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Respiratory System
- An organ system that rhythmically takes in air and expels it from the body
- Supplying the bodyw ith oxygen and expelling carbon dioxide that it generates
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Primary Organs of Respiratory System
- Nose
- Pharnynx
- Larynx
- Trachea
- Bronchi
- Lungs
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Upper-Respiratory Tract
- The airway from the nose through the larynx
- Organs in the head and neck
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Lower-Respiratory Tract
The airway from the trachea through the lungs
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Nose
Warms, cleanses, and humidifies inhaled air, detects odors, and serves as a resonating chamber that amplifies the voice
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Inferior Nose
- Supported by the lateral and alar cartilages
- -Forms the flared portions
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Nasal Cavity
Divided into right and left halves called nasal fossae
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Nasal Septum
- The dividing wall
- Composed of bone and hyaline cartilage
- Vomer forms inferior part
- Septal cartilage forms the anterior part
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Palate
- Separates the nasal cavity from the oral cavity
- Allows you to breathe while chewing food
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Vestibule
- Lined with stratified squamous epithelium
- Stiff guard hairs that block insects and debris from entering the nose
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Olfactory Epithelium
- Covers a small area of the roof of the nasal fossa and adjacent parts of the septum and superior concha
- Odors are detected by these sensory cells
- Ciliated pseudostratified columnar epithelia
- -Cilia are immobile and serve to bind odor molecules
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*******Respiratory Epithelium
- Covers the rest of the nasal cavity, except vestibule
- Ciliated pseudostratified columnar epithelia
- -Cilia is mobile, propels mucus toward pharynx
- Goblet cells secrete mucus
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******Nasal Mucosa
- Contains mucous glands
- Supplement the mucus produced by the goblet cells
- Inhaled dust, pollen, bacteria, and other foreign matter stick to the mucus and are swallowed
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Lamina Propria
- Well populated by lymphocytes and plasma cells that mount immune defenses against inhaled pathogens
- Contains large blood vessels that help warm the air
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Inferior Concha
- Has a venous plexus called the erectile tissue
- Every 30-60 minutes, the tissue on one side swells with blood and restricts airflow through that fossa
- Air is directed through the other fossa, allowing the engorged side time to recover from drying
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Pharynx
- A muscular funnel from the choanae to the larynx
- Nasopharynx
- Oropharynx
- Laryngopharynx
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Nasopharynx
- Receives the auditory tubes from the middle ears and hosues the paryngeal tonsil
- Inhaled air turns 90° downward as it passes through
- Large particles cannot make the turn because of inertia
- -They collide with the posterior wall and stick to the mucosa near the tonsil
- Passes only air
- Lined by pseudostratified columnar epithelium
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Oropharynx
- A space between the posterior margin of the soft palate and the epiglottis
- Passes air, food, and drink
- Lined by stratified squamous epithelium
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Laryngopharynx
- Esophagus begins at this point
- Passes air, food, and drink
- Lined by stratified squamous epithelium
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Larynx
- Keeps food and drink out of the airway
- Evolved the additional role of sound production in many animals
- Superior opening is guarded by the epiglottis
- Has vestibular folds
- Does not produce speech
- -Words are formed by the pharynx, oral cavity, tongue, and lips
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******Epiglottis
- At rest, it stands almost vertically
- During swallowing, extrinsic muscles of larynx pull larynx up towards it
- Tongue pushes it down to meet larynx
- Closes the airway and directs food and drink into the esophagus
- Does not help in forming speech
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Extrinsic Muscles of Larynx
Connect it to the hyoid bone and elevate it during swallowing
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Intrinsic Muscles of Larynx
- Control the vocal cords by pulling on the corniculate and arytenoid cartilages, causing the cartilages to pivot
- Air forced between the adducted vocal cords vibrates them, producing a high-pitched sound
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Males Larynx
- Longer and thicker
- Vibrate more slowly
- Produce lower-pitched sounds
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Trachea
- Supported by 16-20 C-shaped rings of hyaline cartilage
- -Keep it from collapsing when you inhale
- -Gap allows room for the esophagus to expand as food passes by
- Trachealis muscles contract or relax to adjust airflow
