Respiratory system

Respiratory system
A complete, schematic view of the human respiratory system with their parts and functions.
Latin	systema respiratorium

The respiratory system (or ventilatory system) is a biological system consisting of specific organs and structures used for the process of respiration in an organism. The respiratory system is involved in the intake and exchange of oxygen and carbon dioxide between an organism and the environment.

In air-breathing vertebrates, respiration takes place in the respiratory organs called lungs. The passage of air into the lungs to supply the body with oxygen is known as inhalation, and the passage of air out of the lungs to expel carbon dioxide is known as exhalation; this process is collectively called breathing or ventilation. In humans and other mammals, the anatomical features of the respiratory system include include trachea, bronchi, bronchioles, lungs, and diaphragm. Molecules of oxygen and carbon dioxide are passively exchanged, by diffusion, between the gaseous external environment and the blood. This exchange process occurs in the alveoli air sacs in the lungs.^[1]

In fish and many invertebrates, respiration takes place through the gills. Other animals, such as insects, have respiratory systems with very simple anatomical features, and in amphibians even the skin plays a vital role in gas exchange. Plants also have respiratory systems but the directionality of gas exchange can be opposite to that in animals. The respiratory system in plants also includes anatomical features such as holes on the undersides of leaves known as stomata.^[2]

Comparative anatomy and physiology

Horses

Horses are obligate nasal breathers which means that they are different from many other mammals because they do not have the option of breathing through their mouths and must take in oxygen through their noses.

Elephants

The elephant is the only animal known to have no pleural space. Rather, the parietal and visceral pleura are both composed of dense connective tissue and joined to each other via loose connective tissue.^[3] This lack of a pleural space, along with an unusually thick diaphragm, are thought to be evolutionary adaptations allowing the elephant to remain underwater for long periods of time while breathing through its trunk which emerges as a snorkel.^[4]

Birds

The respiratory system of birds differs significantly from that found in mammals, containing unique anatomical features such as air sacs. The lungs of birds also do not have the capacity to inflate as birds lack a diaphragm and a pleural cavity. Gas exchange in birds occurs between air capillaries and blood capillaries, rather than in alveoli.

Reptiles

The anatomical structure of the lungs is less complex in reptiles than in mammals, with reptiles lacking the very extensive airway tree structure found in mammalian lungs. Gas exchange in reptiles still occurs in alveoli however, reptiles do not possess a diaphragm. Thus, breathing occurs via a change in the volume of the body cavity which is controlled by contraction of intercostal muscles in all reptiles except turtles. In turtles, contraction of specific pairs of flank muscles governs inspiration or expiration.^[5]

Amphibians

Both the lungs and the skin serve as respiratory organs in amphibians. The skin of these animals is highly vascularized and moist, with moisture maintained via secretion of mucus from specialized cells. While the lungs are of primary importance to breathing control, the skin's unique properties aid rapid gas exchange when amphibians are submerged in oxygen-rich water.^[6]

Fish

In most fish, respiration takes place through gills. (See also aquatic respiration.) Lungfish, however, do possess one or two lungs. The labyrinth fish have developed a special organ that allows them to take advantage of the oxygen of the air.

Anatomy in invertebrates

Insects

Most insects breath passively through their spiracles (special openings in the exoskeleton) and the air reaches the body by means of a series of smaller and smaller pipes called 'trachaea' when their diameter is relatively large and 'tracheoles' when their diameter is very small. Diffusion of gases is effective over small distances but not over larger ones, this is one of the reasons insects are all relatively small. Insects which do not have spiracles and trachaea, such as some Collembola, breathe directly through their skins, also by diffusion of gases.^[7] The number of spiracles an insect has is variable between species, however they always come in pairs, one on each side of the body, and usually one per segment. Some of the Diplura have eleven, with four pairs on the thorax, but in most of the ancient forms of insects, such as Dragonflies and Grasshoppers there are two thoracic and eight abdominal spiracles. However in most of the remaining insects there are less. It is at this level of the tracheoles that oxygen is delivered to the cells for respiration. The trachea are water-filled due to the permeable membrane of the surrounding tissues. During exercise, the water level retracts due to the increase in concentration of lactic acid in the muscle cells. This lowers the water potential and the water is drawn back into the cells via osmosis and air is brought closer to the muscle cells. The diffusion pathway is then reduced and gases can be transferred more easily.

