Cardiac muscle is an involuntary striated muscle tissue specific to the heart and is responsible for the heart’s ability to pump blood. The average human heart, beating at 72 beats per minute, will beat approximately 2. 5 billion times during an average 66 year lifespan, and pumps approximately 4. 7-5. 7 litres of blood per minute. It weighs approximately in females and in males. Structure The structure of the heart can vary among the different animal species. Cephalopods have two “gill hearts” and one “systemic heart”.
In vertebrates, the heart lies in the anterior part of the body cavity, dorsal to the gut. It is always surrounded by a pericardium, which is usually a distinct structure, but may be continuous with the peritoneum in jawless and cartilaginous fish. Hagfish, uniquely among vertebrates, also possess a second heart-like structure in the tail. It is located anterior to the vertebral column and posterior to the sternum. It is enclosed in a double-walled sac called the pericardium. The pericardium’s outer wall is called the parietal pericardium and the inner one the visceral pericardium.
Between them there is some pericardial fluid which functions to permit the inner and outer walls to slide easily over one another with the heart movements. Outside the parietal pericardium is a fibrous layer called the fibrous pericardium which is attached to the mediastinal fascia. This sac protects the heart and anchors it to the surrounding structures. The outer wall of the human heart is composed of three layers; the outer layer is called the epicardium, or visceral pericardium since it is also the inner wall of the pericardium. The middle layer is called the myocardium and is composed of contractile cardiac muscle.
The inner layer is called the endocardium and is in contact with the blood that the heart pumps. Also, it merges with the inner lining of blood vessels and covers heart valves. The human heart has four chambers, two superior atria and two inferior ventricles. The atria are the receiving chambers and the ventricles are the discharging chambers. During each cardiac cycle, the atria contract first, forcing blood that has entered them into their respective ventricles, then the ventricles contract, forcing blood out of the heart. The pathway of the blood consists of a pulmonary circuit and a systemic circuit which function simultaneously.
Deoxygenated blood from the body flows via the vena cava into the right atrium, which pumps it through the tricuspid valve into the right ventricle, whose subsequent contraction forces it out through the pulmonary valve into the pulmonary arteries leading to the lungs. Meanwhile, oxygenated blood returns from the lungs through the pulmonary veins into the left atrium, which pumps it through the mitral valve into the left ventricle, whose subsequent strong contraction forces it out through the aortic valve to the aorta leading to the systemic circulation. In fish
Primitive fish have a four-chambered heart, but the chambers are arranged sequentially so that this primitive heart is quite unlike the four-chambered hearts of mammals and birds. The first chamber is the sinus venosus, which collects deoxygenated blood, from the body, through the hepatic and cardinal veins. From here, blood flows into the atrium and then to the powerful muscular ventricle where the main pumping action will take place. The fourth and final chamber is the conus arteriosus which contains several valves and sends blood to the ventral aorta.
The ventral aorta delivers blood to the gills where it is oxygenated and flows, through the dorsal aorta, into the rest of the body. . thus, only in birds and mammals are the two streams of blood – those to the pulmonary and systemic circulations – permanently kept entirely separate by a physical barrier. In the human body, the heart is usually situated in the middle of the thorax with the largest part of the heart slightly offset to the left, although sometimes it is on the right, underneath the sternum. The heart is usually felt to be on the left side because the left heart is stronger .
The left lung is smaller than the right lung because the heart occupies more of the left hemithorax. The heart is fed by the coronary circulation and is enclosed by a sac known as the pericardium; it is also surrounded by the lungs. The pericardium comprises two parts: the fibrous pericardium, made of dense fibrous connective tissue, and a double membrane structure containing a serous fluid to reduce friction during heart contractions. The heart is located in the mediastinum, which is the central sub-division of the thoracic cavity.
The mediastinum also contains other structures, such as the esophagus and trachea, and is flanked on either side by the right and left pulmonary cavities; these cavities house the lungs. The apex is the blunt point situated in an inferior direction. A stethoscope can be placed directly over the apex so that the beats can be counted. It is located posterior to the 5th intercostal space just medial of the left mid-clavicular line. In normal adults, the mass of the heart is 250–350 grams, or about twice the size of a clenched fist, but an extremely diseased heart can be up to 1000 g in mass due to hypertrophy.
It consists of four chambers, the two upper atria and the two lower ventricles. Functioning In mammals, the function of the right side of the heart is to collect de-oxygenated blood, in the right atrium, from the body and pump it, through the tricuspid valve, via the right ventricle, into the lungs so that carbon dioxide can be exchanged for oxygen. This happens through the passive process of diffusion. The left side collects oxygenated blood from the lungs into the left atrium. From the left atrium the blood moves to the left ventricle, through the bicuspid valve, which pumps it out to the body .
On both sides, the lower ventricles are thicker and stronger than the upper atria. The muscle wall surrounding the left ventricle is thicker than the wall surrounding the right ventricle due to the higher force needed to pump the blood through the systemic circulation. Starting in the right atrium, the blood flows through the tricuspid valve to the right ventricle. Here, it is pumped out the pulmonary semilunar valve and travels through the pulmonary artery to the lungs. From there, oxygenated blood flows back through the pulmonary vein to the left atrium.
