NEET MDS Lessons
Physiology
Ventilation simply means inhaling and exhaling of air from the atmospheric air into lungs and then exhaling it from the lung into the atmospheric air.
Air pressure gradient has to exist between two atmospheres to enable a gas to move from one atmosphere to an other.
During inspiration: the intrathoracic pressure has to be less than that of atmospheric pressure. This could be achieved by decreasing the intrathoracic pressure as follows:
Depending on Boyle`s law , the pressure of gas is inversely proportional to the volume of its container. So increasing the intrathoracic volume will decrease the intrathoracic pressure which will allow the atmospheric air to be inhaled (inspiration) . As decreasing the intrathoracic volume will increase the intrathoracic pressure and causes exhaling of air ( expiration)
So. Inspiration could be actively achieved by the contraction of inspiratory muscles : diaphragm and intercostal muscles. While relaxation of the mentioned muscles will passively cause expiration.
Contraction of diaphragm will pull the diaphragm down the abdominal cavity ( will move inferiorly) , and then increase the intrathoracic volume ( vertically) . Contraction of external intercostal muscle will pull the ribs upward and forward which will additionally increase the intrathoracic volume ( transversely , the net result will be increasing the intrathoracic volume and decreasing the intrathoracic pressure.
Relaxation of diaphragm will move it superiorly during expiration, the relaxation of external intercostal muscles will pull the ribs downward and backward , and the elastic lungs and chest wall will recoil. The net result is decreasing the intrathoracic volume and increasing intrathoracic pressure.
All of this occurs during quiet breathing. During forceful inspiration an accessory inspiratory muscle will be involved ( scaleni , sternocleidomastoid , and others) to increase negativity in the intrathoracic pressure more and more.
During forceful expiration the accessory expiratory muscles ( internal intercostal muscles and abdominal muscles ) will be involved to decrease the intrathoracic volume more and more and then to increase intrathoracic pressure more and more.
The pressure within the alveoli is called intralveolar pressure . Between the two phases of respiration it is equal to the atmospheric pressure. It is decreased during inspiration ( about 1 cm H2O ) and increased during expiration ( about +1 cm H2O ) . This difference allow entering of 0.5 L of air into the lungs.
Intrapleural pressure is the pressure of thin fluid between the two pleural layers . It is a slight negative pressure. At the beginning of inspiration it is about -5 cm H2O and reachs -7.5 cm H2O at the end or inspiration.
At the beginning of expiration the intrapleural pressure is -7.5 cm H2O and reaches -5 cmH2O at the end of expiration.
The difference between intralveolar pressure and intrapleural pressure is called transpulmonary pressure.
Factors , affecting ventilation :
Resistance : Gradual decreasing of the diameter of respiratory airway increase the resistance to air flow.
Compliance : means the ease , which the lungs expand.It depends on both the elastic forces of the lungs and the elastic forces , caused by the the surface tension of the fluid, lining the alveoli.
Surface tension: Molecules of water have tendency to attract each other on the surface of water adjacent to air. In alveoli the surface tension caused by the fluid in the inner surface of the alveoli may cause collapse of alveoli . The surface tension is decreased by the surfactant .
Surfactant is a mixture of phospholipids , proteins and ion m produced by type II pneumocytes.
Immature newborns may suffer from respiratory distress syndrome , due to lack of surfactant which is produced during the last trimester of pregnancy.
The elastic fibers of the thoracic wall also participate in lung compliance.
There are three types of muscle tissue, all of which share some common properties:
- Excitability or responsiveness - muscle tissue can be stimulated by electrical, physical, or chemical means.
- contractility - the response of muscle tissue to stimulation is contraction, or shortening.
- elasticity or recoil - muscles have elastic elements (later we will call these their series elastic elements) which cause them to recoil to their original size.
- stretchability or extensibility - muscles can also stretch and extend to a longer-than-resting length.
The three types of muscle: skeletal, cardiac, and visceral (smooth) muscle.
Skeletal muscle
It is found attached to the bones for movement.
cells are long multi-nucleated cylinders.
The cells may be many inches long but vary in diameter, averaging between 100 and 150 microns.
All the cells innervated by branches from the same neuron will contract at the same time and are referred to as a motor unit.
Skeletal muscle is voluntary because the neurons which innervate it come from the somatic or voluntary branch of the nervous system.
That means you have willful control over your skeletal muscles.
Skeletal muscles have distinct stripes or striations which identify them and are related to the organization of protein myofilaments inside the cell.
Cardiac muscle
This muscle found in the heart.
It is composed of much shorter cells than skeletal muscle which branch to connect to one another.
These connections are by means of gap junctions called intercalated disks which allow an electrochemical impulse to pass to all the connected cells.
