08 Respiration and Circulation - part 12 - Heart

 

08 Respiration and Circulation - part 12 - Heart

Heart :

• Main pumping organ of the circulatory system. 
• Reddish brown in colour.
• Hollow, muscular organ, roughly the size of one’s fist. 
• Average weight is about 300gm in males and 250gm in females.
• Conical in shape and lies in mediastenum- i.e. the space between two lungs.
• Broader at upper end (base) and conical at lower end (apex). 
• Conical end is slightly tilted to left side and rests above the diaphragm.
• Heart is enclosed in a membranous sac called pericardium.
  
   
• Pericardium is formed of two main layers - 

1. Outer fibrous pericardium and 
2. Inner serous pericardium.

Serous pericardium:

• Soft, moist and elastic. 
• Formed of squamous epithelium.
• Further divisible into two layers as - 

1. Parietal layer and  
2. Visceral layer. 

• Parietal and viscerallayers of serous pericardium are separated by a pericardial space. 
• This space is filled with pericardial fluid (about 50ml)

Pericardial fluid

• Acts as a shock absorber .
• Protects the heart from mechanical injuries. 
• Keeps the heart moist and acts as lubricant.

Heart wall :

• Heart is mesodermal in origin. 
• Its wall is formed of three layers- 

1. Outer epicardium
2. Middle myocardium and 
3. Inner endocardium.

Epicardium  :

• Thin and formed of a single layer of flat squamous epithelium resting on basement membrane. 
•  protective in function

Myocardium :

• Middle thick layer formed of cardiac muscles.
• Responsible for contraction and relaxation of heart.

Endocardium :

• Single thin layer formed of squamous epithelium. 
• protective in function

External structure of heart :

• Human heart is four chambered.
• Two superior chambers are called atria (auricles) and inferior two are called ventricles. 
• Externally, the atria are separated from ventricles by a transverse groove called coronary sulcus or atrioventricular groove.
• Two ventricles are externally separated from each other by two grooves, the anterior and posterior inter-ventricular sulci. 
• Coronary arteries and coronary veins run through these sulci. 
• Pulmonary trunk arising from right ventricle and aorta from left ventricle are present on anterior surface of heart. 
• The pulmonary trunk bifurcates into right and left pulmonary arteries. 
• Aorta (systemic aorta) is divisible into three regions as - 

1. Ascending aorta
2. Systemic arch /aortic arch and 
3. Descending aorta. 

• The Ligamentum arteriosum joins pulmonary trunk and aortic arch. 
• It is the remnant of an embryonic duct called ductus arteriosus. 
• The aortic arch gives out three arteries viz -  
  

1. brachiocephalic (innominate) artery
2. Left common carotid and 
3. Left subclavian.

• The right atrium recieves superior and inferior vena cava along its dorsal surface.
• Pulmonary veins open into left atrium along the dorsal surface of heart.

Internal structure of heart:

Atria :

• Thin-walled receiving chambers of heart. 
• Separated from each other by inter-auricular septum. 
• Interauricular septum has an oval depression called fossa ovalis. 
• It is a remnant of the embryonic aperture called foramen ovalis.
• Superior vena cava (precaval), inferior vena cava (postcaval) and coronary sinus open into the right atrium. 
• Opening of the postcaval is guarded by a Eustachian valve.
• Thebesian valve guards the opening of coronary sinus into right atrium. 
• Four pulmonary veins open into the left atrium.
• These openings are without valves. 
• Both the atria open into the ventricles of their respective sides by atrioventricular apertures. 
• These openings are guarded by cuspid valves. 
• The tricuspid valve is present in the right AV aperture and bicuspid valve (mitral valve) is present in the left AV aperture.
• Heart valves help in maintaining a unidirectional flow of blood. 
• They also avoid back flow of blood.
  

Ventricles : 

• These are inferior, thick-walled pumping chambers of the heart. 
• The right  and left ventricles are separated by an interventricular septum. 
• Wall of the left ventricle is more muscular and about 3-times thicker than the right ventricle. 
• Inner surface of the ventricles shows several ridges called columnae carnae or trabeculae carnae.
• columnae carnae divide the lumen of ventricle into small pockets or fissures. 
• The lumen of ventricles also shows inelastic fibers called chordae tendinae. 
• The right ventricle opens into the pulmonary aorta and left ventricle opens into the aorta. 
• These openings are guarded by three semilunar valves each. 
• These valves prevent the backward flow of blood into the ventricles.

Functon of chordae tendinae :

• Attach the bicuspid and tricuspid valves to the ventricular wall (papillary muscles).
• Regulate their opening and closing.

Pumping action of heart :

• Main pumping organ of the circulatory system. 
• The pumping action is brought about by a rhythmic contraction and relaxation of the cardiac muscles or heart muscles. 
• Contraction of heart muscles is systole and relaxation of heart muscles is diastole.
• A single systole followed by diastole makes one heart beat. 
• The heart beats 70 to 72 times per minute. This is called heart rate.
• During each heart beat ventricles pump about 70 ml of blood this is called stroke volume.
• It means heart pumps about 72 (heart rate) x 70 ml (stroke volume) = 5040 ml ≈ 5 liters of blood per minute this is called cardiac output.

Conducting tissue of heart:

• The human heart is myogenic i.e. the heart is capable of generating a cardiac contraction independent of nervous input. 
• It also shows auto rhythmicity i.e. it can generate its own rhythm by specialized muscles. 
• A specialized cardiac musculature called the nodal tissue is distributed in the heart. 
• A part of this nodal tisse is present in the upper right corner of the right atrium. It is called SA Node or Sinoatrial node. 
• It lies at the base of opening of superior vena cava. 
• Another mass of nodal tissue, the modified muscular fibers also called autorhythmic fibers (conducting tissue) 
• autorhythmic fibers control the beating rate of heart. 
• Conducting (nodal) tissue consists of - 

1. SA node
2. AV node
3. Bundle of His and 
4. Purkinje fibers.

Conducting system of the heart :

SA node (sinu-atrial node) :

• Present in the right atrium. 
• Acts as pacemaker of heart because it has the power of generating a new wave of contraction and making the pace of contraction. 
• Passes the contraction to the left ventricle and also to the AV node.

AV node (atrio-ventricular node)  :

• Present in the right atrial wall near the base of interatrial septum. 
• Acts as pace setter of heart.

Bundle of His/ Tawara branches  :

• Start from AV node and pass through interventricular septum. 
• Forms two branches, the right and left bundles, one for each ventricle.
• These branches form network in ventricular walls and these are called Purkinje fibers.
• Spread impulses in ventricles

Purkinje fibers :

• Spread impulses in ventricles. 
• As a result both the ventricles contract simultaneously.