Respiratory System: Gas Exchange and Partial Pressure

Gas Exchange and Respiratory Systems

• Gas exchange involves the uptake of O2 and disposal of CO2 through respiratory surfaces, facilitated by net diffusion from areas of higher to lower partial pressure.
• Tracheal systems in insects consist of a network of branching tubes throughout the body, requiring ventilation to meet O2 demands.

Respiratory Media and Surfaces

• Respiratory media include air and water, with O2 availability being lower in water, necessitating greater efficiency in obtaining O2.
• Animals require large, moist respiratory surfaces for gas exchange, which can include skin, gills, tracheae, and lungs.

Mammalian Respiratory System

• The mammalian respiratory system consists of a system of branching ducts that conveys air to the lungs.
• Nostrils filter, warm, humidify, and sample inhaled air for odors.
• Pharynx directs air to the lungs and food to the stomach.
• Larynx contains vocal cords where exhaled air passes over to create sounds.

Gas Exchange and Lung Function

• Alveoli are air sacs at the tips of bronchioles where gas exchange occurs.
• Oxygen diffuses through the moist epithelium into capillaries, while carbon dioxide diffuses from capillaries across the epithelium into the air space.
• Surfactants coat the surface of alveoli to prevent collapse.
• Lung volume increases as the rib muscles and diaphragm contract.

Breathing and Control

• Breathing is the process of ventilating the lungs through inhalation and exhalation.
• Control of breathing in humans is usually regulated by involuntary mechanisms in the medulla oblongata of the brain.
• The rate and depth of breathing respond to pH changes in the cerebrospinal fluid.

Coordination of Circulation and Gas Exchange

• Blood arriving in the lungs has a low partial pressure of O2 and a high partial pressure of CO2 relative to air in the alveoli.
• Oxygen diffuses into the blood, and CO2 diffuses into the air in the alveoli.
• In tissue capillaries, partial pressure gradients favor diffusion of O2 into interstitial fluids and CO2 into the blood.

Respiratory Adaptations in Other Animals

• Amphibians, such as frogs, ventilate their lungs by positive pressure breathing.
• Birds have 8 or 9 air sacs that function as bellows, keeping air flowing through the lungs with highly efficient ventilation.
• Diving mammals conserve oxygen by changing buoyancy, decreasing blood supply to muscles, and deriving ATP in muscles from fermentation once oxygen is depleted.

Respiratory Pigments

• Hemoglobin is a protein that transports oxygen, greatly increasing the amount of oxygen that blood can carry.
• Hemocyanin is found in arthropods and many molluscs, using copper as the oxygen-binding component.
• Diving mammals also store oxygen in their muscles in myoglobin proteins.

Carbon Dioxide and Hemoglobin

• Carbon dioxide is transported in blood plasma, bound to hemoglobin, and also reacts with water to form H2CO3, which dissociates into H+ and bicarbonate ions (HCO3-).
• Hemoglobin plays a minor role in transport of CO2 and assists in buffering the blood.
• In the lungs, the relative partial pressures of CO2 favor the net diffusion of CO2 out of the blood.