Respiration and Gas Exchange in Mammals

9. Respiration and gas exchange in mammals

• Bio 152, MCPHS University

• Jennifer Wade, Ph.D.

9.1 Mammalian Respiratory Anatomy

Components of Respiration

• Ventilation: Exchange of air between lungs and atmosphere.

• Exchange of O2and CO2 between lungs and blood.

• Transport of O2$and$CO2 by blood.

• Exchange ofO2$and$CO2 between blood and cells.

The Mammalian Respiratory System

• Nasal cavity

• Pharynx

• Larynx

• Trachea

• Primary bronchus

• Secondary bronchus

• Tertiary bronchus

• Bronchiole

• Terminal bronchiole

• Pulmonary vein

• Pulmonary artery

• Capillary

• Alveolar duct

• Alveolus

• Alveolar sac

• Diaphragm

Trachea and Bronchi

• The trachea and bronchi are held open by rings of cartilage.

• Includes the trachea, larynx, primary bronchi, secondary bronchi, and tertiary bronchi

Bronchioles

• Bronchioles contain smooth muscle that can adjust their diameter.

• Asthma causes constriction of bronchiole smooth muscle, leading to difficulty breathing.

Alveoli

• Alveoli are the site of gas exchange.

• Type I alveolar cell

• Type II alveolar cell

  • Alveolar fluid with surfactant

• Alveolar macrophage

• Respiratory membrane

  • Erythrocyte in pulmonary capillary

  • Capillary endothelial cell

  • Capillary basement membrane

  • Interstitial space

  • Epithelial basement membrane

Mucus and Mucociliary Escalator

• Mucus traps inhaled particles.

• Mucociliary escalator: mucus traps inhaled debris, cilia move it up to the pharynx.

• Saline (water + NaCl) thins the mucus.

• Patients with cystic fibrosis have genetic mutations that affect their ability to make saline, which causes repeated respiratory infections.

9.2 Ventilation

Spirometry

• Spirometry can measure lung volumes.

• Tidal Volume (TV)

• Inspiratory Reserve Volume (IRV)

• Expiratory Reserve Volume (ERV)

• Residual Volume (RV)

• Inspiratory Capacity (IC)

• Vital Capacity (VC)

• Functional Residual Capacity (FRC)

• Total Lung Capacity (TLC)

Breathing

• Breathing results from changes in pressure in the chest cavity.

• The diaphragm (and some other muscles) adjusts the size of the chest cavity.

• For gases, pressure and volume are inversely correlated.

• $P<em>1V</em>1 = P<em>2V</em>2$

• Increasing lung volume decreases pressure, causing air to enter the lungs.

Breathing Muscles

• Diaphragm

• Intercostal muscles between the ribs help the chest cavity expand widthwise.

• Abdominal muscles can also assist, especially for forced exhalation.

• All are skeletal muscle, although usually controlled involuntarily!

Homeostatic Control of Breathing

• Breathing is homeostatically controlled by the brain.

• $CO<em>2 + H</em>2O \rightarrow H<em>2CO</em>3$ (carbonic acid)

• Sensors in the carotid arteries (brain) and aorta (body) sense blood pH.

• Decreased pH is the main stimulus for breathing!

• Blood oxygen levels are also monitored and contribute to breathing rate, but they are less important!

Free Divers

• Free divers hyperventilate before diving.

• Hyperventilation lowers $CO_2$ levels.

• This increases the time you can go without taking a breath.

Surfactant

• Surfactant decreases the work of breathing.

• Lungs are coated with water; hydrogen bonds between water molecules make it harder to stretch the lungs.

• Surfactant fluid decreases surface tension on the inside of alveoli that could cause them to collapse.

• Surfactant production by type II alveolar cells normally begins around 25 weeks gestation and is complete by 34 weeks.

• Infant respiratory distress syndrome: lack of surfactant leads to tendency of alveoli to collapse easily; often fatal.

9.3 Gas Exchange

Thought Question

• In the lungs, oxygen travels from alveoli into the blood and carbon dioxide does the opposite.

• In tissues, oxygen flows OUT of the bloodstream and carbon dioxide flows IN.

• How do these gases "know" where to go?

Partial Pressures

• $P = (P_{atm}) \times (percent content in mixture)$

• $P_{atm}$, the atmospheric pressure, is the sum of all of the partial pressures of the atmospheric gases added together.