Insects were once believed to exchange gases with the environment continuously by the simple diffusion of gases into the tracheal system. More recently, however, large variation in insect ventilatory patterns have been documented and insect respiration appears to be highly variable. Some small insects do demonstrate continuous respiration and may lack muscular control of the spiracles. Others, however, utilize muscular contraction of the abdomen along with coordinated spiracle contraction and relaxation to generate cyclical gas exchange patterns and to reduce water loss into the atmosphere. The most extreme form of these patterns is termed discontinuous gas exchange cycles (DGC).^[8]

Molluscs

Molluscs generally possess gills that allow exchange of oxygen from an aqueous environment into the circulatory system. These animals also possess a heart that pumps blood which contains hemocyaninine as its oxygen-capturing molecule. Hence, this respiratory system is similar to that of vertebrate fish. The respiratory system of gastropods can include either gills or a lung.

Physiology in mammals

Ventilation

In respiratory physiology, ventilation (or ventilation rate) is the rate at which gas enters or leaves the lung. It is categorized under the following definitions:

Measurement	Equation	Description
Minute ventilation	tidal volume * respiratory rate[1][2]	the total volume of gas entering the lungs per minute.
Alveolar ventilation	(tidal volume – dead space) * respiratory rate [1]	the volume of gas per unit time that reaches the alveoli, the respiratory portions of the lungs where gas exchange occurs.
Dead space ventilation	dead space * respiratory rate[3]	the volume of gas per unit time that does not reach these respiratory portions, but instead remains in the airways (trachea, bronchi, etc.).

Control

Ventilation occurs under the control of the autonomic nervous system from parts of the brain stem, the medulla oblongata and the pons. This area of the brain forms the respiration regulatory center, a series of interconnected brain cells within the lower and middle brain stem which coordinate respiratory movements. The sections are the pneumotaxic center, the apneustic center, and the dorsal and ventral respiratory groups. This section is especially sensitive during infancy, and the neurons can be destroyed if the infant is dropped and/or shaken violently. The result can be death due to "shaken baby syndrome".^[9]

The breathing rate increases with the concentration of carbon dioxide in the blood, which is detected by peripheral chemoreceptors in the aorta and carotid artery and central chemoreceptors in the medulla. Exercise also increases respiratory rate, due to the action of proprioceptors, the increase in body temperature, the release of epinephrine, and motor impulses originating from the brain.^[10] In addition, it can increase due to increased inflation in the lungs, which is detected by stretch receptors.

Inhalation

Inhalation is initiated by the diaphragm and supported by the external intercostal muscles. Normal resting respirations are 10 to 18 breaths per minute, with a time period of 2 seconds. During vigorous inhalation (at rates exceeding 35 breaths per minute), or in approaching respiratory failure, accessory muscles of respiration are recruited for support. These consist of sternocleidomastoid, platysma, and the scalene muscles of the neck. Pectoral muscles and latissimus dorsi are also accessory muscles.

Under normal conditions, the diaphragm is the primary driver of inhalation. When the diaphragm contracts, the ribcage expands and the contents of the abdomen are moved downward. This results in a larger thoracic volume and negative pressure (with respect to atmospheric pressure) inside the thorax. As the pressure in the chest falls, air moves into the conducting zone. Here, the air is filtered, warmed, and humidified as it flows to the lungs.

During forced inhalation, as when taking a deep breath, the external intercostal muscles and accessory muscles aid in further expanding the thoracic cavity. During inhalation the diaphragm contracts.

Exhalation

Exhalation is generally a passive process; however, active or forced exhalation is achieved by the abdominal and the internal intercostal muscles. During this process air is forced or exhaled out.

The lungs have a natural elasticity: as they recoil from the stretch of inhalation, air flows back out until the pressures in the chest and the atmosphere reach equilibrium.^[11]

During forced exhalation, as when blowing out a candle, expiratory muscles including the abdominal muscles and internal intercostal muscles, generate abdominal and thoracic pressure, which forces air out of the lungs.

