NEET MDS Lessons
Physiology
Control of processes in the stomach:
The stomach, like the rest of the GI tract, receives input from the autonomic nervous system. Positive stimuli come from the parasympathetic division through the vagus nerve. This stimulates normal secretion and motility of the stomach. Control occurs in several phases:
Cephalic phase stimulates secretion in anticipation of eating to prepare the stomach for reception of food. The secretions from cephalic stimulation are watery and contain little enzyme or acid.
Gastric phase of control begins with a direct response to the contact of food in the stomach and is due to stimulation of pressoreceptors in the stomach lining which result in ACh and histamine release triggered by the vagus nerve. The secretion and motility which result begin to churn and liquefy the chyme and build up pressure in the stomach. Chyme surges forward as a result of muscle contraction but is blocked from entering the duodenum by the pyloric sphincter. A phenomenon call retropulsion occurs in which the chyme surges backward only to be pushed forward once again into the pylorus. The presence of this acid chyme in the pylorus causes the release of a hormone called gastrin into the bloodstream. Gastrin has a positive feedback effect on the motility and acid secretion of the stomach. This causes more churning, more pressure, and eventually some chyme enters the duodenum.
Intestinal phase of stomach control occurs. At first this involves more gastrin secretion from duodenal cells which acts as a "go" signal to enhance the stomach action already occurring. But as more acid chyme enters the duodenum the decreasing pH inhibits gastrin secretion and causes the release of negative or "stop" signals from the duodenum.
These take the form of chemicals called enterogastrones which include GIP (gastric inhibitory peptide). GIP inhibits stomach secretion and motility and allows time for the digestive process to proceed in the duodenum before it receives more chyme. The enterogastric reflex also reduces motility and forcefully closes the pyloric sphincter. Eventually as the chyme is removed, the pH increases and gastrin and the "go" signal resumes and the process occurs all over again. This series of "go" and "stop" signals continues until stomach emptying is complete.
The Stomach :
The wall of the stomach is lined with millions of gastric glands, which together secrete 400–800 ml of gastric juice at each meal. Three kinds of cells are found in the gastric glands
- parietal cells
- chief cells
- mucus-secreting cells
Parietal cells : secrete
Hydrochloric acid : Parietal cells contain a H+ ATPase. This transmembrane protein secretes H+ ions (protons) by active transport, using the energy of ATP.
Intrinsic factor: Intrinsic factor is a protein that binds ingested vitamin B12 and enables it to be absorbed by the intestine. A deficiency of intrinsic factor as a result of an autoimmune attack against parietal cells causes pernicious anemia.
Chief Cells : The chief cells synthesize and secrete pepsinogen, the precursor to the proteolytic enzyme pepsin.
Secretion by the gastric glands is stimulated by the hormone gastrin. Gastrin is released by endocrine cells in the stomach in response to the arrival of food.
Plasma: is the straw-colored liquid in which the blood cells are suspended.
|
Composition of blood plasma |
|
|
Component |
Percent |
|
Water |
~92 |
|
Proteins |
6–8 |
|
Salts |
0.8 |
|
Lipids |
0.6 |
|
Glucose (blood sugar) |
0.1 |
Plasma transports materials needed by cells and materials that must be removed from cells:
- various ions (Na+, Ca2+, HCO3−, etc.
- glucose and traces of other sugars
- amino acids
- other organic acids
- cholesterol and other lipids
- hormones
- urea and other wastes
Most of these materials are in transit from a place where they are added to the blood
- exchange organs like the intestine
- depots of materials like the liver
to places where they will be removed from the blood.
- every cell
- exchange organs like the kidney, and skin.
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
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.
Vital Capacity: The vital capacity (VC) is the maximum volume which can be ventilated in a single breath. VC= IRV+TV+ERV. VC varies with gender, age, and body build. Measuring VC gives a device for diagnosis of respiratory disorder, and a benchmark for judging the effectiveness of treatment. (4600 ml)
Vital Capacity is reduced in restrictive disorders, but not in disorders which are purely obstructive.
The FEV1 is the % of the vital capacity which is expelled in the first second. It should be at least 75%. The FEV1 is reduced in obstructive disorders.
Both VC and the FEV1 are reduced in disorders which are both restrictive and obstructive
Oxygen is present at nearly 21% of ambient air. Multiplying .21 times 760 mmHg (standard pressure at sea level) yields a pO2 of about 160. Carbon dioxide is .04% of air and its partial pressure, pCO2, is .3.
With alveolar air having a pO2 of 104 and a pCO2 of 40. So oxygen diffuses into the alveoli from inspired air and carbon dioxide diffuses from the alveoli into air which will be expired. This causes the levels of oxygen and carbon dioxide to be intermediate in expired air when compared to inspired air and alveolar air. Some oxygen has been lost to the alveolus, lowering its level to 120, carbon dioxide has been gained from the alveolus raising its level to 27.
Likewise a concentration gradient causes oxygen to diffuse into the blood from the alveoli and carbon dioxide to leave the blood. This produces the levels seen in oxygenated blood in the body. When this blood reaches the systemic tissues the reverse process occurs restoring levels seen in deoxygenated blood.
Transport of Carbon Dioxide
A. Dissolved in Blood Plasma (7-10%)
B. Bound to Hemoglobin (20-30%)
1. carbaminohemoglobin - Carb Dioxide binds to an amino acid on the polypeptide chains
2. Haldane Effect - the less oxygenated blood is, the more Carb Diox it can carry
a. tissues - as Oxygen is unloaded, affinity for Carb Dioxide increases
b. lungs - as Oxygen is loaded, affinity for Carb Dioxide decreases, allowing it to be released
C. Bicarbonate Ion Form in Plasma (60-70%)
1. Carbon Dioxide combines with water to form Bicarbonate
CO2 + H2O <==> H2CO3 <==> H+ + HCO3-
2. carbonic anhydrase - enzyme in RBCs that catalyzes this reaction in both directions
a. tissues - catalyzes formation of Bicarbonate
b. lungs - catalyzes formation of Carb Dioxide
3. Bohr Effect - formation of Bicarbonate (through Carbonic Acid) leads to LOWER pH (H+ increase), and more unloading of Oxygen to tissues
a. since hemoglobin "buffers" to H+, the actual pH of blood does not change much
4. Chloride Shift - chloride ions move in opposite direction of the entering/leaving Bicarbonate, to prevent osmotic problems with RBCs
D. Carbon Dioxide Effects on Blood pH
1. carbonic acid-bicarbonate buffer system
low pH → HCO3- binds to H+
high pH → H2CO3 releases H+
2. low shallow breaths → HIGH Carb Dioxide → LOW pH (higher H+)
3. rapid deep breaths → LOW Carb Dioxide → HIGH pH (lower H+)