This causes the cells to form a functional network called a syncytium in which the cells work as a unit. Many cardiac muscle cells are myogenic which means that the impulse arises from the muscle, not from the nervous system. This causes the heart muscle and the heart itself to beat with its own natural rhythm.
But the autonomic nervous system controls the rate of the heart and allows it to respond to stress and other demands. As such the heart is said to be involuntary.
Visceral muscle is found in the body's internal organs and blood vessels.
It is usually called smooth muscle because it has no striations and is therefore smooth in appearance. It is found as layers in the mucous membranes of the respiratory and digestive systems.
It is found as distinct bands in the walls of blood vessels and as sphincter muscles.
Single unit smooth muscle is also connected into a syncytium similar to cardiac muscle and is also partly myogenic. As such it causes continual rhythmic contractions in the stomach and intestine. There and in blood vessels smooth muscle also forms multiunit muscle which is stimulated by the autonomic nervous system. So smooth muscle is involuntary as well
The hypothalamus is a region of the brain. It secretes a number of hormones.
- Thyrotropin-releasing hormone (TRH)
- Gonadotropin-releasing hormone (GnRH)
- Growth hormone-releasing hormone (GHRH)
- Corticotropin-releasing hormone (CRH)
- Somatostatin
- Dopamine
All of these are released into the blood, travel immediately to the anterior lobe of the pituitary, where they exert their effects.
Two other hypothalamic hormones:
- Antidiuretic hormone (ADH) and
- Oxytocin
travel in neurons to the posterior lobe of the pituitary where they are released into the circulation.
An anti-diruetic is a substance that decreases urine volume, and ADH is the primary example of it within the body. ADH is a hormone secreted from the posterior pituitary gland in response to increased plasma osmolarity (i.e., increased ion concentration in the blood), which is generally due to an increased concentration of ions relative to the volume of plasma, or decreased plasma volume.
The increased plasma osmolarity is sensed by osmoreceptors in the hypothalamus, which will stimulate the posterior pituitary gland to release ADH. ADH will then act on the nephrons of the kidneys to cause a decrease in plasma osmolarity and an increase in urine osmolarity.
ADH increases the permeability to water of the distal convoluted tubule and collecting duct, which are normally impermeable to water. This effect causes increased water reabsorption and retention and decreases the volume of urine produced relative to its ion content.
After ADH acts on the nephron to decrease plasma osmolarity (and leads to increased blood volume) and increase urine osmolarity, the osmoreceptors in the hypothalamus will inactivate, and ADH secretion will end. Due to this response, ADH secretion is considered to be a form of negative feedback.
The Nervous System Has Peripheral and Central Units
- The central nervous system (CNS) is the brain and spinal column
- The peripheral nervous system (PNS) consists of nerves outside of the CNS
- There are 31 pairs of spinal nerves (mixed motor & sensory)
- There are 12 pairs of cranial nerves (some are pure sensory, but most are mixed)
The pattern of innervation plotted on the skin is called a dermatome
The Nervous System Has Peripheral and Central Units
- The central nervous system (CNS) is the brain and spinal column
- The peripheral nervous system (PNS) consists of nerves outside of the CNS
- There are 31 pairs of spinal nerves (mixed motor & sensory)
- There are 12 pairs of cranial nerves (some are pure sensory, but most are mixed)
The pattern of innervation plotted on the skin is called a dermatome
1. Automatic control (sensory) of respiration is in - brainstem (midbrain)
2. Behavioral/voluntary control is in - the cortex
3. Alveolar ventilation -the amount of atmospheric air that actually reaches the alveolar per breath and that can participate in the exchange of gasses between alveoli and blood
4. Only way to increase gas exchange in alveolar capillaries - perfusion-limited gas exchange
5. Pulmonary ventiliation not effected by - concentration of bicarbonate ions
6. Central chemoreceptors - medulla - CO2, O2 and H+ concentrations
7. Peripheral chemoreceptors - carotid and aortic bodies- PO2, PCO2 and pH
8. Major stimulus for respiratory centers - arterial PCO2
9. Rhythmic breathing depends on
1. continuous (tonic) inspiratory drive from DRG (dorsal respiratory group)
2. intermittent (phasic) expiratory input from cerebrum, thalamus, cranial nerves and ascending spinal cord sensory tracts
10. Primary site for gas exchange - type I epithelial cells for alveoli
Functions of the nervous system:
1) Integration of body processes
2) Control of voluntary effectors (skeletal muscles), and mediation of voluntary reflexes.
3) Control of involuntary effectors ( smooth muscle, cardiac muscle, glands) and mediation of autonomic reflexes (heart rate, blood pressure, glandular secretion, etc.)
4) Response to stimuli
5) Responsible for conscious thought and perception, emotions, personality, the mind.