Chapter 15 The Respiratory System
Conduction Zone • Organs include: • Nose, pharynx, trachea, bronchi, and bronchioles
RESPIRATORY ZONE • Alveoli and the capillary network within the lungs
Pulmonary Ventilation: the movement of air between the external environment and the air sacs within the lungs. • External respiration: gas molecules diffuse between the air sacs and the capillaries that surround them. • Internal Respiration: movement of oxygen and carbon dioxide between the bloodstream and body cells.
Pharynx is divided into three segments: • Nasopharynx: the superior part—It receives the internal nares and the openings to the two auditory tubes, which extend to the middle ear. • Oropharynx:the portion of the pharynx that you can see when you look into a mirror with your mouth open. • Laryngopharynx: below the level of the tongue—unites with the larynx in the neck.
Larynx: the voicebox short passageway that connects the pharynx with the trachea. It houses the vocal cords, and provides for the production of sound. • Thyroid cartilage--known as the “Adam’s apple”. • Cricoid cartilage--located below the thyroid cartilage. • Epiglottic cartilage--or epiglottis
False vocal cords-- the mucous membrane lining contains two pairs of folds that extend inward from the walls in a horizontal direction. The upper pairs are supported by skeletal muscle fibers that help move the larynx upward during swallowing. They do not function in sound production. • The lower folds are the true vocal cords.
TRACHEA • The WINDPIPE, about 12 cm long (4.5 inches) • It extends downward into the thoracic cavity, where it divides into the right and left bronchi. • Is internally lined with ciliated mucous membrane.
Contains large numbers of mucus-secreting cells . • This tissue is called PSCC, or pseudo-stratified ciliated columnar, epithelium.
BRONCHIAL TREE • The distal end of the trachea splits into the right and left primary bronchi. • Each primary bronchus extends a short distance from the trachea to a lung. • The left bronchus at a sharper angle than the right. As a result of this anatomical difference, materials accidentally inhaled usually end up in the right bronchus.
Each primary bronchus is very similar in structure to the trachea. • Its walls are supported by incomplete rings of cartilage and smooth muscle, and it is internally lined with ciliated mucous membrane. • The small tubes are called bronchioles and are very numerous within each lung.
The bronchioles continue to divide into yet smaller tubes, called alveolar ducts, which terminate as round, microscopic pouches known as alveoli. • There are about 300 to 500 million alveoli in the lungs of an average adult. • Each lung has an internal surface area about 80 times greater than the external body surface area, or about the area of a tennis court.
Each alveolus consists of a microscopic air space surrounded by a thin wall. This wall separates one alveolus from another, and nearby capillaries. • It is composed of a single layer of squamous epithelium.
Interspersed between epithelial cells are specialized cells that secrete a layer of detergentlike lipid molecules called surfactant. • The surfactant normally lines the inner surface of the alveolar wall, along with a thin layer of watery fluid.
The fluid is required to keep the alveolar surface moist, which is necessary for the diffusion of gases to proceed through the alveolar wall. • The water in this fluid exerts a strong attractive force (called surface tension) that causes alveolar walls to be drawn together and collapse when air leaves the alveolar chamber during expiration.
The surfactant counteracts this collapsing force, enabling the alveoli to inflate quickly after an expiration. • Without surfactant, the surface tension would be so great that it would require an enormous amount of muscular effort to reopen the alveoli.
Premature infants born before the seventh month of gestation have not yet developed the ability to produce surfactant. They risk death by suffocation because of the exhaustive effort required to reopen the alveoli during each breath. (treatment--incubators or respiratory tents) • This is known as RDS or hyaline membrane disease- HMD
Respiratory membrane--wall of an alveolus and the capillary wall taken together
LUNGS • The narrow superior portion of each lung is called the apex, and the broad inferior portion is the base. • Two layers of serous membrane surround each lung. • Collectively referred to as the pleurae. • The outer layer, the parietal pleura, lines the thoracic wall and mediastinum.
The visceral pleura surrounds the lung and is firmly attached to its outer surface. • Between the two pleural layers surrounding each lung is a potential space, called the pleural cavity. • It contains a thin film of fluid produced by serous cells in the pleurae.
The pleurae fluid lubricates the surfaces of the two pleural membranes to reduce friction as the lungs expand and contract during breathing. • If fluid production is reduced or the membranes become swollen, a painful condition called pleurisy results, in which the membranes scrape against each other with every breath.
The right lung contains three lobes and is larger than the left, which contains only two lobes. • The lines of division between lobes extend completely through the lung and are called fissures. • Each lobe is supplied by a major branch of the bronchial tree and is enclosed by connective tissue.
MECHANICS OF BREATHING • Breathing, or pulmonary ventilation, provides for an exchange of air between the external environment and the spaces within the alveoli of each lung. • The physical variable that provides the pressure gradient is volume.
At rest, the air pressure inside the alveoli of the lungs is about the same as normal air pressure. (760 mm of Hg) • An increase in volume of the lungs causes the air pressure in the alveoli to drop about 1 to 3 mm of Hg below the external air pressure. • Although this pressure gradient seems small, it is large enough for air to be drawn into the air passageways and into the alveoli
The first step in inspiration is the contraction of the respiratory muscles. • The diaphragm, which presses downward into the abdominal cavity; • And the external intercostal muscles between the ribs, which raise the ribs and elevate the sternum. The result is the expansion of the thoracic cavity.
