AIPMT-NEET Biology Aspirants, read out the next AIPMT-NEET Biology Study material/ Notes of Structure of human heart. In this we will learn about layers of heart wall and chambers of heart, important for AIPMT-NEET Biology.
Human circulatory system consists of heart, blood and blood vessels. Heart has unique properties to circulate /pump blood throughout the body and it works continuously without rest.
- Study of normal heart and the diseases associated with it is called as Cardiology (cardio= heart).
- Heart is a small muscular cone-shaped organ approx. the size of the closed fist. It measures about 12 cm in length, 9cm in width and 6cm in thickness. Heart weighs about 250 – 300 gm in adult female/male.
- Heart has mesodermal origin and it is located in mediastinum/ thoracic cavity resting on diaphragm near the midline of body but slightly tilted towards left side.
NOTE: Mediastinum is the region between (sternum – vertebral column), (first rib to diaphragm) and between both lungs. Mediatinum is the division of the thoracic cavity.
- Heart is surrounded by double- walled membranous bag called as Pericardium.It helps in protection of heart.
- Pericardium has 2 parts : Outer fibrous pericardium and Inner serous pericardium. Fibrous pericardium has dense irregular connective tissue. It prevents overstretching of heart and anchors heart in mediastinum. Serous pericardium again is divided into 2 layers around heart- outer Parietal layer and inner Visceral layer of serous pericardium. Parietal layer is fused to fibrous pericardium while visceral layer forms the external/outer layer of the heart wall called as Epicardium.
- Space between parietal and visceral layers of serous pericardium is called as Pericardial cavity and it enclose the fluid called as Pericardial fluid.
Wall of the heart has 3 layers : outer layer Epicardium, middle layer Myocardium and inner layer Endocardium.
- Epicardium (discussed above) is composed of mesothelium and adipose tissue layer that surround the heart. Adipose tissue has maximum thickness over ventricles. Epicardium also contains blood vessels that supply blood to the myocardium. Epicardium provides slippery and smooth texture to the heart.
- Myocardium is composed of cardiac muscle tissue and is responsible for pumping action of the heart. Myocardium makes upto 95% portion of all 3 layers of the heart wall.
- Endocardium is composed of endothelium and it forms the lining of the heart chambers and valves of the heart. It reduces surface friction as blood passes through heart.
NOTE: Inflammation of the pericardium is called as Pericarditis. Occur due to increase in pericardial fluid.
CHAMBERS OF HEART
Human heart has 4 chambers: Atria – 2 small upper/ superior receiving chambers (atria means entry halls) and Ventricles – 2 large lower/inferior pumping chambers.
Right atrium- Each atrium has pouch-like structure on its anterior surface called as auricle (resembles dog’s ear). Auricle helps to increase the blood holding capacity of atrium. So, right atrium has right auricle. Anterior wall of right atrium has muscular ridges called as pectinate muscles, while posterior wall is smooth.
- Right and left atria are separated by thin muscular wall called as Inter-atrial septum. This septum has an oval depression in it called as fossa ovalis (remnant of foramen ovale). Foramen ovale is an opening in the inter-atrial septum of fetal heart which normally closes soon after birth.
- Right auricle is separated from right ventricle by a septum made of thick fibrous tissue called as right atrio-ventricular septum. This septum has opening guarded by a valve called as tricuspid valve.
- Right atrium receives blood from 3 veins: superior vena cava, inferior vena cava and coronary sinus. Blood passes from right auricle to right ventricle through tricuspid valve, named so as it contains 3 cusps/muscular flaps. Tricuspid valve also called as right atrio-ventricular valve, as it is present on right atrio-ventricular septum.
Right ventricle- It forms major anterior surface of the heart. Internally it has series of muscular ridges called as trabeculae carneae, made up of cardiac muscle fibres. It helps in conduction system of the heart.
- Tendon-like cords called as chordae tendineae are connected to tricuspid valve on one side and papillary muscles (cone shaped trabeculae carneae) on other.
- Right and left ventricles are separated by the thick septum called as Inter-ventricular septum.
- Blood comes in right ventricle from right atrium through tricuspid valve and then passes into large artery called as pulmonary trunk/ pulmonary artery through pulmonary valve/or pulmonary semilunar valve. Pulmonary trunk further gets divided into right and left pulmonary arteries.
Left atrium – It forms the major portion of the base of the heart. Left atrium has left auricle in which pectinate muscles are present. Both anterior and posterior wall of left atrium are smooth.
- Left atrium receives blood from lungs by four pulmonary veins. Blood then passes to left ventricle through a valve present on left atrio-ventricular septum called as bicuspid valve (two cusps) or mitral valve (as it resembles bishop’s hat which is 2 sided). This valve is also called as left atrioventricular valve.
Left ventricle– It forms the apex of the heart and it is the thickest chamber of the heart.
- Left ventricle receives blood from left atrium through bicuspid valve and then it passes blood into ascending aorta through semilunar valve/or aortic semilunar valve.
- From ascending aorta, a branch arises called as coronary artery which supplies some blood to the heart wall.
- Remaining blood goes into arch of the aorta and descending aorta (thoracic aorta and abdominal aorta), which supplies blood throughout the human body.
- Like right ventricle, it also has trabeculae carneae, chordate tendineae and papillary muscles.
NOTE: 1) Veins always return blood to the heart, while arteries always take blood away from the heart.
2) Ductus arteriosus – it is a temporary blood vessel present during fetal life which shunts blood directly from pulmonary trunk into aorta. As a result only a small amount of blood enters into fetal lungs, which are non-functional before birth.
3) Ligamentum arteriosum – Ductus arteriosus closes soon after birth leaving a remnant called as ligamentum arteriosus. It connects aorta and pulmonary trunk.
4) Myocardium (middle layer of heart wall) thickness of both atria and ventricles depend on the function of each chamber. Atria have thin wall as they have to supply blood under less pressure to adjacent ventricles only. While ventricles have thicker walls than atria as they deliver blood over greater distances and under high pressure. Left ventricle has thicker wall (thickest of the four) than right ventricle as left ventricle has to pump blood throughout the body at high pressure while right ventricle pumps blood only to lungs .
5) Lumen of right ventricle is crescent shaped while that of left ventricle is circular in shape.
6) Valves of heart allows only one-way flow of blood, i.e., from the atria to the ventricles and from the ventricles to the pulmonary trunk or aorta. These valves prevent any backward flow of blood by closing as the blood let through them.