Cambridge International AS & A Level Biology Coursebook PDF

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ity rs ve y op ni U C ge w ie id ev br am -R -C s es Chapter 8 y Pr op Transport in ity C rs w ie ve y ev op ni R U C mammals ge w ie id ev br am -R -C s es y Pr op ity C rs w ie ve y ev op ni R U C ge w ie id ev br am -R LEARNING INTENTIONS -C s In this chapter you will learn how to: es y describe the structure of the mammalian circulatory system Pr op explain how the structures of arteries, arterioles, veins, venules and capillaries are related to their functions ity C describe and explain the structure and functions of blood, including the transport of oxygen and carbon dioxide rs w ie make diagrams of blood vessels and blood cells from slides, photomicrographs or electron micrographs ve y ev describe the formation and functions of tissue fluid op ni explain the structure and function of the heart R U C describe the cardiac cycle and its control. e w g ie id ev br am -R -C s es Copyright Material - Review Only - Not for Redistribution  ity rs ve y 8 Transport in mammals op ni U C ge w BEFORE YOU START ie id In your group, make lists of: ev br am -R – the different kinds of blood vessel in the mammalian circulatory system – the different components of blood. -C s es Write two facts about each of the items in your lists – for example, what their function is, and how they y are adapted to carry it out. Pr op Be ready to share your ideas with others in the class. ity C rs w ie ve ARTIFICIAL HEARTS y ev op ni Each year, about 18 million people worldwide die R U from cardiovascular disease, more than from any C other disease. ‘Cardiovascular’ means to do with the ge w heart and the circulatory system, and many of these ie id deaths are due to the heart failing to work normally. ev br In many countries, medical help is available for am people with a failing heart, ranging from treatment -R with drugs to major heart surgery. But, until recently, -C the only hope for some heart patients was a heart s transplant. However, the number of people needing es a new heart is much greater than the number of y Pr op hearts available. Many people wait years for a heart transplant, and many die from their heart disease ity C before they get a new heart. rs w Petar Bilic (not his real name) thought that he was ie ve going to add to that statistic. The muscle in both y ev op ni of his ventricles had deteriorated so much that his Figure 8.1: An artificial heart. R U heart was only just keeping him alive. No suitable C heart could be found for a transplant. ge Biomedical engineers continue to make progress w Petar was very lucky. In recent years, biomedical in developing new types of heart that should work ie id engineers have developed a pumping device called for much longer – perhaps long enough for its ev br a ‘total artificial heart’ (Figure 8.1). Petar’s heart was owner to live out a long life without any need for a am heart transplant. And, while the first artificial hearts -R completely removed, and an artificial heart put in its place. Petar was able to go home within a few were designed to fit into the body of an adult man, -C weeks of his operation. The plan is that the artificial smaller ones are now available, which can be used s for women and children. es heart will keep him alive until a real heart is available y for transplant. Some patients have lived for almost Pr Question for discussion op 5 years with their artificial heart in place. However, living with an artificial heart is not easy. An artificial What do you think might be the advantages and ity C heart needs an energy supply, and this is often disadvantages of using an artificial heart rather than rs w provided with a battery that the patient carries in a a heart transplant, to treat a person whose own ie ve backpack. heart is failing? y ev op ni R U C e w g ie id ev br am -R 193 -C s es Copyright Material - Review Only - Not for Redistribution  ity rs ve y CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK op ni U C ge Key lungs 8.1 Transport systems w oxygenated blood ie id deoxygenated blood ev br in animals am RA right atrium pulmonary circulation -R LA left atrium Most animals are far more active than plants. They must move to find their food, because they cannot make their RV right ventricle -C RA LA s own. Movement requires energy, for example for muscle LV left ventricle es RV LV contraction and the transmission of nerve impulses. This y Pr systemic op energy comes from glucose and other substances, which are broken down in respiration inside each individual cell. circulation ity C The most efficient form of respiration – which releases rs w the most