Your heart is located between your lungs in the middle of your chest, behind and slightly to the left of your breastbone sternum. A double-layered membrane called the pericardium surrounds your heart like a sac. The outer layer of the pericardium surrounds the roots of your heart's major blood vessels and is attached by ligaments to your spinal column, diaphragm, and other parts of your body.
The heart weighs between 7 and 15 ounces to grams and is a little larger than the size of your fist. In fact, each day, the average heart beats , times, pumping about 2, gallons 7, liters of blood. The inner layer of the pericardium is attached to the heart muscle. A coating of fluid separates the two layers of membrane, letting the heart move as it beats. Your heart has 4 chambers. The upper chambers are called the left and right atria, and the lower chambers are called the left and right ventricles.
A wall of muscle called the septum separates the left and right atria and the left and right ventricles. The vagus nerve is a long, wandering nerve that emerges from the brainstem and provides parasympathetic stimulation to a large number of organs in the thorax and abdomen, including the heart. The ventricles are more richly innervated by sympathetic fibers than parasympathetic fibers. Sympathetic stimulation causes the release of the neurotransmitter norepinephrine also known as noradrenaline at the neuromuscular junction of the cardiac nerves.
This shortens the repolarization period, thus speeding the rate of depolarization and contraction, which results in an increased heart rate. It opens chemical or ligand-gated sodium and calcium ion channels, allowing an influx of positively charged ions. The heart is the first functional organ to develop and starts to beat and pump blood at about three weeks into embryogenesis.
This early start is crucial for subsequent embryonic and prenatal development.
The heart derives from splanchnopleuric mesenchyme in the neural plate which forms the cardiogenic region. Two endocardial tubes form here that fuse to form a primitive heart tube known as the tubular heart. This places the chambers and major vessels into the correct alignment for the developed heart. Further development will include the septa and valves formation and remodelling of the heart chambers.
By the end of the fifth week the septa are complete and the heart valves are completed by the ninth week. Before the fifth week, there is an opening in the fetal heart known as the foramen ovale. The foramen ovale allowed blood in the fetal heart to pass directly from the right atrium to the left atrium, allowing some blood to bypass the lungs. Within seconds after birth, a flap of tissue known as the septum primum that previously acted as a valve closes the foramen ovale and establishes the typical cardiac circulation pattern.
Anatomy of the Human Heart - Physiopedia
A depression in the surface of the right atrium remains where the foramen ovale was, called the fossa ovalis. The embryonic heart begins beating at around 22 days after conception 5 weeks after the last normal menstrual period, LMP. It starts to beat at a rate near to the mother's which is about 75—80 beats per minute bpm. The embryonic heart rate then accelerates and reaches a peak rate of — bpm early in the early 7th week early 9th week after the LMP. There is no difference in female and male heart rates before birth.
The heart functions as a pump in the circulatory system to provide a continuous flow of blood throughout the body. This circulation consists of the systemic circulation to and from the body and the pulmonary circulation to and from the lungs. Blood in the pulmonary circulation exchanges carbon dioxide for oxygen in the lungs through the process of respiration. The systemic circulation then transports oxygen to the body and returns carbon dioxide and relatively deoxygenated blood to the heart for transfer to the lungs.
The right heart collects deoxygenated blood from two large veins, the superior and inferior venae cavae. Blood collects in the right and left atrium continuously. The inferior vena cava drains the blood from below the diaphragm and empties into the back part of the atrium below the opening for the superior vena cava. Immediately above and to the middle of the opening of the inferior vena cava is the opening of the thin-walled coronary sinus. The blood collects in the right atrium. When the right atrium contracts, the blood is pumped through the tricuspid valve into the right ventricle.
As the right ventricle contracts, the tricuspid valve closes and the blood is pumped into the pulmonary trunk through the pulmonary valve. The pulmonary trunk divides into pulmonary arteries and progressively smaller arteries throughout the lungs, until it reaches capillaries. As these pass by alveoli carbon dioxide is exchanged for oxygen. This happens through the passive process of diffusion. In the left heart , oxygenated blood is returned to the left atrium via the pulmonary veins.
It is then pumped into the left ventricle through the mitral valve and into the aorta through the aortic valve for systemic circulation. The aorta is a large artery that branches into many smaller arteries, arterioles , and ultimately capillaries. In the capillaries, oxygen and nutrients from blood are supplied to body cells for metabolism, and exchanged for carbon dioxide and waste products.
