# Respiration and Gas Exchange

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Respiration and Gas Exchange
Lecture #17 Respiration and Gas Exchange

Partial Pressure each gas in a mixture of gases exerts its own pressure = partial pressure partial pressures denoted as “p” applies to gases in air and gases dissolved in liquids total pressure is sum of all partial pressures atmospheric pressure (760 mm Hg) = pO2 + pCO2 + pN2 + pH2O to determine partial pressure of O2-- multiply 760 by % of air that is O2 (21%) = 160 mm Hg

Respiratory Media respiratory media – either air or water
conditions for gas exchange depend on this media air is less dense and easier to move over respiratory surfaces it is easy to breathe air but humans only extract 25% of the O2 out of the air they breathe O2 is plentiful in air – is always 21% of the earth’s atmosphere by volume gas exchange from water is much more demanding amount of O2 dissolved in water varies with the conditions of the water warmer and saltier – less O2 but it is always less than what is found in air 40 times more O2 in air than in water!! water is also more dense and viscous – requires considerably more energy to move over the respiratory surface

Respiratory Surfaces ventilation = movement of the respiratory medium over the respiratory surface O2 and CO2 exchange is by diffusion and occurs across a moist surface rate of diffusion determined by three things: 1. surface area 2. thickness of respiratory membrane (e.g. alveolar wall + capillary wall) 3. diffusion coefficient – CO2 20X higher vs. O2 i.e. diffusion is faster when the area for diffusion is large and the distance is short

Respiratory Surfaces simple animals – every cell is close enough to the external environment – gases diffuse quickly across the body surface sponges, cnidarians and flatworms some animals have modified their skin to act as a respiratory organ – dense network of capillaries below the surface earthworms and some amphibians like frogs however this is not true for larger animals – development of more complex structures like gills and lungs

Gills fish gas exchange
to exchange enough O2 – fish must pass large quantities of water across the gill surface water flows in the mouth and out the operculum (slit-like opening in the body wall) flows over the gills most fishes have a pumping mechanism to move water into the mouth and pharynx and out through the opercula some elasmobranchs and open ocean bony fishes (e.g. tuna) – keep their mouth open during swimming – ram ventilation gills are supported by gill arches – contain larger arteries and veins (branchial artery and vein) 2 gill filaments extend from each arch and are made up of plates called lamellae each lamella contains extensive capillary beds Gills Gill arch Operculum Water flow Blood vessels Gill arch Gill filaments O2-poor blood Water flow Blood flow Lamella O2-rich blood

gas exchange across the lamellae – countercurrent or parallel exchange depending on the fish
parallel exchange – the blood flows in the same direction as the water through the gills exchange will stop once the difference between water and blood O2 levels disappears countercurrent exchange – the blood and water flow in opposite directions there always exists a small gradient so that oxygen flows into the blood from the water Counter-current exchange Parallel exchange

amphibian gas exchange: requires a moist surface
skin can function as a respiratory organ through cutaneous respiration the majority of its total respiration gas exchange also occurs along the moist surfaces of the mouth and pharynx – buccopharyngeal respiration

amphibian gas exchange:
contribution of cutaneous and buccopharyngeal respiration to total gas exchange is relatively constant so their rate cannot be increased if metabolic rate goes up an alternate means of increasing respiration is required so amphibians also possess lungs pulmonary ventilation occurs through a buccal pump mechanism muscles of the mouth and pharynx create a positive pressure to force air into the lungs

Tracheal System of Insects
the most common terrestrial respiratory system air tubes that branch throughout the body largest tubes are called tracheae – open to the outside branch into smaller tubes = tracheoles – deliver air directly to the cells of the tissues passive movement of air into the tracheae and diffusion brings in enough O2 to support cellular respiration larger insects with higher energy requirements – must ventilate air and out of the tracheae – through body movements produced by muscles Tracheae Air sacs External opening Trachea Air sac Tracheole Body cell

Terrestrial Animals & the Lung
lungs are localized, regional respiratory organs subdivided into numerous lobes, lobules and broncho-pulmonary segments these divisions are supplied by a series of branching tubes lungs are supplied by the circulatory system – blood comes from the right side of the heart the amphibian lung is quite small – most respiration is done by the skin most reptiles, all birds and all mammals – respiration done lungs

The Lung Primary bronchi supply each lung
Secondary bronchi supply each lobe of the lungs (3 right + 2 left) Tertiary bronchi splits into successive sets of intralobular bronchioles that supply each bronchopulmonary segment ( right = 10, left = 8) IL bronchioles split into Terminal bronchioles -> these split into Respiratory Bronchioles each RB splits into multiple alveolar ducts which end in an alveolar sac