energy from a given amount of glucose – is rest of ie ve aerobic respiration, and this requires good supplies of y body ev oxygen. Supplying oxygen to respiring tissues is one of right side of body left side of body op ni the most important functions of an animal’s transport R U C system. At the same time, waste products such as carbon Figure 8.2: The general plan of the mammalian transport ge dioxide can be removed. system, viewed as though looking at someone facing you. It w is a closed double circulatory system. ie id Very small animals may be able to get enough oxygen ev br to their cells by diffusion, especially if they are not Blood is pumped out of the left ventricle into the aorta particularly active. In a jellyfish, for example, oxygen am -R (Figure 8.3), and travels from there to all parts of the simply diffuses into its body from the seawater around body except the lungs. It returns to the right side of it, and then to the respiring cells. Carbon dioxide -C s the heart in the vena cava. This is called the systemic diffuses in the opposite direction. Because no cell is very es circulation. far from the surface, each cell gets an adequate amount y Pr of oxygen quickly enough for its needs. The blood is then pumped out of the right ventricle into op the pulmonary arteries, which carry it to the lungs. The But in larger animals, such as mammals, diffusion is not ity C final part of the journey is along the pulmonary veins, sufficient. A transport system is needed to distribute rs w which return it to the left side of the heart. This is called oxygen quickly to all the body cells, and to remove their the pulmonary circulation. ie ve waste products. Mammals have greater requirements y ev for oxygen than most other animals because they use op ni respiration to generate heat inside their bodies, to help KEY WORDS R U C to keep their body temperature constant. circulatory system: a system that carries fluids ge w around an organism’s body ie id 8.2 The mammalian ev closed blood system: a circulatory system made br up of vessels containing blood am -R circulatory system double circulation: a circulatory system in which -C the blood passes through the heart twice on one s Figure 8.2 shows the general layout of the main es complete circuit of the body transport system of mammals – the blood system or y Pr systemic circulation: the part of the circulatory op circulatory system. It is made up of a pump – the heart – and a system of interconnecting tubes – the blood system that carries blood from the heart to all of ity C vessels. The blood always remains within these vessels, the body except the gas exchange surface, and rs then back to the heart w and so the system is known as a closed blood system. ie ve Put your finger onto the left ventricle in Figure 8.2. Use pulmonary circulation: the part of the circulatory y ev op ni your finger to follow the journey of the blood around system that carries blood from the heart to the body. You will find that the blood travels twice R the gas exchange surface and then back to U C through the heart on one complete ‘circuit’. This is the heart e w called a double circulation. g ie id ev br am -R 194 -C s es Copyright Material - Review Only - Not for Redistribution  ity rs ve y 8 Transport in mammals op ni U C ge The pressure in the systemic circulation is considerably gills w higher than in the pulmonary circulation. You can read ie id about blood pressure in the circulatory system later in ev br this chapter. am -R ventricle jugular vein carotid artery -C s subclavian vein subclavian artery es atrium y pulmonary aorta Pr op vein ity C rest of heart rs w body ie ve y ev pulmonary artery Figure 8.4: The general plan of the transport system of op ni vena cava a fish. R hepatic vein U lung C liver ge hepatic artery w hepatic kidney 8.3 Blood vessels ie id portal vein gut ev br renal artery There are three main types of vessel making up the renal vein am circulatory system. Figure 8.5 shows these vessels in -R mesenteric artery iliac vein transverse section. Vessels carrying blood away from the -C heart are known as arteries, while those carrying blood s iliac artery towards the heart are veins. Small arteries are called es arterioles, and small veins are venules. Linking arterioles y Pr op Figure 8.3: The positions of some of the main blood and venules, taking blood close to almost every cell in vessels in the human body. the body, are tiny vessels called capillaries. ity C rs w KEY WORDS Question ie ve y artery: vessel with thick, strong walls that carries ev op ni 1 Figure 8.4 shows the general layout of the high-pressure blood away from the heart R circulatory system of a fish. U C With a