B3.1 GAS EXCHANGE

outline gas exchange in organisms

• gas exchange is a vital function in all organisms

  • transport of oxygen into an organism for aerobic respiration

  • transport of carbon dioxide out as a product of cell respiration and excreted as harmful waste (high conc of CO2 increases internal acidity

• size incresases = SA:V ratio decreases

  • small organisms have sufficient gas exchange by simple diffusion as the distance between the exterior and their center is small

  • large organisms need body systems for sufficient gas exchange as the distance between the exterior and center is large

outline the properties of gas-exchange surfaces

• large surface area:

  • more surface area available for gas exchange

• highly permeable:

  • gas exchange occurs via diffusion (phospholipid bilayers are permeable to CO2 and O2)

• thin tissue layer:

  • provides short diffusion distances

  • lungs have single layer cell between air and capillaries

• moist:

  • water helps gases dissolve on exchange surfaces of cells

  • lung cells secrete moist layer on the interior to

explain the maintenance of concentration gradients at exchange surfaces in mammals

• for sufficient exchange, concentration gradients must be maintained

  • outside: high O2, low CO2

  • inside: low O2, high CO2

• adaptations to help maintain the steep conc gradeint include

  • dense networks of blood vessels: high SA of blood for increased exchange of gases

  • continuous blood flow: movement maintains low O2 and high CO2 internally

  • ventilation with air for lungs and with water for gills: movement maintains high O2 and low CO2 externally

outline the adaptations of mammalian lungs for gas exchange

1. branched network of bronchioles (high SA)

   • left/right bronchus branch into numerous bronchioles which branch into many alveoli, increasing surface area for gas exchange

2. high surface area (numerous alveoli)

   • alveoli walls composed of 2 cell types

     • type I pneumocyte cells

       • single cell layer

       • extremely thin specialised cell structure for short diffusion distance

     • type II pneumocytes’

       • numerous secretory vesicles which secrete a surfactant

       • water and compounds to break surface tension and lungs from collapsing

       • moist layer allows for gases to dissolve

3. extensive capillary beds

   • many capillaries surround the alveoli (dense network), creating a v short diffusion distance between alveolar space and the blood as it is only a single cell wall thick

   • high surface area for gas exchange, high blood flow to maintain concentration gradients

explain the process of ventilation of the lungs

• ventilation (breathing) : the movement of air into and out of the lungs

• muscle contractions change the volume of the lungs

  • increasing volume, decreasing gas pressure (inhalation, inspiration)

  • decreasing volume, increasing gas pressure (exhalation, expiration)

    • inverse relationship

• air moves along pressure gradient → high to low pressure

QUIET:

inspiration:

• diaphragm contracts (go down) and flattens, increasing the volume of the lungs

• external intercostal muscles contract to bring the rib cage up and out, increasing lung volume

• internal intercostal muscles are relaxed, decreasing pressure in the lungs

expiration:

• diaphragm relaxes (goes up) moving upwards, decreasing the volume of the lungs

• external intercostal muscles relax to bring the rib cage down and in, decreasing the lung volume

• internal intercostal muscles contract, increasing the internal pressure in the lungs

outline the structure of hemoglobin

• protein in red blood cells

• composed of 4 polypeptide chains and 4 iron containing heme groups

• binds to and transports:

  • O2 (from lungs to respiring tissues)

  • CO2 (from respiring tissues to lungs)

explain the term : oxygen affinity

Oxygen affinity' refers to the ability of hemoglobin to bind oxygen at a specific partial pressure of oxygen, influenced by factors like temperature, pH, and organic phosphate concentration

explain the role of hemoglobin in cooperative binding

• binding of an oxygen molecule to a heme group changes the hemoglobin conformation

  • increases hemoglobins affinity to oxygen so the next oxygen molecules binds more easily

  • higher affinity in oxygen rich areas promotes oxygen loading

• release of oxygen molecule changes hemoglobin conformation

  • decreases the hemoglobins affinity to oxygen

  • lower affinity in oxygen deficient areas (respiring tissues, eg muscles) promotes oxygen unloading

using the oxygen dissociation curve, explain the affinity of hemoglobin for oxygen at different oxygen concentrations in adults

• low oxygen saturation in hemoglobin at low oxygen levels in tissue

• high oxygen saturation in hemoglobin at high oxygen levels in tissue

SIGMOIDAL (S) SHAPED CURVE:

• binding of first O2 molecule is difficult due to low affinity at low partial pressure of oxygen (pO2)

• binding of successive O2 molecules are easier due to increase affinity at increasing partial pressure of oxygen (pO2) due to cooperative binding

• maximum binding reached when hemoglobin is fully saturated, affinity plateaus at high partial pressure of oxygen

compare fetal hemoglobin (HbF) and adult hemoglobin (HbA)

• HbF’s 4 polypeptide chains = 2 alpha chains + 2 gamma chains

• HbA’s 4 polypeptide chains = 2 alpha chains + 2 beta chains

  • gamma polypeptide chains have a higher affinity to oxygen than beta chains, resulting in a higher affinity to oxygen in HbF than HbA

    • allows for developing fetus to obtain oxygen from mothers hemoglobin in bloodstream in the placenta

explain the fetal oxygen dissociation curve

• as fetal hemoglobins have a higher affinity to oxygen that adult hemoglobin, the fetal graph shifts to the left

  • #nobabyleftbehind

• at lower partial pressure oxygen, the fetal hemoglobin will load oxygen easier than adult hemoglobin

• fetal hemoglobin will load oxygen when adult hemoglobin unloads oxygen

explain how an increase in CO2 correlates with an increased dissociation of oxygen

• carbon dioxide can also bind to and transport on hemobglobin

  • CO2 binds at an allosteric site on the hemoglobin and not with the heme group

• carbon dioxide can also cause the Bohr effect:

  • CO2 decreases blood pH due to the presence of carbon, resulting in conformational change so the binding of oxygen becomes less favorable

  • allows for oxygen unloading in areas of high partial pressure of carbon dioxide and low partial pressure of oxygen (respiring tissues)

• Bohr shift: oxygen dissociation curve shifts to the RIGHT due to decreased affinity to O2

  • respiring tissues produce high amounts of CO2 and require high amounts of oxygen

bohr shift describes hemoglobins decrease in affinity to oxygen leadig to an increase in oxygen unloading at respiring tissues