NEET MDS Lessons
Physiology
Carbon Dioxide Transport
Carbon dioxide (CO2) combines with water forming carbonic acid, which dissociates into a hydrogen ion (H+) and a bicarbonate ions:
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3−
95% of the CO2 generated in the tissues is carried in the red blood cells:
- It probably enters (and leaves) the cell by diffusing through transmembrane channels in the plasma membrane. (One of the proteins that forms the channel is the D antigen that is the most important factor in the Rh system of blood groups.)
- Once inside, about one-half of the CO2 is directly bound to hemoglobin (at a site different from the one that binds oxygen).
- The rest is converted — following the equation above — by the enzyme carbonic anhydrase into
- bicarbonate ions that diffuse back out into the plasma and
- hydrogen ions (H+) that bind to the protein portion of the hemoglobin (thus having no effect on pH).
Only about 5% of the CO2 generated in the tissues dissolves directly in the plasma. (A good thing, too: if all the CO2 we make were carried this way, the pH of the blood would drop from its normal 7.4 to an instantly-fatal 4.5!)
When the red cells reach the lungs, these reactions are reversed and CO2 is released to the air of the alveoli.
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
Neural Substrates of Breathing
A. Medulla Respiratory Centers
Inspiratory Center (Dorsal Resp Group - rhythmic breathing) → phrenic nerve→ intercostal nerves→ diaphragm + external intercostals
Expiratory Center (Ventral Resp Group - forced expiration) → phrenic nerve → intercostal nerves → internal intercostals + abdominals (expiration)
1. eupnea - normal resting breath rate (12/minute)
2. drug overdose - causes suppression of Inspiratory Center
B. Pons Respiratory Centers
1. pneumotaxic center - slightly inhibits medulla, causes shorter, shallower, quicker breaths
2. apneustic center - stimulates the medulla, causes longer, deeper, slower breaths
C. Control of Breathing Rate & Depth
1. breathing rate - stimulation/inhibition of medulla
2. breathing depth - activation of inspiration muscles
3. Hering-Breuer Reflex - stretch of visceral pleura that lungs have expanded (vagal nerve)
D. Hypothalamic Control - emotion + pain to the medulla
E. Cortex Controls (Voluntary Breathing) - can override medulla as during singing and talking
Chemical Controls of Respiration
A. Chemoreceptors (CO2, O2, H+)
1. central chemoreceptors - located in the medulla
2. peripheral chemoreceptors - large vessels of neck
B. Carbon Dioxide Effects
1. a powerful chemical regulator of breathing by increasing H+ (lowering pH)
a. hypercapnia Carbon Dioxide increases ->
Carbonic Acid increases ->
pH of CSF decreases (higher H+)- >
DEPTH & RATE increase (hyperventilation)
b. hypocapnia - abnormally low Carbon Dioxide levels which can be produced by excessive hyperventilation; breathing into paper bag increases blood Carbon Dioxide levels
C. Oxygen Effects
1. aortic and carotid bodies - oxygen chemoreceptors
2. slight Ox decrease - modulate Carb Diox receptors
3. large Ox decrease - stimulate increase ventilation
4. hypoxic drive - chronic elevation of Carb Diox (due to disease) causes Oxygen levels to have greater effect on regulation of breathing
D. pH Effects (H+ ion)
1. acidosis - acid buildup (H+) in blood, leads to increased RATE and DEPTH (lactic acid)
E. Overview of Chemical Effects
Chemical Breathing Effect
increased Carbon Dioxide (more H+) increase
decreased Carbon Dioxide (less H+) decrease
slight decrease in Oxygen effect CO2 system
large decrease in Oxygen increase ventilation
decreased pH (more H+) increase
increased pH (less H+) decrease
Hormones are carried by the blood throughout the entire body, yet they affect only certain cells. The specific cells that respond to a given hormone have receptor sites for that hormone.
This is sort of a lock and key mechanism. If the key fits the lock, then the door will open. If a hormone fits the receptor site, then there will be an effect. If a hormone and a receptor site do not match, then there is no reaction. All of the cells that have receptor sites for a given hormone make up the target tissue for that hormone. In some cases, the target tissue is localized in a single gland or organ. In other cases, the target tissue is diffuse and scattered throughout the body so that many areas are affected.
Hormones bring about their characteristic effects on target cells by modifying cellular activity. Cells in a target tissue have receptor sites for specific hormones. Receptor sites may be located on the surface of the cell membrane or in the interior of the cell.
In general those protein hormones are unable to diffuse through the cell membrane and react with receptor sites on the surface of the cell. The hormone receptor reaction on the cell membrane activates an enzyme within the membrane, called adenyl cyclase, which diffuses into the cytoplasm. Within the cell, adenyl cyclase catalyzes or starts the process of removal of phosphates from ATP to produce cyclic adenosine monophosphate or c AMP. This c AMP activates enzymes within the cytoplasm that alter or change the cellular activity. The protein hormone, which reacts at the cell membrane, is called the first messenger. c Amp that brings about the action attributed to the hormone is called the second messenger. This type of action is relatively rapid because the precursors are already present and they just needed to be activated in some way.
Events in gastric function:
1) Signals from vagus nerve begin gastric secretion in cephalic phase.
2) Physical contact by food triggers release of pepsinogen and H+ in gastric phase.
3) Muscle contraction churns and liquefies chyme and builds pressure toward pyloric sphincter.
4) Gastrin is released into the blood by cells in the pylorus. Gastrin reinforces the other stimuli and acts as a positive feedback mechanism for secretion and motility.
5) The intestinal phase begins when acid chyme enters the duodenum. First more gastrin secretion causes more acid secretion and motility in the stomach.
6) Low pH inhibits gastrin secretion and causes the release of enterogastrones such as GIP into the blood, and causes the enterogastric reflex. These events stop stomach emptying and allow time for digestion in the duodenum before gastrin release again stimulates the stomach.
Respiratory system plays important role in maintaining homeostasis . Other than its major function , which is supplying the cells with needed oxygen to produce energy and getting rid of carbon dioxide , it has other functions :
1 Vocalization , or sound production.
2 Participation in acid base balance .
3 Participation in fluid balance by insensible water elimination (vapors ).
4 Facilitating venous return .
5 Participation in blood pressure regulation : Lungs produce Angiotensin converting enzyme ( ACE ) .
6 Immune function : Lungs produce mucous that trap foreign particles , and have ciliae that move foreign particles away from the lung. They also produce alpha 1 antitrepsin that protect the lungs themselves from the effect of elastase and other proteolytic enzymes