FB 1034-Biology II - Respiration and Gas Exchange Notes

• Gaseous exchange: Taking up $O<em>2$ and discharging $CO</em>2$. Linked to ATP production in cellular respiration.

• Respiratory & circulatory systems provide $O<em>2$ and remove $CO</em>2$.

• Respiratory medium: Source of $O<em>2$. Air (21% $O</em>2$), water (variable $O_2$ levels, less than air).

Gaseous Exchange in Animals

Respiratory Systems in Animals

Respiratory Surface

• Gas exchange occurs here by diffusion. Fick’s Law of Diffusion governs this:

  • \frac{dV}{dt} = \frac{A \* D \* (P1 – P2)}{T} (Rate of gas transfer is proportional to area and partial pressure difference, inversely proportional to thickness).

  • $CO<em>2$ diffuses 20x faster than $O</em>2$ due to higher solubility.

• Characteristics for maximizing gas exchange:

  1. Large surface area: Larger area = greater diffusion.

  2. Thin: One cell thick for rapid diffusion.

  3. Moist: Gases dissolve in water to diffuse.

  4. Good blood supply: Efficient gas transport.

  5. Good ventilation gradient: Continuous gas delivery.

Types of Respiratory Surfaces

• Depends on organism size, environment, metabolic demands.

Body Surface

• Protists/unicellular organisms: Gas exchange over entire surface.

• Simple animals (sponges, cnidarians, flatworms): Cells near environment.

• Earthworms/amphibians: Outer skin with capillary network for gas exchange. Limited to water/damp places. High surface area to volume ratio.

Gills

• Outfoldings of body surface in aquatic animals.

  • Advantage (water): Keeps membranes moist.

  • Disadvantage (water): Low $O_2$ concentrations.

• Ventilation: Increases flow over respiratory surface.

• Fish gills: Water enters mouth, passes slits, flows over gills.

• Countercurrent exchange: Enhances gas exchange; removes >80% of $O_2$ from water.

• Gills are unsuited for terrestrial animals: water loss by evaporation and collapse of structure.

Tracheal System

• Adaptation of terrestrial animals.

  • Advantages (air): Higher $O_2$ concentration, faster diffusion, less energy for ventilation.

  • Disadvantages: Water loss due to evaporation reduced by folding respiratory surface into the body.

• Insects: Air tubes (tracheae) branch throughout body. Tracheoles extend to cell surface for gas exchange.

• Open circulatory system: Does not transport $O<em>2$ and $CO</em>2$.

• Ventilation: Rhythmic body movements (esp. flight).

Lungs

• Restricted to one location. Circulatory system transports gases.

• Spiders, terrestrial snails, vertebrates.

• Amphibians: Small lungs or rely on skin diffusion.

• Reptiles/mammals: Rely entirely on lungs.

• Birds: Air sacs increase respiratory efficiency; one-way air flow.

• Lung size/complexity correlates with metabolic rate.

• Mammals: Negative pressure breathing pulls air into lungs.

Human Respiratory System

• Lungs in thoracic cavity; spongy texture, moist epithelium.

• Air enters nostrils, is filtered/warmed/humidified/sampled.

• Nasal cavity → pharynx → larynx (voice box). Epiglottis covers larynx during swallowing.

• Larynx → trachea (cartilage rings) → bronchi → bronchioles.

• Epithelium lining has cilia and mucus. Bronchioles lead to alveoli (air sacs).

• Alveolar surface area =
  $\approx$ 100 m2; $O<em>2$ dissolves in film and diffuses to capillaries; $CO</em>2$ diffuses opposite.

Role of Partial Pressure Gradient

• Gas diffuses from high to low partial pressure.

• Atmospheric pressure = 760 mm Hg.

  • Partial pressure of $O_2$ is
    $\approx$ 160 mm Hg.

  • Partial pressure of $CO_2$ is 0.23 mm Hg.

• In lungs: $CO<em>2$ diffuses from blood to air; $O</em>2$ diffuses from air to blood.

• In tissue capillaries: $O<em>2$ diffuses out of blood; $CO</em>2$ diffuses into blood.

Respiratory Pigments

• Support energy metabolism.

  • Hemocyanin: Copper-based in arthropods/molluscs; bluish blood.

