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During flow of blood through capillaries, some water soluble substances move out in the
space between cells of tissues. This fluid released out is called interstitial fluid or tissue fluid.
It is similar to the blood but has fewer blood proteins, less calcium and phosphorus and high
glucose concentration.
• It is a colourless fluid containing specialized lymphocytes that provide immune
response to body.
• Main function of lymph is to provide immunity, carry proteins and fats molecules and
transport oxygen, food materials, hormones etc.
Circulatory Pathways
All vertebrates have a muscular chambered heart. Fish - 2 chambered heart
Amphibian and Reptiles (except crocodile) - 3 chambered heart.
Crocodile, Birds and Mammals - 4 chambered heart.
Human Circulatory System - consists of 4 chambered muscular heart, closed branching
blood vessels and circulatory fluid blood.
Heart is the mesodermally derived muscular organ, present in thoracic cavity between the
two lungs protected by double membrane of pericardium.
• The upper two chamber is called atria and lower two chambers are called ventricles.
Interatrial septum separate the right and left atrium and thick walled inter ventricle
septum separate the ventricles.
• The opening between right atrium and right ventricle is guarded by a three muscular
flaps called tricuspid valve. Bicuspid or mitral valve guards the left atrium and
ventricle.
• The opening of right and left ventricle to pulmonary artery and aorta respectively is
controlled by semilunar valve.
• The nodal tissue present on upper right corner of right atrium is called SAN (sino-
atrial node) and those on lower left corner of right atrium is called AVN ( atrio•
ventricular node).
• The purkinje fibres along with right and left bundles form the bundle of HIS. The
nodal musculature has ability to generate action potential.
• SAN generate maximum number of action potential and is responsible for rhythmic
contraction of heart. Therefore it is called pace maker.
Cardiac Cycle
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