As the thoracic cavity expands, the parietal pleurae, which are attached to its inner wall, are pulled outward. • This increases the volume of the pleural cavity, causing the pressure within this space to drop. • The sudden decrease in pleural cavity pressure draws the visceral pleurae and the attached lung surfaces outward.
As the surface of the lungs is pulled outward, the internal volume of the alveoli within it expands. • This decreases the alveolar pressure by about 1 to 3 mm of Hg, thereby establishing a pressure gradient between the alveoli of the lungs and the external atmosphere.
As a result, air rushes into the alveoli, restoring equilibrium.
Inspiration Summary • 1. The diaphragm and external intercostal muscles contract. • 2. The thoracic cavity expands. • 3. Pleural cavity pressure decreases. • 4. The lung surface is pulled outward, causing the lung volume to increase. • 5. Alveolar pressure falls below atmospheric pressure. • 6. Air rushes into the alveoli.
EXPIRATION • Contraction of the respiratory muscles initiates the process of inspiration by expanding the thoracic cavity. • Unlike inspiration, expiration is a passive process, since it does not rely upon muscle contraction.
It relies on the ability of the lungs and thoracic wall to recoil, like a rubber band does after it is stretched. • Once inspiration is complete, the diaphragm and external intercostal muscles relax.
As this occurs, the internal diameter of the alveoli gradually decreases as their elastic walls recoil, pushing air outward and into the air passageways. • Their total collapse is prevented by a slightly lower pressure in the plural cavity, which keeps the alveoli slightly inflated even after expiration.
The presence of surfactant in the inner walls of the alveoli prevents them from adhering to one another if they come into contact. • Should this fail because of pressure leaks, a pneumothorax would occur, causing the lung to collapse. • This dangerous condition is called atelectasis, (obstructs the movement of air)
Expiration Summary • 1. The diaphragm and external intercostal muscles relax. • 2. The thoracic cavity decreases in size. • 3. Pleural cavity pressure increases. • 4. Alveolar pressure becomes greater than atmospheric pressure. • 5. Air flows out of the alveoli.
RESPIRATORY VOLUME • A spirometer measures respiratory volume. • For a healthy adult, normal quiet breathing moves about 500 ml of air into and out of the lungs with each breath. This is called the tidal volume. • The amount of air that can be inhaled forcibly over the tidal volume is the inspiratory reserve volume (IRV).
The inspiratory reserve volume normally averages about 3100 ml. • The maximum amount of air that can be forcibly exhaled after a tidal expiration is about 1200 ml and is called expiratory reserve volume (ERV).
The volume of air remaining in the lungs after a forced expiration averages 1200 ml. This is called residual volume (RV). • The total amount of exchangeable air is found by adding the tidal volume, inspiratory reserve volume, and expiratory reserve volume TV+IRV+ERV = VC (vital capacity)
Another value, known as the total lung capacity (TLC), is the sum of the vitalcapacity and the residual volume. It averages about 6000 ml of air. • VC+RV = TLC
Respiratory Volumes Summary • Tidal volume -amount of air moving into or out of the lungs during quiet breathing. • Inspiratory reserve volume- maximum amount of air that can be inhaled forcibly over the tidal volume. • Expiratory reserve volume- maximum amount of air that can be exhaled forcibly over the tidal volume.
Residual volume- amount of air remaining in the lungs following a forced expiration. • Vital capacity- the total amount of exchangeable air, determined by the sum of the tidal volume, inspiratory reserve volume, and expiratory reserve volume.
Total lung capacity- the total amount of air contained in the fully inflated respiratory system, determined by the sum of the vital capacity and the residual volume.
Exchange of Gases • External respiration is the exchange of gases between the alveoli and the bloodstream. • External respiration relies on the fact that there is always a greater partial pressure of oxygen in the alveoli than there is in the blood plasma.
Internal respiration is the exchange of gases between capillaries in the body (other than in the lungs) and body cells.
Control of Breathing • The main source of control is through the respiratory center in the brain. • It is also affected by chemical changes in the blood, the degree of stretch of the lungs, and a person’s mental state. • Breathing is a rhythmic, involuntary process that is controlled by a group of neurons in the brain stem known as the respiratory center. (in the medulla oblongata and pons)
The Medullary Rhythmicity Center contains two groups of neurons extending the length of the medulla, known as the: • dorsal respiratory group • and the ventral respiratory group.
The dorsal group controls the basic rhythm of breathing. • Its neurons produce bursts if impulses in a cyclic manner, which travel to the diaphragm and external intercostal muscles and stimulate their contraction. • After several seconds of activity, the neurons cease abruptly.
They remain inactive during expiration and then begin another burst of activity to stimulate another inspiration sequence. • This cycle of activity and inactivity repeats itself over and over again to produce rhythmic, tidal breathing.
The neurons of the ventral respiratory group are inactive during normal tidal breathing. • They are activated when the need arises to breath more forcefully.
The impulses they generate travel to the diaphragm and external intercostal muscles to stimulate forceful inspiration, and to other thoracic and abdominal muscles to stimulate forceful expiration.
Factors That Affect Breathing • The most important include chemical changes in the blood, and the degree of stretch of the lungs, and a person’s mental state. • The concentrations of certain chemicals in the blood vary when physical demands on the body change.