Heart and circulation in vertebrates :
- Fishes: have 2 chambered hearts with one atrium and one ventricle.
- Amphibian and reptilian (except crocodile) has three chambered heart with two atria and one ventricle.
- Crocodiles, birds and mammals possesses a 4-chambered heart with two atria and two ventricles
- In fishes the two chambered heart pumped deoxygenated blood to the gills for oxygenation and then circulated to the body. (single circulation)
- In amphibians and reptilians the left atrium receives oxygenated blood from the lungs and right atrium receives deoxygenated blood from the body. Blood from the atria pumped into the ventricle from which the mixed blood pumped into the body. (Incomplete double circulation).
- In birds and mammals oxygenated and deoxygenated blood received by left and right atria respectively passed into ventricle of their side. The ventricles pump it out without any mixing up. (double circulation)
HUMAN CIRCULATORY SYSTEM :
Heart :
- Originated from embryonic mesoderm.
- Situated in the thoracic cavity, in between two lungs, slightly tilted towards left.
- It has the size of the clenched fist.
- Heart is covered by a double walled bag, pericardium.
- Our heat is four chambered, two relatively smaller upper chamber called atria and two lower larger chamber called ventricles.
- Two atria are separated by thin muscular wall called inter-atrial septum.
- A thick walled inter-ventricular septum separates two ventricles.
- Atrium and ventricle of same side is separated by a thick fibrous tissue called the atrio-ventricular septum.
- Each of atrio-ventricular septa is provided with an opening through which the atrium and ventricle of same side are connected, called atrio-ventricular opening.
- Right atrio-ventricular opening is guarded by tricuspid valve.
- Left atrio-ventricular opening is guarded by bicuspid or mitral valve.
- The right ventricle opens into systemic aorta and left ventricle opens into pulmonary aorta.
- Both the aorta is guarded by semilunar valves.
- The valves in the heart allow unidirectional flow of blood i.e. from atria to ventricles and from ventricles to their respective aorta.
Conducting system of human heart :
- The entire heart is made of cardiac muscles.
- The wall of the ventricle is much thicker than the atria.
- A patch of nodal tissue is present in the right upper corner of the right atrium called the Sino-atrial node(S A Node).
- Another nodal tissue present in the posterior to the inter-ventricular septum called A V Node (Atrio-ventricular node).
- A bundle of nodal fibres , atrio-ventricular bundle ( AV bundle) continued as A V bundle through the inter-ventricular septum and divided into right and left A V bundle, also called bundle of His.
- The bundle of His gives rise to profuse branches to the wall of the ventricles called perkinji fibres.
- S A node generates the force of contraction for auto rhythmicity of heart, hence called pace maker of the heart.
- Our heart normally beats 70-75 times in minutes (average of 72 beats per minutes).
Cardiac cycle :
- The cyclic events takes place in each heart beat is called one cardiac cycle.
- Lets starts with all the four chambers of heart are in a relaxed state i.e. in joint diastole.
- As the tricuspid and bicuspid valves are open, blood from the pulmonary veins and vena cava flows into the left and right ventricles respectively through left and right atria.
- Semilunar valves are closed at this stage.
- SAN generates the action potential which stimulates contraction of both atria, called atrial systole.
- This increases the blood flow from atria to their respective ventricles by 30 %.
- The action potential from SAN passed to AVN and then to perkinji fibres through AV bundles. This initiates ventricular systole. The atria undergo relaxation (diastole).
- During ventricular systole the intra-ventricular blood pressure increases that lead to closing of tricuspid and bicuspid valves leads to production of first heart sound called lub sound.
- Further increase in pressure leads to opening of semilunar valves.
- Oxygenated blood from the left atrium pumped into systemic aorta and deoxygenated blood from the right atrium pumped into the pulmonary aorta.
- Ventricular systole followed by ventricular diastole.
- Intra-ventricular blood pressure decreases leads to closing of semilunar valves causing second heart sound (dub).
- As the ventricular pressure declines further there is opening of bicuspid and tricuspid valves, blood from the atria flows into the ventricles freely.
- The ventricle and atria relaxed simultaneously called joint diastole.
- This sequential event in the heart which cyclically repeated called cardiac cycle.
- The heart beats 72 times per minutes.
- Each cardiac cycle takes 0.8 sec to complete.
- During a cardiac cycle the ventricles pumped 70 ml blood to the aorta called stroke volume.
- Stoke volume multiplied by heart rate (heart beat per min.) gives the cardiac output.
- Cardiac out put for human heart is 5000 ml.