  • Hemoglobin: Iron-based in red blood cells. Four subunits, each with a heme group. Carries four $O_2$ molecules. $Hb + 4O<em>2 \rightleftharpoons HbO</em>8$

Oxygen Transport and Bohr Effect

• Low $O_2$ solubility in water.

Oxygen Dissociation Curve for Hemoglobin

• $O*2$ saturation (%) vs. partial pressure of $O<em>2$ ($PO</em>2$). Sigmoid Curve = Advantage: Hemoglobin releases more $O<em>2$ to the tissues. Shows how readily hemoglobin acquires and releases $O</em>2$.

• Steep slope: Slight change in $PO<em>2$ causes substantial $O</em>2$ loading/unloading found in body tissues.

• High $pO<em>2$ : Hemoglobin is almost fully saturated; picks up $O</em>2$ in lungs.

• Small plateau: Minimal effect on % saturation; $pO<em>2$ drops slightly, but hemoglobin does not lose much $O</em>2$.

• Significant effect of $pO<em>2$: Hemoglobin gives up most of its $O</em>2$.

The Bohr Shift

• A shift to the right of the oxygen-hemoglobin dissociation curve due to an increase in carbon dioxide or acid in the blood.

• Hemoglobin conformation is sensitive to pH, $CO<em>2$, temperature. Lower pH (higher $CO</em>2$) lowers $O_2$ affinity (Bohr shift). Higher temperature shifts curve right.

• Active tissue: Lowers pH, induces hemoglobin to release more $O<em>2$, by increasing the rate of respiration and increase of $CO</em>2$ released.

• Hemoglobin efficiently uptakes $O<em>2$ when $CO</em>2$ is low (e.g., in lungs).

• Myoglobin: $O<em>2$ store in muscle fibers; releases $O</em>2$ at very low pressures.

• Fetal Hemoglobin: Dissociation curve offset to the left of maternal hemoglobin with High affinity for $O_2$, so it can take $O*2$ from maternal hemoglobin.

Carbon Dioxide Transport

• Hemoglobin transports $CO_2$ and buffers blood pH.

  • 7% in solution.

  • 23% binds to hemoglobin.

  • 70% as bicarbonate ions.

• $CO<em>2$ from cells diffuses into blood plasma and RBCs. Reacts with water (carbonic anhydrase) to form $H</em>2CO_3$.

Control of Breathing

• Centers in medulla oblongata and pons. Medulla sets basic rhythm.

  • Inspiratory center increases rate. Expiratory center cuts off inspiratory activity.

  • Pons controls transition from inhalation to exhalation.

  • Negative feedback via stretch receptors prevents over-expansion.

• Medulla monitors blood $CO<em>2$ level; chemoreceptors detect changes in pH (increase in $CO</em>2$). Increases breathing depth/rate when there is High $CO_2$ levels.

• $O_2$ levels have little effect unless markedly low.

Lung Volumes

• Use a spirometer to measure inhaled/exhaled air volume.

• Terms:

  • Tidal Volume (TV): Normal breathing volume.

  • Inspiratory Reserve Volume (IRV): Max volume inspired from normal inspiration.

  • Expiratory Reserve Volume (ERV): Max volume expired from normal expiration.

  • Residual Volume (RV): Volume remaining after max expiration.

  • Inspiratory Capacity (IC): Max volume inspired from resting level.

  • Vital Capacity (VC): Max volume exhaled after max inspiration.

  • Functional Residual Capacity (FRC): Volume remaining after normal expiration.

  • Total Lung Capacity (TLC): Total lung volume after max inspiration.

Respiratory Diseases

• Asthma: Bronchiolar constriction, mucus, breathing difficulty due to Constriction of smooth muscles in the bronchiolar and bronchial wall, excess mucus secretion and insufficient recoil of the alveoli.. Caused by allergy/emotional upset.

• Pneumonia: Alveoli filled with fluid, caused by chemical, bacteria (Streptococcus), viruses, protozoa or fungi.

• Tuberculosis: Mycobacterium tuberculosis damages lungs.

• Lung Cancer: Inhaled irritants → abnormal growth; for Cigarette smokers have 20 times more risk than non-smokers.

• COVID-19: Affects upper/lower respiratory tracts. Can cause pneumonia/ARDS.