By Dr. Thomas Burnell and Dr. Bethany Turner
Next Lesson - The Cardiac Cycle
Core
The Heart
The Heart is a muscular organ in the thoracic cavity that pumps blood through vessels in the body. It consists of four chambers: two atria and two ventricles. The heart is split into a left and a right side, with each side having one atrium and one ventricle. The interventricular septum divides the heart into the left and right ventricles, and the interatrial septum divides the left and right atria.
- The right atrium receives deoxygenated blood from the body via the superior and inferior vena cava. The right atrium then pumps the blood into the right ventricle.
- The right ventricle receives blood from the right atrium and then pumps blood into the pulmonary trunk which then goes to the lungs.
- The left atrium receives oxygenated blood from the lungs via the four pulmonary veins and then pumps blood into the left ventricle.
- The left ventricle receives blood from the left atrium and then pumps blood into the aorta. This blood goes round the systemic circulation (rest of the body) and returns to the right atrium.
Diagram - Labelled structures of the heart. Note the direction of flow
Creative commons source by Wapcaplet [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
The atria are connected to the ventricles by the atrioventricular valves:
- On the right side is the tricuspid valve, formed from three leaflets.
- On the left side is the mitral valve, formed from two leaflets.
Chordae tendineae are connective tissue cords that attach the leaflets of the atrioventricular valves to the papillary muscles within the ventricles. They prevent the valves from prolapsing (turning inside-out allowing blood to flow the wrong way) into the atria during systole - when the ventricles are contracting and pressures inside become high. Papillary muscles found on the walls of the ventricles anchor the chordae tendineae to the walls to aid them in preventing prolapse.
The ventricles are connected to arteries by the semilunar valves:
- On the right side is the pulmonary valve, formed from three leaflets.
- On the left side is the aortic valve, formed from three leaflets.
Problems with the valves may cause turbulent blood flow which can be heard through a stethoscope as murmurs.
Diagram - The valves of the heart. Note the chordae tendineae and papillary muscles, whose roles are to prevent valvular prolapse
SimpleMed original by Dr. Bethany Turner
The pericardium is a fibrous sac which surrounds the heart and the great vessels (aorta and pulmonary trunk). It is composed of three layers:
- The fibrous layer is the outermost of these layers. It is tough and stiff.
- The serous layer consists of an outer parietal layer and inner visceral layer, both of which are made of mesothelium.
- The parietal layer is adhered to the fibrous layer
- The visceral layer is adhered to the myocardium (muscle of the heart). This layer is also known as the epicardium
- The pericardial cavity separates the two serous layers and contains a small amount of pericardial (serous) fluid which lubricates the pericardium to reduce friction.
Diagram - The layers of the Pericardium. The Pericardium acts as a "sac" around the heart, with the pericardial cavity acting as a slippery gap between the layers of the visceral and parietal pleura
Creative commons source by OpenStax College [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)] - Heart Wall
Creative commons source by Connexions [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)] - Serous Membrane
Images combined by Dr. Marcus Judge
Diagram - The structures of the human blood vessels. Note the thicker smooth muscle layer in the arteries to allow them to be resistance vessels and control blood pressure
Creative commons source by OpenStax College, edited by Dr. Thomas Burnell [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)]
Large blood vessels have three layers:
- Tunica intima - consists of endothelium and the internal elastic lamina.
- Tunica media - smooth muscle with elastic fibres. This layer is responsible for vasoconstriction and vasodilation.
- Tunica adventitia (also called externa) - connective tissue consisting of collagen and elastic fibres. It can anchor the vessel to nearby structures.
Arteries contain blood at a high blood pressure, while the veins contain blood at a low blood pressure. This difference is mirrored in the structure of the vessels:
- Arteries have a thicker muscular wall (tunica media) than veins.
- Veins have valves to prevent the backflow of blood.
- Arteries have a smaller lumen because they need to maintain a high blood pressure, however veins have a larger lumen.
Arteries becomes smaller as they travel towards the capillary beds. Capillaries are the smallest blood vessels.
Capillaries consist of a single layer of flattened endothelial cells surrounded by a basement membrane. Exchange of substances occurs via intercellular clefts and, in some capillary types, specialised pores called fenestrations.
In general there are three types of capillaries:
- Continuous - mostly uninterrupted layer of endothelium with some small gaps in between cells (intercellular cleft). The basement membrane is completely uninterrupted.
- Fenestrated - the endothelium contains a number of small holes (fenestrations) as well as intercellular clefts.
- Sinusoid - only found in liver, spleen and bone marrow. There are large intercellular gaps between the cells of the endothelium and the basement membrane is incomplete.
Diagram - The different types of capillary and their features
Creative commons source by OpenStax College [Public domain] [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
Capillaries converge to form veins which then travel towards the heart, becoming larger and able to carry more blood.
The left and right coronary arteries arise from the left and right aortic sinuses which are found on the wall of the aorta just above the leaflets of the aortic valves. The coronary arteries are the first branches of the aorta.
The right coronary artery comes around the heart to the posterior aspect where it becomes the posterior descending (in some people, the posterior descending artery arises from the left circumflex). On the way it gives off the sino-atrial nodal branch, and right marginal branch.
The left coronary artery branches into the left anterior descending and left circumflex arteries on the medial aspect of the heart. From here, the circumflex comes around to the back of the heart, and gives off the left marginal branch.
Different aspects of the heart are supplied by certain coronary arteries:
- Lateral aspect - circumflex artery
- Inferior aspect - right coronary
- Anterior aspect - left anterior descending
- Septum - left anterior descending
The coronary veins (great, middle and small coronary veins) drain into the coronary sinus which empties into the right atrium.
Diagram - The arterial supply to the heart
Creative commons source by Coronary.pdf: Patrick J. Lynch, medical illustratorderivative work [1]: Fred the Oyster (talk)adaption and further labeling: Mikael Häggström [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
Diagram - The venous supply to the heart
SimpleMed original by Dr. Bethany Turner
Blood is pumped out of the left ventricle into the aorta. The aorta ascends, forms an arch before descending into the abdomen. Three branches come off the arch: the brachiocephalic trunk, left common carotid artery and left subclavian artery which supply the head and upper limbs.
The brachiocephalic trunk further divides into the right subclavian artery (which supplies the right upper limb) and the right common carotid artery (which supplies the head).
The aorta descends into the thorax as the thoracic aorta then into the abdomen as the abdominal aorta. On its way it gives off branches to the organs. The aorta then bifurcates usually around the level of L4 into the left and right common iliac arteries which supply the lower limbs.
Venous blood returns to the heart via veins which follow a similar course to the arteries. Venous blood drains into the inferior vena cava and superior vena cava which empty into the right atrium.
Diagram - The major systemic arteries of the body
Creative commons source by OpenStax College [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)]
Diagram - The major systemic veins of the body
Creative commons source by OpenStax College [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)]
Edited by: Dr. Marcus Judge and Dr. Ben Appleby
In this article
Learn heart chambers, valves, coronary circulation and the cardiac conduction system with clear diagrams and a medical student quiz.
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