NEET MDS Lessons
Physiology
The small intestine
Digestion within the small intestine produces a mixture of disaccharides, peptides, fatty acids, and monoglycerides. The final digestion and absorption of these substances occurs in the villi, which line the inner surface of the small intestine.
This scanning electron micrograph (courtesy of Keith R. Porter) shows the villi carpeting the inner surface of the small intestine.
The crypts at the base of the villi contain stem cells that continuously divide by mitosis producing
- more stem cells
- cells that migrate up the surface of the villus while differentiating into
- columnar epithelial cells (the majority). They are responsible for digestion and absorption.
- goblet cells, which secrete mucus;
- endocrine cells, which secrete a variety of hormones;
- Paneth cells, which secrete antimicrobial peptides that sterilize the contents of the intestine.
All of these cells replace older cells that continuously die by apoptosis.
The villi increase the surface area of the small intestine to many times what it would be if it were simply a tube with smooth walls. In addition, the apical (exposed) surface of the epithelial cells of each villus is covered with microvilli (also known as a "brush border"). Thanks largely to these, the total surface area of the intestine is almost 200 square meters, about the size of the singles area of a tennis court and some 100 times the surface area of the exterior of the body.
Incorporated in the plasma membrane of the microvilli are a number of enzymes that complete digestion:
- aminopeptidases attack the amino terminal (N-terminal) of peptides producing amino acids.
- disaccharidasesThese enzymes convert disaccharides into their monosaccharide subunits.
- maltase hydrolyzes maltose into glucose.
- sucrase hydrolyzes sucrose (common table sugar) into glucose and fructose.
- lactase hydrolyzes lactose (milk sugar) into glucose and galactose.
Fructose simply diffuses into the villi, but both glucose and galactose are absorbed by active transport.
- fatty acids and monoglycerides. These become resynthesized into fats as they enter the cells of the villus. The resulting small droplets of fat are then discharged by exocytosis into the lymph vessels, called lacteals, draining the villi.
Platelets
Platelets are cell fragments produced from megakaryocytes.
Blood normally contains 150,000 to 350,000 per microliter (µl). If this value should drop much below 50,000/µl, there is a danger of uncontrolled bleeding. This is because of the essential role that platelets have in blood clotting.
When blood vessels are damaged, fibrils of collagen are exposed.
- von Willebrand factor links the collagen to platelets forming a plug of platelets there.
- The bound platelets release ADP and thromboxane A2 which recruit and activate still more platelets circulating in the blood.
- (This role of thromboxane accounts for the beneficial effect of low doses of aspirin a cyclooxygenase inhibitor in avoiding heart attacks.)
ReoPro is a monoclonal antibody directed against platelet receptors. It inhibits platelet aggregation and appears to reduce the risk that "reamed out" coronary arteries (after coronary angioplasty) will plug up again.
GENERAL SOMATIC AFFERENT (GSA) PATHWAYS FROM THE BODY
Pain and Temperature
Pain and temperature information from general somatic receptors is conducted over small-diameter (type A delta and type C) GSA fibers of the spinal nerves into the posterior horn of the spinal cord gray matter .
Fast and Slow Pain
Fast pain, often called sharp or pricking pain, is usually conducted to the CNS over type A delta fibers.
Slow pain, often called burning pain, is conducted to the CNS over smaller-diameter type C fibers.
Touch and Pressure
Touch can be subjectively described as discriminating or crude.
Discriminating (epicritic) touch implies an awareness of an object's shape, texture, three-dimensional qualities, and other fine points. Ability to recognize familiar objects simply by tactile manipulation.
The conscious awareness of body position and movement is called the kinesthetic sens
Crude (protopathic) touch, lacks the fine discrimination described above and doesn't generally give enough information to the brain to enable it to recognize a familiar object by touch alone.
Subconscious Proprioception
Most of the subconscious proprioceptive input is shunted to the cerebellum.
Posterior Funiculus Injury
Certain clinical signs are associated with injury to the dorsal columns.
As might be expected, these are generally caused by impairment to the kinesthetic sense and discriminating touch and pressure pathways.
They include
(1) the inability to recognize limb position,
(2) astereognosis,
(3) loss of two-point discrimination,
(4) loss of vibratory sense, and
(5) a positive Romberg sign.
Astereognosis is the inability to recognize familiar objects by touch alone. When asked to stand erect with feet together and eyes closed, a person with dorsal column damage may sway and fall. This is a positive Romberg sign.
Characteristics of Facilitated Diffusion & Active Transport - both require the use of carriers that are specific to particular substances (that is, each type of carrier can 'carry' one type of substance) and both can exhibit saturation (movement across a membrane is limited by number of carriers & the speed with which they move materials
Red Blood Cells (erythrocytes)
- Women average about 4.8 million of these cells per cubic millimeter (mm3; which is the same as a microliter [µl]) of blood.
- Men average about 5.4 x 106 per µl.
- These values can vary over quite a range depending on such factors as health and altitude.
- RBC precursors mature in the bone marrow closely attached to a macrophage.
- They manufacture hemoglobin until it accounts for some 90% of the dry weight of the cell.
- The nucleus is squeezed out of the cell and is ingested by the macrophage.
RBC have characteristic biconcave shape
Thus RBCs are terminally differentiated; that is, they can never divide. They live about 120 days and then are ingested by phagocytic cells in the liver and spleen. Most of the iron in their hemoglobin is reclaimed for reuse. The remainder of the heme portion of the molecule is degraded into bile pigments and excreted by the liver. Some 3 million RBCs die and are scavenged by the liver each second.
Red blood cells are responsible for the transport of oxygen and carbon dioxide.
4. Emphysema
1. Permanent enlargement of airways with distension of alveolar walls
Thickened Bronchial Submucosa, Edema & Cellular Infiltration (loss of elasticity), Dilation of Air spaces, due to destruction of alveolar walls (Air trapped by obstruction)
2. Lower Respiratory tree destruction
Respiratory Bronchioles, Alveolar ducts, & Alveolar sacs
Types of Emphysema:
1. Centrilobular (Centriacinar) = Respiratory Bronchioles
Rarely seen in non Smokers, More in Men than Women, Found in Smokers with Bronchitis
2. Panlobular (Panacinar)
Hereditary, Single autosomal recessive gene. Deficient in 1-globulin (1-antitrypsin), Protects respiratory tract from neutrophil elastase (Enzyme that distroys lung connective tissue) , Aged persons, Results from Bronchi or Bronchiolar constriction
NOTE: Smoking = Leading cause of Bronchitis, Emphysema
Alveolar Ventilation: is the volume of air of new air , entering the alveoli and adjacent gas exchange areas each minute . It equals to multiplying of respiratory rate by ( tidal volume - dead space).
Va = R rate X (TV- DsV)
= 12 X ( 500-150)
= 4200 ml of air.