MedicoPlexus
Human Anatomy & Histology
110. Circulatory system
The cardiovascular system consists of the heart, blood vessels, and the approximately 5 liters of blood that the blood vessels transport. Responsible for transporting oxygen, nutrients, hormones, and cellular waste products throughout the body, the cardiovascular system is powered by the body’s hardest-working organ — the heart, which is only about the size of a closed fist. Even at rest, the average heart easily pumps over 5 liters of blood throughout the body every minute.
The Heart
The heart is a muscular pumping organ located medial to the lungs along the body’s midline in the thoracic region. The bottom tip of the heart, known as its apex, is turned to the left, so that about 2/3 of the heart is located on the body’s left side with the other 1/3 on right. The top of the heart, known as the heart’s base, connects to the great blood vessels of the body: the aorta, vena cava, pulmonary trunk, and pulmonary veins.
Circulatory Loops
There are 2 primary circulatory loops in the human body: the pulmonary circulation loop and the systemic circulation loop.
Pulmonary circulation transports deoxygenated blood from the right side of the heart to the lungs, where the blood picks up oxygen and returns to the left side of the heart. The pumping chambers of the heart that support the pulmonary circulation loop are the right atrium and right ventricle.
Systemic circulation carries highly oxygenated blood from the left side of the heart to all of the tissues of the body (with the exception of the heart and lungs). Systemic circulation removes wastes from body tissues and returns deoxygenated blood to the right side of the heart. The left atrium and left ventricle of the heart are the pumping chambers for the systemic circulation loop.
Blood Vessels
Blood vessels are the body’s highways that allow blood to flow quickly and efficiently from the heart to every region of the body and back again. The size of blood vessels corresponds with the amount of blood that passes through the vessel. All blood vessels contain a hollow area called the lumen through which blood is able to flow. Around the lumen is the wall of the vessel, which may be thin in the case of capillaries or very thick in the case of arteries.
All blood vessels are lined with a thin layer of simple squamous epithelium known as the endothelium that keeps blood cells inside of the blood vessels and prevents clots from forming. The endothelium lines the entire circulatory system, all the way to the interior of the heart, where it is called the endocardium.
There are three major types of blood vessels: arteries, capillaries and veins. Blood vessels are often named after either the region of the body through which they carry blood or for nearby structures. For example, the brachiocephalic artery carries blood into the brachial (arm) and cephalic (head) regions. One of its branches, the subclavian artery, runs under the clavicle, hence the name subclavian. The subclavian artery runs into the axillary region where it becomes known as the axillary artery.
Arteries and Arterioles: Arteries are blood vessels that carry blood away from the heart. Blood carried by arteries is usually highly oxygenated, having just left the lungs on its way to the body’s tissues. The pulmonary trunk and arteries of the pulmonary circulation loop provide an exception to this rule – these arteries carry deoxygenated blood from the heart to the lungs to be oxygenated.
Arteries face high levels of blood pressure as they carry blood being pushed from the heart under great force. To withstand this pressure, the walls of the arteries are thicker, more elastic, and more muscular than those of other vessels.
Capillaries: Capillaries are the smallest and thinnest of the blood vessels in the body and also the most common. They can be found running throughout almost every tissue of the body and border the edges of the body’s avascular tissues. Capillaries connect to arterioles on one end and venules on the other.
Capillaries carry blood very close to the cells of the tissues of the body in order to exchange gases, nutrients, and waste products. The walls of capillaries consist of only a thin layer of endothelium so that there is the minimum amount of structure possible between the blood and the tissues. The endothelium acts as a filter to keep blood cells inside of the vessels while allowing liquids, dissolved gases, and other chemicals to diffuse along their concentration gradients into or out of tissues.
Veins and Venules: Veins are the large return vessels of the body and act as the blood return counterparts of arteries. Because the arteries, arterioles, and capillaries absorb most of the force of the heart’s contractions, veins and venules are subjected to very low blood pressures. This lack of pressure allows the walls of veins to be much thinner, less elastic, and less muscular than the walls of arteries.
