“Hoppy” Heart Homework Assignment


P-2 Class 17


EMSM 250


5/19/03


The human heart is a hollow, cone shaped organ that is relatively the size of a person’s fist. The apex of the heart is tilted obliquely and pointed towards the left hip and it rests on the diaphragm at approximately at the level of the fifth intercostal space, which is about the nipple line (Marieb 309). The heart weighs around 300 grams (10 oz.) in an adult. The heart is near the middle of the thoracic cavity in the mediastinum, which is the space between the vertebral column and the sternum. The heart is more left of the body’s midline and this is the reason for the left lung to only have two lobes to accommodate space for the heart. Its dimensions are about 12cm (5 in.) long, 9cm (3 ˝ in.) wide at its widest point, and 6cm (2 ˝ in.) thick. The wide upper and posterior portion of the heart, opposite of the apex is called the base, which is mostly formed by the atria (Tortora and Grabowski 592).


Starting with the outermost of cardiac tissues the pericardium comes first and consists of three layers that act as a protective “sack” for the heart. The pericardium holds the heart in the proper position for function but also allows the right amount of movement for contraction and changing conditions such as rate and force. The outermost layer of the pericardium is the fibrous pericardium, which is an inelastic, tough fibrous connective tissue. This tissue encases the heart completely and only opens where it is attached to blood vessels entering and leaving the heart. “The fibrous pericardium prevents overstretching of the heart, provides protection, and anchors the heart in the mediastinum” (Tortora and Grabowski 592). The next layer of the pericardium is the serous pericardium, which is a thinner and more delicate double membrane that surrounds the heart. The outer of this double membrane tissue is called the parietal layer and is fused to the fibrous pericardium. The inner membrane of the serous pericardium is called the visceral layer or also known as the epicardium, which adheres tightly to the muscle of the heart. Between these two layers of the serous pericardium is a film of serous fluid or pericardial fluid, which lubricates the two membranes enabling the heart to move with minimal friction. The space between the visceral and parietal layers is the pericardial cavity where the pericardial fluid resides (Tortora and Grabowski 592).


Inside the pericardium is the actual heart itself, which is composed of three layers making up the heart wall. “The three layers of the heart wall from outermost are the epicardium (external layer), myocardium (middle layer) and the endocardium (inner most layer)” (Tortora and Grabowski 592). The epicardium also called the visceral layer of the serous pericardium and is the thin and transparent outer layer of the heart wall. The visceral layer gets its smooth, slippery texture from the composition of mesothelium and delicate connective tissue that surrounds the outside of the heart. The middle layer, myocardium, is cardiac muscle tissue and gives the heart its pumping action. Cardiac muscle fibers are involuntary, striated and branched. Cardiac muscle fibers “swirl” around the heart diagonally and form two networks atrial and ventricular. All cardiac muscle fibers are directly connected to neighboring fibers by transverse thickenings of the sarcolemma or better known as intercalated discs. Within these discs are gap junctions, which are electrical synapses that allow muscle action potentials to spread across to each fiber. This results in the whole atrial network contracting as a unit and the ventricular as another. The intercalated discs also contain demosomes, which act as reinforcing spot-welds and prevent adjacent cardiac fibers from pulling apart during vigorous contractions. Finally the endocardium is the smooth and thin lining within the heart and also covers the valves. The endocardium also goes on to line large blood vessels associated with the heart and throughout the cardiovascular system (Tortora and Grabowski 594).


The heart has four cavities or compartments called chambers that receive and pump out blood with the assistance of valves. The top two chambers are called the atria and receive blood from the superior and inferior vena cavas or from the pulmonary veins. The left and right atriums have