The Alveolus Respiratory bronchioles branch into multiple alveolar ducts alveolar ducts end in a grape-like cluster = alveolar sac each grape = alveolus Pharynx Larynx (Esophagus) Trachea Right lung Bronchus Bronchiole Diaphragm (Heart) Capillaries Left lung Dense capillary bed enveloping alveoli (SEM) 50 m Alveoli Branch of pulmonary artery (oxygen-poor blood) pulmonary vein (oxygen-rich Terminal bronchiole Nasal cavity

Alveolus one cell thick - site of gas exchange by simple diffusion
surrounded by a capillary bed fed by a pulmonary arteriole and collected by a pulmonary venule deoxygenated blood flows over the alveolus picks up O2 and the oxygenated blood leaves the alveolus -> heart Type I alveolar cells: simple squamous cells where gas exchange occurs Type II alveolar cells (septal cells): secrete alveolar fluid containing surfactant Alveolar dust cells: wandering macrophages remove debris

Ventilation & Breathing
ventilation = movement of the respiratory medium over the respiratory surface amphibians – use positive pressure breathing inflate their lungs by forcing air into them mammals – use negative pressure breathing change the volume of the lungs to either increase or decrease air pressure within it – moves the air in and out birds – unique mechanism involving negative pressure breathing

Birds respiratory system is designed to be efficient and to provide the flight muscles with enough oxygen external nares located in the bill – draws air in – eventually enters into the bronchii bronchi connect to air sacs that occupy much of the body & to the lungs lung does not contain alveoli – but contains parabronchii – tiny channels for gas exchange inspiration and expiration results from increasing and decreasing the volume of the thorax and from the expansion and compression of the air sacs bird actually uses two rounds of inhalation/exhalation to move a volume of air through its respiratory system Anterior air sacs Posterior Lungs 1 mm Airflow Air tubes (parabronchi) in lung Second inhalation First inhalation 3 2 4 1 Second exhalation First exhalation

Birds 1st inhalation – air moves into the posterior/abdominal air sacs
1st exhalation – posterior air sac contracts – forces air into the lungs for additional gas exchange 2nd inhalation – air passes from the lungs into the anterior air sacs; new air moves into the posterior air sacs 2nd exhalation – anterior air sacs contract and air moves out of body; posterior air sacs contract and a new volume of air moves in to lung due to this arrangement – birds have a near continuous movement of O2 rich air over the respiratory surfaces of the lungs Anterior air sacs Posterior Lungs 1 mm Airflow Air tubes (parabronchi) in lung Second inhalation First inhalation 3 2 4 1 Second exhalation First exhalation

Mammalian Breathing to understand mammalian ventilation - must understand the physical relationship between the lungs and the thoracic cavity Pleural cavity is potential space between ribs & lungs the lungs do not fill the entire pleural cavity pressure of air inside the lungs is greater than the pressure in the pleural cavity lungs and thoracic cavity are lined with membranes Visceral pleura covers lungs Parietal pleura lines ribcage & covers upper surface of diaphragm

Respiratory pressures
two different pressures need to be considered 1. atmospheric (barometric) pressure caused by the weight of air on objects on the Earth’s surface 2. intrapulmonary (intra-alveolar) pressure pressure within the lungs (within each alveolus) when not ventilating – pressure of air inside the lungs = pressure of air outside the body ventilation happens because of a pressure gradient between AP and IP

Mammalian Ventilation: Boyle’s law
Inhalation - the diaphragm drops and the rib cage swings up and out – the thoracic cavity increases in volume fluid adhesion holds the visceral and parietal pleural membranes together so when the parietal the movement of the thoracic cavity “pulls” the lungs with it this expands the lungs in volume – air pressure in the lung (i.e. IP) drops below atmosphere (i.e. AP) Rib cage expands. Air inhaled. Lung Diaphragm Boyle’s law: As the size of closed container decreases, pressure inside is increased As the size of a closed container increases, pressure decreases

Mammalian Ventilation: Boyle’s law
Exhalation – the diaphragm comes back up and the rib cage swings back down – the thoracic cavity decreases in volume PLUS – elastic recoil of the lung tissue decreases volume lung volume decreases and the air pressure within the lungs increases vs. atmospheric air moves out to equilibrate Air exhaled. Rib cage gets smaller.

Mammalian Ventilation: Boyle’s law
additional muscles can be used to increase and decrease the volume of the thoracic cavity more than normal other animals use the rhythmic movement of organs in their abdomen to increase breathing volumes Air exhaled. Rib cage gets smaller.