Veins rely on gravity, inertia, and the force of skeletal muscle contractions to help push blood back to the heart. To facilitate the movement of blood, some veins contain many one-way valves that prevent blood from flowing away from the heart. As skeletal muscles in the body contract, they squeeze nearby veins and push blood through valves closer to the heart.
111.Heart
The heart is a hollow muscular pump, which lies in the middle mediastinum. On its surface, it has several distinctive features, which are of anatomical and clinical importance.
In this article, we shall look at the surface anatomy of the heart.
Orientation and Surfaces
The heart does not have a straightforward orientation. Once you have grasped the orientation of the heart, it is much easier to comprehend the rest of its anatomy.
It is described by many texts as “a pyramid which has fallen over”. The apex of this pyramid pointing in an anterior-inferior direction.
In its typical anatomical orientation, the heart has 5 surfaces, formed by different internal divisions of the heart:
Anterior (or sternocostal) – Right ventricle.
Posterior (or base) – Left atrium.
Inferior (or diaphragmatic) – Left and right ventricles.
Right pulmonary – Right atrium.
Left pulmonary – Left ventricle.
Borders
Separating the surfaces of the heart are its borders. There are four main borders of the heart:
Right border – Right atrium
Inferior border – Left ventricle and right ventricle
Left border – Left ventricle (and some of the left atrium)
Superior border – Right and left atrium and the great vessels
Sulci of the Heart
The heart is a hollow structure. On the interior, it is divided into four chambers. These divisions create grooves on the surface of the heart – these are known as sulci.
The coronary sulcus (or atrioventricular groove) runs transversely around the heart – it represents the wall dividing the atria from the ventricles. The sinus contains important vasculature, such as the right coronary artery.
The anterior and posterior interventricular sulci can be found running vertically on their respective sides of the heart. They represent the wall separating the ventricles.
Pericardial Sinuses
The pericardial sinuses are not the same as ‘anatomical sinuses’ (such as the paranasal sinuses). They are passageways formed the unique way in which the pericardium folds around the great vessels.
The oblique pericardial sinus is a blind ending passageway (‘cul de sac’) located on the posterior surface of the heart.
The transverse pericardial sinus is found superiorly on the heart. It can be used in coronary artery bypass grafting
Chest X-ray Anatomy
Heart size and contours
Key points
The heart size is assessed as the cardiothoracic ratio (CTR)
A CTR of >50% is abnormal – PA view only
The left hemidiaphragm should be visible behind the heart
The hemidiaphragm contours do not represent the lowest part of the lungs
Heart size is not assessed by absolute measurement, but rather in relation to the total thoracic width, and is expressed as a ratio. Cardiothoracic ratio (CTR) = Cardiac Width : Thoracic Width A CTR of greater than 1:2 (50%) is considered abnormal. This, however, assumes the projection is Posterior-Anterior (PA), and that cardiac size is not exaggerated by factors such as patient rotation or an incomplete breath in.
Assessing the heart
The heart size should be considered on every chest X-ray, but the cardiothoracic ratio (CTR) can only be assessed confidently if a posterior – anterior (PA) view has been acquired. The rule is, if an anterior – posterior (AP) view has been taken, then the heart should not be called enlarged even if the CTR is >50%. This is because an AP view exaggerates the heart size. If the CTR is <50% on an AP view, then clearly the heart size is within normal. If the heart contours are not clearly seen, this may be because of increase in density of the adjacent lung. The lingula of the upper lobe of the left lung, wraps over the left ventricle, and so loss of definition of the left heart border may be related to disease in this area of lung. On the right, the middle lobe is located adjacent to the right atrium, and therefore loss of definition of the right heart border may be due to increased density caused by disease in this lung lobe.
112. chambers and valves
The atria of the heart receive blood from the great vessels and pump it into the ventricles. In the human heart, there are two atria, one left and one right.
