T OPIC 6.4 G AS EXCHANGE Essential idea: The lungs are actively ventilated to ensure that gas exchange can occur passively. Nature of science: Obtain evidence for theories— epidemiological studies have contributed to our understanding of the causes of lung cancer. (1.8)

U NDERSTANDINGS Ventilation maintains concentration gradients of oxygen and carbon dioxide between air in alveoli and blood flowing in adjacent capillaries. Type I pneumocytes are extremely thin alveolar cells that are adapted to carry out gas exchange. Type II pneumocytes secrete a solution containing surfactant that creates a moist surface inside the alveoli to prevent the sides of the alveolus adhering to each other by reducing surface tension. Air is carried to the lungs in the trachea and bronchi and then to the alveoli in bronchioles. Muscle contractions cause the pressure changes inside the thorax that force air in and out of the lungs to ventilate them. Different muscles are required for inspiration and expiration because muscles only do work when they contract.

A PPLICATIONS AND SKILLS : Application: Causes and consequences of lung cancer. Application: Causes and consequences of emphysema. Application: External and internal intercostal muscles, and diaphragm and abdominal muscles as examples of antagonistic muscle action. Skill: Monitoring of ventilation in humans at rest and after mild and vigorous exercise. (Practical 6)

RESPIRATION The act of breathing (ventilation) cellular respiration converts the energy in nutrients into the ATP requires a steady supply of oxygen and generates carbon dioxide as a waste product circulatory system works in close harmony with the respiratory system The circulatory system extracts oxygen from the air in your lungs, carries it within diffusing distance of each cell, and then picks up carbon dioxide for release from the lungs

THREE REQUIREMENTS THAT FACILITATE DIFFUSION ? Respiratory surfaces remain moist so that gases can diffuse across cell membranes cells lining respiratory surfaces are very thin, to facilitate diffusion of gases through them Respiratory systems have a sufficiently large area in contact with the environment to allow adequate gas exchange

A N O VERVIEW OF G AS E XCHANGE IN H UMANS Fig O2O2 O2O2 CO 2 alveoli (air sacs) Oxygenated blood Deoxygenated blood Gases move in and out of the lungs by breathing O 2 and CO 2 are exchanged in the lungs by diffusion Gases dissolved in the blood are transported by the circulatory system 2 3 O2O2 O2O2 1 CO 2 left ventricle left atrium O 2 and CO 2 are exchanged in the tissues by diffusion 4 O2O2 right atrium right ventricle CO 2 O2O2

(a) Human respiratory system(b) Alveoli with capillaries bronchiole pulmonary venule alveoli capillary network pulmonary veins pulmonary artery diaphragm nasal cavity pharynx oral cavity epiglottis larynx esophagus trachea bronchi bronchioles rings of cartilage pulmonary arteriole T HE H UMAN R ESPIRATORY S YSTEM Fig. 33-7

H OW D OES THE H UMAN R ESPIRATORY S YSTEM W ORK ? The conducting portion of the respiratory system carries air to the lungs The epiglottis prevents food from entering the larynx when swallowing During normal breathing, the epiglottis is tilted upward, allowing air to flow into the larynx During swallowing, the epiglottis folds downward and covers the larynx, directing substances into the esophagus

THE RESPIRATORY SYSTEM CARRIES AIR TO THE LUNGS Inhaled air travels past the larynx into the trachea, a flexible tube whose walls are reinforced with semicircular bands of stiff cartilage trachea splits into two bronchi, one leading to each lung Inside lung, each bronchus branches repeatedly into ever small tubes called bronchioles Bronchioles lead to microscopic alveoli, the tiny air sacs where gas exchange occurs

G AS EXCHANGE OCCURS IN THE ALVEOLI Alveoli cluster around the end of each bronchiole like a bunch of grapes, providing a huge surface area for diffusion, totaling about 1,500 square feet A network of capillaries covers most of the alveolar surface The walls of the alveoli consist of a single thin layer of epithelial cells The respiratory membrane, through which gases diffuse, consists of epithelial cells of the alveoli and the endothelial cells that form the wall of the capillary across which gas exchange occurs

A LVEOLI IN LUNGS ARE WELL ADAPTED FOR GAS EXCHANGE Because the alveolar walls and the adjacent capillary walls are each only one cell thick, gases must diffuse only a short distance to move between the air and the blood The alveoli are coated with a thin layer of watery fluid containing surfactant, which prevents the alveolar surfaces from sticking together and collapsing when air is exhaled Gases dissolve in this fluid as they pass in and out of the alveolar air

from the pulmonary artery alveolar membrane respiratory membrane surfactant fluid to the pulmonary vein (air)CO 2 O2O2 capillary Oxygen diffuses into the red blood cells Carbon dioxide diffuses into the alveolus G AS E XCHANGE B ETWEEN A LVEOLI AND C APILLARIES Fig. 33-9

O XYGEN AND CARBON DIOXIDE ARE TRANSPORTED IN BLOOD USING DIFFERENT MECHANISMS Blood picks up oxygen from the air in the lungs and supplies it to the body tissues, simultaneously absorbing CO 2 from the tissues and releasing it into the lungs These exchanges occur because diffusion gradients favor them In the lungs, O 2 is high and CO 2 is low, whereas in body cells, CO 2 is high and O 2 is low

H OW D OES THE H UMAN R ESPIRATORY S YSTEM W ORK ? Air is inhaled actively and exhaled passively Breathing occurs in two stages Inhalation, when air is drawn into the lungs the chest cavity is enlarged diaphragm is contracted, which pulls it downward, and the rib muscles contract, lifting the ribs up and outward Exhalation, when air is expelled from the lungs the muscles that cause inhalation are relaxed As the diaphragm relaxes, it domes upward; at the same time, the ribs fall down and inward Both of these movements decrease the size of the chest cavity and force air out of the lungs

Air moves in Air moves out Rib cage contracts Lungs compress Diaphragm relaxes upward Diaphragm contracts downward Rib cage expands Lungs expand (a) Inhalation(b) Exhalation T HE M ECHANICS OF B REATHING Fig

B REATHING RATE IS CONTROLLED BY THE RESPIRATORY CENTER OF THE BRAIN The respiratory center is located in the medulla portion of the brain, just above the spinal cord Nerve cells in the respiratory center generate cyclic action potentials that cause contractions (followed by passive relaxation) of respiratory muscles The respiratory center receives input from several sources and adjusts the breathing rate and volume to meet the body’s changing needs

B REATHING RATE IS CONTROLLED BY THE RESPIRATORY CENTER OF THE BRAIN Breathing rate can be primarily modified by CO 2 receptors located in the medulla that adjust the breathing rate to maintain a constant low level of CO 2 in the blood, while also ensuring that O 2 levels remain adequate As a backup system, there are also O 2 receptors in the aorta and carotid arteries that stimulate the respiratory center to increase the rate and depth of breathing if O 2 levels in the blood drop