Respiration PDF

How is muscle contraction fuelled? ATP is essentially the energy currency of the body. It is the breakdown of ATP that releases energy which the body's...

How is muscle contraction fuelled? ATP is essentially the energy currency of the body. It is the breakdown of ATP that releases energy which the body's tissues such as muscle can use All animals need oxygen to metabolize nutrients to generate cellular energy 1 How do we supply our cells with oxygen and get rid of the carbon dioxide External respiratory processes must meet the demands of the size of the animal and the rate of metabolism through diffusion and bulk transport. Every cell relies on the exchange of gases (diffusion) between the extracellular environment and the intracellular environment and these gases are obtained/expelled during breathing (external respiration). We gain O2 during inhalation and exhale the by-product of metabolism, CO2 during exhalation 2 The steps of Respiration 1. Ventilation Bulk transport When gases move from an area of high pressure to an area of low pressure, this is known as 'bulk flow. During a breathing cycle, air moves in and out of the lungs by bulk flow Diffusion 2. Gas exchange Internal O2 and CO2 are transported by 3. Gas transport bulk internal circulation between the respiratory surface and all tissues transport Diffusion 4. Gas exchange NOTE: We will not be discussing cellular respiration in this module. You covered 5. Cellular respiration this in Biology 124 3 1  Gas exchange Diffusion Diffusion LUNG BLOOD TISSUES PUMP Vertebrates have 2 diffusion barriers O2 and CO2 are transferred passively across body surfaces by diffusion. At the alveoli, O2 moves down its concentration gradient into the Pulmonary vein while CO2 diffuses from the Pulmonary artery into the alveoli. At the tissues, O2 diffuses from the blood (high PO2) into the external environment and into the cell (lower PO2 and high CO2) and CO2 diffuses into the circulatory system 4 We need to understand atmospheric pressure and partial pressure At sea level, the weight of the atmosphere ‘We live submerged at supports a the bottom of an ocean column of mercury of air which is known 760mm high to have weight’. 5 Atmospheric and Partial Pressures Dry air contains 78% N, 21% O2, 0.03% C02 + argon The pressure of a specific gas in a mixture is called its partial pressure and is denoted by P Partial pressure is calculated by multiplying the fractional composition of that gas by the atmospheric pressure Gas Percent in air Partial Pressure At sea level N2 78% 593 mmHg At sea level PO2 = 760 x 21% = 160mm Hg O2 21% 160 mmHg PCO2 = 760 x 0.03 = 0.2mm Hg CO2 0.03% 0.2 mmHg 6 2 Gas Laws Exchange of O2 and CO2 is purely passive and depends on the behaviour of gases Dalton’s Law of Partial Pressures Gases in a mixture exert their own pressures as if other gases are not present 7 Gas Laws Pulling up increases the volume and decreases the Pushing down pressure decreases the volume and increases the pressure Boyle’s Law The increase of the volume of a given quantity of gas results in a decrease in pressure Particles collide more with the walls in the smaller space – it is this that we measure as “pressure exerted by the gas” Pulmonary ventilation is the process of breathing, which is driven by pressure differences between the lungs and the atmosphere. Boyle's law describes the relationship between volume and pressure. 8 Gas Laws Increase pressure Henry’s Law The quantity of gas that dissolves at a given temperature is proportional to the partial gas pressure of that gas in the gas phase liquid More gas molecules are soluble at higher pressure 9 3 Gas Laws Fick’s Law of Diffusion Difference in partial pressure of gas on either side of barrier to diffusion Rate of diffusion can be optimised by natural selection 1) Increasing A 2) Increasing Δp (P2-P1) 3) Decreasing d R = D x A Δp/d R = rate of diffusion D = diffusion constant A = area over which diffusion takes place Δp = concentration difference between the 2 sides d = the diffusion distance 10 Diffusion rates Unicellular organisms and less advanced invertebrates have high surface area to volume ratio ➔ diffusion sufficient Body size =  diffusion distances and  surface area to volume Surface area to volume maintained by a change in shape (flattening, folding) but the integument might not provide enough surface area for exchange and animals evolved special organs for gaseous