Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Module 3 3.1.1 Exchange Surfaces By Ms Cullen All living things need to obtain oxygen and nutrients, and excrete waste substances into the fluid environment which surrounds them. • For single-celled animals their fluid surroundings will be the habitat in which they live, usually fresh or salt water. • In a multicellular organism, single cells are surrounded by tissue fluid or extracellular fluid, with which they exchange materials. Single-celled Organisms • A small animal like an amoeba has a large surface area to volume ratio. • This means all parts of its body are in close contact with the external environment. • As a result it can exchange all the substances it needs through diffusion with its fluid environment. Small Multicellular Organisms • In small multicellular organisms the shape of the organism or parts of the organism also aids diffusion. • A good example of this is jellyfish. Inside their body they have a large cavity filled with water, which in effect doubles their surface area contact with the water. • Flat worms have a flattened shape to increase surface area. • Flowering plants have flattened leaves with large internal air spaces. Large Multicellular Organisms • Large multicellular organisms have a low surface area relative to their body volume. • Most large organisms also have waterproof and gas proof skin. • To aid diffusion they require specialised exchange surface areas. For example alveoli, villi. • The exchange of substances will be by either passive diffusion or active transport. Rate of Diffusion • Rate of diffusion depends on the concentration gradient of the internal and external environment, the exchange surface and the distance travelled. • It can be calculated by using Fick’s Law: Rate of diffusion = surface area x difference in concentration thickness of the exchange surface • The general rules are; the greater the surface area, the greater the concentration gradient, the thinner the separating layers, the faster the rate of diffusion. Complete Practical Activity 10 ‘The effect of SA:Vol ratio on rate of diffusion’ Villi in small intestine – large surface area aids absorption Alveoli in lungs Large surface area aids gaseous exchange The structure and function of a mammalian gaseous exchange system The pulmonary system (ventilation/breathing*) • Air is drawn into the lungs via the nasal and buccal cavities, which are separated by the palate to allow feeding and breathing at the same time. • Inspired air is warmed, particularly when inhaled via the nasal cavity. • Q - Why is this useful? The pulmonary system; upper • Nasal hairs trap dust particles and some microbes. • The epiglottis covers the TRACHEA when swallowing to prevent food from entering. • Q - What is the importance of these mechanisms? • The larynx contains vocal cords, which are adjusted as air passes over them to produce sounds. The upper pulmonary system The pulmonary system; thorax • The THORAX with its THORACIC CAVITY in humans is the area between the bottom of the neck and the DIAPHRAGM. • It contains the LUNGS and the heart (with associated structures), and some important membranes, all of which are protected by the RIB CAGE. • Q – Why is it important that the lungs are contained in an enclosed cavity? The thoracic cavity • Q – What major organs are contained in the thoracic cavity? • Q – Why is the heart situated in the thoracic cavity in close proximity to the lungs? The pulmonary system; full The pulmonary system; trachea • The TRACHEA is supported by C-shaped cartilage rings. This is important for keeping the airway open when thoracic pressure falls. • It is lined with CILIATED EPITHELIAL CELLS and MUCUS secreting GOBLET CELLS. • Q – What is the importance of mucus & cilia? A cross section through the trachea. The luman is lined with ciliated columnar epithelium with Goblet cells. Cartilage in the Trachea 1.Hyoid bone 2. Thyroid cartilage 3. Cricoid cartilage 4. Tracheal cartilages Cartilage is C-shaped The pulmonary system; ribs, intercostal muscles and diaphragm • The ribs protect the lungs and heart and have EXTERNAL and INTERNAL INTERCOSTAL muscles between them. • The external muscles contract to lift the RIB CAGE upwards and outwards during INSPIRATION (relaxing for EXPIRATION). • INTERNAL INTERCOSTAL MUSCLES aid expiration. • At the bottom of the thoracic cavity is a strong domeshaped MUSCULAR DIAPHRAGM, which can increase the volume of the thorax when contracted (pulled down/flat). Ventilation The pulmonary system; pleural membranes & cavity • The lungs are surrounded by PLEURAL MEMBRANES (pleurae). • Between the 2 membranes is a PLEURAL CAVITY, which contains pleural fluid and is kept at negative pressure