- Pseudostratified columnar epithelium composed of mucus secreting goblet cells, ciliated cells, and short basal stem cells
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Mucociliary Escalator
- Mucus traps inhaled particles
- Upward beating of cilia drives the debris-laden mucus towards the pharynx
- Swallowed
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Adventitia
- Outermost layer of trachea
- Fibrous connective tissue
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Carina
Directs the airflow to the right and left bronchi
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Hilum
- Slit through which the lung receives the main bronchus, blood vessels, lymphatics, and nerves
- These structures constitute the root of the lung
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Left Lung
- Has a superior and inferior lobe
- Single oblique fissure
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Right Lung
- Has a superior, middle, and inferior lobe
- Horizontal fissure that separates the superior and middle lobes
- Oblique fissure separates the middle and inferior lobes
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Right Main Bronchus
- Inhaled foreign objects lodge here more often
- Gives off three branchs
- -Superior, middle, and inferior lobar bronchi
- -One to each lobe of right lung
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Left Main Bronchus
Gives off superior and inferior lobar bronchi to the two lobes of the left lung
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Lobar Bronchi
- Branch into segmental bronchi
- -10 in the right and 8 in the left
- -Each ventilates a functionally independent unit of lung tissue called a broncopulmonary segment
- Supported by overlapping crescent-shaped cartilaginous plates
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Main Bronchi
- Supported by C-shaped rings of hyaline cartilage
- Lined with ciliated pseudostratified columnar epithelium
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Bronchial Tree
- Has a lot of elastic connective tissue
- -Contributes to the recoil that expels air from the lungs in each respiratory cycle
- Branches of the pulmonary artery follow it on their way to the alveoli
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Bronchioles
- Continuations of the airway that lack supportive cartilage
- Portion of the lung ventilated by one is called a pulmonary lobule
- Have ciliated cuboidal epithelium
- Well-developed layer of smooth muscle
- Divided into 50-80 terminal bronchioles
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Terminal Bronchioles
- The final branches of the conducting division
- Have no mucous glands or goblet cells
- Have cilia so that mucus draining into them from the higher passages can be driven back, preventing congestion of them and the alveoli
- Gives off two or more respiartory bronchioles
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Respiratory Bronchioles
- Have alveoli budding from their walls
- The beginning of the respiartory division because of their alveoli participate in gas exchange
- Have scanty smooth muscle, and smallest are nonciliated
- Divides into 2-10 elongated alveolar ducts
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Alveolar Ducts
- Thin-walled passages
- Smaller divisions
- Have nonciliated simple squamous epithelium
- End in alveolar sacs
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Alveolar Sacs
Grapelike clusters of alveoli arrayed around a central space called the atrium
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*******Conducting Division
- Where there are no alveoli
- Tissue walls are too thick for any exchange of oxygen or carbon dioxide with blood
- Nasal cavity -> pharynx -> trachea -> main bronchus -> lobar bronchus -> segmental bronchus -> bronchiole -> terminal bronchiole
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Respiratory Division
- Have alveoli along the walls
- Engage in gas exchange
- Respiratory bronchiole -> alveolar duct -> atrium -> alveolus
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Lung
- Spongy mass composed of 150 million little sacs, alveoli
- Provide a large surface for gas exchange
- Do not ventilate themselves
- Have only smooth muscle in the walls of the bronchi and bronchioles
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Squamous (Type I) Alveolar Cells
- Cover about 95% of the alveolar surface area
- Thinness allows for rapid gas diffusion between the air and blood
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Great (Type II) Alveolar Cells
- Cover about 5% of the alveolar surface area
- Round to cuboidal
- Cover less surface area
- Repair the alveolar epithelium when squamous cells are damaged
- Secrete pulmonary surfactant
- -Without it, the walls of a deflating alveolus wound cling together making it hard to reinflate them on the next inhalation
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Pulmonary Surfactant
A mixture of phospholipids and protein that coats the alveoli and smallest bronchioles and prevents them from collapsing when one exhales
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Alveolar Macrophages (Dust Cells)
- Most abundant cells in lungs
- Wander the lumens of the alveoli and connective tissue between them
- Keep the alveoli free of debris by phagocytizing dust particles that escape the mucus in higher parts
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Respiratory Membrane