exchange ( A;  d) 11 But as animals become more complex the integument no longer provides sufficient surface area to rely on diffusion and we see special respiratory organs evolving in many animal groups Insect trachea Fish gills Amphibian lung Reptile lung Mammal lung Bird lung 12 4 Respiratory organs 1) Gills Large surface areas Highly efficient Respiratory Organs Countercurrent exchange 13 Why is the countercurrent flow of water and blood so effective? Countercurrent oxygen exchange means the blood flows through the gills in the opposite direction as the water flowing over the gills. As the blood flows through the gills and gains oxygen from the water, it encounters increasingly fresh water with a higher oxygen concentration that is able to continuously offload oxygen into the blood. The low-oxygen blood just entering the gill, meets low-oxygen water but the water has more O2 and so O2 moves from water to blood along the entire length of the gill. In a countercurrent system, equilibrium is not reached, so gas exchange continues, increasing efficiency. In contrast, in a concurrent system, equilibrium is reached and thus exchange is inefficient. 14 Respiratory organs Terrestrial respiratory organs 2) Trachea Insects have fairly high surface area to volume ratio ➔ network of air-filled passages transferring O2 directly to cells Air is piped directly from the environment through a series of passages directly to cells without the need of a circulatory system O2 and CO2 diffuse 10 000 times more rapidly in air than water or blood 15 5  Amphibian lungs Lungs Positive pressure breathing (air is forced by positive pressure into the lungs by a buccal pump – don’t have a diaphragm to create a negative pressure gradient and so must force air into lungs ) Inspiration Expiration Nostrils open Nostrils close Buccal cavity expands Nostrils open Buccal cavity expands Glottis opens Lungs contract Glottis closes Buccal cavity contracts Buccal cavity contracts Lungs expand Drawing air in: the frog lowers the floor of Eliminating CO2: the floor of the mouth its mouth and the buccal cavity expands moves down, drawing the air out of the allowing air to enter the enlarged cavity. lungs and into the mouth. Finally the nostrils The nostrils then close and the air in the are opened and the floor of the mouth moved buccal cavity is forced into the lungs by up pushing the air out of the nostrils. contraction of the floor of the mouth Reptile lungs Mostly negative pressure breathing (see mammalian lungs) 16 Respiratory organs 3) Mammalian Lungs Human lungs contain about 300 million alveoli per lung. Total surface area for diffusion is ~75m 2 Branched system of passages ending in alveoli Respiratory efficiency is sacrificed for reduced evaporation Lungs (internal gas exchange system) developed during the water-land transition as an adaption to air-breathing. Mammals continually lose water to the atmosphere (and through respiration) but the lungs consist of a series of branching pipes that end in small air sacs called alveoli and this reduces water loss via evaporation, crucial to avoiding desiccation. Gases can only cross cell membranes when they are dissolved in an aqueous solution, thus respiratory surfaces must be moist. 17 Breathing in mammals Advantages Internal lungs help reduce water loss and maintain a moist gas exchange surface. This allows mammals to inhabit a greater variety of terrestrial habitats. The large surface area to volume ratio of lungs allows mammals to get quite big The blood system surrounds each alveoli and reduces the distance dissolved gasses have to diffuse. This also enables mammals to grow to larger sizes. Disadvantages Tidal ventilation – air comes in and flows out along same route and not all of the air can be forced out of the lungs (residual volume). Inhaled oxygen mixes with this “stale” air. The tidal functioning of the lungs does not allow a countercurrent gas exchange mechanism to develop (i.e. air and blood flowing in opposite directions – see gills) Oxygen cannot diffuse across the cartilaginous trachea/trachioles = anatomical dead space O2 concentration in the lungs < ambient O 2 concentration due to the presence of water vapour and the residual volume in lungs 18 6  Adaptive features of the lung: Surfactants and mucous There is a thin layer of fluid lining lungs, which generates surface tension – lung inflation difficult Cells of alveoli produce surfactants which interferes