Gas exchange

The major function of the respiratory system is gas exchange between the external environment and an organism's circulatory system. In humans and other mammals, this exchange facilitates oxygenation of the blood with a concomitant removal of carbon dioxide and other gaseous metabolic wastes from the circulation.^[12] As gas exchange occurs, the acid-base balance of the body is maintained as part of homeostasis. If proper ventilation is not maintained, two opposing conditions could occur: respiratory acidosis, a life threatening condition, and respiratory alkalosis.

Upon inhalation, gas exchange occurs at the alveoli, the tiny sacs which are the basic functional component of the lungs. The alveolar walls are extremely thin (approx. 0.2 micrometres). These walls are composed of a single layer of epithelial cells (type I and type II epithelial cells) close to the pulmonary capillaries which are composed of a single layer of endothelial cells. The close proximity of these two cell types allows permeability to gases and, hence, gas exchange. This whole mechanism of gas exchange is carried by the simple phenomenon of pressure difference. When the air pressure is high inside the lungs, the air from lungs flow out. When the air pressure is low inside, then air flows into the lungs.

Immune functions

Airway epithelial cells can secrete a variety of molecules that aid in the defense of lungs. Secretory immunoglobulins (IgA), collectins (including Surfactant A and D), defensins and other peptides and proteases, reactive oxygen species, and reactive nitrogen species are all generated by airway epithelial cells. These secretions can act directly as antimicrobials to help keep the airway free of infection. Airway epithelial cells also secrete a variety of chemokines and cytokines that recruit the traditional immune cells and others to site of infections.

Most of the respiratory system is lined with mucous membranes that contain mucosal-associated lymphoid tissue, which produces white blood cells such as lymphocytes.

Metabolic and endocrine functions of the lungs

In addition to their functions in gas exchange, the lungs have a number of metabolic functions. They manufacture surfactant for local use, as noted above. They also contain a fibrinolytic system that lyses clots in the pulmonary vessels. They release a variety of substances that enter the systemic arterial blood and they remove other substances from the systemic venous blood that reach them via the pulmonary artery. Prostaglandins are removed from the circulation, but they are also synthesized in the lungs and released into the blood when lung tissue is stretched. The lungs also activate one hormone; the physiologically inactive decapeptide angiotensin I is converted to the pressor, aldosterone-stimulating octapeptide angiotensin II in the pulmonary circulation. The reaction occurs in other tissues as well, but it is particularly prominent in the lungs. Large amounts of the angiotensin-converting enzyme responsible for this activation are located on the surface of the endothelial cells of the pulmonary capillaries. The converting enzyme also inactivates bradykinin. Circulation time through the pulmonary capillaries is less than one second, yet 70% of the angiotensin I reaching the lungs is converted to angiotensin II in a single trip through the capillaries. Four other peptidases have been identified on the surface of the pulmonary endothelial cells.

Vocalization

The movement of gas through the larynx, pharynx and mouth allows humans to speak, or phonate. Vocalization, or singing, in birds occurs via the syrinx, an organ located at the base of the trachea. The vibration of air flowing across the larynx (vocal cords), in humans, and the syrinx, in birds, results in sound. Because of this, gas movement is extremely vital for communication purposes.

Temperature control

Panting in dogs, cats and some other animals provides a means of controlling body temperature. This physiological response is used as a cooling mechanism.

Coughing and sneezing

Irritation of nerves within the nasal passages or airways, can induce coughing and sneezing. These responses cause air to be expelled forcefully from the trachea or nose, respectively. In this manner, irritants caught in the mucus which lines the respiratory tract are expelled or moved to the mouth where they can be swallowed. During coughing, contraction of the smooth muscle narrows the trachea by pulling the ends of the cartilage plates together and by pushing soft tissue out into the lumen. This increases the expired airflow rate to dislodge and remove any irritant particle or mucus.

