Gas Exchange and Partial Pressure

Gas Exchange

• Gas exchange involves the uptake of O2 from the environment and the release of CO2 back to it

Partial Pressure

• Partial pressure is the pressure exerted by a single gas within a mixture of gases
• At sea level, atmospheric pressure is 760 mm Hg
• Oxygen accounts for 21% of the atmosphere, resulting in a partial pressure of 160 mm Hg (P02), while carbon dioxide (PC02) has a partial pressure of 0.29 mm Hg
• Partial pressures are applicable to gases dissolved in liquids like water
• Each gas dissolved in water has a partial pressure equivalent to its partial pressure in the air
• Oxygen is significantly less soluble in water compared to air

Air vs. Water as Respiratory Media

• Air (at sea Level): O2 Partial Pressure is 160mm, O2 Concentration is 210 ml/L, Density is 0.0013 kg/L, Viscosity is 0.02 cP
• Water (at 20°C): O2 Partial Pressure is 160mm, O2 Concentration is 7 ml/L, Density is 1 kg/L, Viscosity is 1 cP
• Air-to-Water ratio: O2 Partial Pressure is 1:1, O2 Concentration is 30:1, Density is 1:770, Viscosity is 1:50

Respiratory Media

• Breathing air is relatively easy and efficient in humans, with an efficiency of about 25%
• Oxygen availability in water is less than in air for a given volume
• Obtaining oxygen from water demands greater efficiency and expends more energy; for example, fish have an efficiency of about 80%

Respiratory Surfaces

• Gas exchange across respiratory surfaces occurs through diffusion
• Diffusion rate is directly proportional to surface area and inversely proportional to the distance it travels
• Respiratory surfaces differ across animals and may include the skin, gills, tracheae, and lungs
• Earthworms, amphibians, and some reptiles can use skin as a respiratory organ

Gills

• Gills, found in aquatic animals like marine worms, crayfish and sea stars, are outfoldings that create large surface areas for gas exchange
• Ventilation involves moving the respiratory medium over the respiratory surface
• Aquatic animals achieve ventilation either by swimming or moving water over their gills, often involving coordinated mouth movements

Fish Gills

• Fish gills employ a countercurrent exchange system, where blood flows in the opposite direction of water passing over the gills
• This ensures blood is always less saturated with O2 than the water it encounters
• 80% of the dissolved O2 in the water is removed as it passes over the respiratory surface

Insect Tracheal Systems

• Insects possess a tracheal system, a network of branching tubes throughout the body that delivers O2 directly to cells, bypassing the circulatory system
• Larger insects must actively ventilate their tracheal system by opening and closing external openings to meet O2 demands

Lungs

• Lungs, found in mammals, amphibians, reptiles and birds, are internal infoldings of the body surface
• The circulatory system is responsible for transporting gases between the lungs and the rest of the body
• Lungs also evolved in organisms with open circulatory systems, like spiders and snails
• The size and complexity of lungs are linked to an animal's metabolic rate

Mammalian Respiratory System

• Mammals like humans, use a system of branching ducts to convey air to the lungs
• Air inhaled through the nostrils undergoes filtering, warming, humidification, and odor sampling
• The pharynx directs air toward the lungs and food towards the stomach
• During swallowing, the larynx moves upward, and the epiglottis covers the glottis to prevent food from entering the trachea
• Exhaled air passes over the vocal cords in the larynx, creating sounds

Process of Breathing

• Air flows sequentially through the pharynx, larynx, trachea, bronchi, and bronchioles before reaching the alveoli, where gas exchange takes place
• The epithelium of the air ducts, lined with cilia and mucus, moves particles up to the pharynx, cleaning the respiratory system
• This "mucus escalator" allows particles to be swallowed into the esophagus

Alveoli

• Gas exchange occurs in alveoli, which are air sacs located at the tips of bronchioles
• The total surface area of the alveoli is 50 times that of the skin
• Oxygen diffuses through the moist film of the epithelium into capillaries, while carbon dioxide diffuses from capillaries into the air space
• Alveoli lack cilia and are susceptible to contamination, so they're patrolled by white blood cells
• Alveoli are subject to surface tension
• Surfactants coat the alveoli to reduce surface tension
• Preterm babies often lack surfactant, making them vulnerable to respiratory distress syndrome, treated with artificial surfactants

