Respiratory System Flashcards
Describe the two ways by which the respiratory system can be sub-divided.
1) The respiratory system can be divided into the upper and lower tract, where the upper tract includes all aspects concerned with respiration and food intake (nasal and oral cavity and pharynx), and the lower tract concerning only air (trachea and below).
2) It can also be divided into physiological divisions - the conducting and respiratory zones. The conducting zone conducts air towards the lungs, and includes the nose, pharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles. The respiratory zone is the functional zone involved in respiration, and includes the alveolar ducts and sacs.
Describe the gross anatomy, histology, and functions of the nose and nasopharynx.
Air travels through the nostrils, into the nose, and subsequently to the nasopharynx, where the nasal and oral cavities connect with the pharynx.
The nasal cavity can be divided into the olfactory region (located superiorly and lined with olfactory epithelium), the respiratory region (lined with respiratory epithelium), and the vestibule (area around the external opening to the nasal cavity - lined with skin and hairs). Paranasal sinuses add resonance to the voice, and reduce the weight of the skull. Nasal conchae (AKA turbinates) are three projections of bone from the lateral walls of the cavity, and the meatuses (area between the conchae) create vortexes of air to increase the time of contact and surface area for contact between air with mucosa and olfactory receptors (allows for heating and moistening of air by capillaries).
Describe the gross anatomy, histology, and functions of the pharynx
The pharynx is a tube consisting of three muscles - the superior, middle, and inferior pharyngeal constrictors (stacked like plant pots). The pharync can be divided into the nasopharynx (proximal to the nasal cavity and lined with respiratory epithelium), the oropharynx (mid-region which spans from the soft palate to the epiglottis, and lined with stratified squamous epithelium), and the laryngopharynx (most distal part of the pharynx, from the epiglottis to the oesophagus).
Describe the gross anatomy, histology, and functions of the larynx.
The larynx is a primarily cartilaginous structure which produces sound by acting upon exhaled columns of air through vocal folds. It is comprised of 9 cartilages - the epiglottis, thyroid, cricoid, 2x arytenoid, 2x cuneiform, and 2x corniculate).
Vocal folds are comprised of true and false vocal cords - true vocal cords are covered with stratified squamous epithelium, false are covered with respiratory epithelium.
Describe the gross anatomy, histology, and functions of the trachea.
The trachea is around 12cm long and 2.5cm wide, and lies anterior to the oesophagus. It proximally continues from the cricoid cartilage of the larynx, and distally splits into the left and right primary bronchi.
C-shaped cartilages keep the trachea open, the last of which is called the carina (where it bifurcates, and is sensitive to mechanical stimuli - cough reflex). Trachealis muscle lies posteriorly, allowing food boli from the oesophagus to extend inward as it passes.
Describe the gross anatomy, histology, and functions of the bronchial tree and alveoli.
The trachea bifurcates into two primary bronchi which are supported by cartilage rings, which then divide into secondary (lobar) bronchi supported by cartilage plates. There are 2 secondary bronchi in the left, and 3 in the right lung.
These secondary bronchi divide into tertiary (segmental) bronchi, which have segments of cartilage (one tertiary bronchus for each lung segment).
Tertiary bronchi then divide into bronchioles, which have smooth muscle walls, and then terminal bronchioles which are lined with simple cuboidal epithelium.
Terminal bronchioles terminate at the respiratory bronchioles which comprise the alveolar ducts, leading to the alveoli.
Many alveoli make up the alveolar sacs, and have a very large surface area (surrounded by a network of blood capillaries for gas exchange).
Simple squamous epithelium in the alveoli minimises distance to maximise gas exchange between capillaries and alveoli. Type I pneumocytes (P1) are involved in gas exchange, and type II (P2) secrete surfactant to reduce surface tension during respiration.
Compare the gross anatomies of the right and left lungs.
The lungs occupy the majority of the thoracic cavity, and are separated in the midline by the mediastinum. The left lung has two lobes (inferior and superior) and one fissure (line dividing them - oblique fissure) due to the cardiac notch. The right lung has three lobes (superior, middle and inferior), and two fissures (oblique and horizontal).
Both lungs have hilum structures, where bronchi, pulmonary arteries, pulmonary veins, lymphatics, and nerves all enter and leaving. These are located posteriorly, facing inwards towards the mediastinum.
Identify and describe the membrane which surrounds the lungs.
The lungs are surrounded by a continuous membrane called the pleural membrane, which is divided into two layers - the parietal and visceral pleura. The visceral pleura makes contact with lungs, and the parietal with the thoracic wall - the pleural cavity between the two layers is filled with pleural fluid to reduce friction and prevent adherence between the two layers.
Describe the structures and mode of function of the muscles of respiration.
1) The diaphragm is the most significant muscle of respiration - it is a dome-shaped, musculotendinous sheet which separates the thorax from the abdomen (also has a central tendon with a hole in it where the oesophagus, vena cava, and aorta pass through).
When the diaphragm contracts, it increases the volume of the thoracic cavity, drawing in air by decreases interpulmonary pressure. Expiration at rest occurs via elastic recoil of the diaphragm, and so no energy is required when exhaling (but is required when inhaling).
The diaphragm is innervated by the phrenic nerve (C3-5).
Intercostal muscles lie between the ribs, in intercostal spaces. These form three layers, and are responsible for elevating the ribs during inspiration. As well as these, accessory muscles of respiration include the sternomastoid, scalene, pectoralis, and serratus muscles.
