure to hold up to 70% of the total blood volume and therefore serve as a blood reservoir. Veins have unidirectional valves which maintains directionality of blood flow, with smooth muscle in their walls which serves to contract the veins during exercise, increasing the blood pressure, thus driving more blood back into the heart.