The blood does not make direct contact with the tissue cells.
The blood is retained in the blood vessels.
A closed system is very responsive to the change needs of the organs and is highly efficient.
Double Circuit
Pulmonary Circuit: deoxygenated blood flows from the heart to the lungs, oxygen is taken on and carbon dioxide is excreted, oxygenated blood flows from the lungs back to the heart.
Systemic Circuit: oxygenated blood flows from the heart to the organ systems of the body, oxygen is delivered and carbon dioxide is taken on, deoxygenated blood flow from the organs systems back to the heart.
The double circuit does not allow mixing of oxygenated and deoxygenated blood. Therefore oxygen supply is highly efficient.
Portal System
A portal blood vessel has a set of capillaries at each end.
The blood flows from one set of capillaries along the portal vessel to the other set of capillaries.
The hepatic portal vein carries blood rich in absorbed nutrients from the capillaries in the alimentary canal to capillaries in the liver.
The Heart
Textbook Diagram: structure of the heart.
The heart is located in the thoracic cavity between the lungs protected by the rib cage.
The heart is a double pump.
The right side collects deoxygenated blood from all parts.
The right side pumps deoxygenated to the lungs for oxygenation and excretion of CO2.
The left side collects oxygenated blood from the lungs and pumps it to all parts.
The right and left side fill and empty in unison.
Each side pumps the same volume of blood.
The wall of the left ventricle is about three times thicker than that of the right ventricle.
The left ventricle needs more cardiac muscle to give the blood a much stronger push.
The oxygenated blood has to be driven a far greater distance.
Heart action
Deoxygenated blood from the inferior and superior vena cava flows into the right atrium and on into the right ventricle.
Oxygenated blood from the pulmonary veins flows into the left atrium and on into the left ventricle.
The right and left ventricles fill with blood.
Then the atria now fill with blood.
The pacemaker (sino-atrial node) in the right atrium generates a nerve impulse causing the atria to contract.
Contraction of the atria adds extra blood to the ventricles.
The transmission of the impulse to the ventricles is delayed giving time for them to receive the additional blood.
The impulse enters the ventricles at the atrio-ventricular node and travels through the septum to the ventricles.
The ventricles contract and force the blood towards the openings of the arteries, pulmonary artery and aorta.
The cuspid valves close (lub sound) preventing blood returning to the atria.
The semilunar valves are pushed open by the inrushing blood.
The elastic arteries expand quickly taking the blood.
When the ventricles stop contracting the arteries recoil elastically.
The elastic recoil squeezes the blood in the artery forcing it further away from the heart.
The closure of the semilunar valves prevents blood flowing back into the ventricles from the arteries.
The blood flows completely along the artery into the distant capillaries.
Cardiac cycle
Diastole: relaxed cardiac muscle — the heart fills with blood under low pressure from the veins.
Systole: cardiac muscle contracting — the chambers of the heart are emptying of blood.
Atrial Systole: contraction and emptying of the atria supplying extra blood to the ventricles.
Ventricular Systole: contraction and emptying of the ventricles ejecting blood from the heart into the arteries.
Cardiac Muscle
The muscle making up the heart is called cardiac muscle.
It is myogenic, i.e., stimulates itself to contract — contracts without any external stimulation.
It is an involuntary, strong muscle that does not fatigue.
The Pacemaker
It is a small area of cardiac muscle in the wall of the right atrium.
It lies close to the entry of the superior vena cava.
Its automatic rhythmic contraction starts each cardiac cycle.
Two nerves from the medulla oblongata connect to it influencing its rate of contraction.
One nerve quickly accelerates the heart rate and the other can quickly reduces it back to resting rate.