so the lungs follow the movement of the rib cage. • The inner (visceral) pleura is attached to the lungs and the outer (parietal) pleura is attached to the wall of the chest The pulmonary system; pleural membranes & cavity • The pleural fluid lubricates the membranes so they can slide against each other with ease during ventilation allowing the lungs to move ‘friction-free’ against the wall of the thorax. • The pleural membranes also separate the lungs; so if one is punctured the other can still function. • Pleurisy (pleuritis) is a an inflammation, often from infection, of the pleural membranes. • It leads to painful breathing and disruption to the negative pressure system. Pleural membranes The pulmonary system; bronchi, bronchioles & alveoli • The trachea divides into 2 BRONCHI (singular = BRONCHUS), which are also held open, under low thoracic pressure, by rings of cartilage. • The bronchi divide into many BRONCHIOLES, which are less than 1mm thick and generally contain no cartilage. • Bronchioles terminate in air sacs called ALVEOLI, which are the site of GAS EXCHANGE. Bronchi and bronchioles are surrounded by a layer of smooth muscle, which is located between the cartilage and epithelium. This can contract limiting the amount of air to & from the alveoli. Alveoli Elastic fibres form the bulk of the connective tissue present in the walls of the alveoli. These dilate (widen) the airways. Alveoli (singular = alveolus) • Alveoli are highly specialised for gas exchange with adaptations that speed up the rate of DIFFUSION. • They have a LARGE SURFACE AREA. • They have an EXTREMELY THIN EXCHANGE SURFACE. • The epithelial layer is ONE CELL THICK. • There is a STEEP CONCENTRATION GRADIENT between their contents and their surrounding capillaries. Gas exchange at alveoli • Alveolar septal cells secrete a phospholipid SURFACTANT; lowering the surface tension of the water lining them. • This prevents alveolar collapse. • Oxygen diffuses across the alveolar epithelium then across the capillary endothelium and combines with the HAEMOGLOBIN of RED BLOOD CELLS. • Haemoglobin has a high affinity for oxygen thus making this process more efficient. Gas exchange at alveoli • The oxygen diffuses into the capillary down it’s concentration gradient. • Carbon dioxide diffuses from the blood plasma the opposite way down it’s concentration gradient and is breathed out. Gas exchange at alveoli • The alveoli have an extensive blood supply. • De-oxygenated blood is supplied to the lungs by branches of the PULMONARY ARTERY, which branch again into capillaries. • Oxygenated blood is carried via capillaries to branches of the PULMONARY VEIN from where it will be taken to the left atrium of the heart. • Can you remember the many ways the alveoli are adapted for gas exchange? Gas exchange at alveoli Comparison of inhaled and exhaled air (%) Inhaled Alveolar Exhaled O2 20.95 13.80 16.40 CO2 0.04 5.50 4.00 N2 79.01 80.70 79.60 H2O variable saturated Saturated Notes When inhaling & exhaling… Inspiration/ Inhalation External intercostal muscles Diaphragm Thorax volume Thorax pressure (pressure on lungs) Air movement Expiration/ exhalation Inhalation Expiration Hmmm…Interesting • A drop in O2 concentration in the blood has almost no effect on the rate of ventilation. It is usually changes in carbon dioxide concentration that affect ventilation rate. • If you are healthy it is not possible to stop breathing. You could hold your breath and become unconscious, but the body will resume breathing by itself. • Lungs also act as a shock absorber for the heart. • Lungs can filter out small blood clots formed in veins. Useful Websites: • http://www.smm.org/heart/lungs/vascular.ht m • http://www.footprintsscience.co.uk/alveoli.htm Measuring Lung Capacity The Breathing Cycle (measured by a spirometer) The Breathing Cycle (measured by a spirometer) • Tidal Volume: breathing rate normally at rest. A person usually takes in and expels ½ a litre of air during each respiratory cycle. • Ventilation Rate: the volume of air breathed per minute. ventilation rate = tidal volume x frequency of inspirations. During muscular exercise the frequency and depth of breathing increases resulting in a greater ventilation rate. • Inspiratory Rate: In a deep breath, you can take approx. 