Gaseous Exchange and Transport in Mammals PDF

Gaseous exchange and transport in mammals GASEOUS EXCHANGE SYSTEMS Cellular (internal) respiration is the process within cells th...

Gaseous exchange and transport in mammals GASEOUS EXCHANGE SYSTEMS Cellular (internal) respiration is the process within cells that generates usable energy in the form of ATP (spread 6.1). Mammals obtain most of their energy from the aerobic respiration of food molecules (chapter 6), although they can respire anaerobically for short periods. e Oxygen is essential for aerobic respiration. It is obtained from the air and is transported in the blood. e Carbon dioxide is produced as a waste product of respiration. It is cethe end of this spread you should be transported in the blood, and is breathed out into the air. able to: e A gaseous exchange system (sometimes called external respiration) is ¢ describe the main structures of the therefore needed to exchange oxygen and carbon dioxide between the mammalian respiratory system air and the blood. * explain the importance of lungs to « In mammals this gaseous exchange takes place in the lungs. mammals. e For gaseous exchange to happen there must be a continous flow of air =| Factoflife o> into and out of the lungs. This is called ventilation. It is achieved by the breathing movements of the chest. Although lungs are the main organs of gaseous exchange among terrestrial Why do mammals need a gaseous exchange system? vertebrates, and gills among aquatic All the cells of a mammal, even those deep in the centre of body, need a vertebrates, some vertebrates breathe through their skin. These continous supply of oxygen and must be able to eliminate carbon dioxide. include amphibians, sea snakes, Most animals have evolved as land animals. Their skin is adapted to and amphibious fish. They all live conserve water and is unsuitable as a surface for gaseous exchange. In in moist habitats and have skin that addition, mammals are too large to rely on simple diffusion to exchange is well supplied with blood vessels (highly vascularised). The common gases. It would take too long for oxygen to reach the central cells from rocky shore fish Blennius pholis, for the skin (see spreads 4.7 and 4.13). Mammals have evolved specialised example, can survive several hours internal organs — the lungs — to enable them to exchange gases but out of water as long as its body without losing too much water. surface is kept moist. Adult frogs usually breathe through their lungs The alveoli are the site of gaseous exchange and skin, but the South American The lungs consist of a system of tubes of ever-decreasing size (figure frog Telmatobius culeus, which lives permanently submerged in the waters 2) which end in microscopic bulbous sacs called alveoli. This is where of Lake Titacaca, breathes exclusively gaseous exchange takes place. The alveoli have the following adaptations through the skin. The surface area of for efficient gas exchange (spread 7.3): its skin is enlarged by loose folds. In mammals, little if any oxygen is taken e very thin walls up across the skin. However, some ¢ a moist inner surface bats can lose carbon dioxide through their skin. These bats have large, thin, ¢ a huge combined surface area hairless, and highly vascularised wing e arich blood supply - each alveolus is surrounded by capillaries. membranes which provide a large surface area (figure 1). The lungs are deep within the chest. This minimises water loss from the alveolar surface, but means that air must be moved in and out of the lungs so that the alveoli are in contact with a constantly changing supply of air (spread 7.2). The human respiratory system The human resiratory system is shown in figure 2. ¢ Air enters the human airway through either the nose or the mouth. Air entering via the nose cavity is filtered by hairs in the nasal passages, warmed by contact with the tissues in the nasal cavity, and moistened by cells in the mucous membrane of the nasal cavity. Mucous membranes Figure 1 The Australian ghost bat excretes carbon dioxide through its skin line much of the airway. These membranes contain globlet cells (figure as well as through its lungs. At 27°C, 2) which secrete mucus, a slimy material rich in glycoproteins. If an as much as 11.5 per cent of the carbon irritating substance is breathed in, this can stimulate a sneeze. The dioxide excreted by a bat can be lost through the skin, but this figure drops to irritant is ejected at speeds exceeding 160 km h!. 0.4 per cent at 18°C. 142 advanced BIOLOGY  |ASEOUS EXCHANGE ANSPORT IN MAMMAL Goblet cells — Nasal cavity secrete mucus Buccal cavity ‘ Mucus-filled Epiglottis : ; Pharynx goblet cell Glottis dip Ciliated epithlium Oesophagus Ciliated Larynx cells Trachea Ring of cartilage Bronchiole F Ribs Right bronchus Smooth Left lung muscle -— Bronchioles Ha Pleural cavity External Inner é pleural membrane intercostal (visceral pleura) muscles Outer pleural membrane (parietal pleura Internal i. intercostal Abdominal cavity muscles Flattened cuboid Diaphragm Alveoli epithelial cells Figure 2 The human respiratory system. The trachea, bronchi, and bronchioles contain smooth muscle which can contract to constrict the tubes. They also contain elastic fibres which allow the tubes to expand during inhalation and contribute to the lungs’ elastic recoil during exhalation. e Air entering the respiratory system through the mouth into the buccal cavity is not warmed or moistened as much as air passing through the nose, and it is not filtered at all. ¢ The nasal and buccal cavities lead into the pharynx, a tube that conducts both food and air. When food is swallowed a flap of tissue called the epiglottis closes over the glottis. This is a reflex action which prevents food from going down the trachea (windpipe). e The larynx is a box-shaped structure just above the trachea. The flow of air in and out of the respiratory system makes the vocal cords (folds of mucous membrane) vibrate, producing sounds. The sound is varied by changing the position and tension of the cords. e Air passes through the larynx into the trachea. This single tube forms the major airway. The trachea is held open by horseshoe-shaped rings QUICK CHECK of cartilage. Without them the trachea