Development

Humans and mammals

The respiratory system lies dormant in the human fetus during pregnancy. At birth, the respiratory system becomes fully functional upon exposure to air, although some lung development and growth continues throughout childhood.^[13] Pre-term birth can lead to infants with under-developed lungs. These lungs show incomplete development of the alveolar type II cells, cells that produce surfactant. The lungs of pre-term infants may not function well because the lack of surfactant leads to increased surface tension within the alveoli. Thus, many alveoli collapse such that no gas exchange can occur within some or most regions of an infant's lungs, a condition termed respiratory distress syndrome. Basic scientific experiments, carried out using cells from chicken lungs, support the potential for using steroids as a means of furthering development of type II alveolar cells.^[14] In fact, once a pre-mature birth is threatened, every effort is made to delay the birth, and a series of steroid shots is frequently administered to the mother during this delay in an effort to promote lung growth.^[15]

Disease

Disorders of the respiratory system can be classified into four general areas:

• Obstructive conditions (e.g., emphysema, bronchitis, asthma)
• Restrictive conditions (e.g., fibrosis, sarcoidosis, alveolar damage, pleural effusion)
• Vascular diseases (e.g., pulmonary edema, pulmonary embolism, pulmonary hypertension)
• Infectious, environmental and other "diseases" (e.g., pneumonia, tuberculosis, asbestosis, particulate pollutants):

Coughing is of major importance, as it is the body's main method to remove dust, mucus, saliva, and other debris from the lungs. Inability to cough can lead to infection. Deep breathing exercises may help keep finer structures of the lungs clear from particulate matter, etc.

The respiratory tract is constantly exposed to microbes due to the extensive surface area, which is why the respiratory system includes many mechanisms to defend itself and prevent pathogens from entering the body.

Disorders of the respiratory system are usually treated internally by a pulmonologist and Respiratory Therapist.

Plants

Plants use carbon dioxide gas in the process of photosynthesis, and exhale oxygen gas as waste. The chemical equation of photosynthesis is 6 CO₂ (carbon dioxide) and 6 H₂O (water) and that makes 6 O₂ (oxygen) and C₆H₁₂O₆ (glucose). What is not expressed in the chemical equation is the capture of energy from sunlight which occurs. Photosynthesis uses electrons on the carbon atoms as the repository for that energy. Respiration is the opposite of photosynthesis. It reclaims the energy to power chemical reactions in cells. In so doing the carbon atoms and their electrons are combined with oxygen forming a gas which is easily removed from both the cells and the organism. Plants use both processes, photosynthesis to capture the energy and respiration to use it.

Plant respiration is limited by the process of diffusion. Plants take in carbon dioxide through holes on the undersides of their leaves known as stoma or pores. However, most plants require little air.^{[citation needed]} Most plants have relatively few living cells outside of their surface because air (which is required for metabolic content) can penetrate only skin deep. However, most plants are not involved in highly aerobic activities, and thus have no need of these living cells.

References

External links

The Wikibook Human Physiology has a page on the topic of: The respiratory system

The Wikibook Anatomy and Physiology of Animals has a page on the topic of: Respiratory System

• A high school level description of the respiratory system
• Introduction to Respiratory System
• Science aid: Respiratory System A simple guide for high school students
• The Respiratory System University level (Microsoft Word document)
• Lectures in respiratory physiology by noted respiratory physiologist John B. West (also at YouTube)

Library resources about Respiratory system

Resources in your library

v t e Human systems and organs TA 2–4: MS Skeletal system Bone Carpus Collar bone (clavicle) Thigh bone (femur) Fibula Humerus Mandible Metacarpus Metatarsus Ossicles Patella Phalanges Radius Skull (cranium) Tarsus Tibia Ulna Rib Vertebra Pelvis Sternum Cartilage Joints Fibrous joint Cartilaginous joint Synovial joint Muscular system Muscle Tendon Diaphragm TA 5–11: splanchnic/ viscus mostly Thoracic Respiratory system URT Nose Nasopharynx Larynx LRT Trachea Bronchus Lung mostly Abdominopelvic Digestive system+ adnexa Mouth Salivary gland Tongue upper GI Oropharynx Laryngopharynx Esophagus Stomach lower GI Small intestine Appendix Colon Rectum Anus accessory Liver Biliary tract Pancreas GU: Urinary system Kidney Ureter Bladder Urethra GU: Reproductive system Female Uterus Vagina Vulva Ovary Placenta Male Scrotum Penis Prostate Testicle Seminal vesicle Endocrine system Pituitary Pineal Thyroid Parathyroid Adrenal Islets of Langerhans TA 12–16 Circulatory system Cardiovascular system peripheral Artery Vein Lymphatic vessel Heart Lymphatic system primary Bone marrow Thymus secondary Spleen Lymph node CNS equivalent Glymphatic system Nervous system Brain Spinal cord Nerve Sensory system Ear Eye Integumentary system Skin Subcutaneous tissue Breast Mammary gland Blood (Non-TA) Myeloid Myeloid immune system Lymphoid Lymphoid immune system General anatomy: systems and organs, regional anatomy, planes and lines, superficial axial anatomy, superficial anatomy of limbs