Respiratory Volumes and Capacities
inspiratory capacity (IC) = max. amnt of air taken in after a normal exhalation, 3500 ml vital capacity = max. amnt of air capable of inhaling, IRV + TV + ERV = 4600 ml total lung capacity = VC + RV = 6000ml (TV) = amnt of air that enters or exits the lungs 500 ml per inhalation functional residual capacity = ERV + RV, 2300 ml inspiratory reserve volume (IRV) = IC + TV, 3000 ml residual volume (RV) = amnt of air left in lungs after forced expiration = 1200 ml expiratory reserve volume (ERV) = amnt of air forcefully exhaled, 1100 ml

Control of Breathing controlled by three clusters of neurons that make up the Respiratory Center 1. medullary rhythmicity area – in the medulla oblongata controls the rate and depth of breathing 2. pneumotaxic area – in the pons shortens the breath 3. apneustic area – in the pons prolongs the breath detects changes in the pH of the CSF surrounding the brain

CO2 is the major determinant for breathing rate
the major determinant of CSF pH is the blood’s pH the major determinant of blood pH is the dissolution of CO2 into the plasma CO2 combines with the water of the plasma to create carbonic acid carbonic acid dissociates into H+ ions (pH) and bicarbonate ions (HCO3-)

Carotid arteries Aorta Homeostasis: Blood pH of about 7.4 CO2 level
Figure 42.29 Homeostasis: Blood pH of about 7.4 CO2 level decreases. Stimulus: Rising level of CO2 in tissues lowers blood pH. Response: Rib muscles and diaphragm increase rate and depth of ventilation. Carotid arteries Aorta Sensor/control center: Cerebrospinal fluid Medulla oblongata neurons in carotid and aortic arch sense drop in blood pH medulla detects drop in CSF pH Figure Homeostatic control of breathing.

Respiratory pigments CO2 dissolves in the water of the plasma
but O2 dissolves poorly in plasma reduces the amount of O2 that the blood can carry so there is the need for a respiratory pigment to bind oxygen hemocyanin – respiratory pigment of molluscs, arthopods, annelids has copper as it’s oxygen binding element hemoglobin used by most other animals uses iron to bind oxygen acts as an “oxygen sponge” allows for the transport of significant amounts of O2 in the blood

Hemoglobin comprised of 4 proteins called globin
each globin has a heme group each heme group has an iron-containing pigment at its core each iron atom binds one O2 molecule as one heme binds one O2 – the other three increase their affinity for their O2 “partners” as one heme releases its O2 – the other three lose their affinity for their O2 so each Hb can carry four O2 molecules

Hemoglobin & O2 (b) pH and hemoglobin dissociation PO (mm Hg) 20 40 60
(a) PO and hemoglobin dissociation at pH 7.4 Tissues during exercise Tissues at rest Lungs PO (mm Hg) 20 40 60 80 100 O2 unloaded to tissues during exercise O2 saturation of hemoglobin (%) (b) pH and hemoglobin dissociation PO (mm Hg) 2 20 40 60 80 100 Hemoglobin retains less O2 at lower pH (higher CO2 concentration) pH 7.2 pH 7.4 O2 saturation of hemoglobin (%) Bohr shift: low pH decreases the affinity of Hb for O2

CO2 transport CO2 produced by tissue cells & diffuses into the plasma
Body tissue Capillary wall Interstitial fluid Plasma within capillary CO2 transport from tissues CO2 produced CO2 H2O H2CO3 Hb Red blood cell Carbonic acid Hemoglobin (Hb) picks up CO2 and H+. H+ HCO3 Bicarbonate To lungs to lungs Hemoglobin releases Alveolar space in lung CO2 produced by tissue cells & diffuses into the plasma over 90% of CO2 then diffuses into the RBC some CO2 combines with Hb most CO2 reacts with the cytosol inside the RBC to form carbonic acid – catalyzed by the enzyme carbonic anhydrase dissociation of carbonic acid into H+ and HCO3- Hb binds the H+ ions and prevents the Bohr shift most of the HCO3- diffuses out of the RBC into the plasma in the lungs – Hb releases the H+ ion – it combines with the HCO3- to reform carbonic acid carbonic acid breaks up into H2O and CO2; CO2 is released by Hb CO2 diffuses into the alveolar air

Diving Mammals humans can hold their breath for no more than 3 minutes
seals – can dive to m and can hold their breath for close to 20 minutes some whales can reach depths of 1500m and stay submerged for close to 2 hours evolutionary adaptations: 1. ability to store large amounts of O2 in their muscle mass 2. adaptations to conserve O2 – little effort to swim and their buoyancy allows them to change depths easily 3. regulatory mechanisms routes blood to the brain, spinal cord, eyes, adrenal glands – shut off in other areas during a dive