would collapse during breathing 1 List the structures through _ out, when the external atmospheric pressure is higher than the which air passes on its way pressure inside the trachea. The gaps in the cartilage rings allow the from the nose to the alveoli. trachea to be flexible so that food can pass easily down the oesophagus 2 Give two reasons why which runs behind the trachea. mamunals need lungs, rather * The trachea is lined with mucous membrane containing ciliated than exchanging gases epithelium. The epithelial cells have microscopic hair-like extensions through the skin. called cilia. These beat in a wave-like manner, moving mucus, dust, and microorganisms upwards and out of the lungs. Food for thought - ° The trachea subdivides into two main branches, the right and left Ventilation is a form of mass bronchus. The bronchi are narrower than the trachea but have a transport (also known as mass similar structure. Each bronchus divides repeatedly into smaller tubes flow) in which particles of called bronchioles. Larger bronchioles are lined by complete rings of matter are transferred in bulk cartilage which collapse quite easily. The smallest bronchioles have no from one place to another by cartilage (this enables them to constrict completely) and are lined with being carried in a fluid. Nicotine within cigarettes can stop cilia flattened cuboidal epithelium. Some gaseous exchange takes place through these bronchioles. from beating. Suggest how this can affect the ventilation of the © The bronchioles lead ultimately to numerous alveoli. This is where lungs. most of the gaseous exchange takes place. advanced BIOLOGY  VENTILATION By the end of this spread you should be In mammals, ventilation (breathing) is the flow of air into and out of able to: the lungs. It results in the mass transport of gases, and liquid or solid | © explain how air gets in and out of the particles suspended in air, to and from the respiratory exchange surfaces | lungs in the alveoli. An efficient ventilation mechanism is essential for survival. * define the lung volumes. The mechanism of ventilation Gases flow from regions of high pressure to regions of low pressure. Fact of life When the total gas pressure inside the alveoli is equal to the pressure ae STAT aene un ai of the surrounding atmosphere, no movement of gas is possible. For be — a Nae | inhalation (breathing in or inspiration) to occur, the gas pressure in the alveoli must be less than that in the atmosphere. For exhalation aoeoT (breathing out or expiration), the gas pressure in the alveoli must be ns greater than that in the atmosphere. Inhalation Humans inhale by enlarging the thoracic (chest) cavity, which enlarges the lungs as well. This reduces the gas pressure in the alveoli, creating a pressure gradient which draws air into the lungs. The expansion of the thoracic cavity is brought about by the combined movements of the rib cage (ribs and sternum) and diaphragm. The rib cage is moved by the two sets of intercostal muscles between the ribs (figure la). e¢ During inhalation the external intercostal muscles contract. This draws the rib cage upwards and outwards and causes the sternum to move outwards and forwards. ¢ The diaphragm also contracts: the central portion of this sheet of muscle moves downwards. The lungs cannot expand on their own. The inner pleural membrane, which covers the surface of the lungs, is closely linked to the outer pleural membrane, which lines the inside of the thorax. Only a thin layer of fluid separates the two. Expansion of the thorax therefore causes the lungs to expand. (a) Inhalation (b) Exhalation External intercostal Air in Vertebral Airin Sternum ~ Air out Internal intercostal Air out muscles move moves muscles move ribs Sternum ribs upwards upwards. downwards and | moves and outwards and inwards downwards forwards N and backwards contracts Diaphra pay (moves down) upwards ~ The volume of the thoracic cavity is The volume of the thoracic cavity is increased, drawing air into the lungs reduced, forcing the air out of the lungs Figure 1 Movements that ventilate the lungs. Inhalation requires considerable work. The active contraction of the intercostal muscles and the diaphragm has to provide enough force to overcome a series of resistances. These include: ¢ the recoil of elastic tissue of the lungs and thorax (their resistance to being stretched) * the frictional resistance of air as it passes through the hundreds of thousands of small bronchioles leading into the alveoli ° the resistance created by surface tension at the fluid-gas interfaces in the alveoli. 114 advanced BIOLOGY  EOUS EXCHANGEA SPORT IN MAMMA Arm and pen attached to Revolving drum (k the chamber lid g erain eymegraph) on which a trace is drawn as 5.0 the lid moves up and down I Air-tight chamber consisting of a Inspiratory Inspiratory Vital reserve volume perspex lid floating on water. The 4.0— capacity chamber is filled with oxygen at capacity the beginning of the experiment. Tidal xs SHOES volume _ o Expiratory ‘S capacity 2 Expiratory reserve volume capacity S$ 20- lum - io>) = =) _ onal’ 1.05 Residual volume ¥ Person breathes in and out of Soda lime canister removes carbon the air-tight chamber causing it dioxide from the exhaled air 0 to move up and down Time Figure 2 A spirometer records the volume of air breathed Figure 3 Typical human lung volumes measured with a in and out. spirometer. Exhalation Higher centres in forebrain Exhalation is usually a relatively passive process (figure 1b). Enlargement joey Respiratory of the thorax during inhalation stretches the tissues of the thorax and Chemoreceptors x centre in sensitive to carbon hindbrain lungs, which recoil naturally during exhalation. The diaphragm also dioxide levels in the blood Nerves to returns to its resting position by elastic recoil. The muscles are contracted intercostal muscles actively during exhalation only during high rates of ventilation or when there is an obstruction in the airway. The internal intercostal muscles, which act in opposition to the external intercostal muscles, draw the rib Stretch receptors cage downwards and inwards. in the bronchi Phrenic nerve Lung volumes to diaphragm The volume of air inhaled and exhaled can be measured using a Figure 4 The regulation of ventilation spirometer (figures 2 and 3). in mammals. The respiratory centre receives information from various e The volume of air breathed in or out of the lungs per breath is called sources and sends impulses which the tidal volume. It is about 0.5 dm? at rest but varies with each cause the inhalation muscles to contract rhythmically. individual; it increases during exercise. ¢ The vital capacity is the maximum volume of air that can be forcibly Quick CHECK expired after a maximal intake of air. It varies between 3.5 and 6.0 dm? depending on the size and fitness of the person. 