3 litres of air over and above the tidal volume. The Breathing Cycle (measured by a spirometer) • Expiratory Reserve Volume: Extra air expelled (approx 1 litre) after normal expiration. • Vital Capacity: Total amount of expired air after a maximum inspiration (ie. tidal volume plus inspiration and expiration reserve volumes). For an average person this is between 4-5 litres, an athlete over 6 litres. • Residual Volume: 1.5 litres of air remains in the lungs even after maximum respiration. • Of the ½ litre of air inspired only 350cm3 gets into the parts of the lungs where gaseous exchange takes place. The rest remains in the trachea and bronchioles, known as dead space. • Minute Volume (VE) is the product of tidal volume and the rate of ventilation. VE = ventilation rate (breaths per min) x tidal volume (cm3) • The minute volume is basically the amount of air breathed in and out per minute. It increases during exercise – the tidal volume increases first and then the ventilation rate increases. VC – Vital Capacity IRV – Inspiratory Reserve Volume TV – Tidal Volume ERV – Expiratory Reserve Volume IC – Inspiratory Volume RV – Residual Volume FRC – Functional Residual Capacity (volume of air that stays in lungs when breathing at rest) The Peak Flow • The Peak Flow is the maximum rate at which air flows out of the lungs. Usually 400-600dm3 per min. • Peak Flow varies with age, sex and size. The structure and function of a other gaseous exchange systems Gas exchange in bony fish • Fish have very specialised gaseous exchange surfaces because water is 1000x denser than air, 100x more viscous and has a much lower oxygen content. • The skin of bony fish is covered with impermeable scales and therefore gas exchange has to occur through gills. • Fish maintain a flow of water in one direction over the gills. • The gills are contained in a cavity with a covering operculum (bony flap) which helps maintain water flow over the gills. Gas exchange in bony fish • Each gill is supported by a bony gill arch and has 2 stacks of thin lamellae which lie on top of each other like pages in a book. • The lamellae need to be kept apart by water to prevent them sticking together. • The upper and lower surface of each lamellae have projections called gill plates, these increase the surface area further. Gas exchange in bony fish • Blood flows from an afferent vessel in each gill and back through an efferent vessel. • Water passes in the opposite direction to blood flow. This ensures blood meets water with a higher concentration of oxygen then it’s own. • The effect of this is that oxygen diffuses out of the water where it is in high concentration, into the blood, where it is in a low concentration. • This is known as the counter current mechanism. Gas exchange in bony fish • Bony fish ventilate their gills. • How? • Muscular contractions cause water to move into the buccal cavity through the mouth, the pharynx, over the gills and out through the valves of the opercular. • Most fish use muscles to change the volume of their buccal cavity, pharynx, gill cavity and opercular cavity. • As the volume of a cavity becomes less this increases the pressure and squeezes the water to where there is less pressure. Gas exchange in bony fish How are gills adapted? • Large surface area • Good blood supply • Thin layers Gas exchange in insects • Insects have a segmented body and a rigid exoskeleton. The exoskeleton is covered by a layer of wax which is impermeable to water and gases. • Instead gases diffuse through spiracles, these are small holes, which are located in the thorax and abdomen. • The spiracles lead to a network of large tubs called tracheae, which branch into smaller tubes called tracheoles. • These tracheoles grow between and even into the insect’s body cells. • The outer tracheae are covered in chitin which keep the tracheae open, but as a result they are impermeable to gases. • The tracheoles are permeable to gases as they are elongated single cells with no chitin. • Carbon dioxide diffuses from the cells to the tracheoles and oxygen diffuses from the tracheoles to the cells. Gas exchange in insects Gas exchange in insects Gas exchange in insects • Most gaseous exchange occurs by diffusion. • The diffusion distance must be short. The distance between the spiracle and the tracheoles must be short or the insect will die, this explains why insects are relatively small. • The tiny tracheoles provide a large surface area and gases dissolve in moisture on the walls of the tracheoles. At the end of the tracheoles there is tracheal fluid which limits the penetration of air for diffusion. • Insects can ventilate their tracheoles; by compressing their bodies they squeeze air from the tracheal tubes. This is then replaced with fresh air when the body returns to its normal size. • When insects are active, lactic acid accumulates in their muscles so that water moves by osmosis from the lining of the tracheoles into the muscle cells. • This movement of water increases the free volume within the tracheoles which, if the spiracles are open, will cause fresh air to move deeper into the tracheoles, as there is a larger surface area for diffusion to occur.