Why must we use our nose to breathe, instead of our mouth ? How does air get into our lungs ?

Chapter 20 Gaseous Exchange

§20.1 Respiratory Surfaces §All aerobic organisms must obtain regular supplies of oxygen from their environment and return to it the waste gas carbon dioxide. §The movement of these gases between the organism and its environment is called gaseous exchange. §Gaseous exchange always occurs by diffusion over part or all of the body surface - the respiratory surface.

Large organisms: use special respiratory structures, eg. Gills in fishSkin in frogLungs in mammals

In order to maintain the maximum possible rate of diffusion respiratory surfaces have a number of characteristics: §1. Large surface area to volume ratio This may be the body surface in small organisms or infoldings of the surface such as lungs, gills §2. Permeable §3. Thin - Diffusion is only efficient over very short distances, e.g. 1 mm §Rate of diffusion is inversely proportional to the square of the distance between the concentrations on the two sides of the respiratory surface.

§4. Moist - since oxygen and carbon dioxide diffuse in solution form §5.Efficient transport system - This is necessary to maintain a diffusion gradient and may involves a vascular system. § Diffusion is proportional to: §surface area x difference in concentration thickness of membrane §Organisms can obtain their gases from the air or from water.

TABLE 20.1: Water and Air as Respiratory Media PropertyWaterAir Oxygen content O2 Diffusion Rate Density Viscosity

TABLE 20.1: Water and Air as Respiratory Media PropertyWaterAir Oxygen contentLess than 1% O2 Diffusion Rate Density Viscosity

TABLE 20.1: Water and Air as Respiratory Media PropertyWaterAir Oxygen contentLess than 1%21% O2 Diffusion Rate Density Viscosity

TABLE 20.1: Water and Air as Respiratory Media PropertyWaterAir Oxygen contentLess than 1%21% O2 Diffusion RateLow Density Viscosity

TABLE 20.1: Water and Air as Respiratory Media PropertyWaterAir Oxygen contentLess than 1%21% O2 Diffusion RateLowHigh Density Viscosity

TABLE 20.1: Water and Air as Respiratory Media PropertyWaterAir Oxygen contentLess than 1%21% O2 Diffusion RateLowHigh DensityDensity of water>1000 than that of air Viscosity

TABLE 20.1: Water and Air as Respiratory Media PropertyWaterAir Oxygen contentLess than 1%21% O2 Diffusion RateLowHigh DensityDensity of water>1000 than that of air Viscosity Water much greater,about 1000 times than that of air

20.2 Mechanisms of Gaseous Exchange §As animals increase in size most of their cells are some distance from the surface and cannot receive adequate oxygen. §Many larger animals also have an increased metabolic rate which increases their oxygen demand. §These organisms need to develop specialized respiratory surfaces such as gills & lungs. §These surfaces allow gases to enter and leave the body more rapidly.

Small organisms §Small organisms have a large surface area to volume ratio and do not require specialized structures for gaseous exchange. §In amoeba, gases diffuse over their whole surface. Obelia have all their cells in contact with water.

§Platylelminthes rely on diffusion over the whole body surface. §All these organisms must live in water from which they obtain dissolved oxygen; they would rapidly desiccate in a terrestrial environment.

Flowering plants

§Plants have a low metabolic rate, requiring less energy per unit volume than animals. §Unicellular algae employ the whole body surface for gaseous exchange. In flowering plants : §Gases pass through stomata in leaves and green stems §Woody stems have lenticels

§Lenticels on woody tree trunk

§Within the plant oxygen diffuses through the intercellular air spaces & moist cell walls into the respiring cells; with carbon dioxide moves in the opposite direction §Rate of photosynthesis (producing oxygen; absorbing carbon dioxide) is affected by light intensity, §thus varying the amount of these gases during the day

Insects §Gases enter and leave through pores called spiracles. §Each spiracle is surrounded by hairs which help to retain water vapour and may be closed by muscular valves. §Respiring cells give out carbon dioxide which accumulates to stimulate the chemoreceptors to open the spiracles. §Spiracles open into tubes called tracheae which are supported by rings of chitin to prevent collapse.