- Barrier between the alveolar air and blood
- Consists of only squamous alveloar cell, squamous endothelial cell of the capillary, and their shared basement membrane
- Low capillary blood pressure prevents the rupture of this
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Visceral Pleura
- Serous membrane
- Extends into the fissures
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Parietal Pleura
- At the hilum
- Visceral pleura turns back on itself
- Adheres to the mediastinum, inner surface of ribcage, and superior surface of diaphragm
- Pulmonary ligament connects it to the diaphragm
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Pleural Cavity
- Space between the parietal and visceral pleurae
- Does not contain a lung, but wraps around it
- Contains nothing but a film of slippery pleural fluid
- Only a potential space, no room between the membranes
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Pleurae and Pleural Fluid Functions
- Reduction of friction
- Creation of pressure gradient
- Compartmentalization
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Reduction of Friction
Pleural fluid acts as a lubricant that enables the lungs to expand and contract with minimal friction
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Creation of Pressure Gradient
The pleurae play a role in the creation of one that expands the lungs when one inhales
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Compartmentalization
The pleurae, mediastinum, and pericardium compartmentalize the thoracic organs and prevent infections of one organ from spreading easily to neighboring organs
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Smooth Muscle of Lungs
Adjusts the diameter of the airway and affects the speed of airflow, but it does not expand or shrink the lungs or create the airflow
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Air Flows In
- Increase the volume and lower the pressure in the thoracic cavity
- External intercostals raise up, widen thoracic cavity
- Diaphragm descends and increases depth of thoracic cavity
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Air Flows Out
Reduce thoracic volume and raise pressure
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Forced Expiration
- Internal intercostals depresses ribs 1-11, narrow thoracic cavity
- Diaphragm ascends and reduces depth of thoracic cavity
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Neural Control of Breathing
- Depends on repetitive stimuli from the brain
- Ceases if the nerve connections to the thoracic muscles are severed or if the spinal cord is severed high on the neck
- Skeletal muscles cannot contract without nervous stimulation
- Involves the well-orchestrated action of multiple muscles, requiring a central coordinating mechanism
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2 Breathing Control Methods of Brain
- Cerebral and conscious, enabling us to inhale or exhale at will
- Unconscious and automatic
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Autonomic, unconscious cycle of breathing
- Controlled by 3 pairs of respiratory centers in the reticular formation of the medulla oblongata and pons
- One of each on the right and left sides of brainstem
- Ventral respiratory group
- Dorsal respiratory group
- Pontine respiratory group
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*****Ventral Respiratory Group
- Primary generator of the respiratory rhythm
- -Receives input from the dorsal one to modify
- An elongated nucleus in the medulla with 2 commingled webs of neurons
- -Inspiratory and expiratory neurons
- -Each form a reverberating neural circuit
- Controls the automatic, unconscious cycle of breathing
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Ventral Respiratory Group: Inspiratory Neurons
- In quiet breathing, it fires for about 2 seconds at a time
- Issues nerve signals to intergrating centers in the spinal cord
- Output from the spinal centers travels by phrenic nerves
- From intercostal nerves to the external intercostal muscles
- Contraction of these muscles enlarges the thoracic cage causing inspiration
- As long as these are firing, the inhibit the expiratory neurons
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Ventral Respiratory Group: Expiratory Neurons
- Inhibit inspiratory neurons
- Elastic recoil of the thoracic cage expels air from the lungs
- Normally last 3 seconds
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Dorsal Respiratory Group
- An integrating center that receives input from several sources: a respiratory center in the pons, a chemosensitive center of the anterior medulla oblongata, chemoreceptors in certain major arteries, and stretch and irritant receptors in the airway
- Issues outpu to the VRG that modifies the respiratory rhythm to adapt to varying conditions
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Pontine Respiratory Group
- Each side of the pons
- Modifies the rhythm of the VRG
- Receives input from higher brain centers including the hypothalmus, limbic system, and cerebral cortex
- Issues output to both the DRG and VRG
- Hastens or delays the transition from inspiration to expiration
- Adapts breathing to special circumstances
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Central Chemoreceptors
- Brainstem neurons that respond to changes in the pH of the cerebrospinal fluid
- Concentrated on each side of the medulla oblongata
