Chapter 23

• A respiratory exchange membrane is a thin layer of tissue consisting typically of one or two simple epithelia that separates the internal tissues of the animal from the environmental medium.
• External respiration, or breathing, is the process by which oxygen is transported to the gas-exchange membrane fro, the environmental medium and by which CO2 is transported away from the membrane into the environmental medium.
• Ventilation is bulk flow of air or water to and from the gas-exchange membrane during breathing.
• Not all animals employ ventilation, breathing can occur by diffusion rather than convection.
• Oxygen always crosses the gas-exchange membrane by diffusion.
• This means that for oxygen to enter an animal from the environment, the partial pressure of oxygen on the inside of the gas exchange membrane must be lower than that on the outside.
• The fact that oxygen enters animals by diffusion explains why the area and thickness of the gas-exchange membrane play critical roles in oxygen acquisition.
• The rate of diffusion across a membrane increases in proportion to the the area of the membrane.
• The rate of diffusion across a membrane increases as the membrane thickness decreases.
• Diffusion is the principal mechanism of CO2 diffusion, but active transfer also occurs.
• The identifies breathing organs, the lings and gills, take up almost all oxygen and void almost all CO2.
• Gills are respiratory structures that are evaginated from the body and surrounded by the environmental medium.
• Lungs are respiratory structures that are invaginated into the body and contain the environmental medium.
• Branchial is associated with gills and pulmonary is associated with lungs.
• Ventilation is active if the animal creates the ventilatory currents of air or water that flow to and from the gas-exchange membrane, using forces of suction or positive pressure that it generates by use of metabolic energy.
• Ventilation is passive if environmental air or water currents directly or indirectly induce flow to and from the gas-exchange membrane.
• Active ventilation is more reliable, controllable, and vigorous than passive ventilation.
• The oxygen utilization coefficient during breathing is the percentage of oxygen in an inhaled medium that an animal removes before exhaling the medium.
• Countercurrent gas exchange has the highest ability to establish a high oxygen partial pressure in blood exiting the breathing organ.
• Air breathers tend to have a much higher CO2 partial pressure in their systemic arterial blood than water breathers.
• The gill surface of most fish of a given body size is similar to the lung surface area of amphibians and reptiles of the same size.
• Mammals and birds have much more lung surface area, helping to meet their far higher needs fro gas exchange.
• The barrier between the blood and the air/water in the breathing organs is notably thin in mammals and birds.
• The skin can account for 25% or more of gas exchange in some fish, turtles, reptiles and up to 100% in some amphibians.
• THe skin is a minor contributor to gas exchange in mammals and birds.
• The breathing muscles of vertebrates are skeletal muscles activated by motor-neuron impulses.
• The breathing rhythm originates in a central pattern generator in the brainstem.
• The secondary lamellae are the principle sites of gas exchange in fish gills.
• Countercurrent gas exchange occurs in the lamellae of fish gills.
• Water flow across the gills is essentially unidirectional.
• Water flow across the gills is driven by a buccal pressure pump and an opercular suction pump that act in an integrated rhythm.
• The buccal pump drives water across the gills when the opercular pump drives is being empties of water and the opercular pump sucks water sucks water across the gills while the buccal pump is being filled with water.
• Some fish turn to ram ventilation when swimming fast enough.
• Some fish, such as tunas, are obligate ram ventilators and must swim all the time to avoid suffocation.
• A lowered oxygen partial pressure in the blood is a more potent stimulus for increased ventilation in fish than an elevated CO2 partial pressure.
• The lungs of mammals consist of dendritically branching airways that end blindly in small, thin-walled, well-vascularized outpocketings called the alveoli.
• The airway exhibits 23 levels of branching in the human adult lung, giving rise to 500 million alveoli.
• The airways in a mammalian lung are categorized as conducting and respiratory airways.
• Conducting airways have little gas exchange with the blood.
• Respiratory airways us where most gas exchange with the blood takes place.