It then travels through the mitral valve to the left ventricle, from where it is pumped through the aortic semilunar valve to the aorta. The aorta forks and the blood is divided between major arteries which supply the upper and lower body. The blood travels in the arteries to the smaller arterioles and then, finally, to the tiny capillaries which feed each cell. The deoxygenated blood then travels to the venules, which coalesce into veins, then to the inferior and superior venae cavae and finally back to the right atrium where the process began.
The heart is effectively a syncytium, a meshwork of cardiac muscle cells interconnected by contiguous cytoplasmic bridges. This relates to electrical stimulation of one cell spreading to neighboring cells. Some cardiac cells are self-excitable, contracting without any signal from the nervous system, even if removed from the heart and placed in culture. Each of these cells have their own intrinsic contraction rhythm. A region of the human heart called the sinoatrial node, or pacemaker, sets the rate and timing at which all cardiac muscle cells contract. The SA node generates electrical impulses, much like those produced by nerve cells.
Because cardiac muscle cells are electrically coupled by inter-calculated disks between adjacent cells, impulses from the SA node spread rapidly through the walls of the artria, causing both artria to contract in unison. The impulses also pass to another region of specialized cardiac muscle tissue, a relay point called the atrioventricular node, located in the wall between the right atrium and the right ventricle. Here, the impulses are delayed for about 0. 1s before spreading to the walls of the ventricle. The delay ensures that the artria empty completely before the ventricles contract.
Specialized muscle fibers called Purkinje fibers then conduct the signals to the apex of the heart along and throughout the ventricular walls. The Purkinje fibres form conducting pathways called bundle branches. This entire cycle, a single heart beat, lasts about 0. 8 seconds. The impulses generated during the heart cycle produce electrical currents, which are conducted through body fluids to the skin, where they can be detected by electrodes and recorded as an electrocardiogram . The events related to the flow or blood pressure that occurs from the beginning of one heartbeat to the beginning of the next is called a cardiac cycle.
The SA node is found in all amniotes but not in more primitive vertebrates. In these animals, the muscles of the heart are relatively continuous and the sinus venosus coordinates the beat which passes in a wave through the remaining chambers. Indeed, since the sinus venosus is incorporated into the right atrium in amniotes, it is likely homologous with the SA node. In teleosts, with their vestigial sinus venosus, the main centre of coordination is, instead, in the atrium. The rate of heartbeat varies enormously between different species, ranging from around 20 beats per minute in codfish to around 600 in hummingbirds.
From splanchnopleuric mesoderm tissue, the cardiogenic plates develop cranially and laterally to the neural plates. In the cardiogenic plates, two separate angiogenic cell clusters form on either side of the embryo. The cell clusters coalesce to form an endocardial heart tube continuous with a dorsal aorta and a vitteloumbilical vein. As embryonic tissue continues to fold, the two endocardial tubes are pushed into the thoracic cavity, begin to fuse together, and complete the fusing process at approximately 22 days.
The human embryonic heart begins beating at around 22 days after conception, or five weeks after the last normal menstrual period . The first day of the LMP is normally used to date the start of the gestation . The human heart begins beating at a rate near the mother’s, about 75–80 beats per minute . The embryonic heart rate then accelerates linearly by approximately 100 BPM during the first month to peak at 165–185 BPM during the early 7th week after conception, . This acceleration is approximately 3. 3 BPM per day, or about 10 BPM every three days, which is an increase of 100 BPM in the first month.
The regression formula which describes this linear acceleration before the embryo reaches 25mm in crown-rump length, or 9. 2 LMP weeks, is: the Age in days EHR+6. After 9. 1 weeks after the LMP, it decelerates to about 152 BPM during the 15th week post LMP. After the 15th week, the deceleration slows to an average rate of about 145 BPM, at term. There is no difference in female and male heart rates before birth. History of discoveries The valves of the heart were discovered by a physician of the Hippocratean school around the 4th century BC, although their function was not fully understood.
On dissection, arteries are typically empty of blood because blood pools in the veins after death. Ancient anatomists subsequently assumed they were filled with air and served to transport it around the body. Philosophers distinguished veins from arteries, but thought the pulse was a property of arteries themselves. Erasistratos observed that arteries cut during life bleed. He ascribed the fact to the phenomenon that air escaping from an artery is replaced with blood which entered by very small vessels between veins and arteries.
Thus he apparently postulated capillaries, but with reversed flow of blood. The Greek physician Galen knew blood vessels carried blood and identified venous and arterial blood, each with distinct and separate functions. Growth and energy were derived from venous blood created in the liver from chyle, while arterial blood gave vitality by containing pneuma and originated in the heart. Blood flowed from both creating organs to all parts of the body, where it was consumed and there was no return of blood to the heart or liver.
The heart did not pump blood around, the heart’s motion sucked blood in during diastole and the blood moved by the pulsation of the arteries themselves. Galen believed the arterial blood was created by venous blood passing from the left ventricle to the right through ‘pores’ in the interventricular septum, while air passed from the lungs via the pulmonary artery to the left side of the heart. As the arterial blood was created, “sooty” vapors were created and passed to the lungs, also via the pulmonary artery, to be exhaled.