v t e Head and neck, upper RT: Nose (TA A06.1, TH H3.05.01, GA 10.992) External nose Ala of nose nasal cartilages of the septum Greater alar Lesser alar Lateral nasal Accessory nasal Vomeronasal Nasal cavity Openings Nasal vestibule Nostril Posterior nasal apertures Lateral wall Nasal conchae/meatus: Superior nasal concha Middle nasal concha Inferior nasal concha Superior nasal meatus Middle nasal meatus Inferior nasal meatus Sphenoethmoidal recess Ethmoid bulla Agger nasi Ethmoidal infundibulum Semilunar hiatus Maxillary hiatus Medial wall Nasal septum Vomeronasal organ Mucous membrane Olfactory mucosa Paranasal sinuses Maxillary sinus Sphenoidal sinuses Frontal sinus Ethmoid sinus Naso-pharynx Pharyngeal opening of auditory tube Salpingopharyngeal fold Salpingopalatine fold Torus tubarius Pharyngeal tonsil Pharyngeal recess M: RES anat (n, x, l, c)/phys/devp noco (c, p)/cong/tumr, sysi/epon, injr proc, drug (R1/2/3/5/6/7)

v t e Anatomy: Lower RT respiratory system (TA A06.3–5, TH H3.05.02, GA 11.1084) TB tree Trachea Tracheal rings Annular ligaments of trachea Carina Bronchus main bronchus right left lobar/secondary bronchi eparterial bronchus segmental/tertiary bronchi Lungs General Left lung/Right lung Base/Apex Root/Hilum Superior lobe Lingula of left lung/Middle lobe of right lung Inferior lobe borders: Anterior border Cardiac notch Posterior border Inferior border surfaces: Costal surface Mediastinal surface Cardiac impression Diaphragmatic surface fissures: Oblique fissure Horizontal fissure BP segment Bronchiole: Conducting zone (Terminal bronchiole) Respiratory zone (Respiratory bronchiole Alveolar duct Alveolus Blood-air barrier) Cells Pneumocyte (Type I pneumocyte Type II pneumocyte) Clara cell Goblet cell M: RES anat (n, x, l, c)/phys/devp noco (c, p)/cong/tumr, sysi/epon, injr proc, drug (R1/2/3/5/6/7)

v t e Anatomy, Respiratory system: Thoracic cavity (TA A07, TH H3.05.03, GA 11.1087) Pleurae Parietal pleura Cervical Costal Mediastinal Diaphragmatic Visceral pleura Pulmonary ligament Pleural cavity recesses Costomediastinal Costodiaphragmatic Suprapleural membrane Mediastinum Superior Anterior Middle Posterior General Thoracic wall M: RES anat (n, x, l, c)/phys/devp noco (c, p)/cong/tumr, sysi/epon, injr proc, drug (R1/2/3/5/6/7)

