Human Gas Exchange Notes

Human Gas Exchange

Structures Involved in Gas Exchange

• Nose: Filters, moistens, and warms air.

• Pharynx: Throat; passage for food & air. Dust and mucus are swept back by cilia.

• Larynx: Voice box; if non-gas enters, the cough reflex activates.

• Trachea: Epiglottis covers trachea during swallowing; contains C-shaped ringed cartilage covered by cilia and mucus cells.

• Bronchi → Bronchioles: Two bronchi, which enter the lungs and branch into narrower bronchioles.

• Alveoli: Small sacs at the end of bronchioles, surrounded by blood-carrying capillaries.

  • Where gas exchange between the circulatory system and lungs occurs via diffusion.

  • Coated with surfactant, reducing surface tension.

• Lungs:

  • Left lung: smaller, 2 lobes.

  • Right lung: larger, 3 lobes.

• Pleurae: Membranous cover with 2 layers:

  • Visceral pleura: Lines lung surface.

  • Parietal pleura: Lines inside of chest cavity.

  • Intrapleural space: Between the 2 layers.

• Diaphragm: Lower border of the thoracic cavity.

• Intercostal muscles: Aid in breathing.

Cells of Respiration

• Epithelium functions to moisten the tract, protect from pathogens, and aid in gas exchange.

• Trachea and bronchi have pseudostratified cells.

• 3 main types:

  1. Goblet cells: Secrete mucus.

  2. Basal cells: Produce new cells.

  3. Cilia cells: Sweep debris trapped by mucus.

Mechanics of Breathing

Inhalation

1. Diaphragm moves down (contracts).

2. Volume in the lungs increases.

3. Pressure inside of the lungs decreases.

4. Air flows in.

Exhalation

1. Diaphragm rises (relaxes).

2. Volume in the lungs decreases.

3. Pressure inside of the lungs increases.

4. Air rushes out.

Chemistry of Gas Exchange

• $CO<em>2$ is more soluble in blood than $O</em>2$.

• Most $CO<em>2$ in blood is transported as $HCO</em>3^-$ (bicarbonate ion) in plasma.

• The conversion of $CO<em>2$ into $HCO</em>3^-$ is catalyzed by the enzyme carbonic anhydrase via the following reaction: $CO<em>2 + H</em>2O \rightleftharpoons H<em>2CO</em>3 \rightleftharpoons H^+ + HCO_3^-$

• Bicarbonate ($HCO_3^-$): Main buffer molecule in blood.

  • "Buffering" maintains a certain pH to function under physiological ideals.

• Hemoglobin: A protein structure with 4 polypeptide subunits complexed around an Fe atom, similar to chlorophyll with Mg.

Control of Respiration

• Medulla oblongata: Respiratory control center of the brain.

• Control of diaphragm (through phrenic nerve):

  • $\uparrow$ partial pressure $CO_2$ = medulla oblongata stimulates rate of ventilation = diaphragm signaled to contract = lungs inflate = thoracic pressure decreases

• Chemoreceptors: Measure concentrations of specific gases and acids and use that information to tell the brain what changes in respiration need to be made.

  • Central chemoreceptors (in medulla): indirectly monitor [$H^+$] in the cerebrospinal fluid.

  • Peripheral chemoreceptors (in carotid arteries and aorta): function to monitor the atrial concentrations of $CO<em>2$, $O</em>2$, and pH via $H^+$.

Dissociation Curve

• A left shift on the oxygen dissociation curve results in oxygen being held more tightly by hemoglobin.

• A right shift involves physiological states where tissues need more oxygen.

• Mnemonic “CADET, face right!” to remember the factors ($CO_2$, Acid, 2,3-DPG, Exercise, and Temperature) that shift the oxygen dissociation curve to the right. A left shift occurs when the factors from the right shift are reversed in their direction.

Efficient Respiration

• Fish: The gills of fish provide a large surface area that allows for highly efficient respiration.

• Countercurrent exchange occurs as water outside the fish and blood inside the fish slowly pass each other = max diffusion of $O<em>2$ into the blood and $CO</em>2$ into water.