In this article, we shall look at the anatomy of the atria, their anatomical landmarks, and the clinical correlations.
Right Atrium
The right atrium receives deoxygenated blood from the superior and inferior vena cava. It also receives drainage from the coronary veins via the coronary sinus. The atrium pumps this blood the right ventricle.
The interior surface of the right atrium can be divided into two parts, each with a distinct embryological origin. They are divided by a smooth muscular ridge, called the crista terminalis:
Posterior to the crista terminalis: This part receives blood from the vena cavae. It has smooth walls and is derived from the embryonic sinus venosus.
Anterior to the crista terminalis: This part is derived from the primitive atrium. Internally, it has muscular walls, formed by pectinate muscles. It also contains the right auricle.
The right auricle (also known as the right atrial appendage) is located on the anteromedial portion of the right atrium, overlapping the root of the aorta. It acts to increase the capacity of the right atrium.
Interatrial Septum
The two atria are separated by a solid muscular wall – the interatrial septum.
Within the right atrium, the interatrial septum has an oval shaped depression, called the fossa ovalis. It is the closed form of the foramen ovale, a valve present in the fetal heart.
Left Atrium
The left atrium receives oxygenated blood from the pulmonary veins. In the anatomical position, it forms the posterior border of the heart. Blood is pumped from the left atrium into the left ventricle, via the mitral valve.
Much like the right, the left atrium can be divided embryologically:
Inflow portion – This part receives blood from the pulmonary veins. Its internal surface is smooth. It is derived from the pulmonary veins themselves
Outflow portion – This part is located anteriorly, where the blood flows into the ventricles. It contains the pectinate muscles and the left auricle. It is derived from the embryonic atrium.
The Right Ventricle
The right ventricle receives deoxygenated blood from the right atrium and pumps it into the pulmonary artery.
The ventricle is triangular in shape and forms the majority of the anterior border of the heart.
The interior of the inflow part of the right ventricle is covered by irregular muscular structures, called trabeculae carneae. They give the ventricle a ‘sponge-like’ appearance. There are three types of trabeculae carneae:
Attached to the ventricle wall at both ends (forming a bridge)
Attached to the ventricle wall along their length (forming a ridge)
Attached to the ventricle wall at their base, and to chordae tedineae at the other end. These are known as papillary muscles.
There are three papillary muscles in the right atrium. They are attached to chordae tendineae (fibrous cords), which are in turn attached to the tricuspid valve. By contracting, the papillary muscles ‘pull’ on the chordae tendineae and prevent the valve from regurgitating during ventricular contraction.
There is a large muscular structure that runs between the interventricular septum and the anterior wall of the right ventricle. It is called the moderator band. It has an important conductive function, containing right bundle branches (for more information, see the Conduction Pathway of the Heart)
The outflow part (leading to the pulmonary artery) is located in the superior aspect of the ventricle. It is called the conus arteriosus and is derived from the embryonic bulbus cordis. It is visibly different from the rest of the ventricle, having smooth walls, with no trabeculae carneae.
Interventricular Septum
The interventricular septum separates the two ventricles.
The septum is made up of a membranous part (superiorly) and a muscular part (inferiorly)
The muscular part forms the majority of the septum. It is the same thickness as the left ventricular wall. The membranous part is thinner, and part of the fibrous skeleton of the heart. It is in the membranous section of the septum where many of the septal defects occur.
Externally, the interventricular septum can be located, as it spans between the anterior and posterior interventricular grooves.
Left Ventricle
The left ventricle receives oxygenated blood from the left atrium and expels it into the aorta. It is located anteriorly to the left atrium, contributing to the anterior aspect of the heart, and forming the apex.
To pump blood into the aorta, and around the body, the ventricle must create a high pressure. As such, it has thicker muscular walls than the right ventricle.
In the left ventricle there are two papillary muscles, which are larger than those found in the right. They are attached to the chordae tendinae, which in turn attach to the mitral valve.