Breathing

• Breathing, the alternate inhalation and exhalation of air, ventilates the lungs
• Amphibians such as frogs use positive pressure breathing to force air down the trachea

Avian Breathing

• Birds utilize air sacs to maintain a continuous flow of air through the lungs
• Air passes through the lungs in only one direction
• Air flows through parabronchi, tiny channels that serve as the gas exchange surface
• Two cycles of inhalation and exhalation are needed for air to pass entirely through the system of lungs and air sacs
• Ventilation in birds is highly efficient, as incoming air doesn't mix with outgoing air, important for flying at high altitudes

Mammalian Breathing

• Mammals employ negative pressure breathing to ventilate their lungs, essentially pulling air into the lungs
• Lung volume increases when rib muscles and the diaphragm contract, expanding the thoracic cavity
• Exhalation is passive, as the thoracic cavity relaxes
• Muscles in the neck, back, and chest can assist in increasing the volume of the thoracic cavity during exercise

Lung Volumes in Mammals

• Tidal volume, about 500 ml in resting humans, represents the volume of air inhaled with each breath
• Vital capacity, the maximum tidal volume, ranges from 3.4 L in women to 4.8 L in men
• Residual volume refers to the air remaining in the lungs after exhalation, increasing with age
• Mammals do not completely empty their lungs during exhalation
• During inhalation, incoming air mixes with residual air, resulting in lower oxygen partial pressure (P02) in alveoli compared to the atmosphere
• This explains why mammals do not function as efficiently as birds at high altitudes

Control of Breathing

• Breathing is regulated by involuntary mechanisms
• Breathing control centers reside in the medulla oblongata of the brain which adjusts the rate and depth of breathing in response to pH changes in the cerebrospinal fluid
• The bicarbonate buffer system helps maintain blood pH
• Sensors in the aorta and carotid arteries monitor O2 and CO2 concentrations in the blood
• These sensors signal the breathing control centers to respond as needed
• Effective breathing control requires ventilation is matched to blood flow through alveolar capillaries
• During exercise, increased breathing rate should be coupled to increased cardiac output

Adaptations and Respiratory Pigments

• The blood must transport large quantities of O2 and CO2 to meet the metabolic demands of many organisms
• Respiratory pigments, proteins that transport oxygen, can greatly increase the oxygen-carrying capacity of blood, from 4.5 to 200 ml per liter

Respiratory Pigments

• Arthropods and many molluscs utilize hemocyanin, with copper as the oxygen-binding component
• Most vertebrates and some invertebrates rely on hemoglobin
• Hemoglobin is contained within erythrocytes
• A single hemoglobin molecule can carry four molecules of oxygen, one molecule for each iron-containing heme group
• The hemoglobin dissociation curve illustrates a small change in the partial pressure of oxygen can lead to a significant change in oxygen delivery, indicated by a steep slope

Hemoglobin

• CO2 produced during cellular respiration lowers blood pH and decreases the affinity of hemoglobin for O2, known as the Bohr shift
• Hemoglobin plays only a minor role in carbon dioxide transport and aids in blood buffering

Carbon Dioxide Transport

• Only about 7% of CO2 from respiring cells diffuses directly into the blood to be transported in the blood plasma
• The remainder diffuses into erythrocytes and reacts with water to form H2CO3, which dissociates into H+ and bicarbonate ions (HCO3-)
• Most HCO3- diffuses to blood plasma and about 5% binds to hemoglobin
• Most H+ binds to hemoglobin and other proteins minimizing change in blood pH

Lungs & CO2

• In the lungs, the relative partial pressures of CO2 favor the net diffusion of CO2 out of the blood

Diving Mammals

• Diving mammals possess evolutionary adaptations that allow them to perform extraordinary feats
• Weddell seals can remain underwater for 20 minutes to an hour while Cuvier’s beaked whales can dive to 2,900 m and stay submerged for over 2 hours
• Diving mammals have a high blood to body volume ratio and stockpile O2, using it slowly
• Deep-diving air breathers, such as diving mammals, conserve oxygen
• They changing their buoyancy to glide passively, reducing heart rate and O2 consumption and routing blood to vital tissues.
• They also shut off blood supply to muscles and deriving ATP in muscles from fermentation once oxygen is depleted