- pH reflects the CO2 level in the blood
- By regulating respiration to maintain a stable pH, the respiratory centers also ensure a stable blood CO2 level
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Peripheral Chemoreceptors
- Located in the carotid and aortic bodies of large arteries above heart
- Respond to O2 and CO2 content of blood, but most of all pH
- Carotid bodies communicate with brainstem by way of glossopharyngeal nerves
- Aortic bodies by way of vagus nerves
- -Sensory fibers enter the medulla and synapse with neurons of the DRG
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Stretch Receptors
- Found in the smooth muscle of the bronchi and bronchioles and in the visceral pleura
- Respond to inflation of lungs and signal the DRG by vagus nerves
- Excessive inflation triggers inflation reflex
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Inflation Reflex
A protective somatic reflex that strongly inhibits the I neurons and stops inspiration
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Irritant Receptors
- Nerve endings amid the epithelial cells of the airway
- Respond to smoke, dust, pollen, chemical fumes, cold air, and excess mucus
- Transmit signals by way of vagus nerves to the DRG
- DRG returns signals to the respiratory and bronchial muscles
- Results in shallower breathing, breath-holding, or coughing
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Alveolar Ventilation
- Air that actually enters the alveoli becomes available for gas exchange
- In a state of relaxation, parasympathetic stimulation keeps airway somewhat constricted, minimizing dead space, ventalating more alveoli
- In a state of arousal, sympathetic nervous system dilates the airway, increasing airflow
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Anatomical Dead Space
- The conducting division where air cannot exchange gases with blood
- Typically about 1mL per pound of body weight
- In some diseases, it can be greater because some alveoli lack blood flor or because the pulmonary membrane is thickened by edema or fibrosis
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Physiological Dead Space
The sum of anatomical dead space and any pathological alveolar dead space that may exist
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******Air
Consists of about 78.6% nitrogen, 20.9% oxygen, 0.04% carbon dioxide
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Alveolar Gas Exchange
- Air in the alveolus is in contact with the film of water covering the alveolar epithelium
- Oxygen must dissolve in the water and pass through the respiratory membrane separating the air from the bloodstream to get in the blood
- Carbon dioxide must pass the other way and diffuse out of the water film into the alveolar air to leave the blood
- Back-and-forth traffic of O2 and CO2 across the respiratory membrane
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Carbon Dioxide
Transported in 3 forms: carbonic acid, carbamino compounds, and dissolved gas
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Carbonic Acid
90% is hydrated to form this which then dissociates into bicarbonate and hydrogen ions
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Carbamino Compounds
- 5% binds to amino groups of plasma proteins and hemoglobin to form this
- Carbon dioxide does not compete with oxygen because CO2 and O2 bind to different sites on a hemoglobin molecule
- -Oxygen to heme moiety and CO2 to polypeptide chains
- Hemoglobin can transport both at the same time
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Dissolved Gas
- 5% of CO2 is carried in the blood by this
- Carbonated soft drinks and sparkling wines
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*******Carbon Dioxide Loading
- Aerobic respiration produces a molecule of CO2 for every molecule of O2 it consumes
- CO2 diffuses into the bloodstream, where it is carried in 3 forms
- Most of it reacts with water to form bicarbonate
- -Occurs slowly in the blood plasma, but faster in the RBCs where it's catalyzed by the enzyme carbonic anydrase
- -Most common form
- Chloride-bicarbonate exchanger pumps most of the HCO3- out of RBC in exchange for Cl- from the blood plasma
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4 Factors of Adjustment of Oxygen for Individual Tissues
- Ambient PO2
- Temperature
- The Bohr effect
- BPG
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Ambient PO2
- Since an active tissue consumes oxygen rapidly, this of its tissue fluid remains low
- A low one releases more oxygen
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Temperature
- When it rises, it promotes oxygen unloading
- Active tissues are warmer and extract more oxygen from the blood passing through them
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Bohr Effect
- Active tissues generate extra CO2, which raises the H+ concentration and lowers the pH of blood
- Hydrogen ions weaken the bond between hemoglobin and oxygen, promoting oxygen unloading
- Less pronounced at high PO2 present in lungs, pH has little effect on pulmonary oxygen
- More pronounced at low PO2 present in systemic capillaries
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*******BPG
- Erythrocytes have no mitochondria and meet their energy needs by anaerobic fermentation
- Binds to hemoglobin and promotes oxygen unloading
- Elevated body temperature stimulates this sythesis, as do thyroxine, growth hormone, testosterone, and epinephrine
