Biology Chapter on Gas Exchange

Introduction to Gas Exchange

• Lungs are large, paired, spongy organs located in the thoracic cavity (chest).
• Each lung is covered by a pleural membrane.
• The right lung has three lobes, while the left lung has two lobes.
• Air travels through the following structures:
  • Nostrils
  • Nasal cavities
  • Pharynx
  • Larynx
  • Trachea
  • Bronchi
  • Bronchioles
  • Alveoli
• Humans cannot survive for more than a few minutes without oxygen.

Respiratory Pigment

• Most organisms need respiratory pigments to transport oxygen (O2) and carbon dioxide (CO2).
• A respiratory pigment is a molecule that increases the ability to transport respiratory gases.
• Vertebrates utilize two main respiratory pigments:
  • Hemoglobin (found in erythrocytes)
  • Myoglobin (found in muscles)
• Erythrocytes, or red blood cells (RBCs), lack nuclei and organelles to maximize space for hemoglobin.
• RBCs also lack mitochondria.

Hemoglobin

• Hemoglobin is a protein found in red blood cells that carries oxygen and helps transport carbon dioxide.
• It also buffers the blood.
• Hemoglobin is a conjugated protein with a quaternary structure.
• It consists of four polypeptide chains (subunits):
  • Two α-chains
  • Two β-chains
• Each chain is linked to a heme (iron-porphyrin) ring.
• An iron atom is located at the center of each heme ring.
• Each iron atom can bind to one oxygen molecule.
• A single hemoglobin molecule can bind with four oxygen molecules.

Regulation of Gas Exchange in Humans

• Gas exchange is the process of exchanging oxygen and carbon dioxide between an organism and its environment.
• Gas exchange provides oxygen for cellular respiration and removes its waste product, carbon dioxide.
• Gas exchange in humans and other animals with lungs involves three phases:
  • Breathing: The process of inhaling and exhaling air.
  • Transport of gases by the circulatory system: Oxygen and carbon dioxide are transported through the bloodstream.
  • Exchange of gases with body cells: Cells take up oxygen from the blood and release carbon dioxide into the blood.

Negative Pressure Breathing

• In humans and other mammals, ventilation (breathing) occurs through negative pressure breathing.
• Negative pressure breathing is a system where air is pulled into the lungs.
• The contraction of rib muscles and the diaphragm expands the thoracic cavity, reducing air pressure in the alveoli and drawing air into the lungs.

Breathing Control Center

• Breathing control centers in the brain coordinate breathing with body needs by sensing and responding to the pH of the cerebrospinal fluid, which indicates the CO2 level in the blood.
• A drop in blood pH triggers an increase in the rate and depth of breathing.
• The breathing control centers accommodate the increased need for oxygen during exercise.

Transport of Gases

• Blood transports respiratory gases.
• The heart pumps oxygen-poor blood to the lungs, where it picks up oxygen and drops off carbon dioxide.
• Oxygen-rich blood returns to the heart and is pumped to body cells, where it delivers oxygen and picks up carbon dioxide.

Human Fetus and Mother's Blood

• Fetal hemoglobin attracts oxygen more strongly than adult hemoglobin.
• This enhances oxygen transfer from maternal blood in the placenta.
• At birth, rising carbon dioxide in fetal blood stimulates the breathing control center to initiate breathing.

Mechanisms of Gas Exchange in Animals

• Respiratory surfaces must be thin and moist for the diffusion of oxygen and carbon dioxide to occur.
• Some animals use their entire skin as a gas exchange organ.
• In most animals, gills, a tracheal system, or lungs provide large respiratory surfaces for gas exchange.

Respiratory Surfaces and Organs: Gills

• Gills are adapted for gas exchange in aquatic environments.
• They absorb oxygen dissolved in water.
• In fish, gas exchange is enhanced by ventilation (the flow of water over the respiratory surface) and countercurrent exchange (the transfer of oxygen between two fluids flowing in opposite directions).

Respiratory Surfaces and Organs: Trachea

• The tracheal system of insects provides direct exchange between the air and body cells.
• The tracheal system transports oxygen directly to body cells and moves carbon dioxide away from them.
• This system reduces evaporative water loss.
• It is highly efficient system for delivering oxygen and removing carbon dioxide.
• The largest tubes in the tracheal system, called tracheae, connect to external openings spaced along the body.

Respiratory Surfaces and Organs: Lungs

• Lungs are internal structures that function in gas exchange.
• Air contains a much higher concentration of oxygen than does water.
• Air is much lighter and easier to move than water.
• Lungs have a large surface area for gas exchange due to their intricate structure and branching pattern of airways.