1 Which muscles cause the ribs to move during: e The volume of air remaining in the lungs at the end of a maximal expiration is called the residual volume (typically about 1.5 dm’). a inhalation Inhaled air mixes with residual air, keeping the levels of gases in the b exhalation? alveoli relatively constant. 2 What name is given to the ¢ In addition to these lung volumes, physiologists are also interested in volume of air inhaled or the dead space. This is the volume of air taken into the lungs which exhaled per breath? does not take part in gaseous exchange. Food for thought Regulation of the ventilation rate The trachea, bronchi, and their The rate at which a person breathes is called the ventilation rate, often branches are not especially _ expressed as the volume of air breathed per minute (i.e. the minute adapted for gaseous exchange. At ventilation): the end of exhalation these tubes Ventilation rate = tidal volume x number of breaths per minute contain ‘used’ air. The volume of this ‘used’ air, called the dead Changes in the circulation of blood and the ventilation rate ensure space, is about 0.15dm?. When that the blood always supplies sufficient oxygen to meet the needs of the a person inhales, this ‘used’ air tissues. During exercise, for example, breathing becomes both deeper and is drawn back into the lungs. faster, so that the ventilation rate increases — to as much as 200 dm? per Therefore, out of 0.5 dm} of air minute in a top-class athlete. inhaled, only about 0.35 dm? of Respiratory centres in the hindbrain (the medulla oblongata, see fresh air enters the lungs. The spread 10.13) control the rate and depth of breathing. They have an dead space thus makes up about intrinsic rhythmic activity which keeps the movements of ventilation one-third of the tidal volume at going automatically. The basic rhythm of breathing is modified by inputs rest. Suggest how the proportion from stretch receptors in the bronchi, other receptors sensitive to carbon of dead space to tidal volume dioxide levels in the blood, and higher centres of the brain. This ensures changes during exercise. that the ventilation rate meets the demands of specific situations (figure 4). advanced BIOLOGY 115  GASEOUS EXCHANGE IN THE ALVEOLI By the end of this spread you should be Minimising evaporation able to: Gaseous exchange (the diffusion of oxygen into the body and carbon — ¢ describe the structure and function of dioxide out of it) takes place mainly in the alveoli. Each alveolus is a tiny alveoli sac with a wall just one cell thick. Alveoli vary in diameter from * explain how they are adapted to 75 jim to 300 ym. Fluid from the cytoplasm passes through the cell maximise the rate of diffusion. surface membrane onto the surface of alveolar cells, keeping the alveoli moist. However, it is essential that these membranes do not dry out, otherwise gases would not be able to diffuse across them. Mammals minimise water loss by having their lungs inside the body. Here the alveoli can be kept moist without losing too much water during breathing (figure la). h J hi The details of diffusion Shoe rae “his SRS ¢ The blood is the transport medium that carries oxygen and carbon i od dioxide between the lungs and the body cells. These gases are exchanged between the air and the blood in the alveoli. ¢ Alveoli are in close contact with a vast network of blood capillaries Ee by chet eeeSe Pe ERY eee OAR ECbrosa teerie Leben stberretse (figure 1b); each alveolus has its own blood supply. ® Gases dissolve in the fluid on the cell surface membrane and diffuse Table 1 Composition of inspired through the thin walls of the alveolus and its neighbouring capillaries (atmospheric), alveolar, and expired air. into the blood. Oxygen enters the blood in this way. Carbon dioxide Note that expired air still contains quite a high proportion of oxygen, enough to leaves the blood and diffuses into the air in the alveolus. save someone’s life by mouth-to-mouth resuscitation. Composition of alveolar air The gas in alveoli does not have the same composition as atmospheric Percentage composition by volume air. Each breath brings a fresh supply of air into the lungs, which mixes with the gases already in the respiratory tract (the residual volume). In Gas Inspired Alveolar Expired normal quiet breathing, about 0.5 dm? of air is drawn into the lungs with air each breath. Of this, about 0.35 dm? reaches the alveoli and mixes with Oxygen 20.95 the residual volume of gas (about 1.5 dm?). It is this mixture of gases that Carbon 0.04 forms the alveolar air involved in gaseous exchange. Table 1 shows the dioxide Nitrogen 79.01 Figure 1 The structure and position of alveoli, and gaseous exchange by diffusion across the wall of an alveolus. By the time the blood leaves the alveolus, it is almost saturated with oxygen and has lost much of its carbon dioxide. (b) Structure of alveoli and blood supply (c) Diffusion across the surface of an alveolus Bronchiole Pulmonary artery Pulmonary vein Blood out High : in oxygen esas low in carbon dioxide Equilibrium reached between air and blood Respiratory surface y Gaseous (within lung) 7x exchange by // diffusion Oxygen combines with haemoglobin Blood in in red blood ceils High in carbon dioxide Body surface Alveoli Capillary network low in oxygen advanced BIOLOGY  3EOUS EXCHANG YSPORT IN MAMM, relative proportions of the main respiratory gases at various points of the respiratory tract. In comparison with alveolar air, the blood arriving in the lungs from the rest of the body has a relatively high concentration of carbon dioxide and a relatively low concentration of oxygen. Both gases diffuse