NEET MDS Lessons
Physiology
The bulk of the pancreas is an exocrine gland secreting pancreatic fluid into the duodenum after a meal. However, scattered through the pancreas are several hundred thousand clusters of cells called islets of Langerhans. The islets are endocrine tissue containing four types of cells. In order of abundance, they are the:
- beta cells, which secrete insulin and amylin;
- alpha cells, which secrete glucagon;
- delta cells, which secrete somatostatin, and
- gamma cells, which secrete a polypeptide of unknown function.
Beta Cells
Beta cells secrete insulin in response to a rising level of blood sugar
Insulin affects many organs. It
- stimulates skeletal muscle fibers to
- take up glucose and convert it into glycogen;
- take up amino acids from the blood and convert them into protein.
- acts on liver cells
- stimulating them to take up glucose from the blood and convert it into glycogen while
- inhibiting production of the enzymes involved in breaking glycogen back down (glycogenolysis) and
- inhibiting gluconeogenesis; that is, the conversion of fats and proteins into glucose.
- acts on fat (adipose) cells to stimulate the uptake of glucose and the synthesis of fat.
- acts on cells in the hypothalamus to reduce appetite.
Diabetes Mellitus
Diabetes mellitus is an endocrine disorder characterized by many signs and symptoms. Primary among these are:
- a failure of the kidney to retain glucose .
- a resulting increase in the volume of urine because of the osmotic effect of this glucose (it reduces the return of water to the blood).
There are three categories of diabetes mellitus:
- Insulin-Dependent Diabetes Mellitus (IDDM) (Type 1) and
- Non Insulin-Dependent Diabetes Mellitus (NIDDM)(Type 2)
- Inherited Forms of Diabetes Mellitus
Insulin-Dependent Diabetes Mellitus (IDDM)
IDDM ( Type 1 diabetes)
- is characterized by little or no circulating insulin;
- most commonly appears in childhood.
- It results from destruction of the beta cells of the islets.
- The destruction results from a cell-mediated autoimmune attack against the beta cells.
- What triggers this attack is still a mystery, although a prior viral infection may be the culprit.
Non Insulin-Dependent Diabetes Mellitus (NIDDM)
Many people develop diabetes mellitus without an accompanying drop in insulin levels In many cases, the problem appears to be a failure to express a sufficient number of glucose transporters in the plasma membrane (and T-system) of their skeletal muscles. Normally when insulin binds to its receptor on the cell surface, it initiates a chain of events that leads to the insertion in the plasma membrane of increased numbers of a transmembrane glucose transporter. This transporter forms a channel that permits the facilitated diffusion of glucose into the cell. Skeletal muscle is the major "sink" for removing excess glucose from the blood (and converting it into glycogen). In NIDDM, the patient's ability to remove glucose from the blood and convert it into glycogen is reduced. This is called insulin resistance. NIDDM (also called Type 2 diabetes mellitus) usually occurs in adults and, particularly often, in overweight people.
Alpha Cells
The alpha cells of the islets secrete glucagon, a polypeptide of 29 amino acids. Glucagon acts principally on the liver where it stimulates the conversion of glycogen into glucose (glycogenolysis) which is deposited in the blood.
Glucagon secretion is
- stimulated by low levels of glucose in the blood;
- inhibited by high levels, and
- inhibited by amylin.
The physiological significance of this is that glucagon functions to maintain a steady level of blood sugar level between meals.
Delta Cells
The delta cells secrete somatostatin. Somatostatin has a variety of functions. Taken together, they work to reduce the rate at which food is absorbed from the contents of the intestine. Somatostatin is also secreted by the hypothalamus and by the intestine.
Gamma Cells
The gamma cells of the islets secrete pancreatic polypeptide. No function has yet been found for this peptide of 36 amino acids.
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.
Structure and function of skeletal muscle.