- All of these hormones promote oxygen unloading to the tissues
- Ereythrocytes low in it do not unload 02 very well
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Squamous-Cell Carcinoma
- Basal cells of the bronchial epithelium multiply
- Ciliated pseudostratified epithelium transforms into the stratified squamous type
- As the dividing epithelial cells invade underlying tissues of the bronchial wall, the bronchus develops bleeding lesions
- Dense swirled masses of keratin appear in the lung parenchyma and replace functional respiratory tissue
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Adenocarcinoma
Originates in the mucus glands of the lamina propria
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Small-Cell Carcinoma
- Clusters of cells that resemble oat grains
- Originates in the main bronchi but invades the mediastinum and metastasizes quickly to other organs
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Lung Cancer
- Over 90% of tumors originate in the mucous membranes of large bronchi
- As the tumor invades the bronchial wall and grows around it, it compresses the airway and may cause collapse of more distal parts of the lung
- Growth of tumor produces a cough with blood
- Common sites of metastasis are the pericardium, heart, bones, liver, lymph nodes, and brain
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Digestive System
- Organ system that processes food, extracts nutrients, and eliminates the residue
- Ingestion
- Digestion
- Absorption
- Compaction
- Defecation
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Mechanical Digestion
- The physical breakdown of food into smaller particles
- Achieved by the cutting and grinding action of the teeth and the churning contractions of the stomach and small intestine
- Exposes more food surface to the action of digestive enzymes
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*********Chemical Digestion
- A series of hydrolysis reactions that break dietary macromolecules into their monomers: polysaccharides into monosaccharides, proteins into amino acids, fats into monoglycerides and fatty acids, and nucleic acids into nucleotides
- Carried out by digestive enzymes produced by the salivary glands, stomach, pancreas, and small intestine
- Some nutrients are already present and are absorbed without being digested: vitamins, free amino acids, minerals, cholesterol, and water
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Digestive Tract
Includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine
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Accessory Organs
Teeth, tongue, salivary glands, liver, gallbladder, and pancreas
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********Tissue Layers of Digestive Tract
- Most inner to most outer:
- Lamina propria
- Muscularis mucosae
- Mucosa
- Submucosa
- Muscularis externa
- Serosa
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Tissue Layers of Digestive Tract: Mucosa
- Epithelium, simple columnar, but stratified squamous in oral cavity to the esophagus and in lower anal canal
- Lamina propria, loose connective tissue
- Muscularis mucosae, thin layer of smooth muscle
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Tissue Layers of Digestive Tract: Muscularis Externa
- Inner circular layer encircle the tract
- -Some areas, it's thickened to form sphincters
- Outer longitudinal layer
- Responsible for the motility that propels food and residue through the digestive tract
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Tissue Layers of Digestive Tract: Serosa
- Areolar tissue
- Mesothelium, simple squamous
- Pharynx, most of the esophagus, and rectume have none, but are surrounded by fibrous connective tissue called adventitia
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Muscularis Mucosae
- Tenses the mucosa, creating grooves and ridges that enhance its surface area and contact with food
- Improves the efficiency of digestion and nutrient absorption
- Has an abundance of lymphocytes and lymphatic nodules
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Tissue Layers of Digestive Tract: Submucosa
A thicker layer of loose connective tissue containing blood vessels and lymphatics, a nerve plexus, and in some places, glands that secrete lubricating mucus into the lumen
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Enteric Nervous System
- Nervous network found in the esophagus, stomach, and intestines
- Regulates digestive tract motility, secretion, and blood flow
- Can function independently from the CNS
- Composed of 2 networks:
- Submucosal plexus
- Myenteric plexus
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Myenteric Plexus
- Parasympathetic ganglia and nerve fibers between layers of muscularis exterma
- Parasympathetic preganglionic fibers terminate in the ganglia of here
- Postganglionic fibers arising here also pass through the muscularis externa's inner circular layer and contribute to the submucosal plexus
- Controls peristalsis and other contractions
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Submucosal Plexus
Controls movements of the muscularis mucosae and glandular secretion of the mucosa
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******Mesenteries
- Hold the abdominal viscera in their proper relationship to each other and prevent the small intestine from becoming twisted and tangled by its own contractions or changes in body position