partner, discuss: vein: vessel with relatively thin walls that carries ge low-pressure blood back to the heart w a how this system differs from the circulatory ie id system of a mammal arteriole: small artery ev br b why the mammalian transport system may be venule: small vein am -R able to deliver more oxygen more quickly to capillary: the smallest blood vessel, whose role is the tissues than the fish’s transport system to deliver oxygen and nutrients to body tissues, -C s c how these differences could relate to the and to remove their waste products es different requirements of a fish and a mammal. y Pr Be ready to share your ideas with the rest of the class. op ity C rs w ie ve y ev op ni R U C e w g ie id ev br am -R 195 -C s es Copyright Material - Review Only - Not for Redistribution  ity rs ve y CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK op ni U C ge Transverse section (TS) through small artery w inner layer, which is the endothelium Arteries in different parts of the body ie id (a very smooth, single layer of cells – vary in their structure. Arteries near ev br squamous epithelium) the heart have especially large am numbers of elastic fibres in the middle -R relatively narrow lumen layer, as shown here. In other parts of the body, the middle layer contains middle layer, containing elastic fibres, less elastic tissue and more smooth -C s collagen fibres and smooth muscle muscle. es outer layer, containing collagen y Pr fibres and some elastic fibres op TS through capillary 7 μm ity C wall made of rs w TS through small vein endothelium, ie ve one cell thick y ev lumen, just big enough op ni for a red cell to squeeze R U inner later, like that along C of the artery ge w relatively large lumen ie id ev br middle layer, very thin, containing some smooth am 0.7 mm -R muscle and elastic fibres outer layer, mostly -C s collagen fibres es y Figure 8.5: The tissues making up the walls of arteries, capillaries and veins. Pr op ity C Arteries and arterioles KEY WORDS rs w The function of arteries is to transport blood, swiftly and endothelium: a tissue that lines the inner surface ie ve at high pressure, to the tissues. y of a structure such as a blood vessel ev op ni Artery walls are very strong and elastic. Blood leaving squamous epithelium: one or more layers of R U the heart is at a very high pressure. Blood pressure in the C thin, flat cells forming the lining of some hollow human aorta may be around 120 mmHg, which can also ge structures, e.g. blood vessels and alveoli w be stated as 16 kPa. The thickness and composition of ie id the artery wall enables it to withstand this pressure. smooth muscle: a type of muscle that can ev contract steadily over long periods of time br Both arteries and veins have walls made up of three am -R layers (Figures 8.5 and 8.6): IMPORTANT -C an inner layer, which is made up of a layer of s endothelium (lining tissue) consisting of a layer es of flat cells (squamous epithelium) fitting together Blood pressure is still measured in the old units y of mmHg, even though kPa is the SI unit. The Pr op like jigsaw pieces, plus a layer of elastic fibres; the endothelium is very smooth, minimising friction abbreviation mmHg stands for ‘millimetres of ity C with the moving blood mercury’, and refers to the distance which a column of mercury is pushed up the arm of a rs w a middle layer containing smooth muscle, collagen U-tube. 1 mmHg is equivalent to about 0.13 kPa. ie ve and elastic fibres y ev op ni an outer layer containing elastic fibres and collagen R U fibres. C e w g ie id ev br am -R 196 -C s es Copyright Material - Review Only - Not for Redistribution  ity rs ve y 8 Transport in mammals op ni U C ge w ie id ev br am -R -C s es y Pr op ity C rs w ie ve y ev op ni R U C ge w ie id ev br am -R Figure 8.6: Photomicrograph of an artery (left) and a vein (right) (×110). -C s es Arteries have the thickest walls of any blood vessel. are not entirely effective in achieving this: if you feel y The aorta, the largest artery, has an overall diameter your pulse in your wrist, you can feel the artery, even at Pr op of 2.5 cm close to the heart, and a wall thickness of this distance from your heart, being stretched outwards about 2 mm. The composition of the wall provides great with each surge of blood from the heart. ity C strength and resilience. The middle layer, which is by far As arteries divide into smaller vessels, the proportion rs w the thickest part of the wall, contains a large amount of of muscle in their walls increases and the proportion ie ve elastic fibres. These allow the wall to stretch as pulses of of elastic tissue decreases. They are now muscular y ev blood surge through at high