Chapter 42 (6)

The process that ventilates lungs is __, the alternating inhalation and exhalation of air. A variety of mechanisms for moving air in and out of lungs have evolved.

An amphibian ventilates its lungs by __, inflating the lungs with forced airflow. During the first stage of inhalation, muscles lower the floor of an amphibian's oral cavity, drawing in air through its nostrils. Next, with the nostrils and mouth closed, the floor of the oral cavity risess, forcing air down the trachea. During exhalation, air is forced back out by the elastic recoil of the lungs and by compression of teh muscular body wall. When male frogs puff themselves up in aggressive or courtship displays, they disrupt this breathing cycle, taking in air several times without allowing any release.

Unlike amphibians, mammals employ __- pulling, rahter than pushing, air into their lungs using muscle contraction to actively expand the thoracic caivty, mammals lower air pressure in their lungs below that of the air outside their body. Because gas flows from higher to lower pressure, air rushes through the nostrils and mouth and down the breathing tubes to the alveoli. Durign exhalation, the muscles controlling the thoracic cavity relax, and the volume of the cavity is reduced. The increasesd air pressur ein the alveoli forces air up the brreathing tubes and out of the body. HTus, inhalation is always __ and requires work, whereas exhalation is usually __.

Expanding the thoracic cavity during inhalation involves the animal's rib muscles nad hte __, a sheet of skeletal muscle that forms the bottom wall of the cavity. Contracting the rib muscles expands the rib cage, the front wall fo the thoracic cavity, by pulling the ribs upward and thernum outward. At the same time, the __ contracts, expanding the thoracic cavity downward.

Within the thoracic cavity, a __ surrounds the lungs. What does the inner layer of htis membrane adhere to? outer layer?

A thin space filled with fluids separates the two layers. __ in the fluid causes the two layers to stick together. They can slide smoothly past each other, but can't be pulled apart easily. THe volume of the thoracic cavity and the ovlume of hte lungs changes in unison.

Depending on activity level, additional muscles may be recruited to aid breathing, The rib muscles and __ are sufficient to change lung voluem when a mammal is at rest. During exercise, other muscles of the neck, back and chest increase the volume of the thoracic cavity by raising the rib cage.

THe volume of air inhaled and exhaled with eac breath is called __. It averages about 500 mL in resting humans. THe __ during max inhalation/ exhalation is the __, which is about 3.4 L and 4.8 L for college-age women and men.

The air that remains after a forced exhalation is called the __. As we age, our lungs lose their resilence, and __ increases at the expense of vital capacity.

True or False:
Because the lungs in mammals do not completely empty with each breath, adn because inhalation occurs through the same airways as exhalation, each inhalation mixes fresh air w/ oxygen- depleted residual air. As a result, the max P_o2 in alveoli is always considerably less than in the atmosphere.
True False

__ is both more efficient and more complex in birds. When birds breathe, they passs air over the gas exchange surface in only one direction. Furthermore, incoming fresh air doesn't mix with air that has already carried out __. To bring fresh air to their lungs, birds use eight or nine air sacs situated on either side of the lungs.

THe air sacs don't function directly in gas exchange but act as bellows that keep air flowing through the lungs. Instead of alveoli, which are dead ends, the sites of gas exchange in bird lungs are tiny channels called __. Passage of air through the entire system- lungs and air sacs- requires __ cycles of inhalation and exhalation. In some passageways, the direction in which air moves alternates. Within the __, air always flows in the same direction.

True or False:
Beacuse the air in a bird' slungs is renewed with every exhalation, the max P_O2 in the lungs is higher in birds than in mammals.
True False

Networks of neurons that regulate breathing, called __, are located in two brain regions, the __ and __.

Control circuits int he medulla establisht he __, while neurons in the pons regulate its __.
- When you breathe deeply, a negative-feedback mechanism prevents the lungs from __: During inhalation, sensors that detect stretching of the lung tissue send nerve impulses to the control circuits in the medulla, inhibiting further inhalation.

In regulating breathing, the __ uses the pH of the surrounding tissue fluid as an indicator of blood CO2 concentration. THe reason pH can be used in this way is that blood CO2 is the main determinant of the pH of __, the fluid surrounding the brain and spinal cord. Carbon dioxide diffuses fromt eh blood to the __, where it reacts with water and forms __. The __ can then dissociate into a __ and a __.

CO2 + H2O <--> H2CO3<--> HCO3- + H+

Increased metabolic activity lowers pH by increasing the concentration of __ in teh blood. In response, the medulla's control circuits increase the depth and rate of breathing. Both remain high until the excess CO2 is eliminated in exhaled air and pH returens to a normal value.

The O2 concentration in the blood usually has little effect on the __. Hoever, when the O2 level drops very low, O2 sensors in the aorta nad the carotid artersies in the neck and singlas to the __, which respond by increasing hte breathing rate.

True or False:
Breathing control is effective only if it is coordinated with control of the cardiovascular system so that ventilation is matched to blood flow through alveolar capillaries.
True False