- Provide passage for the blood vessels and nerves that supply the digestive tract
- Contain many lymph nodes and lymphatic vessels
- Serous membranes that suspend the stomach and intestines from the abdominal walll
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Posterior (Dorsal) Mesentery
- A translucent 2-layered membrane extending to the digestive tract
- When it reaches an organ, the 2 layers separate and pass around opposite sides forming the serosa
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Anterior (Ventral) Mesentary
- May hang freely in the abdominal cavity or attach to the anterior abdominal wall or other organs
- Two layers come together on far side of organ and continue as another sheet of tissue
- Lesser omentum
- Greater omentum
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Lesser Omentum
Extends the short distance from the liver to the right superior margin of stomach
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Greater Omentum
- Hangs like an apron from the left inferior margin of stomach, loosely covering the small intestine
- Turns back on itself and passes upward, behind the superficial layer, forming a deep pouch
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Omenta
- Contain many lymph nodes and lymphatic vessels, blood vessels, and nerves
- Adhere to perforations or inflammed areas of the stomach or intestines, contribute immune cells to the site, and isolate infections that might otherwise give rise to peritonitis
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Mouth
- Ingestion, taste, and other sensory responses to food, mastication, chemical digestion, swallowing, speech, and respiration
- Lined with stratified squamous epithelium
- -Keratinized in areas subject to the greatest food abrasion (gums and hard palate)
- -Nonkeratinized in other areas
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*******Tongue
- Manipulates food between the teeth while it avoids being bitten
- Can extract food particles from teeth after a meal, and is sensitive enough to feel a stray hair
- Covered with nonkeratinized stratified squamous epithelium
- Bumps called lingual papillae- taste buds
- Vallate papillae
- Terminal papillae- behind vallate
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Mastication
Breaks food into pieces small enough to be swallowed and exposes more surface to the action of digestive enzymes
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Saliva
- Moistens and cleanses mouth, inhibits bacterial growth, dissolves molecules to stimulate taste buds, digests a little starch and fat, and makes swallowing easier by making a bolus
- Hypotonic solution
- Mucus
- Electrolytes
- Lysozyme
- Immunoglobulin A
- Salivary amylase
- Lingual lipase
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Lysozyme
Enzyme that kills bacteria
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Immunoglobulin A
Antibacterial antibody
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Salivary Amylase
An enzyme that begins starch digestion in mouth
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Lingual Lipase
Enzyme that begins fat digestion in the mouth
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*******Intrinsic Salivary Glands
- Indefinite number of small glands dispersed among the other oral tissues
- Secrete saliva at a fairly constant rate
- Contains ligual lipase and lysozyme
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*******Extrinsic Salivary Glands
- 3 pairs of larger organs located outside of the oral mucosa
- -Parotid glands
- -Submandibular glands
- -Sublingual glands
- Secrete 1-1.5L of saliva per day
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Parotid Glands
- Located beneath the skin
- Mumps is an inflammation and swelling caused by a virus
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********Submandibular Glands
Duct empties into the mouth at a papilla on the side of the ligual frenulum, near the lower central incisors
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Sublingual Glands
- Located in the floor of the mouth
- Have multiple ducts that empty into the mouth posterior to the papillae of the submandibular ducts
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*******Lower Esophageal Sphincter
- Prevents stomach acid from regurgitating into the esophagus
- Protects the esophageal mucosa from the erosive effect of stomach acid
- Heartburn
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Stomach: Lesser Curvature
Margin that extends for the short distance from esophagus to duodenum along the medial to superior aspect, facing the liver
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Stomach: Greater Curvature
Longer margin from esophagus to duodenum on the lateral to inferior aspect
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Stomach
- Divided into 4 regions
- -Cardiac region, fundus, body, pyloric region
- Receives parasympathetic nerve fibers from the vagus nerves and sympathetic fibers from the celiac ganglia
- Supplied with blood by branches of the celiac trunk
- All blood drained enters the hepatic portal circulation and filters through the liver before returning to the heart
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Pyloric Sphincter
Regulates the passage of chyme into the duodenum
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Mucous Cells