pressure. Arteries further op ni arteries. Muscular arteries take blood from an elastic away from the heart have fewer elastic fibres in the R U artery and deliver it close to its final destination. The C middle layer but have more muscle fibres. type of muscle in their walls is smooth muscle, which is ge w Arteries that have a lot of elastic tissue in their middle able to contract slowly and steadily to alter the internal ie id layer – such as the aorta – are called elastic arteries. diameter of the artery and therefore control the volume ev br The function of an elastic artery is to carry blood from of blood that can flow through it. the heart on the first part of its journey towards its am -R Muscular arteries divide to form even smaller vessels final destination. The elasticity of these artery walls is called arterioles. These also contain a lot of smooth important in allowing them to stretch, which reduces -C muscle in their walls. Their narrowness provides s the likelihood that they will burst. This elasticity also es has another very important function. Blood is pumped y KEY WORDS Pr out of the heart in pulses, rushing out at high pressure op as the ventricles contract, and slowing as the ventricles elastic arteries: relatively large arteries, which ity C relax. The artery walls stretch as the high-pressure blood have a lot of elastic tissue and little muscle tissue surges into them, and then recoil inwards as the pressure rs w in their walls drops. Therefore, as blood at high pressure enters an ie ve artery, the artery becomes wider, reducing the pressure muscular arteries: arteries that are closer to the y ev op ni a little. As blood at lower pressure enters an artery, the final destination of the blood inside them than R U artery wall recoils inwards, giving the blood a small elastic arteries, with more smooth muscle in their C ‘push’ and raising the pressure a little. The overall effect walls which allows them to constrict and dilate e w is to ‘even out’ the flow of blood. However, the arteries g ie id ev br am -R 197 -C s es Copyright Material - Review Only - Not for Redistribution  ity rs ve y CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK op ni U C ge platelet red blood cell throughout every tissue in the body except the brain, w inner layer, cornea and cartilage. Such networks are sometimes ie id containing called capillary beds. ev br endothelial am The small size of capillaries is of great importance in -R cells allowing them to bring blood as close as possible to elasc fibres in each group of cells in the body. A human capillary is -C s the inner approximately 7 μm in diameter, about the same size as es layer a red blood cell (Figures 8.5, 8.8 and 8.9). The walls of y Pr middle layer, capillaries are extremely thin because they are made up op containing of a single layer of endothelial cells. As red blood cells ity C smooth carrying oxygen squeeze through a capillary, they are muscle ssue brought to within as little as 1 μm of the cells outside the rs w capillary that need the oxygen. ie ve outer layer, y ev op ni containing collagen R U C ge Figure 8.7: Transmission electron micrograph (TEM) of a w small artery. ie id ev br resistance to blood flow, causing it to slow down, which am -R provides extra time for exchange of gases and nutrients as the blood flows through the capillaries in the tissues. -C s The walls of arterioles have a nerve supply. Nerve es impulses from the brain can cause their smooth muscle y Pr op to contract, narrowing the arteriole. This is called vasoconstriction. This can be used to reduce blood flow ity C to a particular area and divert it to other tissues. When rs w the muscle relaxes, the diameter of the arteriole widens. red blood cell endothelial cell ie ve This is called vasodilation. The smooth muscle can also y ev respond to hormones in the blood. Figure 8.8: Photomicrograph of a blood capillary op ni containing red blood cells (dark red) (×900). The cells of R U C KEY WORDS the endothelium are very thin, except where there is a ge nucleus (red). w vasoconstriction: the narrowing of a muscular ie id artery or arteriole, caused by the contraction of In most capillaries, there are tiny gaps between the ev br the smooth muscle in its walls individual cells that form the endothelium. As you will see am -R vasodilation: the widening of a muscular artery later in this chapter, these gaps are important in allowing or arteriole, caused by the relaxation of the some components of the blood to seep through into the -C s smooth muscle in its walls spaces between the cells in all the tissues of the body. es By the time blood reaches the capillaries, it has already y Pr lost much of