Describe the mechanics of breathing (including volume and pressure changes which occur during a breath).
Between inspiration and expiration, the alveolar and barometric pressures are both said to be zero. Pleural pressure is negative to prevent the lungs from collapsing. Transpulmonary pressure is the difference in pressure between the alveoli and the pleural cavity.
When the inspiratory muscles contract, an increase in thoracic volume results in pleural pressure becoming more negative, and transpulmonary pressure increasing, and the resulting negative alveolar pressure compared to barometric results in air flowing into the alveoli. End inspiration occurs when then the muscles stop contracting (and thus the volume of the lungs stops increasing, meaning that pressure is no longer changing), and alveolar pressure becomes equal to barometric pressure again.
During expiration, the thoracic volume decreases, and the pleural pressure increases, reducing the transpulmonary pressure. The thorax and lungs recoil, compressing the air in the alveoli such that the alveolar pressure becomes greater than the barometric pressure, and air is expelled.
Explain the process of gas exchange in the lungs.
At the interface between the alveolar sacs and the pulmonary capillaries, type I pneumocytes and capillary endothelium share a basement membrane. Gas exchange by diffusion is optimised by the presence of fenestrated (incomplete), simple squamous epithelium. Gases move down their pressure gradients to be exchanged between the blood and the air - O2 enters the blood and CO2 leaves.
Describe how oxygen and carbon dioxide are transported in the blood.
Oxygen is carried through the blood in two main ways - dissolved or bound to Hb. The amount of oxygen dissolved in the blood is proportional to its partial pressure (usually 0.003ml/100ml blood for every mmHg of PO2). Therefore, in arterial blood (100mmHg), there is 0.3ml of dissolved O2 per 100ml blood. As this does not meet the body’s tissue requirements, protein carriers are required to transport oxygen to target tissues.
Hb is composed of 2 alpha and 2 beta chains, and within each chain is a haem group. Each haem group contains iron in its reduced ferrous form, which is the site of O2 binding. Each RBC contains around 280 million Hb molecules.
When PO2 is at 100mmHg (as in systemic circulation), around 100% of haemoglobin is saturated. A drop in partial pressure down to 60mmHg has little effect on the % saturation of Hb, however, after this the relationship steepens (fundamental for our Hb to release oxygen into tissues).
CO2 is carried in the blood in 3 ways - 7% is dissolved in the blood, 93% diffuses into RBCs (of this, 23% is bound to Hb, and 70% is converted to carbonic acid which breaks down into hydrogen and bicarbonate ions). Bicarbonate ions move out of RBCs in exchange for chloride ions - this is key in the homeostasis of blood pH. In pulmonary capillaries, hydrogen joins bicarbonate to form carbonic acid, which is then dehydrated to form water and CO2 for diffusion into expired gas.
Discuss peripheral and central chemoreceptor’s roles in influencing control of ventilation
1) Peripheral chemoreceptors are held outwith the CNS - they are small, highly vascularised bodies found in the aortic arch and carotid sinuses (within the bifurcation of the carotid artery) and are paired with baroreceptors. When stimulated these receptors send signals to a nucleus in the brainstem called the nucleus tractus solitarius via the vagus nerve (from aortic arch) or the carotid sinus & glossopharyngeal nerve (from carotid sinuses). These peripheral chemoreceptors tend to respond to a decrease in arterial PO2 (hypoxia). This information is integrated in the brain, which modulates breathing to restore PO2 levels. If arterial PO2 drops below 60mmHg, there will be progressive hyperventilation. PO2 plays very little part in the control of breathing - PCO2 is far more influential.
2) Central chemoreceptors are clusters of neurons within the brainstem which are activated upon hypercapnia (high PCO2) or pH decrease. Upon stimulation, information is relayed int he brain, and the brain modulates breathing to restore PCO2 levels. Very small changes in PCO2 result in very profound effects on ventilation - on a moment-to-moment basis, CO2 concentration modulates breathing more than O2 concentration.
Explain the role of mechanoreceptors in respiratory contorl.
In the respiratory system, mechanoreceptors detect inflation of lungs and movement of the chest wall - they are responsible for integrating the respiratory pattern with other movements (posture, locomotion, etc). Repiratory mechanoreceptors are located throughout the respiratory tree. When stimulated, neural signals are sent to the brain via the vagus nerve, and the nucleus tractus solitarius. The brain integrates information received by mechanoreceptors and adjusts ventilation accordingly.
Explain the control of breathing rhythm by the brainstem.
Information relating to mechanical and chemical environment of the respiratory system is relayed afferently to the nucleus tractus solitarius (AKA the dorsal respiratory group) of the brainstem which is then sent to the ventral respiratory group - clusters of neurons in the brainstem which integrate this information and generate the rhythm of breathing.
Some neurons fire during inspiration, and others fire during expiration, in a rhythmic firing pattern. Neurons always fire at the same phase, and can therefore be denoted as inspiratory and expiratory neurons.
Respiration rhythm is generated by the brainstem by the ventral respiratory group, because the rhythm that they fire fire in have their efferent outputs on respiratory muscles. The pathway for this travels down the spinal cord, and the then exits via the phrenic nerve (C3-5) which innervates the diaphragm. Thoracic nerves innervate the intercostal muscles, and lumbar nerves innervate the abdominal muscles.