down their concentration gradients and equalise between the blood and the air. The blood leaving the lungs therefore has a similar composition to that of expired air (figure Ic). Partial gas pressures Table 1 gives the percentage composition of gases. However, partial pressures are usually used to compare the proportions of gases in a mixture. The partial pressure of a gas in a mixture of gases is the pressure exerted by that gas. It is usually measured in kilopascals (kPa). For example, at sea level the total atmospheric pressure is 101.3 kPa. The atmosphere contains 21 per cent oxygen, which therefore has a partial Air exhaled pressure of 755 x 101.3 kPa, that is 21.3 kPa. The partial pressure of oxygen is abbreviated as p(O,). Water lining the inner When air is Maximising the rate of diffusion exhaled the surface of alveolus Diffusion between the blood and the alveoli obeys Fick’s law (see spread walls of the alveolus are 4.7). The rate of diffusion is maximised by a number of adaptations. drawn inwards ¢ Good ventilation and an efficient circulatory system maintain steep Hydrophobic concentration gradients. With each breath, air carrying carbon dioxide is expelled from the lungs and fresh supplies of oxygen are brought in. With each heart beat, blood high in oxygen is transported to all the body’s tissues and blood high in carbon dioxide is returned to the lungs. e The large number of alveoli provide a very large surface area. There are about 300 million alveoli in a pair of human lungs. Spread out they The surfactant would fill half a tennis court, about 50 times the total surface area of (phospholipids) the skin. prevents the alveolar Hydrophilic membranes from head immersed ¢ The alveolar and capillary walls are very thin and close together, sticking together in water minimising the diffusion path. The alveoli are lined by squamous Figure 2 Surfactant prevents the alveoli epithelium. The cells of this epithelium are flattened so the alveolar collapsing during exhalation. membrane is only about 0.2 pm thick. Capillary walls are also thin and the lumen is so narrow that red blood cells have to squeeze their way through. This brings haemoglobin (the protein that transports most of QUICK CHECK the oxygen around the body) very close to alveolar air. The combined 1 List the adaptations of effect of the large surface area of the lungs and the very thin alveolar that make them suitable for membranes allows very high rates of gaseous exchange by diffusion. gaseous exchange. Lung surfactant 2 If the aircontains 0.04 per cent carbon dioxide, calculate _ Alveoli must be kept open for their extensive surface area to be used for gaseous exchange. However, the inner surface of the-alveoli is lined with a watery film p(CO,). Assume that the total _ pressure ofthe air is 101.3 kPa. which exerts a surface tension. If there was nothing between this film and the alveolar air, the alveoli would collapse as the water molecules stick together, Food for thought : especially when air is exhaled. To prevent this, a mixture of phospholipid Suggest why each cubic molecules called lung surfactant occupies the space between the watery film on centimetre volume of a frog lung the inner surface of the alveoli and the air within them. Lung surfactant reduces has a total gaseous exchange the surface tension so that the alveoli remain open and available for gaseous surface of about 20 cm?, whereas exchange (figure 2). a cubic centimetre volume of Lung surfactant is especially important at birth when a baby’s lungs must inflate a mouse lung has a gaseous to take the first breath of its life. This first breath is 15 to 20 times more difficult to exchange surface of about take than subsequent breaths. However, once expanded the alveoli become lined 800 cm?. with surfactant, making breathing much easier. Premature babies may not have enough surfactant to keep their alveoli expanded. They may be given synthetic surfactant to help them breathe more easily. advanced BIOLOGY  THE CARDIOVASCULAR SYSTEM OB JE ie oe Ve ES | By the end of this spread you should be Why do mammals need a cardiovascular system? | able to: Mammals are relatively large and highly organised, with different body regions adapted to carry out different functions. For example, cells in the small intestine are adapted to absorb food; those in the alveolus are ie and a closed circulatory system adapted for gaseous exchange. These specialised regions depend on each ame ee a other: intestinal cells need oxygen acquired by the alveoli; the alveoli need |¢ discuss the structure and functions of nutrients acquired by the small intestine. This interdependence demands arteries, capillaries, and veins. an efficient transport system. Substances can move by diffusion across exchange surfaces (such peeee| Fact oflife pues as the gut and the alveolar walls), and in and out of individual cells. i William Harvey was physician to However, mammals are too large to rely solely on diffusion to move King Charles | of England. Harvey materials throughout their bodies. Over long distances substances |calculated that the heart beats more are transported more efficiently by mass flow: the bulk movement of than 1000 times in 30 minutes, and that each beat of the heart pumps substances from one area to another due to differences in pressure. The _ about 60g of blood. He concluded that blood vascular system provides the most important means of mass flow the body does not take in sufficient in mammals. Many substances are carried in the blood, which is pumped _ fluid to replace this blood, so it in special vessels continuously and fairly rapidly to all parts of the body. ' cannot be used up or lost from the body. Therefore the blood that leaves Open circulatory systems the heart must come back to it. He Some small animals have an open circulatory system in which blood ‘suggested that blood circulates the body in closed vessels. He stated: is not confined to vessels. For example, in arthropods blood flows freely _ ‘| frequently and seriously bethought over tissues, through spaces known collectively as the haemocoel. Blood me, and long revolved in my mind, flow is slow and at low pressure with little control over its distribution. what might be the quantity of blood... | began to think whether there might — Closed circulatory systems not be a motion, as it were, in a circle.’ Large mammals have a closed circulatory system, which consists of the heart, arteries, arterioles (narrow thin-walled arteries), capillaries, venules (small veins), and veins. Figure 2 shows the relationship between these vessels and compares their structures. e Arteries carry blood away from the heart. ° Capillaries are microscopic thin-walled structures made only of squamous endothelium (a layer of flattened epithelial cells); they form networks in various parts of the body. e Veins carry blood towards the heart. In a closed circulatory system, the blood can be at a much higher pressure than in an open circulatory system (important, for example, for ultrafiltration in the kidney) and there is much more control over its distribution. Vessels can be widened (vasodilation) or narrowed (vasoconstriction) by contraction of smooth muscle. This allows blood to be shunted to areas where it is needed. For example, during exercise blood is shunted from the intestine to respiring skeletal muscles. The double circulatory system of mammals Figure 1 William Harvey (1578-1657). Mammals have a double circulatory system (figure 3). This means that His skilful dissections combined with in each complete circuit of the body blood flows through the heart twice. accurate observations and careful reasoning led him to the idea that blood ¢ The pulmonary circulation transports blood between the heart and is enclosed in vessels and circulates the the lungs; the systemic circulation carries blood between the heart body — a closed circulatory system. and all other parts of the body. ¢ Each organ has a major artery which supplies it with blood and most have a major vein taking blood back to the heart. Exceptions are the stomach and small intestines. The blood from these organs is carried by the hepatic portal vein to the liver where the digested food molecules are processed. The blood is transported back to the heart by the hepatic vein. The single circulatory system of fish Fish have a single circulatory system — blood flows through the heart only once for each complete circuit of the body. Blood is pumped from advanced BIOLOGY  Blood pumped Blood flows Backflow of blood closes ____ y from the heart to the heart semi-lunar valve in vein ~ Upward pressure of blood forces valve open and blood flows towards the heart Capillaries Endothelium Collagen fibres (squamous endothelium) Collagen fibres Endothelium 7-10 um approx. Endothelium Se one om © Smooth muscle, Lumen elastic and Smooth muscle, Lumen fibrous tissues elastic and To rest of body fibrous tissues From rest of body ¢ Thick muscular wall ¢ No muscle ¢ Thin muscle wall ¢ More elastic than fibrous tissue ¢ No elastic or fibrous tissue e More fibrous than elastic allows distension of the artery ¢ Relatively large lumen tissue for protection and development of pulse wave ¢ Relatively large lumen ¢ Blood under low pressure ° Relatively small lumen ¢ No valves ¢ Blood under low pressure ¢ Blood under high pressure ¢ Semilunar valves maintain e Valves in aorta and pulmonary artery one-way flow of blood Figure 2 Schematic diagram comparing an artery, a capillary, and a vein. Jugular vein Carotid artery the heart to the gills, and then flows directly to the rest Head and neck of the body. The pressure drops dramatically as blood Subclavian vein Subclavian artery leaves the gills, so the blood flow to vital organs is Upper limbs both slower and at a lower pressure than in a double Pulmonary Pulmonary artery vein circulatory system. A single circulatory system would be unsuitable for mammals because their kidneys Heart Vena cava Right Left cannot function efficiently at low pressures. atrium atrium Right Left Moving the blood through the system ventricle ventricle As in any mass flow system, the blood vascular Hepatic vein Hepatic artery system transports fluid from high-pressure areas to Liver low-pressure areas — down pressure gradients. The Hepatic porta! vein Gastric and pressure gradients are produced in three main ways: Stomach and mesenteric arteries intestines ¢ by the pumping action of the heart Renal vein Renal artery * contractions of skeletal muscles squeeze blood along veins | Genital vein Genital artery © inspiratory movements of the thorax reduce the Gonads pressure inside the thoracic cavity, which helps to lliac vein : Hi Lower limbs Senay. draw blood back to the heart. Valves prevent backflow and ensure that the blood Figure 3 Schematic diagram of the mammalian double flows in one direction only. circulatory system. advanced BIOLOGY “LS  STRUCTURE OF THE HEART en NOns WeeMoe Ta WOES | } The mammalian heart is a remarkable organ. From birth to death it able to: | pumps blood continuously around the body. In a healthy person, the heart pumps continuously 24 hours a day without tiring. The heart of a ] ¢ describe the structure and function of a | mammalian heart. & | resting person pumps more than 10 000 dm of blood per day, enough to fill a small swimming pool. | j{ The pumping action of the heart is of vital importance as the circulation eet of blood supports all the living cells of the body. If the human brain is deprived of blood, an individual loses consciousness within three to five 5, pear. seconds; after 15 to 20 seconds the body begins to go into convulsions; after maar only a few minutes, parts of the brain may become permanently damaged. SES ce ba a" The human heart The human heart (figure 1) is about the size of a clenched fist and lies Eeaa in the chest cavity between the two lungs. It is encapsulated by a double eT es layer of tough inelastic membranes which form the pericardium. A fluid (the pericardial fluid) is secreted between the membranes allowing them to move easily over each other. The pericardium protects the heart from overexpansion caused by elastic recoil when it is beating very fast. The walls of the heart consist mainly of a special type of muscle called cardiac muscle (spread 11.3, figure 1). Cardiac muscle is found only in the heart. Unlike other muscle, it never fatigues. However, it does not tolerate lack of oxygen or lack of nutrients and soon dies if its supply of blood is cut off. (See