the pressure originally supplied to it by the op contraction of the ventricles. Blood pressure continues ity C Capillaries to drop as it passes through the capillaries. As blood rs enters a capillary from an arteriole, it may have a w The arterioles themselves continue to branch, eventually pressure of around 35 mmHg or 4.7 kPa; by the time it ie ve forming the tiniest of all blood vessels, capillaries. y reaches the far end of the capillary, the pressure will have ev The function of capillaries is to take blood as close as op ni dropped to around 10 mmHg or 1.3 kPa. possible to all cells, allowing rapid transfer of substances R U C between cells and blood. Capillaries form a network e w g ie id ev br am -R 198 -C s es Copyright Material - Review Only - Not for Redistribution  ity rs ve y 8 Transport in mammals op ni U C ge Question several leg muscles. Whenever you tense (contract) these w muscles, they squeeze inwards on the veins in your legs, ie id 2 Suggest why there are no blood capillaries in temporarily raising the pressure within them. ev br the cornea of the eye. How might the cornea be am This squeezing, in itself, would not help to push the -R supplied with oxygen and nutrients? blood back towards the heart; blood would just squidge up and down as you walked. To keep the blood flowing -C capillary wall, s nucleus of made of a single layer in the right direction, veins contain half-moon valves, es endothelial cell of endothelial cells or semilunar valves, formed from their endothelium y (Figure 8.10). These valves allow blood to move Pr op towards the heart, but not away from it. So, when ity C you contract your leg muscles, the blood in the veins is squeezed up through these valves, but cannot pass rs w down through them. ie ve y ev op ni to heart R U C ge w Pressure in ie the vein is semilunar valve, id produced preventing flow ev br by skeletal of blood away am -R muscles from heart contracting mitochondrion red blood cell -C close to it. s in endothelial in the lumen es cell of the capillary y Pr Figure 8.10: Longitudinal section (LS) through part of a op Figure 8.9: TEM of a transverse section (TS) through a small small vein. capillary (approximately ×4500). ity C rs w ie ve Veins and venules KEY WORD y ev op ni As blood leaves a capillary bed, the capillaries gradually semilunar valve: a half-moon shaped valve, R U join with one another, forming larger vessels called such as the ones in the veins and between the C ventricles and arteries ge venules. These join to form veins. The function of veins w is to return blood to the heart. ie id By the time blood enters a vein, its pressure has dropped ev br to a very low value. In humans, a typical value for Blood pressure in the am -R venous blood pressure is about 5 mmHg or less. This very low pressure means that there is no need for veins circulatory system -C to have thick walls. They have the same three layers as s es arteries, but the middle layer is much thinner and has You have seen how blood leaves the heart at high y far fewer elastic fibres and muscle fibres. Pr pressure, and then gradually loses this pressure as it op The low blood pressure in veins creates a problem: passes through muscular arteries, arterioles, capillaries, ity C how can this blood be returned to the heart? Think venules and veins. This happens in both systems – about how blood can return to your heart from your the systemic system and the pulmonary system. The rs w feet when you are standing up. Unaided, the blood in pressure of blood leaving the heart is much greater in ie ve your leg veins would sink and accumulate in your feet. y ev op ni However, many of the veins run within, or very close to, R U C e w g ie id ev br am -R 199 -C s es Copyright Material - Review Only - Not for Redistribution  ity rs ve y CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK op ni U C ge 120 w ie id ev 100 br Blood pressure / mmHg am -R 80 -C s es 60 y Pr op 40 ity C rs w 20 ie ve y ev 0 op ni aorta arteries arterioles capillaries venules veins venae pulmonary arterioles capillaries venules pulmonary cavae arteries veins R U C ge Systemic circulation Pulmonary circulation w ie id Figure 8.11: Blood pressure in different regions of the human circulatory system. ev br am -R the systemic system than in the pulmonary system. Who has written the best reasons? What These blood pressure changes are shown in Figure 8.11. makes these the best? -C s If you think your table can be improved, make es changes to it. Questions y Pr op 3 Suggest reasons for each of the following. ity C a Normal venous pressure in the feet is about 8.4 Tissue fluid rs w 25 mmHg. When a soldier stands motionless ie ve at attention, the