with the surface tension forces Premature babies do not produce surfactants yet and thus they experience respiratory stress and sometimes need to be put onto a respirator Other cells produce mucous which capture dirt and micro- organisms inhaled Cilia move mucous + debris to pharynx where it is swallowed. This phenomenon is called the mucous escalator 19 How do we breathe? Mammals use negative pressure breathing Atmospheric pressure = 760mmHg Atmospheric pressure = 760mmHg Alveolar pressure Alveolar pressure = 758mmHg = 762mmHg Diffusion & bulk flow Video 18 in the Video repository on SUNLearn Most vertebrates expand their lungs creating a lower (negative) pressure in the lungs (Boyle’s Law) and so air flows into the low pressure area 20 The basic rhythm of inspiration is determined by nerve impulses from the Respiratory Centre (medulla and pons). Nerve activity results from neurons that are auto-rhythmic ➔ they fire impulses for 2 seconds during inspiration and then rest for 3 seconds during expiration (then automatically fire again for 2s and so on). During firing the respiratory muscles + diaphragm contract, when the neuron stops firing, respiratory muscles + diaphragm relax Inspiratory Centre Expiratory Centre Inspiratory Centre Expiratory Centre Active (2s) Inactive (3s) Active Active Diaphragm + Diaphragm + Diaphragm + Internal rib external rib external rib muscles external rib muscles + muscles contract relax, lungs recoil muscles contract abdominal elastically muscles contract Resting Resting Forced Forced inspiration expiration inspiration expiration Quiet breathing Vigorous breathing 21 7  Bird respiration Why is breathing in birds more efficient? Human Lungs Breathing in birds is unidirectional and continuous. Mammals have tidal breathing which is not continuous. 22 Bird ventilation Anterior air sac Cycle 1 Posterior air sac Inspiration 1 Expiration 1 Cycle 2 Inspiration 2 Expiration 2 A volume of air takes 2 cycles to move through the respiratory system but during Inspiration 2, another volume is inhaled so air flow is continuous and unidirectional 23 Bird lungs 1. Small lungs relative to mammals but larger hearts 2. Lungs + number of air sacs 3. Unidirectional flow 4. Contraction and expansion of lungs relatively small 5. Continuous flow = maximizes gas diffusion at gas exchange surfaces Countercurrent 6. Countercurrent type (crosscurrent) flow between blood and air. 24 8  Why can birds fly at high altitudes yet mammals battle at high altitude? Take a moment to think about it Continuous and flow is unidirectional Unidirectional flow of air allows to maximize environmental O2 concentration on external side of exchange surfaces ( diffusion efficiency) PO2 at lung surface = PO2 of environment Air forced across gas exchange surface Countercurrent type flow between blood and air (tidal breathing does not allow for countercurrent exchange). Less sensitive to blood CO2 concentration LUNGS: tidal flow limits the concentration difference between air and blood – PO2 at lung surface = approx 67% that of ambient air (because of residual volume, water vapour and anatomical dead space) 25 You have just inhaled a breath of air at sea level (PO2 = 160mmHg). What would the PO2 be at the alveolar surface? Think We know it is not 160mmHg. Why? The body produces significant amounts of water vapour and it’s presence in the alveoli limit the space available for oxygen Water vapour has a partial pressure of 47mmHg (in essence humidification of inspired air “dilutes” the partial pressure of the inspired air) and at an attitude of 19,200m, to atmospheric pressure is 47mmHg - the lung will be filled with water vapour and no air or O 2 could enter lung O2 mixes with residual air from previous expiratory cycle The PO2 within the alveoli is 100mmHg (more than 1/3 reduction) We breathe to provide a continuous supply of O 2 for pickup by blood and to constantly remove CO2. Recall that gases diffuse passively down a concentration gradient [Δp (P2-P1) in Fick’s Law]. Gas molecules move from a region of high concentration to a region of low concentration (or better stated, from a high partial pressure of that gas to a low partial pressure of that gas. 26 Gaseous exchange in lungs and tissue Low PCO2 High PO2 Gases diffuse from areas of higher partial pressure to areas of lower partial pressure by simple passive diffusion Gas gradients across the pulmonary High PCO2 Low PO2 and systemic capillaries 27 9