§Tracheae divide to form smaller tracheoles extending right into the tissues. §The tracheal system carries oxygen rapidly to the cells and allows the insects to develop high metabolic rates. §The ends of the fine tracheoles are fluid-filled. §As activities increase, fluid will be drawn into the muscle cells to draw air further into the tracheoles in order to increase oxygen supply.

§The system is ventilated by contractions of the abdominal muscles of the insect flattening the body, §thus reduces the volume of the tracheal system. §The volume increases again by elasticity of the body and system to return to original shapes. §Larger insects, e.g. locusts, have some of the tracheae expanded to form air-sacs for blowing air in and out.

Limitations of the tracheal system: 1 Insects cannot attain a large size because it relies entirely on diffusion for the gases to move from the environment to the respiring cells. 2 The chitinous linings of the tracheae must be moulted before the rest of the exoskeleton Bony fish - not required in syllabus

§ Mammals

nasal cavity nose nostril pharynx epiglottis vocal cords larynx cartilage rings trachea bronchiole Intercostal muscle rib alveolus Inner pleural membrane outer pleural membrane pleural fluid left lung bronchus heart diaphragm Structures of the respiratory system:

nasal cavity nose nostril pharynx epiglottis vocal cords larynx cartilage rings trachea bronchiole Intercostal muscle rib alveolus Inner pleural membrane outer pleural membrane pleural fluid left lung bronchus heart diaphragm Structures of the respiratory system:

Mammals §Lungs are the site of gaseous exchange in mammals. §Rib cage encloses and protects the lungs. There are 12 pairs of ribs. §The ribs are moved by a series of intercostal muscles. §Diaphragm separates the thorax and the abdomen.

Regions of the respiratory system §Air passes into the lungs through a series of tubes in the following order:  Nose nostril, hairs (filter dust):  nasal cavity  pharynx  larynx  trachea  bronchi (bronchus)  bronchioles  air sacs  alveoli (alveolus)

Move up the throat Unwanted particles Mucus-secreting cell Mucus Cilia Ciliated epithelium Cells inside the nasal cavity

§Nasal Cavity §Wall is lined with a ciliated epithelium and mucus-secreting cells. §Bacteria & dust, trapped by mucus, are sent towards the throat by the beating cilia. §The mucus is then swallowed or coughed up. §Numerous blood vessels warm and moisten the incoming air. §Olfactory cells give the sense of smell of the incoming air.

Pharynx §belongs to both the respiratory & digestive systems §epiglottis (a cartilage) covers the glottis (opening to larynx) to prevent food from entering the trachea Larynx (voice box) §produces voice when air is forced through its vocal cords

Larynx (voice box) - with cartilage & vocal cords Larynx Vocal cords

§Trachea and Bronchi §further divide into bronchioles which finally end into alveoli §dirt particles & germs are trapped by mucus and sent upwards by its cilia §wall of trachea is strengthened by C-shaped cartilages which keep it open

Alveoli - a respiratory surface with a total area of about 100 m 2 How large are the respiratory surfaces provided by the lungs ? About half the size of a tennis court.

Features for efficient gas exchange 1. Very thin so that gases can diffuse through very quickly 2. A large surface area to diffuse more gases per unit time 3. Moist so that gases can pass through in solution forms 4. An excellent transport system of blood capillaries to transport gases

Vertebral column Intercostal muscles Rib Sternum Lung §protected by the thoracic basket which consists of the vertebrae, ribs, and sternum

§Lungs covered by two pleural membranes which secrete pleural fluid to reduce friction during breathing movements

Outer & inner pleural membranes with pleural fluid reduce friction during breathing.

