• The wall of the artery is thicker: thicker connective tissue layer, thicker mixed layer of muscle and elastic tissue.

• The lumen of the artery is much narrower.

• Arteries do not have valves along their length, veins do.

• Valves in the veins prevent the backflow of blood so the flow is in one correct direction towards the heart.

• Blood flows away from the heart in arteries; blood flows towards the heart in veins.

• Blood pressure in arteries is higher and so also the speed of blood flow.

• Pulse flow in an artery, steady flow in a vein.

Muscle tissue in not present in the capillaries

The muscle layer allows change in the diameter of vessels. This helps to regulate blood flow to and from the tissues in response to their requirement.

Capillaries

• The real work of the blood, exchange with tissue cells, is carried out at the capillaries.

• Capillaries are the microscopic links between arteries veins.

• The capillary wall is one cell thick and somewhat porous — ideal to allow materials to pass in and out.

• All tissue cells very close to a capillary so exchange is very efficient.

• Exchange at the capillaries is by diffusion, mass flow and active transport.

• Blood flow in capillaries is slow giving enough time for effective exchange.

Closed System of Blood Vessels

• 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.

Factors affecting heart rate

• Increase: exercise, increased body temperature, stress, mental excitement, infection.

• Decrease: increased physical fitness, sleep, and mental relaxation.

Coronary circulation

• The blood flowing through the heart does not directly serve the heart.

• Like all other organs the heart muscle has its own blood circuit.

• Two coronary arteries arise from the aorta just beyond its semilunar valve.

• The right coronary artery mostly serves the right atrium and right ventricle.

• The left coronary artery is much larger and supplies the left atrium and left ventricle.

• There is a very extensive capillary network throughout the cardiac muscle.

• The coronary veins collect the blood from the capillaries.

• The coronary veins deliver the deoxygenated blood to the right atrium.

Textbook Diagram: General Circulation and learn the names of the major blood vessels.

Pulse

• This is a wave of vibration that passes from the heart along a systemic artery.

• The vibration is caused by the forceful ejection of blood from the heart.

• The pulse wave travels much faster than the blood.

• The pulse rate is the same as the heart rate.

Blood Pressure

• The blood is pressing against the blood vessel walls.

• Pressure is how concentrated is the push.

• The pressure varies along the circuit – decreasing from artery to arteriole to capillary to venule to vein.

• Pressure is highest at the start of the artery and lowest at the entrance to the atrium.

• Blood pressure is much higher in the aorta than in the pulmonary artery.

• Reference blood pressure is measure in a large artery in the upper arm.

• The pressure need to stop blood flow in this artery is measured at diastole and systole.

• Standard healthy readings: 80 mm Hg diastolic, 120 mm Hg systolic.

Effect of Smoking

• Hardening of the arteries.

• Increased risk of heart disease.

• Raised blood pressure.

• Increased risk of stroke.

Effect of Diet

• Anaemia: lack of iron.

• High Blood Pressure: excessive salt.

• Low Blood Pressure: lack of protein.

Effect of Exercise

• Lower resting heart rate – more efficient heart.

• Dilated arteries – improved blood flow to all the organs.

• Less risk of heart disease.