spread 11.3 for a comparison of cardiac muscle with other types of muscle.) hd * Opie Lehlad Palieee a ~= The heart is divided into a left side and a right side separated by the septum. The septum becomes rigid just before the heart contracts, so that it serves as a fulcrum for the action of heart muscle. Each side of the heart has two chambers: an atrium which receives blood from the veins, Figure 1 The structure of the human heart. and a ventricle which pumps blood into the arteries. Pulmonary artery (to right lung) Aorta (main artery) Superior vena cava (main vein from upper body) —— =p; Pulmonary artery = (to left lung) — ie. Se Semi-lunar valves SL, > Se Left atrium Pulmonary veins Bicuspid valve Right atrium Tendinous cords Tricuspid valve Papillary muscle Inferior vena cava (main vein from lower body) Left ventricle Right ventricle Cardiac muscle Pericardial membranes (pericardium) 120 advanced BIOLOGY  OUS EXCHANGEAN SPORT IN MAMMALS Deoxygenated blood from the systemic veins enters the right atrium and is passed through the tricuspid valve into the right ventricle. This contracts and pumps blood through the pulmonary artery into the lungs. Oxygenated blood returns through the pulmonary vein into the left atrium and then, through the bicuspid valve, into the left ventricle. This has a very thick muscular wall which enables it to contract strongly and exert sufficient pressure to pump blood into the aorta and all the way around the body. The role of the valves The one-way flow of blood is maintained by valves. The tricuspid valve between the right atrium and right ventricle has three flaps; the bicuspid valve has two flaps. These valves allow blood to flow freely from atria to ventricles, but when the blood pressure in the ventricles exceeds that in the atria, the valves prevent backflow from ventricles to atria. A sound is produced at the precise moment the valves shut; this is the ‘lub’ of the ‘lub-dub’ sounds heard when a stethoscope is placed on the chest. Tendinous cords attached to special muscles (papillary muscles) prevent the valves from turning inside out. The papillary muscles do not move the valves, they just increase the tension of the tendinous cords so that they can resist the powerful back pressure of blood. Semi-lunar valves (half-moon shaped valves) prevent backflow in the pulmonary artery and dorsal aorta. Closure of these valves produces the ‘dub’ sound heard through a stethoscope. The venae cavae constrict during each heartbeat so that blood does not flow back into the veins. The heart’s blood supply Because the heart is a very active organ, it has a high demand for oxygen and nutrients. Some of the oxygenated blood leaving the left ventricle goes directly to the heart through the coronary arteries, of which there are usually three. These arteries branch out to supply the thick heart muscle with nutrients and oxygen (figure 2). Disease of these arteries can lead to them becoming blocked, resulting in a heart attack (see spread 16.7). Quick CHECK 1 Draw a flow diagram showing all the main structures through which a red blood cell moves on its passage from the superior vena cava to the dorsal aorta. 2 Which chamber of the heart has the most muscular wall? Give reasons for your answer. Food for thought Endurance athletes commonly have a condition known as ‘athlete’s heart’: the heart muscle becomes enlarged as a result of regular training; the volume ot the chamber of the left ventricle increases; and the muscular wall of the left ventricle becomes thicker. The condition is not detrimental to health because the blood supply to the heart is also improved. Suggest why an enlarged heart in an inactive person may indicate heart disease, such as a faulty valve. Figure 2 The coronary arteries supply nutrients and oxygen to the working heart muscle. advanced BIOLOGY ‘eeze. _ ‘ eacee I a ceeds DEE =: Seen  Left atrium Systole Left ; S ventricle | nal ystole , 165 = Pressure in aorta Semi-lun ‘ valves iy Seranuner ia 12 ak closed. | & = Pressure 5 8- in ventricle n n = "3 Bicuspid Figure 2 Pressure changes in the left | Bicuspid valve side of the heart during the cardiac 4 valve Open cycle. closed Pressure in Note that closure of the valves is a atrium passive process. It depends on the et \ relative pressures either side of the valve. For example, the atrioventricular e Pee eee ees valves (bicuspid and tricuspid valves) es close when the pressure in the Tima/sto j ventricles is higher than the pressure 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 in the atrium. They open when the Atrial systole — Ventricular systole Atrial and ventricular diastole pressuraisihighes(ouhe atuuatnena the ventricle. A typical ECG consists of characteristic waves which correspond to particular events in the cardiac cycle (figure 3). ¢ The P wave is caused by atrial systole. ¢ The ORS wave is caused by ventricular systole. ¢ The T wave coincides with ventricular diastole. ¢ The heart rate can be calculated from the interval between one P wave and the next. If disease disrupts the heart’s conduction system the ECG is changed. ECGs are therefore used to diagnose cardiovascular disease. The doctor can tell what has happened to the heart from the pattern of the ECG. R [SRM Sal OTST 1 sala THM are tc wis SS wrnagel 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Time/s Figure 3 An electrocardiogram (ECG) shows the electrical activity in the heart. This trace shows a single cardiac cycle in a healthy heart. 