PHS 301 Respiratory tract Flashcards
Respiratory control, surfactant & dead space
Types of respiratory control
Chemical control mechansim
Nervous control mechanism
What is chemical control of respiration (and stimulation)
carries out by chemoreceptors, these are sensory nerve ending that responds to changes in the chemical constituent of blood
they can be stimulated by:
i. Hypoxia (low O2)
ii. Hypercapnea (increase PCO2)
iii. Increased [H+]
Types of chemoreceptors
i. central
ii. peripheral
Central chemoreceptors
Location, function & stimulant
Central chemoreceptors are situated in the deeper part of the medulla oblongata, close to the dorsal respiratory group of neurons.
They form synapse with the respiratory centres, especially the DRG
Chemoreceptors are in close contact with blood and CSF. They act slowly but effectively and are responsible for 70% to 80% of increased ventilation through the chemical control of respiration.
The main stimulant for central chemoreceptors is the increased [H+] conc.
Whenever [H+] conc. increases in the blood, it cannot stimulate the central chemoreceptors because the hydrogen ions from blood cannot cross the BBB and blood-CSF barrier.
However, if CO2 increases in the blood, it can easily cross the BBB and blood CSF barrier and enter the interstitial fluid of the brain or the CSF.
There CO2 combines with H2O to form carbonic acid: CO2+ H2O → H2CO3 → H+ + HCO3–
Carbonic Acid immediately dissociates into H+ & HCO3-
[H+] stimulates the central chemoreceptors and impulses are passed from the chemoreceptors to the DRG. Resulting in increased ventilation (increased rate and force of breathing).
Because of this, excess CO2 is washed out and respiration is brought back to normal.
PERIPHERAL CHEMORECEPTORS
Peripheral chemoreceptors are situated in the carotid and aortic bodies
The carotid bodies are bilaterally located at the bifurcation of the common carotid artery. The afferent nerve fibers passes through the Herring’s nerve and to the dorsopharyngeal nerve, then to dorsorespiratory area of the medulla. The aortic bodies are located along the arch of the aorta. The afferent nerve fibers pass through the vagi and to the dorsal respiratory area of the medulla.
The main stimulant for the peripheral chemoreceptors is decreased in PO2. Whenever there is a decrease in PO2, peripheral chemoreceptors are stimulated and impulses are sent through the aortic and Herring’s nerve fibres to the respiratory area, especially DRG. The DRG sends impulses to the respiratory muscles. DRG sends impulses and this causes an increase in ventilation, hence hypoxia is being inverted so that the PO2 is increased.
Describe the nervous control of respirations
Respiratory centres are composed of several groups of neurons, which are bilaterally located in the medulla oblongata and pons of the brainstem
Types:
1. Medullary Centres
2. Pontine Centres
MEDULLARY CENTERS
Composed of:
1. Dorsal Respiratory Group of Neurons (DRG)
2. Ventral Respiratory Group of Neurons (VRG)
Dorsal Respiratory Group of Neurons
Dorsal respiratory group of neurons are located in the nucleus of tractus solitarius (solitary tract) in the upper medulla. All the neurons of the dorsal respiratory group are inspiratory and generate inspiratory ramp by their autorhythmic property and are responsible for the basic rhythm of respiration
Ventral Respiratory Group of Neurons
Located anteriorly and laterally to the nucleus of tractus solitarius. They are composed of a nucleus ambiguous and a nucleus retroambiguous. Have both inspiratory and expiratory neurons.
They are normally inactive during quiet breathing and become active during forced breathing.
PONTINE CENTERS are divided into
Pneumotaxic Center
Apneustic Center
Apneustic Center (location and function)
The apneustic center is situated in the reticular formation of lower pons, it increases the depth of inspiration by acting directly on dorsal group neurons.
Pneumotaxic Cente (location and function)
The pneumotaxic centre is situated in the posterolateral part of the reticular formation in the upper pons. It is formed by neurons of medial parabrachial and subparabrachial nuclei. The primary function of pneumotaxic centre is to control the medullary respiratory centres especially the DRG.
It acts through an apneustic centre.
Pneumotaxic centre inhibits the apneustic centre so that the dorsal group neurons are inhibited. Because of this, inspiration stops and expiration starts. Thus, pneumotaxic centre influences the switching between inspiration and expiration.
Influence of the neural respiratory centre
The faster or slower the impulse, the faster or slower the breathing. Different factors can influence the rate at which impulses are released
Firstly, higher centres of the brain allow for the voluntary control of breathing. Also, the higher centres of the brain are important for the perception of pain, emotion and temperature. Hence, all these factors act on the pontine respiratory centres either positive or negative.
The pontine response centre will also be positive or negative, and the medullary response centre and hence stimulate or inhibit the rate of respiration.
Hering-Breuer initiation reflex
TYPES OF DEAD SPACE
Dead space is of two types:
1. Anatomical dead space
2. Physiologic dead space
What is dead space?
Dead space is defined as the part of the respiratory tract, where gaseous exchange does not take place. Air
present in the dead space is called dead space air.
Anatomical dead space (what it contains)
Anatomical dead space extends from the nose up to terminal bronchiole.
It includes the nose, pharynx, trachea, bronchi and branches of bronchi up to terminal bronchioles. These structures serve only as the passage for air movement. Gaseous exchange does not take place in these structures.
Physiologic dead space/ total dead space
Includes anatomical dead space plus two additional volumes.
1. Air in the alveoli, which are non-functioning.
In some respiratory diseases, alveoli do not function because of dysfunction or destruction of the alveolar membrane.
2. Air in the alveoli, which do not receive adequate blood flow.
Gaseous exchange does not take place during inadequate blood supply.
Normal range for dead space
150ml but can become expanded in physiologic conditions
Surfactants
a surface-acting material or agent that reduces the surface tension of a fluid.
A surfactant that lines the epithelium of the alveoli in the lungs is known as a pulmonary surfactant and it decreases the surface tension on the alveolar membrane.
Pulmonary surfactant is secreted by two types of cells:
- Type II alveolar epithelial cells in the lungs, which are called surfactant-secreting alveolar cells or pneumocytes. A characteristic feature of these cells is the presence of microvilli on their alveolar surface.
- Clara cells, which are situated in the bronchioles. These cells are also called bronchiolar exocrine cells
Chemistry of surfactant
Surfactant is a lipoprotein complex formed by lipids, especially phospholipids, proteins and ions. Phospholipids form about 75% of the surfactant. The major phospholipid present in the surfactant is dipalmitoylphosphatidylcholine (DPPC). Other lipid substances of surfactant are
triglycerides and phosphatidylglycerol (PG).
Proteins of the surfactant are called specific surfactant proteins. There are four main surfactant proteins, called SPA, SPB, SPC and SPD.
SPA and SPD are hydrophilic, while SPB and SPC are hydrophobic. Surfactant proteins are vital components of surfactant and the surfactant becomes inactive in the absence of proteins.
Ions present in the surfactant are mostly calcium ions.
Effect of surfactant deficiency
The absence of surfactant in infants causes collapse of the lungs and the condition is called infant respiratory distress syndrome or hyaline membrane disease. The rate of maturation of surfactant can be increased by glucocorticoids. Surfactants can also prevent pulmonary oedema. Cigarette smoking decreases lung surfactant.
Describe the mechanics of respiration (chief muscle of respiration)
The chief muscle of respiration is the diaphragm. It is responsible for 25% of the change in intra-thoracic volume.
The diaphragm arches over the liver, It is attached to the bottom of the thoracic cage as it arches over the liver. And it’s contraction causes an increase in the thoracic cage