The cardiac cycle refers to the sequence of events in which the heart contracts and relaxes with every heartbeat.
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The atria and ventricles work in concert, so in systole when the ventricles are contracting, the atria are relaxed and collecting blood. When the ventricles are relaxed in diastole, the atria contract to pump blood to the ventricles. This coordination ensures blood is pumped efficiently to the body. At the beginning of the cardiac cycle, the ventricles are relaxing.
As they do so, they are filled by blood passing through the open mitral and tricuspid valves. After the ventricles have completed most of their filling, the atria contract, forcing further blood into the ventricles and priming the pump. Next, the ventricles start to contract. As the pressure rises within the cavities of the ventricles, the mitral and tricuspid valves are forced shut.
As the pressure within the ventricles rises further, exceeding the pressure with the aorta and pulmonary arteries, the aortic and pulmonary valves open. Blood is ejected from the heart, causing the pressure within the ventricles to fall.
Simultaneously, the atria refill as blood flows into the right atrium through the superior and inferior vena cavae , and into the left atrium through the pulmonary veins. Finally, when the pressure within the ventricles falls below the pressure within the aorta and pulmonary arteries, the aortic and pulmonary valves close. The ventricles start to relax, the mitral and tricuspid valves open, and the cycle begins again. Cardiac output CO is a measurement of the amount of blood pumped by each ventricle stroke volume in one minute. This is calculated by multiplying the stroke volume SV by the beats per minute of the heart rate HR.
The average cardiac output, using an average stroke volume of about 70mL, is 5. Preload refers to the filling pressure of the atria at the end of diastole, when the ventricles are at their fullest. A main factor is how long it takes the ventricles to fill: if the ventricles contract more frequently, then there is less time to fill and the preload will be less. The force of each contraction of the heart muscle is proportional to the preload, described as the Frank-Starling mechanism. This states that the force of contraction is directly proportional to the initial length of muscle fiber, meaning a ventricle will contract more forcefully, the more it is stretched.
Afterload , or how much pressure the heart must generate to eject blood at systole, is influenced by vascular resistance. It can be influenced by narrowing of the heart valves stenosis or contraction or relaxation of the peripheral blood vessels. The strength of heart muscle contractions controls the stroke volume. This can be influenced positively or negatively by agents termed inotropes. Inotropes that increase the force of contraction are "positive" inotropes, and include sympathetic agents such as adrenaline , noradrenaline and dopamine. The normal rhythmical heart beat, called sinus rhythm , is established by the sinoatrial node , the heart's pacemaker.
Here an electrical signal is created that travels through the heart, causing the heart muscle to contract. The sinoatrial node is found in the upper part of the right atrium near to the junction with the superior vena cava. It travels to the left atrium via Bachmann's bundle , such that the muscles of the left and right atria contract together. This is found at the bottom of the right atrium in the atrioventricular septum —the boundary between the right atrium and the left ventricle. The septum is part of the cardiac skeleton , tissue within the heart that the electrical signal cannot pass through, which forces the signal to pass through the atrioventricular node only.
The Location, Size, and Shape of the Heart
In the ventricles the signal is carried by specialized tissue called the Purkinje fibers which then transmit the electric charge to the heart muscle. The normal resting heart rate is called the sinus rhythm , created and sustained by the sinoatrial node , a group of pacemaking cells found in the wall of the right atrium. Cells in the sinoatrial node do this by creating an action potential.
The cardiac action potential is created by the movement of specific electrolytes into and out of the pacemaker cells.
The action potential then spreads to nearby cells. When the sinoatrial cells are resting, they have a negative charge on their membranes. However a rapid influx of sodium ions causes the membrane's charge to become positive. This is called depolarisation and occurs spontaneously.
All the ions travel through ion channels in the membrane of the sinoatrial cells. The potassium and calcium start to move out of and into the cell only once it has a sufficiently high charge, and so are called voltage-gated.
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Shortly after this, the calcium channels close and potassium channels open, allowing potassium to leave the cell. This causes the cell to have a negative resting charge and is called repolarization. The ions move from areas where they are concentrated to where they are not. For this reason sodium moves into the cell from outside, and potassium moves from within the cell to outside the cell.
Calcium also plays a critical role. Their influx through slow channels means that the sinoatrial cells have a prolonged "plateau" phase when they have a positive charge. A part of this is called the absolute refractory period.