GASEOUS EXCHANGE IN THE ALVEOLI §Pulmonary artery delivers deoxygenated blood to the lungs. §Oxygen from the incoming air diffuses across the walls of the alveoli and the capillaries and passes into the blood because of a higher concentration: §O 2 + haemoglobin  oxyhaemoglobin § Oxygenated blood then goes to the heart through the pulmonary vein

Gaseous Exchange in the alveoli Capillary from pulmonary artery Red blood cell Oxygen diffuses into RBCs Carbon dioxide diffuses into alveolus Capillary to pulmonary vein Film of moisture Epithelium of alveolus (1-cell thick) Owing to concentration differences:

§Carbon dioxide in the form of hydrogen carbonate ions in plasma diffuses to the alveoli because of its higher concentration in blood

MECHANISM OF BREATHING (a) Inspiration

MECHANISM OF BREATHING (a) Inspiration §1) Thoracic basket is raised §2) Diaphragm flattens §3) Volume of thoracic cavity increases §4) Air is drawn into the lungs

(b) Expiration §5) Thoracic basket drops down §6) Diaphragm moves up §7) Volume of thoracic cavity decreases §8) Air is forced out of the lungs

1 Movement of the ribs – external & internal intercostal muscles sternum Rubber band shortened (intercostal muscles contract) Ribs raised upwards & outwards Volume of thoracic cavity & lungs increases Lung air pressure lower than atmosphere Air flows from atmosphere to the lungs Rubber band lengthened (intercostal muscles relax) Ribs fall downwards & inwards V  P  Air flows out of the lungs into the atmosphere Inspiration Expiration vertebral column

2 Movement of the diaphragm Inspiration rub Pleural membranes Pleural fluid Diaphragm muscles contract Diaphragm lowered Lungs expanded Air pressure becomes lower than that of the atmosphere Air drawn in

2 Movement of the diaphragm Expiration Diaphragm muscles relax Diaphragm returns to dome shape Lung returns to original volume Air pressure becomes higher than that of the atmosphere Vertebral column Air expelled

20.3 Control of Ventilation in Man

-Rate and depth of breathing is controlled by the respiratory centre in the medulla oblongata of the hind-brain by changes in blood CO 2 concentration: Blood CO 2 in blood  detected by chemoreceptors  nerve impulses  respiratory centre in medulla

20.3 Control of Ventilation in Man

-Rate and depth of breathing is controlled by the respiratory centre in the medulla oblongata of the hind-brain by changes in blood CO 2 concentration: Blood CO 2 in blood  detected by chemoreceptors  nerve impulses  respiratory centre in medulla  phrenic & thoracic nerves  diaphragm & intercostal muscle contractions  Inspiration

20.3 Control of Ventilation in Man

 stretch receptors in lungs stimulated  vagus  expiratory centre in medulla to switch off the inspiratory centre  expiration takes place

20.3 Control of Ventilation in Man

 stretch receptors in lungs stimulated  vagus  expiratory centre in medulla to switch off the inspiratory centre  expiration takes place  stretch receptors not stimulated  expiratory centre switched off  inspiratory centre switched on  inspiration again

20.3 Control of Ventilation in Man

§The ventral portion of the breathing centre is the inspiratory centre; §the remainder is the expiratory centre §Chemoreceptors in the carotid and aortic bodies of the blood system

§The breathing centre may also be stimulated by impulses from the forebrain resulting in a conscious increase or decrease in breathing rate. §The main stimulus for ventilation is carbon dioxide; §Changes in oxygen concentration have relatively little effect.

§At high altitudes the reduced atmospheric pressure makes it more difficult to load the haemoglobin with oxygen. §In an attempt to obtain sufficient oxygen a mountaineer takes very deep breaths. §This forces more carbon dioxide out of the body and the level of carbon dioxide in the blood therefore falls. §The inspiratory centre is no longer stimulated and breathing becomes increasingly laboured, causing great fatigue.

§Given time, man can adapt to these conditions by excreting more alkaline urine. §This causes the pH of the blood to fall, §i.e. more acidic §the chemoreceptors are stimulated and so is the inspiratory centre.

Lung volumes Volume of air in lung Vital capacity Tidal volume Residue volume Time

20.4 Measurement of Lung Capacity

§Tidal volume is the volume of air breathed in or out during each respiratory cycle §Vital capacity is the total amount of air that can be forcibly inspired or expired §Residue volume is the amount of air that remains in the lungs even after maximum expiration §Ventilation rate is the process of exchanging gases in the lungs/gills with gases from the environment per unit time.