HUMAN CIRCULATORY SYSTEM

Heart

Mesodermally derived organ

Present in the thoracic cavity between lungs, slightly tilted towards the left

Protected by Pericardium, encloses the pericardial fluid

Upper chambers - Atria & Lower chambers - Ventricle

Inter-ventricular septum

Thick-walled

Separates left and right ventricles

Inter-atrial septum

Thin walled

Separates right and left atrium

Atrio-ventricular septum

Thick and fibrous tissue

Separates the right atrium & ventricle & left atrium and ventricle

Mitral/Bicuspid Valve

2 cusps and guards the opening between left atrium and ventricle

Tricuspid Valve

3 cusps and guards the opening between right atrium and ventricle

Semilunar Valves

Guards the opening of pulmonary artery and aorta into the right and right & left ventricle respectively

Cardiac Cycle

Sino-Atrial Node (SAN)

Present in the upper-right corner of the right atrium

Atrio-ventricular Node (AVN)

Present in the lower left corner of the right atrium close to the Atrio-ventricular septum

AV bundle continues form AVN and branches into right and left AV bundle in the Atrio-ventricular septum

The right and left AV bundle give rise to many minute fibres throughout the ventricular musculature called Purkinje Fibres

The entire AV bundle along with the Purkinje fibres and right and left AV bundle is known as Bundle of His

The nodal musculature is auto-excitable, i.e., it generates action potential without any external stimuli

Natural Pacemaker

Generates 70-75 action potential per minute which is responsible for initiating & maintaining rhythmic contractile activity of the heart

1. Joint Diastole

  1. All chambers are in relaxed state
  1. As the mitral & tricuspid valves open, venous blood flows from anterior & posterior vena cava & pulmonary artery to the right & left atrium respectively
  1. The semilunar valves are closed at this stage

2. Atrial Systole

  1. Flow of blood into the atria increases the pressure
  1. SAN generates action potential and stimulates atria to undergo systole
  1. The semilunar valves open while the mitral and tricuspid valves close

3. Ventricular Systole

  1. Blood flows into ventricles, increasing the pressure
  1. Semilunar valves open due to the increase in pressure
  1. Mitral and tricuspid valves close to prevent backflow of blood to atria
  1. The action potential generated by SAN is transmitted through AVN and AV Bundle through the Purkinje Fibres, reaching the ventricles & stimulate them to undergo systole
  1. Then, the ventricular pressure decreases, opening the mitral and tricuspid valves
  1. Simultaneously, the ventricles undergo diastole
  1. Simultaneously, the atria undergo diastole

One cardiac cycle lasts for about 0.8 seconds

Stroke Volume: Volume of blood pumped out by each ventricle during a cardiac cycle. It is approx. 70mL of blood

Cardiac Output: Volume of blood pumped put by each ventricle per minute and averages 5L in a healthy individual

Sounds Produced during Cardiac cycle:

Lub: Closing of tricuspid and bicuspid valves

Dub: Closing of seimlunar valves

Electrocardiograph (ECG)

  1. P-wave: Represents depolarisation or electrical excitation of atria which leads to contraction of atria
  1. QRS complex: Represents depolarisation of ventricles which initiates ventricular contraction
  1. T-wave: Represents repolarisation of ventricles marking the end of systole

Structure of Blood Vessels

1. Tunica Intima

Innermost layer

Lining of squamous endothelium

2. Tunica Media

Middle layer

Layer of smooth muscle and elastic fibres

3. Tunica Externa

Layer of fibrous connective tissue with collagen fibres

Outermost layer

Comparatively thin in veins than arteries