The interior surface of the left ventricle contains trabeculae carnea, in a similar structure to that of the right.
The outflow part of the left ventricle is known as the aortic vestibule. It is smooth-walled, and a derivative of the embryonic bulbus cordis.
The valves of the heart are structures which ensure blood flows in only one direction. They are made up of connective tissue and endocardium (the inner layer of the heart).
There are four valves of the heart, which are divided into two categories:
Atrioventricular valves are located between the atria and the ventricles. They close during relaxation of the atria and contraction of the ventricles.
Semilunar valves are located between the ventricles and the outflow vessels. They close during relaxation of the ventricle and elastic recoil of the outflow vessel. Atrioventricular Valves
The two atrioventricular valves are tricuspid and the mitral valves. They are closed by contraction of small muscles, known as papillary muscles. The papillary muscles are attached to the valves by chordae tendineae (cord-like tendons).
The chordae tendineae can withstand a very high tensile stress and prevent the valve inverting.
The closure of the atrioventricular valves is responsible for the first heart sound.
Tricuspid Valve
The tricuspid valve is located between the right atrium and the right ventricle. The tricuspid valve has three cusps (cusps are ‘flaps’ which come together to shut the valve). These cusps are attached to the fibrous ring of the skeleton of the heart.
The chordae tendinae are attached to the apex of the valves. This valve opens during diastole and closes during systole.
Mitral Valve
The mitral valve is located between the left atrium and the left ventricle. This valve has two cusps (known as the anterior and posterior cusps) and is occasionally known as the bicuspid valve because of this. The mitral valve opens during diastole and closes during systole.
Semilunar Valves
The two semilunar valves are the pulmonary and aortic valves. They are not closed by papillary muscles. Instead, the backflow of blood collects in the cusps, pushing them together, and closing the valve.
The closure of the semilunar valves is responsible for the second heart sound.
Pulmonary Valve
The pulmonary valve is located between the right ventricle and the pulmonary artery. This valve has three cusps attached to a fibrous ring. The pulmonary valve opens during systole and closes during diastole.
When the pulmonary valve closes, it produces the second heart sound. The sound has a higher pitch, shorter duration and lower intensity than the first. In inspiration the second sound can split and closing of the pulmonary valve is delayed (known as P2). This is because blood is drawn into the thorax, resulting in an increased right ventricular pressure.
The aortic valve is located between the left ventricle and the aorta.
Around the aortic valve opening is a fibrous ring where the three cusps of the valve attach. The three cusps are named the posterior cusp, the right coronary cusp and the left coronary cusp
The openings to the coronary arteries are found in the aortic wall just above the right and left coronary cusps. During diastole, when the cusps fill, and the valve closes, blood can enter the coronary circulation via the left and right coronary arteries
During inspiration, the aortic valve can close early, forming a ‘split’ second heart sound. When air enters the lungs, there is a greater negative pressure in the thorax. This causes a decreased return of blood from the lungs. Thus, the aortic valve closes early, creating a split-second heart sound.
113. heart wall.
The heart wall itself can be divided into three distinct layers: the endocardium, myocardium, and epicardium.
In this article, we shall look at the anatomy and clinical relevance of these layers. Endocardium
The innermost layer of the cardiac wall is known as the endocardium. It lines the cavities and valves in the heart.
Structurally, the endocardium is made up of loose connective tissue and simple squamous epithelial tissue – it is similar in its composition to the endothelium which lines the inside of blood vessels.
In addition to lining the inside of the heart, the endocardium also regulates contractions and aids cardiac embryological development.
Subendocardial layer
The subendocardial layer lies between, and joins, the endocardium and the myocardium. It consists of a layer of loose fibrous tissue, containing the vessels and nerves of the conducting system of the heart. The purkinje fibres are located in this layer.
As the subendocardial layer houses the conducting system of the heart, damage to this layer can result in various arrhythmias.
