Structure of Heart

External Structure  :-

• Heart is situated at the ventral side of mediastinal space of thoracic cavity in between the lungs.

• Left lung has cardiac notch, heart is 5 x 3.5 inches in size, weight 300 gm.

• Its triangular superior-broad portion is tilted slightly towards right side.

• Its lower narrow portion is tilted towards left side.

• Heart is enclosed from all the sides by an envelope of two membranes (outer parietal made of white fibrous CT & inner visceral serous layer) called pericardial membrane.

• The narrow space in between these two membranes is called pericardial cavity.

• A fluid is present in this cavity it is called pericardial fluid. 

  Functions of pericardial fluid :-

    1. It prevents the heart from external jerks.

    2. It provides moisture to heart. It prevents the two membranes from collapsing.

    3. It prevents the heart from the bad effect of friction at the time of contraction. 

• The heart of rabbit or man is four chambered 2 auricles and 2 ventricles.

• It is pinkish in colour and conical in shape.

• The broad upper part of heart is called auricular part or base and lower conical part is called ventricular part (its tip is called apex). 

  a. Auricles -

      • Auricular part of heart is smaller and of dark colour.

     • Its walls are thin.

     • Therefore out of these two, right auricular surface is bigger than left auricle.

  b. Ventricles -

    • Ventricular part is broad, muscular and of light colour.

    • Ventricles have thicker walls than auricles.

    • It is oblique or tilted toward Right.

    • It does not reach till the tip or apex of the heart, so the right ventricle is smaller than the left ventricle. 

Internal Structure of heart :-

The internal structure of heart is discussed under following headings.

• Cut section of heart :-

The wall of heart is made of three layers. from outside inwards:

      I. Epicardium : Outermost layer, mesodermal in origin. Made simple squamous epithelium.

      II. Myocardium : Middle layer, thickest, mesodermal. Made of cardiac muscles. which are striated but involuntary.

      III. Endocardium : Innermost layer, endodermal in origin. Made of simple squamous epithelium. 

• Septum :

  a. Inter-auricular septum -

      > It is a partition between the left and right auricles.

      > It is shifted slightly towards left, so the Right auricle is slightly bigger than left.

  b. Inter-ventricular septum -

      > It is a partition dividing the right and left Ventricles.

      > It is shifted towards right.

      > So the left Ventricle is bigger than right.

  C. Auriculo Ventricular septum -

      > It separates the two auricles from the two ventricles.

      > It is shifted upwards towards Auricles. Therefore auricles are smaller than ventricles. 

• Fossa Ovalis :

      > An oval depression (Fossa Ovalis) is present on its Posterior part.

      > It is remnant of foramen ovale present in foetal stage which closes at birth.

      > In foetal circulation the lungs are non functional and by-passed so the blood directly reaches the Lt. Atrium from Rt. Atrium through foramen ovale. 

• Chambers :-

The mammalian heart has four chambers.

  1. Right Auricle or Atrium -

      > Inlets : It receives one S.V.C.,one I.V.C. and one opening of coronary sinus in man.

      > SVC (superior vena cava) = anterior vena cava = pre caval.

      > IVC = inferior vena cava posterior vena cava = post cavals.

      > The SVC & IVC bring impure blood from the upper and lower body parts respectively.

       > The Coronary sinus receives impure blood from the Rt. & Lt. Coronary veins and drains it in the right auricle.

      > Outlets : It is impure blood drains through the right AV foramen into the right ventricle.

  2. Right Ventricle -

      > Inlets : Receives impure blood through right AV foramen from auricle.

      > Outlets : Drains the impure blood into pulmonary artery through which it reaches lungs for oxygenation.

  3. Left Auricle -

      > Inlets - Receives oxygenated blood from lungs via pulmonary vein.

      > Outlet : This pure blood is drained into left ventricle through left AV foramen.

  4. Left Ventricle -

      > Inlets - Receives pure blood through left AV formen from left auricle.

      > Outlets :- Drains pure blood into the Aorta from where it is supplied to systemic organs.

• Valves :-

  ∆ Right Atrium-

      > All its inlets are guarded with valves to prevent backflow of the blood. The SVC opening is said to be guarded by Haversian valve.

      > The IVC which opens below this has its opening guarded by a valve called Eustachian valve.

      > The opening of coronary sinus in Rt. Atrium is guarded by Thebesian valve.

  ∆ Lt. Atrium -

      > At its inlet is pulmonary vein (four veins in man and two in rabbit), these have no guarding valve. 

• Walls :-

  ∆ Auricles :-

    > The inner wall surface here presents a series of transverse muscular ridges called musculi pectinati.

    > They run forwards and downwards towards AV foramen, giving appearance of the teeth of a comb (combed muscles). 

  ∆ Ventricles :-

    > The inner wall is rough due to presence of muscular ridges trabeculae carneae or columnae carnae. 

    > These continue as papillary muscles, whose one end is attached to the ventricular wall and the other end connected to the cusps of AV valves by chordae tendineae.

    > These chordae tendineae are collagenous and inelastic chords one end of which is inserted in the papillary muscles and other end is connected to the flaps of AV valves.

    > These are meant for preventing the pushing of flaps into atrium during ventricular contraction. 

   

• Conducting System of Human Heart :-

  ∆ Sinoatrial node (SA node) :-

    > It is known as the "pacemaker" of the heart.

    > Located in the wall of Rt. Atrium it generates impulses at the rate of about 72 per minute, initiates heartbeat.

  ∆ Internodal pathway :-

    > It is the network of neuromuscular pathway that connects the SA node to the AV node.

  ∆ Atrioventricular node (AV Node) :-

    > It is smaller than SA node and is situated in the atria near AV septum. It is capable of generating impulse at rate of about 40/mt. 

  ∆ Bundle of His (AV Bundle) :-

    > It is the connection between the atrial and ventricular musculature.

    > It begins at the AV node and then divides into left and right branches as it descends down towards ventricles.

  ∆ The Purkinje fibers  :-

    > These are distributed through the endocardium of the ventricles and propagate the impulse in the entire ventricular musculature. 

• Conducting System of Human Heart :-

    > The nodal musculature is autoexcitable.

    > All the heart muscle cells have the generate the electrical impulses (or action potentials) that trigger cardiac contraction, the SAN indicates it, simply because it generates the maximum number of action potentials, i.e. 70-75 min, and is responsible for initiating and maintaining the rhythmic contractile activity of the heart. Therefore it is called the pacemaker of the head.

    > Our heart normally beats 70-75 times in a minute (average 72 beats min). This is called heart rate. 

• Working of Heart :-

    > Heart beat is entirely controlled by nervous supply in arthropods and some annelids.

    > It is called neurogenic heart beat and the heart is called neurogenic heart.

    > In molluscs and vertebrates heart beat originates from a special muscular tissue.

    > Such a heartbeat is called myogenic heart beat and the heart is called myogenic heart. Human heart is myogenic.