CIE Biology A-level - Mega Lecture

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Online Classes : w.megalecture.comCIE Biology A-levelTopic 8: Transport in mammalsNoteswww.pmt.education

Online Classes : w.megalecture.comCirculatory systems can either be open, for instance in insects, or closed, like in fish andmammals where the blood is confined to blood vessels only. Closed circulatory systemscome in two forms, either a single form which consists of a heart with two chambersmeaning the blood passes through the heart once for every circuit of the body, or double,where the heart has four chambers and blood passes through the heart twice for everycircuit of the body.Important structures and their functions Arteries – adapted to carrying blood away from the heart to the rest of the body,thick walled to withstand high blood pressure, contain elastic tissue which allowsthem to stretch and recoil thus smoothing blood flow, contain smooth muscle whichenables them to vary blood flow, lined with smooth endothelium to reduce frictionand ease flow of blood Arterioles – branch off arteries, have thinner and less muscular walls, their role is tofeed blood into capillaries Capillaries – smallest blood vessels, site of metabolic exchange, only one cell thickfor fast exchange of substances. They are adapted for efficient diffusion by having anarrow lumen, a large surface area, and a slow blood flow to allow more time forexchange. Venules – larger than capillaries but smaller than veins Veins – carry blood from the body to the heart, contain a wide lumen to maximumvolume of blood carried to the heart, thin walled as blood is under low pressure,contain valves to prevent backflow of blood, no pulse of blood meaning there’s littleelastic tissue or smooth muscle as there is no need for stretching and recoiling.Tissue FluidTissue fluid is a liquid containing dissolved oxygen and nutrients which serves as a meansof supplying the tissues with the essential solutes in exchange for waste products such ascarbon dioxide. Therefore, it enables exchange of substances between blood and cells.Hydrostatic pressure is created when blood is pumped along the arteries, into arteriolesand then capillaries. This pressure forces blood fluid out of the capillaries to form tissuefluid. Only substances which are small enough to escape through the gap in capillary arecomponents of the tissue fluid – this includes dissolved nutrients and oxygen.The fluid is also acted on by hydrostatic pressure which pushes some of the fluid back intothe capillaries. As both the tissue fluid and blood contain solutes, they have a negativewater potential. However, the potential of tissue fluid is less negative therefore meaningwww.pmt.education

Online Classes : w.megalecture.comthat water moves down the water potential gradient from the tissue fluid to the blood byosmosis.The remaining tissue fluid which is not pushed back into the capillaries is carried back viathe lymphatic system. The lymphatic system contains lymph fluid, similar in content totissue fluid. However, lymph fluid contains less oxygen and nutrients compared to tissuefluid, as its main purpose is to carry waste products. The lymph system also contains lymphnodes which filter out bacteria and foreign material from the fluid with the help oflymphocytes which destroy the invaders as part of the immune system defences.Mammalian heart and cardiac cycleDue to the heart’s ability to initiate its own contraction, it is referred to as myogenic. In thewall of the right atrium there is a region of specialised fibres called the sinoatrial nodewhich is the pacemaker of the heart, as it initiates a wave of electrical stimulation whichcauses the atria to contract at roughly the same time. The ventricles do not start contractinguntil the atria have finished due to thepresence of tissue at the base of theatria which is unable to conduct thewave of excitation. The electrical waveeventually reaches the atrioventricularnode located between the two atriawhich passes on the excitation toventricles, down the bundle of His tothe apex of the heart. The bundle ofHis branches into Purkyne fibres whichcarry the wave upwards. This causesthe ventricles to contract, thusemptying them.There are 3 stages of the cardiac cycle:1) Atrial systole – during atrialsystole the atria contract and thisforces the atrio-ventricular valves openand blood flows into the ventricles.2) Ventricular systole –contraction of the ventricles causesthe atrioventricular valves to close and semi-lunarvalves to open thus allowing blood to leave the left ventricle through the aorta andright ventricle through the pulmonary artery.Figure SEQ Figure \*ARABIC 1 Wikipedia Atrium3) Cardiac diastole – atria and ventricles relax, elastic recoil of the heart lowers thewww.pmt.education

Online Classes : w.megalecture.compressure inside the heart chambers and blood is drawn from the arteries and veinsthus causing semilunar valves in the aorta and pulmonary arteries to close,preventing backflow of blood.HaemoglobinHaemoglobin is a water soluble globular protein which consists of two beta polypeptidechains and a haem group. It carries oxygen in the blood as oxygen can bind to the haem(Fe2 ) group and oxygen is then released when required. Each molecule can carry fouroxygen molecules.The affinity of oxygen for haemoglobin varies depending on the partial pressure of oxygenwhich is a measure of oxygen concentration. The greater the concentration of dissolvedoxygen in cells the greater the partial pressure. Therefore, as partial pressure increases, theaffinity of haemoglobin for oxygen increases, that is oxygen binds to haemoglobin tightly.This occurs in the lungs in the process known as loading. During respiration, oxygen is usedup therefore the partial pressure decreases, decreasing the affinity of oxygen forhaemoglobin. As a result, oxygen is released in respiring tissues where it is needed. Afterthe unloading process, the haemoglobin returns to the lungs where it binds to oxygen again.Dissociation curves illustrate the change in haemoglobin saturation as partial pressurechanges. The saturation of haemoglobin is affected by its affinity for oxygen, therefore inthe case where partial pressure is high, haemoglobin has high affinity for oxygen and istherefore highly saturated, and vice versa.Carbonic anhydrase is an enzyme found in the blood. Its job is to help haemoglobindissociate from oxygen and bind to carbon dioxide to form carbaminohaemoglobin instead.Carbonic anhydrase catalyses a reaction between carbon dioxide and water to producecarbonic acid. Carbonic acid, when in a solution, releases hydrogen ions. When thesehydrogen ions combine with haemoglobin, haemoglobinic acid forms. This encouragesdissociation of oxygen from haemoglobin.Saturation can also have an effect on affinity, as after binding to the first oxygen molecule,the affinity of haemoglobin for oxygen increases due to a change in shape, thus making iteasier for the other oxygen molecules to bind.Fetal heamoglobin has a different affinity for oxygen compared to adult haemoglobin, as inneeds to be better at absorbing oxygen because by the time oxygen reaches the placenta,the oxygen saturation of the blood has decreased. Therefore, fetal haemoglobin must havea higher affinity for oxygen in order for the foetus to survive at low partial pressure.The affinity of haemoglobin for oxygen is also affected by the partial pressure of carbondioxide. Carbon dioxide is released by respiring cells which require oxygen for the processwww.pmt.education

Online Classes : w.megalecture.comto occur. Therefore, in the presence of carbon dioxide, the affinity of haemoglobin foroxygen decreases, thus causing it to be released. This is known as the Bohr effect.When at a high altitude, red blood cell count increases. This is because there are feweroxygen molecules, thus the partial pressure of oxygen decreases. Consequently, more redblood cells are made so that there is more haemoglobin for the oxygen to bind to.www.pmt.education

pressure inside the heart chambers and blood is drawn from the arteries and veins thus causing semilunar valves in the aorta and pulmonary arteries to close, preventing backflow of blood. Haemoglobin Haemoglobin is a water soluble globular protein which consists of two beta polypeptide chains and a haem group.It carries oxygen in the blood as oxygen can bind to the haem

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