3.2

• Circulatory systems can either be open, like in insects, or closed, like in fish and mammals where the blood is confined to blood vessels only.
• In the presence of carbon dioxide, the affinity of haemoglobin for oxygen decreases, causing oxygen to be released, a process known as the Bohr effect.
• Closed circulatory systems come in two forms: single, consisting of a heart with two chambers, and double, with four chambers.
• Important structures and their functions in circulatory systems include arteries, which are adapted to carrying blood away from the heart to the rest of the body, are thick walled to withstand high blood pressure, contain elastic tissue which allows them to stretch and recoil thus smoothing blood flow, contain smooth muscle which enables them to vary blood flow, are lined with smooth endothelium to reduce friction and ease flow of blood.
• Arterioles branch off arteries, have thinner and less muscular walls, and their role is to feed blood into capillaries.
• Capillaries are the smallest blood vessels, site of metabolic exchange, and are only one cell thick for fast exchange of substances.
• Veins carry blood from the body to the heart, contain a wide lumen to maximise volume of blood carried to the heart, are thin walled as blood is under low pressure, contain valves to prevent backflow of blood, and have no pulse of blood meaning there’s little elastic tissue or smooth muscle as there is no need for stretching and recoiling.
• Tissue fluid is a liquid containing dissolved oxygen and nutrients which serves as a means of supplying the tissues with the essential solutes in exchange for waste products such as carbon dioxide.
• Hydrostatic pressure is created when blood is pumped along the arteries, into arterioles and then capillaries, forcing blood fluid out of the capillaries.
• Only substances which are small enough to escape through the gaps in the capillary wall are components of the tissue fluid, including dissolved nutrients and oxygen.
• The fluid is referred to as tissue fluid, as described above.
• The fluid is also acted on by osmotic pressure which pushes some of the fluid back into the capillaries.
• Both the tissue fluid and blood contain solutes, they have a negative water potential, and the tissue fluid is positive in comparison to the blood.
• This causes water to move down the water potential gradient from the tissue fluid to the blood by osmosis.
• The remaining tissue fluid which is not pushed back into the capillaries is carried back via the lymphatic system.
• The lymphatic system contains lymph fluid, similar in content to tissue fluid, but lymph fluid contains less oxygen and nutrients compared to tissue fluid, as its main purpose is to carry waste products.
• Haemoglobin carries oxygen in the blood as oxygen can bind to the haem (Fe 2+) group and oxygen is then released when required.
• The atria do not start contracting until the atrio-ventricular node, located between the two atria, passes on the excitation to ventricles, down the bundle of His to the apex of the heart.
• Dissociation curves illustrate the change in haemoglobin saturation as partial pressure changes.
• Haemoglobin is a water soluble globular protein which consists of two alpha and two beta polypeptide chains each containing a haem group.
• Fetal haemoglobin has a different affinity for oxygen compared to adult haemoglobin, as it needs to be better at absorbing oxygen because by the time oxygen reaches the placenta, the oxygen saturation of the blood has decreased.
• Saturation of haemoglobin is affected by its affinity for oxygen, therefore in the case where partial pressure is high, haemoglobin has high affinity for oxygen and is therefore highly saturated, and vice versa.
• During respiration, oxygen is used up, therefore the partial pressure decreases, causing oxygen to be released in respiring tissues where it is needed.
• After the unloading process, the haemoglobin returns to the lungs where it binds to oxygen again.
• As partial pressure increases, the affinity of haemoglobin for oxygen increases, meaning oxygen binds to haemoglobin tightly.
• The sinoatrial node is the pacemaker of the heart, initiating a wave of electrical stimulation which causes the atria to contract at roughly the same time.
• After binding to the first oxygen molecule, the affinity of haemoglobin for oxygen increases due to a change in shape, making it easier for the other oxygen molecules to bind.
• The affinity of oxygen for haemoglobin varies depending on the partial pressure of oxygen, which is a measure of oxygen concentration.
• The affinity of haemoglobin for oxygen is also affected by the partial pressure of carbon dioxide, which is released by respiring cells that require oxygen for the process to occur.
• There are three stages of the cardiac cycle: Atrial systole, during which the atria contract and force the atrio-ventricular valves open, allowing blood to flow into the ventricles; Ventricular systole, when the contraction of the ventricles causes the atrio-ventricular valves to close, allowing blood to leave the left ventricle through the aorta and right ventricle through the pulmonary artery; and Cardiac diastole, when the atria and ventricles relax, elastic recoil of the heart lowers the pressure inside the heart chambers and blood is drawn from the arteries and veins, causing semilunar v
• The bundle of His branches into Purkyne fibres which carry the wave upwards, causing the ventricles to contract, thus emptying them.
• Each molecule of haemoglobin can carry four oxygen molecules.
• The lymph system also contains lymph nodes which filter out bacteria and foreign material from the fluid with the help of lymphocytes which destroy the invaders as part of the immune system defences.
• Due to the heart’s ability to initiate its own contraction, it is referred to as myogenic.