Skeletal muscles have a belly which contains the cells and which attaches by means of tendons or aponeuroses to a bone or other tissue. An aponeurosis is a broad, flat, tendinous attachment, usually along the edge of a muscle. A muscle attaches to an origin and an insertion. The origin is the more fixed attachment, the insertion is the more movable attachment. A muscle acts to shorten, pulling the insertion toward the origin. A muscle can only pull, it cannot push.
Muscles usually come in pairs of antagonistic muscles. The muscle performing the prime movement is the agonist, the opposite acting muscle is the antagonist. When the movement reverses, the names reverse. For example, in flexing the elbow the biceps brachii is the agonist, the triceps brachii is the antagonist. When the movement changes to extension of the elbow, the triceps becomes the agonist and the biceps the antagonist. An antagonist is never totally relaxed. Its function is to provide control and damping of movement by maintaining tone against the agonist. This is called eccentric movement.
Muscles can also act as synergists, working together to perform a movement. This movement can be different from that performed when the muscles work independently. For example, the sternocleidomastoid muscles each rotate the head in a different direction. But as synergists they flex the neck.
Fixators act to keep a part from moving. For example fixators act as postural muscles to keep the spine erect and the leg and vertebral column extended when standing. Fixators such as the rhomboids and levator scapulae keep the scapula from moving during actions such as lifting with the arms.
The endocrine system along with the nervous system functions in the regulation of body activities. The nervous system acts through electrical impulses and neurotransmitters to cause muscle contraction and glandular secretion and interpretation of impulses. The endocrine system acts through chemical messengers called hormones that influence growth, development, and metabolic activities
Physiology - science that describes how organisms FUNCTION and survive in continually changing environments
Bleeding Disorders
A deficiency of a clotting factor can lead to uncontrolled bleeding.
The deficiency may arise because
- not enough of the factor is produced or
- a mutant version of the factor fails to perform properly.
Examples:
- von Willebrand disease (the most common)
- hemophilia A for factor 8 deficiency
- hemophilia B for factor 9 deficiency.
- hemophilia C for factor 11 deficiency
In some cases of von Willebrand disease, either a deficient level or a mutant version of the factor eliminates its protective effect on factor 8. The resulting low level of factor 8 mimics hemophilia A.
Contractility : Means ability of cardiac muscle to convert electrical energy of action potential into mechanical energy ( work).
The excitation- contraction coupling of cardiac muscle is similar to that of skeletal muscle , except the lack of motor nerve stimulation.
Cardiac muscle is a self-excited muscle , but the principles of contraction are the same . There are many rules that control the contractility of the cardiac muscles, which are:
1. All or none rule: due to the syncytial nature of the cardiac muscle.There are atrial syncytium and ventricular syncytium . This rule makes the heart an efficient pump.
2. Staircase phenomenon : means gradual increase in muscle contraction following rapidly repeated stimulation..
3. Starling`s law of the heart: The greater the initial length of cardiac muscle fiber , the greater the force of contraction. The initial length is determined by the degree of diastolic filling .The pericardium prevents overstretching of heart , and allows optimal increase in diastolic volume.
Thankful to this law , the heart is able to pump any amount of blood that it receives. But overstretching of cardiac muscle fibers may cause heart failure.
Factors affecting contractility ( inotropism)
I. Positive inotropic factors:
1. sympathetic stimulation: by increasing the permeability of sarcolemma to calcium.
2. moderate increase in temperature . This due to increase metabolism to increase ATP , decrease viscosity of myocardial structures, and increasing calcium influx.
3. Catecholamines , thyroid hormone, and glucagon hormones.
4. mild alkalosis
5. digitalis
6. Xanthines ( caffeine and theophylline )
II. Negative inotropic factors:
1. Parasympathetic stimulation : ( limited to atrial contraction)
2. Acidosis
3. Severe alkalosis
4. excessive warming and cooling .
5. Drugs ;like : Quinidine , Procainamide , and barbiturates .
6. Diphtheria and typhoid toxins.