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1.4 The Heart

Arteries/Veins/Cardiac cycle

Red Blood Cells:

-No nucleus so have a large volume inside cell for storing oxygen.

-Biconcave shape for maximised surface area for exchange of gas.

-Filled with oxygen transporting pigment Hb or haemoglobin.

Simplified diagram:

RHS

RA

LA

LHS

RV

LV

Arteries, veins and capillaries

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Arteries: Move oxygenated blood away from the heart and to respiring tissue. Arteries are thicker walled, with elastic structures for stretch and recoil, and blood held under a high pressure (narrow lumen)

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Veins: Move deoxygenated blood from the respiring tissue to the heart, so that the blood can be pumped back to the lungs and be oxygenated. Veins carry blood at a lower pressure and therefore have valves to stop backflow of blood.

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Capillaries: The smallest type of blood vessel, capillaries are typically 1 cell thick and are therefore suitable sites for exchange to occur. Their lumen is big enough to accommodate the width of a red blood cell, but only just!

Main blood vessels in the heart:

-aorta: Main artery leaving the heart. Left ventricle powerful contraction causes high blood pressure in aorta as blood is squeezed around the body.

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-vena cava: Returns the deoxygenated blood from respiring tissues to the right atrium.

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-pulmonary artery: The only artery in the heart to contain deoxygenated blood, the pulmonary artery carries the deoxygenated blood from respiring tissue to the lungs to be oxygenated.

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-pulmonary vein: Returns the oxygenated blood from the lungs to the heart into the left atrium before a second contraction event pumps the blood through the aorta.

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-coronary artery: The artery which supplies the heart myogenic muscle with oxygen so it can respire/make ATP/contract.

The Cardiac Cycle:

1. The RA contains special tissue called the SA (sinoatrial) node which releases a wave of excitation, causing atria to contract simultaneously.

2. Pause before ventricles contract as they fill up with blood (septum at bottom of heart cannot conduct wave of excitation)

3. The wave of excitation passes to the AV node (atrioventricular) which causes the impulse to move along the purkinje fibres and eventually the bundle of His, causing the ventricles to contract.

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Diastole: The relaxation of heart tissue (i.e in cardiac diastole, where the heart relaxes and begins filling with blood.

Systole: The contraction of heart tissue (ventricular systole forces blood out of the aorta)

The coronary artery: This artery runs on the outside of the heart and supplies the myogenic heart muscle tissue with oxygen and glucose for respiration (contraction ATP demanding)

Tissue fluid formation:

-A high hydrostatic pressure is generated in the capillaries as the contraction of the ventricles occurs.

-This increased pressure squeezes components out of the blood such as glucose, oxygen and nutrients.

-This lowers the WP of the newly formed tissue fluid, so water moves back into the blood due to the osmotic gradient.

-Remaining tissue fluid drains into the lymph and is carried in the lymphatic system. The lymphatic system contains many lymphocytes, so clears out any pathogens.

-Lymph contains less oxygen and dissolved nutrients than tissue fluid.

Features of a circulatory system:

  • ​Atria (thin walled with elastic tissue for stretch and recoil) which stretch as they fill with blood.

  • Ventricle (thick, muscular wall for strong contraction and high pressure)

  • Left AV valve is bicuspid

  • Left AV valve is tricuspid

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