• $P<em>{atm} = P</em>{N<em>2} + P</em>{O<em>2} + P</em>{H<em>2O} + P</em>{CO_2} = 760 \text{ mm Hg}$

• The pressure of the atmosphere at sea level is 760 mm Hg. Therefore, the partial pressure of oxygen is:

• $P<em>{O</em>2} = (760 \text{ mm Hg}) \times (0.21) = 160 \text{ mm Hg}$

• and for carbon dioxide:

• $P<em>{CO</em>2} = (760 \text{ mm Hg}) \times (0.0004) = 0.3 \text{ mm Hg}$

Humidity and Altitude

• Humidity and altitude lower atmospheric pressure.

• When the air mixture reaches the lung, it has been humidified. The pressure of the water vapor in the lung does not change the pressure of the air, but it must be included in the partial pressure equation. For this calculation, the water pressure (47 mm Hg) is subtracted from the atmospheric pressure:

• $760 \text{ mm Hg} - 47 \text{ mm Hg} = 713 \text{ mm Hg}$

• and the partial pressure of oxygen is:

• $(760 \text{ mm Hg} - 47 \text{ mm Hg}) \times 0.21 = 150 \text{ mm Hg}$

Gas Flow

• $O<em>2$ and $CO</em>2$ flow from high to low concentration!

• Includes ambient air, oxygenated arteries (upper and lower torso), deoxygenated veins (upper and lower torso), arterial blood, and venous blood

Oxygenation of Blood

• Oxygenation of blood happens very quickly.

• Partial pressure of oxygen reaches 100 mm Hg in about 0.25 seconds in the pulmonary capillary.

Pathologies

• Pathologies can impair exchange

• $Diffusion \propto \frac{\text{surface area } \times \text{ barrier permeability}}{\text{distance}^2}$

• Normal lung

• Emphysema

  • Destruction of alveoli means less surface area for gas exchange.

• Fibrotic lung disease

  • Thickened alveolar membrane slows gas exchange. Loss of lung compliance may decrease alveolar ventilation.

• Pulmonary edema

  • Fluid in interstitial space increases diffusion distance. Arterial $P<em>{CO</em>2}$ may be normal due to higher $CO_2$ solubility in water.

• Asthma

  • Increased airway resistance decreases alveolar ventilation.

9.4 Gas Transport in the Blood

Hemoglobin

• 98% of blood oxygen is bound to hemoglobin

• A hemoglobin molecule is composed of four protein globin chains, each centered around a heme group. In most adult hemoglobin, there are two alpha chains and two beta chains as shown.

• Each heme group consists of a porphyrin ring with an iron atom in the center.

Pulse Oximetry

• Pulse oximetry uses light sensors to measure color of hemoglobin.

• Oxygenated hemoglobin has a bright red color.

• Deoxygenated hemoglobin is more dark red color.

Oxygen Binding

• The amount of oxygen bound to Hb depends on…

• % Saturation of Hb

• Plasma $O_2$

• The amount of hemoglobin

  • Total number of Hb binding sites

  • Hb content per RBC

  • Number of RBCs

Factors Influencing Oxygen Binding

• $O<em>2$ binding to hemoglobin is influenced by pH, $CO</em>2$, temperature

• Carbon monoxide’s (CO) affinity for hemoglobin is much stronger than oxygen’s!

Fetal Hemoglobin

• Fetal hemoglobin binds $O_2$ more tightly.

• Fetal hemoglobin saturation curve is to the left of the maternal hemoglobin saturation curve.

Gas Solubility

• $CO<em>2$ is more water-soluble than $O</em>2$

• When temperature remains constant, the amount of a gas that dissolves in a liquid depends on both the solubility of the gas in the liquid and the partial pressure of the gas.

• [Gas]$<em>{diss}$ = [P$</em>{gas}$] $\times$ a

  • a for oxygen is $(0.03 \text{ mL } O<em>2 / \text{L blood}) / \text{mm Hg } P</em>{O_2}$

  • a for $CO<em>2$ is $(0.7 \text{ mL } CO</em>2 / \text{L blood}) / \text{mm Hg } P<em>{CO</em>2}$

$CO_2$ Transport in Blood

• Dissolved in plasma: 7%

• Converted to bicarbonate ion: 70%

• Bound to hemoglobin: 23%