blood pressure in his feet rises Blood is composed of cells floating in a pale yellow y ev very quickly to about 90 mmHg. liquid called plasma. Blood plasma is mostly water, with op ni a variety of substances dissolved in it. These solutes R b When you breathe in – that is, when the U C include nutrients such as glucose and waste products volume of the thorax increases – blood moves ge such as urea that are being transported from one place w through the veins towards the heart. to another in the body. Solutes also include protein ie id 4 Using the graph in Figure 8.11, describe and molecules, called plasma proteins, which remain in the ev br explain in your own words how blood pressure blood all the time. varies in different parts of the circulatory system. am -R 5 a Construct a table comparing the structure of KEY WORDS -C arteries, veins and capillaries. Include both s es similarities and differences, and give reasons plasma: the liquid component of blood, in which for the differences you describe. y the blood cells float; it carries a very large range Pr op b Compare your table with others. of different substances in solution ity C Are the headings of the rows and columns plasma proteins: a range of several different the same? If not, whose do you think are rs w proteins dissolved in the blood plasma, each with best, and why? ie their own function; many of them are made in ve y Has anyone else thought of a similarity or the liver ev op ni difference that you did not? R U C e w g ie id ev br am -R 200 -C s es Copyright Material - Review Only - Not for Redistribution  ity rs ve y 8 Transport in mammals op ni U C ge As blood flows through capillaries within tissues, some water moves from the capillaries into the tissue fluid w of the plasma leaks out through the gaps between the (Figure 8.12). ie id cells in the walls of the capillary, and flows gently into ev br At the venule end of a capillary bed, the blood pressure the spaces between the cells of the tissues. Almost am inside the capillaries is lower, so there is less tendency -R one-sixth of your body consists of spaces between your for water to be pushed out of the capillaries into the cells. These spaces are filled with this leaked plasma, tissue. The water potential gradient caused by the -C which is known as tissue fluid. s difference in the concentration of dissolved proteins es is still similar to that at the arteriole end. Now, the net y KEY WORD Pr movement of water is from the tissue fluid, back into op tissue fluid: the almost colourless fluid that fills the capillaries. ity C the spaces between body cells; it forms from the Overall, more fluid flows out of capillaries than into rs w fluid that leaks from blood capillaries them, so there is a net loss of fluid from the blood as it ie ve flows through a capillary bed. y ev op ni Tissue fluid is almost identical in composition to blood If blood pressure is too high, too much fluid is forced R U plasma. However, it contains far fewer protein molecules out of the capillaries and may accumulate in the tissues. C than blood plasma, because these are too large to escape This build-up of fluid is called oedema. One of the roles ge w easily through the capillary endothelium. Red blood of arterioles is to reduce the pressure of the blood that ie id cells are much too large to pass through, so tissue fluid enters the capillaries, in order to avoid this. ev br does not contain these, but some white blood cells can Tissue fluid forms the environment of each individual squeeze through and move around freely in tissue fluid. am -Rbody cell. Exchanges of materials between cells and The volume of fluid that leaves the capillary to form the blood occur through the tissue fluid. Within -C tissue fluid is the result of two opposing forces. At the your body, many processes take place to maintain s es arterial end of a capillary bed, the blood pressure inside the composition of tissue fluid at a constant level, to provide an optimum environment in which cells can y the capillary is enough to push fluid out into the tissue. Pr op However, there is a greater concentration of dissolved work. These processes contribute to the overall process proteins in the blood plasma than in the tissue fluid. of homeostasis – that is, the maintenance of a constant ity C This produces a water potential gradient from the tissue internal environment – and include the regulation of rs w fluid into the blood plasma (see Chapter 4, Section 4.5, glucose concentration, water, pH, metabolic wastes and ie ve Movement of substances across membranes). Overall, temperature. y ev op ni R U C cells ge w ie id arterial end of capillary blood venous end of capillary