- Secrete mucus, predominate in the cardiac and pyloric glands
- In gastric glands, they are concentrated in the narrow neck of the gland where it opens into the gastric pit
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Regenerative Cells
- Found in the base of the pit and neck of the gland
- Divide rapidly and produce a continual supply of new cells
- Newly generated cells migrate upwards to the gastric surface as well as downwards into the glands to replace cells that die
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*******Parietal Cells
- Found mostly in the upper half of the gland
- Secrete hydrochloric acid, intrinsic factor, and ghrelin
- Found mostly in the gastric glands, but a few in the pyloric glands
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Chief Cells
- Secrete the enzymes gastric lipase and pepsinogen
- Dominate the lower half of the gastric glands
- Absent from pyloric and cardiac glands
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Enteroendocrine Cells
- Secrete hormones and paracrine messengers that regulate digestion
- Occur in all regions of the stomach, but are most abundant in lower ends of gastric and pyloric glands
- At least 8 kinds, each produces a different chemical messenger
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*******Hydrochloric Acid
- Activates pepsin and lingual lipase
- Breaks up connective tissues and plant cell walls; helping to liquefy food and form chyme
- Converts ingested ferric ions to ferrous ions
- Contributes to nonspecific disease resistance by destroying most ingested pathogens
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Pepsin
- Secreted by chief cells
- Pepsinogen (zymogen)
- Hydrochloric acid removes some amino acids and converts it to this
- Has an autocatalytic effect
- Digests dietary proteins to shorter peptide chains, which then pass to the small intestine, where digestion is complete
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Gastric Lipase
- Secreted by chief cells
- Digests 10-15% of dietary fat in stomach
- Remainder is digested in small intestine
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Intrinsic Factor
- Secreted by parietal cells
- Essential for the absorption of B12 by the small intestine
- Binds B12 and intestinal cells then absorb this complex by receptor-mediated endocytosis
- Without B12, hemoglobin can't be synthesized and anemia develops
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Cephalic Phase
- The stage in which the stomach responds to the sight, smell, taste, or thought of food
- Sensory and mental inputs converge on the hypothalamus, which relays signals to the medulla oblongata
- Vagus nerve fibers stimulate the enteric nervous system of the stomach
- Stomach stimulates gastric secretion
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Gastric Phase
- A period in which swallowed food and semidigested protein activate gastric activity
- Stretches the stomach and raising pH contents
- Stimulated by acetylcholine (parasympathetic nerve fibers), histamine (paracrine secretion), and gastrin (enteroendocrine G cells in pyloric glands)
- -Stimulate parietal cells to secrete HCl and intrinsic factor
- Chief cells secrete pepsinogen in response to gastrin
- Positive feedback loop
- -Protein -> amino acids -> G cells -> more gastrin
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Intestinal Phase
- A stage in which the duodenum responds to arriving chyme and moderates gastric activity through hormones and nervous reflexes
- Stretching of duodenum accentuates vagovagal reflexes that the stomach, and peptides and amino acids in the chyme stimulate G cells to secrete more gastrin that stimulates the stomach more
- Trigger enterogastric reflex
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Liver
- Secretes bile
- 4 lobes
- Round ligament carries blood from umbilical cord to the liver of a fetus
- Hepatic portal vein and bile travel through lesser omentum
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Hepatic Portal Vein
- Liver receives 70% of blood from here
- Receives blood from the stomach, intestines, pancreas, and spleen
- All nutrients absorbed, reach the liver by this route except for lipids
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Hepatic Arteries
- Liver recieves 30% of blood from here
- Exits the aorta at the celiac trunk -> common hepatic artery -> hepatic artery -> right and left hepatic arteries which enter the liver and porta
- Deliver oxygen and other materials to the liver
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Gallbladder
- Store and concentrates bile
- Simple columnar epithelium
- When waste products become concetranted, they may form gallstones
- Biles enters by filling the bile duct, then overflowing into it
- Between meals, it absorbs water and electrolytes from bile and concentrates it
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Pancreas
- Spongy retroperitoneal gland posterior to the greater curvature of the stomach
- Endocrine and exocrine gland
- -Pancreatic islets secrete insulin and glucagon
- Has an accessory duct that allows juices to be released into duodenum even while bile is not
- Trypsinogen, chymotrypsinogen, and procarboxypeptidase
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Trypsinogen
- When secreted into the itnestinal lumen, it is converted to trypsin by enterokinase
- Autocatalytic
- Positive feedback loop
