Respiratory Physiology: Key Concepts

• Key concepts for respiratory physiology are summarized
• Types of respiration are defined

Types of Respiration

• Anaerobic respiration doesn't require oxygen
• Aerobic respiration requires oxygen
• External respiration transports O2 into the body and CO2 out of the body, involving the body and external environment
• Internal respiration transports O2 into cells and CO2 out of cells, involving the body and internal environment
• Cellular respiration consists of intracellular catabolic reactions converting stored energy to ATP

Fick's Law

• Fick's law applies to understanding diffusion of respiratory gases
• Fick's law defines diffusion rate/transport of masses and solutes
• Diffusion depends on concentration gradients and the distance between them
• Adaptations to counter diffusion of gases occur across the respiratory membrane/organ
• Breathing movements include ventilation
• Gases diffuse across the respiratory epithelia and capillary walls
• The circulatory system is responsible for bulk transport of gases

Water vs. Air as Respiratory Mediums

• Higher altitude reduces the inspired pressure of oxygen, but not the percentage of oxygen in the atmosphere
• Water is more viscous and dense and has limited oxygen
• Increased water temperature decreases oxygen solubility
• Oxygen is less soluble in water
• Respiratory systems vary in water and air-breathing animals, including insects
• Water-breathing animals have gills, which are invaginations of the body

Gill Function

• Water moves over the gills, and cilia create a water current for gill ventilation
• Tracheal systems in insects are used for gas exchange
• Birds use air sacs and lungs, while mammals use mammalian lungs
• Fish gills facilitate counter-current gas exchange
• Gills are multifunctional body organs and part of the ventilation system for fish
• Fish have external and internal gills; external gills extend outside the body, while internal gills are located within the body
• Currents of water are directed over the gills

Gill Protection and Countercurrent Flow

• Fish gills utilize countercurrent flow, where blood and water move in opposite directions
• Blood leaving the capillaries has the same oxygen content as fully oxygenated water entering the gills
• Countercurrent flow prevents equilibrium, enabling diffusion due to the concentration gradient

Avian Respiratory System

• The avian respiratory system involves air sacs and lungs, with cross-current gas exchange
• During first inhalation, most oxygen flows directly to the posterior air sacs
• During exhalation, both anterior and posterior air sacs contract
• During the next inhalation, air from the lung moves into the anterior air sac
• During the second exhalation, air from anterior sacs is expelled to the outside through the trachea
• Blood flow runs concurrently, with small amounts of blood coming into contact with air

Respiratory Adaptations

• High-altitude flying birds have respiratory adaptations
• These birds have larger lungs and high capillary density in flight muscles

Human Lung Structure

• Human lungs, including tidal ventilation
• Mammalian lungs have two branching airways: the trachea divides into two primary bronchi
• Primary bronchi branch and re-branch into bronchioles, which then branch into alveoli
• Both lungs are surrounded by a pleural sac

Lung Volumes and Capacities

• Total lung capacity (TLC) is the maximum amount of air that can be held in the lungs
• Tidal Volume (TV) is the volume of air entering and exiting during a single breath
• Functional residual capacity (FRC) is the amount of air left in the lung after exhalation
• Residual volume (RV) is the consistent amount of air in the lungs, which prevents lung collapse
• Vital capacity (VC) is the maximum amount of air that can be moved out during a single breath following maximum inhalation

Tidal Ventilation

• Tidal ventilation involves fresh air mixing with stale leftover air, decreasing the partial pressure of oxygen.

Neural Control of Breathing

• Neural control of breathing in mammals involves peripheral chemoreceptors that monitor carbon dioxide pressures and pH
• A secondary response monitors oxygen pressure
• Lung ventilation is regulated by sensors (chemoreceptors and mechanoreceptors) that detect carbon dioxide, pH, and oxygen
• These sensors signal central controllers (pons, medulla), which activate effectors (respiratory muscles)

Transport of O2 and CO2

• Oxygen is carried in the blood by red blood cells (98%) and plasma (2%)
• Hemoglobin in red blood cells carries most of the oxygen
• Because oxygen is not very soluble in plasma water, hemoglobin is needed to transport oxygen to the tissues
• Hemoglobin is an iron-containing oxygen transport metalloprotein found in almost all vertebrate red blood cells
• 96% of a red blood cell is hemoglobin
• Hemoglobin can bind up to 4 oxygen molecules
• There's a sigmoidal curve relationship between oxygen and hemoglobin
• Oxygen binding in one site increases the affinity of other oxygen-binding sites

Hemoglobin Affinity

• Hemoglobin's affinity for oxygen is affected by temperature, pH (low pH reduces Hb affinity, releasing oxygen), and carbon dioxide.

Gas Exchange

• Oxygen diffuses from blood to interstitial fluid to body cells during O2 and CO2 exchange in lung and systemic capillaries
• 10% of carbon dioxide dissolves in plasma
• 70% of carbon dioxide is converted into hydrogen and bicarbonate ions
• 20% of carbon dioxide combines with hemoglobin
• Carbonic anhydrase, a metalloenzyme with zinc, catalyzes the rapid interconversion of carbon dioxide and water into bicarbonate and hydrogen ions
• This interconversion helps transport carbon dioxide out of tissues and maintain acid-base balance.
• Carbon dioxide is largely transported as bicarbonate ions in plasma, mostly produced in red blood cells with carbonic anhydrase.