123 advanced BIOLOGY  THE CONTROL OF THE HEART By the end of this spread you should be Cardiac output able to: The amount of blood pumped around the body by the heart depends on * define cardiac output two factors: how quickly the heart is beating and the amount of blood it pumps out per beat. These two factors give the cardiac output, which is ¢ describe how to measure the defined as the product of stroke volume (the volume of blood leaving the pulse rate | left ventricle with each beat) and heart rate (number of heart beats per * explain how the action of the heart is minute). controlled. Cardiac output = stroke volume x heart rate In humans, typical values for a sedentary male at rest are a stroke volume of 75 cm? and a heart rate of 70 beats per minute. When exercising, both the stroke volume and the heart rate increase significantly. The heart rate also varies considerably from one individual to another. Males tend to have lower heart rates than females, and well trained endurance athletes tend to have very low resting heart rates, often below 40 beats per minute. An endurance athlete develops strong heart muscles and has a high stroke volume, so that the cardiac output at rest is hyinoue1).Thebeeiy painerea at Neco stimulates theheart about the same as that of a sedentary person with a higher heart rate. The heart rate is often measured by taking the pulse. In most circumstances pulse rate and heart rate are identical, but this is not so if the heartbeat becomes irregular. The pulse is actually a wave of pressure that passes along the arteries, causing them to expand and recoil rhythmically. The pulse can be felt in any artery that passes over a bone close to the body surface (figure 2). Figure 1 An artificial pacemaker fitted Figure 2 One of the best sites for checking the pulse is directly above the base of into the chest cavity. The pacemaker the thumb on the underside of the wrist. This is called the radial pulse because it is sends regular electrical pulses which where the radial artery passes alongside the radius. To count the pulse, press the help keep the heart beating regularly. fingertips of your middle and index fingers gently but firmly against the radial artery. Having a pacemaker fitted is one of the It usually takes a few seconds to become aware of the pulse, and a few seconds more most common types of heart surgery. to become sensitive to the rhythm. Once this is established, an accurate count can be In 2010 in England, more than 40000 made over a 10- or 15-second period, and the pulse rate per minute calculated. people had a pacemaker fitted. Control of cardiac output As cardiac output is the product of stroke volume and heart rate, a change in either of these will affect the amount of blood pumped out of the heart each minute. Although heart muscle has its own inherent rhythm, the heart rate is carefully regulated by the nervous and hormonal systems so that the cardiac output can adapt to the demands of a particular situation. Sensory receptors in the walls of the heart chambers and some blood vessels (e.g. the carotid and aortic sinuses) are sensitive to changes in blood pressure. These receptors convey information to the cardiorespiratory centre in the medulla oblongata at the base of the brain 124 advanced BIOLOGY (see chapter 10). A branch of the vagus nerve, part of the parasympatheti c nervous system, leads directly to the sinoatrial node. The vagus is an inhibitory nerve, and impulses from it slow the heart rate (figure 3). Vagus nerve (slows heart) fy From cardiovascular £4 centres in medulla oblongata of brain SA node Sympathetic nerve (pacemaker) (accelerates heart) Left atrium Right atrium Bundle of His AV node Left ventricle Purkyne tissue Right ventricle Figure 3 The cardiac output is modified by the parasympathetic and sympathetic nervous systems. Branches of the sympathetic nerve have the opposite effect on the heart: impulses from these nerves increase the heart rate. At times of excitement or danger, the sympathetic nervous system also stimulates the release of adrenaline from the adrenal glands. Adrenaline increases both the strength and speed of cardiac contractions (that is, it increases both the stroke volume and the heart rate). Cardiac output varies with the volume of blood returning to the heart (the venous return). When the venous return is high, the walls of the right atrium are stretched and the heart beats faster (the Bainbridge reflex). A high venous return also stretches the walls of the left ventricle, causing the ventricles to contract more strongly, giving a greater stroke volume (the Frank-Starling effect). These responses enable the heart to adjust the strength and rate of its contractions according to the volume of blood passing through it at any given time. 125 advanced BIOLOGY  BLOOD By the end of this spread you should be’| Blood makes up about one-twelfth of the body mass of a mammal. The CT average human has 4-6 dm of blood circulating around the body. Blood “list the main components of blood consists of cellular components suspended in a fluid called plasma. The cellular components are red blood cells, white blood cells, and platelets. |* discuss the functions of blood They can be separated from the plasma by centrifugation (figure 1).. describe the process of blood clotting. Red blood cells, white blood cells, and platelets The red blood cells or erythrocytes of most mammals are round discs, concave on each side (they are biconcave) and have no nucleus. The size varies between species: human red blood cells, for example, are about 8 jm in diameter. They are filled with a red protein called haemoglobin. The main function of haemoglobin is to carry oxygen (spread 7.9). Human red blood cells live for only about 120 days. During that time each cell will have made many thousands of journeys around the body. Because of their short life span, red blood cells have to be replaced continuously, mainly by cells in the bone marrow. Several different types of white blood cell (leukocytes) are involved in defence. Two types, neutrophils and monocytes, can move freely around the body, even penetrating deep into bone. These cells are also known as phagocytes because they engulf foreign substances and other cells such as bacteria. (For more detail on the structure and function of white blood cells, see spread 15.5.) Platelets are fragments of cells broken off from large cells in the bone marrow. They play an important role in blood clotting. Place blood » oeston intube p 4s Centrifuge | Cellular components 45% pacer eg Plasma 55% Cell type Functions Constituent Main functions Salts: Osmotic balance, Sodium buffering, control 2 of membrane core) permeability Calcium Leukocytes Defence and Magnesium (white blood cells) immunity Chloride Hydrogencarbonate Substances transported by blood Nutrients (e.g. glucose, fatty acids, vitamins) Waste products of metabolism Respiratory gases (O, and CO.) Hormones Figure 1 The major components of blood can be eee by centrifugation. P26 advanced BIOLOGY  US EXCHAN RT IN MAMM, Counting blood cells Haemocytometer _a—i The number of cellular components in each cubic millimetre of blood can be estimated using a special slide called a haemocytometer (figure 2). 