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Pancreatic Amylase
Digests starch
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Pancreatic Lipase
Digests fat
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Ribonuclease
Digests RNA and DNA
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Regulation of Pancreatic Juice and Bile
- Acetylcholine
- Cholecystokinin
- Secretin
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Acetylcholine
- Coming from the vagus and enteric nerves
- Stimulates the pancreatic acini to secrete their enzymes even durn cephalic phase of gastric control, before food is swallowed
- Enzymes remain stored in pancreatic acini and ducts in preparation for release later when chyme enters the duodenum
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Cholecystokinin
- Secreted by the mucosa of the duodenum and proximal jejunum
- In response to fats in the small intestine
- Stimulates the pancreatic acini to secrete enzymes
- Induces contractions of the gallbladder and relaxation of the hepatopancreatic sphincter, discharging bile into the duodenum
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Secretin
- Produced in response to the acidity of chyme from the stomach
- Stimulates the ducts of both the liver and pancreas to secrete a lot of sodium bicarbonate
- In pancreas, this flushes the enzymes into the duodenum
- Sodium bicarbonate buffers the HCl arriving from the stomach
- Carbonic acid breaks down to CO2 and H2O
- -CO2 is absorbed in the blood and exhaled
- Salt water is left in small intestine
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Duodenum
- Receives the stomach contents, pancreatic juice, and bile
- Stomach acid is neutralized here
- Fats are physically broken up by the bile acids
- Pepsin is inactivated by elevated pH
- Pancreatic enzymes take over the job of chemical digestion
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Jejunum
Most digestion and nutrient absorption occurs here
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Ileum
- Walls are thinner than jejunum, less muscular
- Prominent lymphatic nodules in clusters called Peyer's patches
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*******Lacteal
- Absorbs most lipids (triglycerides)
- Give its contents a milky appearance
- Core of the villus has a few smooth muscle cells that contract periodically
- -Enhances mixing of chyme in intestinal lumen and milks lymph down here to the larger lymphatics of the submucosa
- Located in small intestine
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Intestinal Crypts
- Numerous pores that open into tubular glands on the floor of the small intestine
- Upper half has enterocytes and goblet cells
- Lower half has dividing stem cells
- Life span of 3-6 days
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Paneth Cells
- Clustered at the base of each crypt
- Secrete lysozyme, phospholipase, and defensins
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Duodenal Glands
Secrete an abundance of bicarbonate-rich mucus that neutralizes stomach acid and shields the mucosa from its erosive effects
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Contractions of Small Intestine Functions
- Mix chyme with intestinal juice, bile, and pancreatic juice, allowing these fluids to neutralize acid and digest nutrients more effectively
- Churn chyme and bring it into contact with the mucosa for contact digestion and nutrient absorption
- Move residue toward the large intestine
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Segmentation
- A movement in which stationary ringlike constrictions appear at several places along the intestne and then relax as new constrictions form elsewhere
- Most common type of intestinal contraction
- Churn the contents
- Causes slow progression of the chyme toward the colon
- Intensity of contractions is modified by nervous and hormonal influences
- Declines when most nutrients have been absorbed, peristalsis begins
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Water
- Digestive tract receives about 9L per day
- -0.7 in food, 1.6 in drink, and 6.7L in gastrointestinal secretions: saliva, gastric juice, bile, pancreatic juice, and intestinal juice
- 8L is absorbed by the small intestine
- 0.8L is absorbed by the large intestine
- 0.2L for fecal output
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Diarrhea
- Occurs when the large intestine absorbs too little water
- When the intestine is irritated by bacteria and feces pass through too quickly for adequate reabsorption, or when the feces contain abnormally high concentrations of a solute that opposes osmotic absorption of water
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Constipation
- Occurs when fecal movement is slow
- Too much water is reabsorbed
- Feces becomes hardened
- Can result from a lack of dietary fiber, exercise, emotional upset, or laxative abuse
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Large Intestine
- Cecum, colon, rectum, and anal canal
- Harbors about 800 species of bacteria called the bacterial flora
- -Mutually beneficial relationship with many of these
- -Provide them with room and board while they provide us with nutrients from our food that we cannot extract on our own
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Appendix
- Attached to the cecum
- Densly populated with lymphocytes
- Significant source of immune cells
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