Myocardium
The myocardium is comprised of cardiac muscle and is a striated involuntary muscle. The myocardium is responsible for contractions of the heart.
Subepicardial layer
The subepicardial layer lies between, and joins, the myocardium and the epicardium.
Epicardium
The epicardium is the outermost layer of the heart, and a layer of the pericardium. It is comprised of connective tissue and fat. The connective tissue secretes a small amount of lubricating fluid into the pericardial cavity.
In addition to the connective tissue and fat, the epicardium is lined by on its outer surface by simple squamous epithelial cells.
The cardiac skeleton , sometimes called the fibrous skeleton of the heart, is the structure of dense connective tissue in the heart that separates the atria from the ventricles. The cardiac skeleton consists of four dense bands of tough elastic tissue, called fibrous rings, that encircle the bases of the pulmonary trunk, aorta, and heart valves. The fibrous skeleton is composed primarily of collagen, a contractile protein. The right and left fibrous rings of the heart surround the atrioventricular (AV) node and arterial openings. The left fibrous ring is thicker and stronger than the right, to support the thicker and more contractile tissue of the left ventricle. The fibrous skeleton surrounds the valves of the heart as well, and separates the atria from the ventricles, with only the AV node and AV bundle having access to both the atria and ventricle.
While not a true skeleton, the fibrous skeleton provides structure and support for the heart and electrically isolates the atria from the ventricles. This electrical separation is essential for cardiac function, as it allows the AV node and AV bundle to delay the wave of depolarization, such that the atria can contract and assist in ventricular filling before the ventricles themselves depolarize and contract. The cardiac skeleton provides an attachment point for the muscle fibers of the atria and ventricles. In addition, it provides structural support to the four heart valves, allowing them to stay open, and is the attachment point for the tendons that support the valve leaflets.
114. conducting system
The pumping action of the heart muscle is controlled by a spontaneous electrical impulse, conducted around the heart by specialized cells. This article will look at the key structures involved in the generation and conduction of that electrical impulse.
Overview of Heart Conduction
The sequence of electrical events during one full contraction of the heart muscle:
An excitation signal (an action potential) is created by the sinoatrial (SA) node.
The wave of excitation spreads across the atria, causing them to contract.
Upon reaching the atrioventricular (AV) node, the signal is delayed.
It is then conducted into the bundle of His, down the interventricular septum.
The bundle of His and the Purkinje fibres spread the wave along the ventricles, causing them to contract.
Detailed below is the anatomy of the individual components involved in the conducting system.
Sinoatrial Node
The sinoatrial node is found in the crista terminalis, where the superior vena cava opens into the right atrium.
This node consists of a specialized collection of neurons. It generates electrical impulses, producing pacemaker action potentials. These spread across the atria of the heart, causing them to contract.
The SA node is sometimes referred to as the ‘primary pacemaker‘ – it is responsible for the initiation of the contraction of the myocardium. It therefore receives a rich neuronal innervation, allowing the CNS to control heart rate.
Atrioventricular Node
The atrioventricular node can be located within the interventricular septum, near the opening of the coronary sinus. It lies inside the triangle of Koch .
This node receives electrical impulses from the atria and allows them to spread to the ventricles. However, it delays the spread of these impulses, so the atria can contract fully and empty.
Bundle of His
The bundle of His is a direct continuation of the AV node.
It carries impulses from the node to the purkinje fibres, which then allow the wave to spread through the ventricles
To allow the conduction to spread to both sides of the heart, the bundle of His splits into two branches. These are the right and left bundle branches, which run along the interventricular septum.
The left bundle branch then divides into the left anterior and posterior fascicles.
Purkinje Fibers
The purkinje fibers are modified cardiac muscle fibers and have an accumulation of glycogen in the central portions of the cell. These fibers are located in the subendocardial layer of ventricular walls (see the Wall of the Heart). They arise from the bundle branches and fascicles from the bundle of His.
The purkinje fibers spread the electrical impulses into the ventricular myocardium.
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