5 The slide holds a known volume of a liquid within an area divided up into y if squares. The cellular components within these squares are counted, and A the total number of each component per cubic millimetre estimated. In each cubic millimetre of blood there are between 5 and 6 million red blood cells, 5000 to 10 000 white blood cells, and 250000 to 400000 platelets. Plasma and blood clotting The straw-coloured plasma is mainly water, but is slightly denser than pure water because it contains many dissolved substances. The plasma is Cells shown only in the main transport medium in the body. In addition to many chemicals, it one small square. By also transports heat from hot regions of the body to cooler regions. convention, A, B, C, and D are regarded Plasma proteins are involved in buffering (keeping body fluids at a as being within the square. E is regarded constant pH) and defence against injury and disease. Fibrinogen plays a as being in the next key role in blood clotting. Removal of this protein from plasma produces square, and is not counted. serum, which does not clot. Figure 2 A haemocytometer used to If a blood vessel is ruptured, it is vital to stem the flow of blood. Most estimate the number of blood cells and mammals can lose up to one-third of their blood without any long-lasting other cells. By knowing the depth of the liquid on the slide (0.1 mm) and the damage, but if a mammal loses more than half of its blood there is little area of each small square (0.0025 mm?) chance of survival. it is possible to estimate the density of the cells on a haemocytomeier. By Blood clotting minimises blood loss following injury. The blood convention, only cells in a square and coagulates to form a solid plug (clot) made of cells trapped in a fibrous touching its top or left-hand side are counted (that is, cells ABCD). Therefore network (figure 3). The clot prevents further blood loss, reduces the risk the density of the cells in the square is of pathogens (harmful microorganisms) entering the body, and provides 4 per 0.00025 mm? or 16000 per mm%. a framework for the repair of damaged tissue. Blood clotting involves a complex series of biochemical reactions. If a blood vessel is damaged, collagen fibres in the vessel wall become exposed to blood. Platelets stick rapidly to the exposed collagen fibres. The platelets release thromboplastins (clotting factors). These clotting factors, with the help of calcium and vitamin K, convert prothrombin (an inactive plasma protein) to thrombin (an active plasma protein). Thrombin acts as an enzyme, catalysing the conversion of soluble fibrinogen into insoluble fibrin. Fibrin forms the network of fibres that Electron micrograph showing red blood traps blood cells and debris to form the clot. cells trapped in a fibrous mesh. (x2000) Blood does not normally clot in intact blood vessels because of the action of a number of anticoagulants such as heparin circulating in the Platelets in contact with collagen fibres exposed in damaged tissue bloodstream. Intact endothelium (the inner lining of blood vessels) also produces molecules which inhibit clotting. Blood clots quickly when Thromboplastins (clotting factors) exposed to air because of the absence of an endothelium and a lack of anticoagulants. Calcium ions Vitamin K Prothrombin ». Thrombin (inactive) (active) en plasma and serum. _ mentioned in this spread which help to buffer the Fibrinogen », Fibrin (soluble) (insoluble) ‘fibrinogen? - Figure 3 Blood clotting: the clotting factors act in series, each factor being converted into its activated form which sell s but the red cells of all other lack nuclei, then activates the next facter in the uibians, reptiles, and birds) have nuclei, and pathway. This produces a cascade effect which has been called a ‘biochemical : : enificance of whether red blood cells have a amplification’ because an initial small owever, non-mammalian red blood cells are reaction leads to a large final reaction. ger than those of mammals. Suggest possible Lack of clotting factors can cause excessive bleeding, as in haemophilia ntages of red blood cells having a nucleus. A (caused by lack of clotting Factor VIII) and haemophilia B or Christmas disease (caused by a deficiency ofclotting Factor IX). advanced BIOLOGY sea A  HAEMOGLOBIN OB JueCe TU SES By the end of this spread you should be Carrying oxygen _ able to: Blood is the main transport medium for respiratory gases. Most of "+ describe the structure of haemoglobin the carbon dioxide excreted by cells is transported in solution as hydrogencarbonate ions (see below). Oxygen does not dissolve well in * explain how haemoglobin transports water and only a very small amount (no more than 20 cm? in humans) oxygen is carried in solution. Most of the oxygen supplying mammalian cells is * interpret oxygen dissociation curves carried around the body by haemoglobin. The total haemoglobin content | for haemoglobin. of blood is about 750 g, which is normally confined within red blood cells (figure 1). Fact oflife” MISC The structure of haemoglobin [ “Respiratory Shane are not confined e¢ Haemoglobin is a conjugated protein (figure 2). i. to the animal kingdom: leguminous e The protein part (called globin) consists of four polypeptide chains. plants such as beans have a red I: oxygen- binding protein pigment These chains are of two types called alpha and beta. They are about “resembling haemoglobin. It is called the same length (about 140 amino acids) but have slightly different lt leghaemoglobin and it occurs in root compositions. ier nodules where it takes up oxygen and i’“maintains an anaerobic environment e Each chain is combined with a non-protein prosthetic group called Kk for nitrogen-fixing bacteria, which are haem. Haem consists of an atom of iron enclosed in a ring structure. _anaerobic. e Each haem group can combine with one molecule of oxygen. This process is called oxygenation. (It is not the same as oxidation, because the iron does not lose any electrons and is not chemically oxidised.)