v t e Respiratory system, physiology: respiratory physiology Lung volumes VC FRC Vt dead space CC PEF calculations respiratory minute volume FEV1/FVC ratio methods of lung testing spirometry body plethysmography peak flow meter nitrogen washout Airways/ventilation (V) positive pressure ventilation breath (inhalation exhalation) respiratory rate respirometer pulmonary surfactant compliance elastic recoil hysteresivity airway resistance bronchial hyperresponsiveness bronchoconstriction/bronchodilation Blood/perfusion (Q) pulmonary circulation hypoxic pulmonary vasoconstriction pulmonary shunt Interactions/ ventilation/perfusion ratio (V/Q) ventilation/perfusion scan zones of the lung gas exchange pulmonary gas pressures alveolar gas equation alveolar–arterial gradient hemoglobin oxygen–haemoglobin dissociation curve (Oxygen saturation 2,3-BPG Bohr effect Haldane effect) carbonic anhydrase (chloride shift) oxyhemoglobin respiratory quotient arterial blood gas diffusion capacity (DLCO) Control of respiration pons pneumotaxic center apneustic center medulla dorsal respiratory group ventral respiratory group chemoreceptors central peripheral pulmonary stretch receptors Hering–Breuer reflex Insufficiency high altitude oxygen toxicity hypoxia M: RES anat (n, x, l, c)/phys/devp noco (c, p)/cong/tumr, sysi/epon, injr proc, drug (R1/2/3/5/6/7)

v t e Pathology of respiratory system (J, 460–519), respiratory diseases Upper RT (including URTIs, Common cold) Head sinuses Sinusitis nose Rhinitis Vasomotor rhinitis Atrophic rhinitis Hay fever Nasal polyp Rhinorrhea nasal septum Nasal septum deviation Nasal septum perforation Nasal septal hematoma tonsil Tonsillitis Adenoid hypertrophy Peritonsillar abscess Neck pharynx Pharyngitis Strep throat Laryngopharyngeal reflux (LPR) Retropharyngeal abscess larynx Croup Laryngitis Laryngopharyngeal reflux (LPR) Laryngospasm vocal folds Laryngopharyngeal reflux (LPR) Vocal fold nodule Vocal cord paresis Vocal cord dysfunction epiglottis Epiglottitis trachea Tracheitis Tracheal stenosis Lower RT/lung disease (including LRTIs) Bronchial/ obstructive acute Acute bronchitis chronic COPD Chronic bronchitis Acute exacerbations of chronic bronchitis Acute exacerbation of COPD Emphysema) Asthma (Status asthmaticus Aspirin-induced Exercise-induced Bronchiectasis unspecified Bronchitis Bronchiolitis Bronchiolitis obliterans Diffuse panbronchiolitis Interstitial/ restrictive (fibrosis) External agents/ occupational lung disease Pneumoconiosis Asbestosis Baritosis Bauxite fibrosis Berylliosis Caplan's syndrome Chalicosis Coalworker's pneumoconiosis Siderosis Silicosis Talcosis Byssinosis Hypersensitivity pneumonitis Bagassosis Bird fancier's lung Farmer's lung Lycoperdonosis Other ARDS Pulmonary edema Löffler's syndrome/Eosinophilic pneumonia Respiratory hypersensitivity Allergic bronchopulmonary aspergillosis Hamman-Rich syndrome Idiopathic pulmonary fibrosis Sarcoidosis Obstructive or restrictive Pneumonia/ pneumonitis By pathogen Viral Bacterial Pneumococcal Klebsiella) Atypical bacterial Mycoplasma Legionnaires' disease Chlamydiae Fungal Pneumocystis Parasitic noninfectious Chemical/Mendelson's syndrome Aspiration/Lipid By vector/route Community-acquired Healthcare-associated Hospital-acquired By distribution Broncho- Lobar IIP UIP DIP BOOP-COP NSIP RB Other Atelectasis circulatory Pulmonary hypertension Pulmonary embolism Lung abscess Pleural cavity/ mediastinum Pleural disease Pleuritis/pleurisy Pneumothorax/Hemopneumothorax Pleural effusion Hemothorax Hydrothorax Chylothorax Empyema/pyothorax Malignant Fibrothorax Mediastinal disease Mediastinitis Mediastinal emphysema Other/general Respiratory failure Influenza SARS Idiopathic pulmonary haemosiderosis Pulmonary alveolar proteinosis M: RES anat (n, x, l, c)/phys/devp noco (c, p)/cong/tumr, sysi/epon, injr proc, drug (R1/2/3/5/6/7)

v t e Prenatal development/Mammalian development of respiratory system (overview) (GA 11.1071, TE E5.5) Upper Nasal placode Lower Laryngotracheal groove Respiratory bud M: RES anat (n, x, l, c)/phys/devp noco (c, p)/cong/tumr, sysi/epon, injr proc, drug (R1/2/3/5/6/7)