NEET MDS Lessons
Physiology
The thyroid gland is a double-lobed structure located in the neck. Embedded in its rear surface are the four parathyroid glands.
The Thyroid Gland
The thyroid gland synthesizes and secretes:
- thyroxine (T4) and
- calcitonin
T4 and T3
Thyroxine (T4 ) is a derivative of the amino acid tyrosine with four atoms of iodine. In the liver, one atom of iodine is removed from T4 converting it into triiodothyronine (T3). T3 is the active hormone. It has many effects on the body. Among the most prominent of these are:
- an increase in metabolic rate
- an increase in the rate and strength of the heart beat.
The thyroid cells responsible for the synthesis of T4 take up circulating iodine from the blood. This action, as well as the synthesis of the hormones, is stimulated by the binding of TSH to transmembrane receptors at the cell surface.
Diseases of the thyroid
1. hypothyroid diseases; caused by inadequate production of T3
- cretinism: hypothyroidism in infancy and childhood leads to stunted growth and intelligence. Can be corrected by giving thyroxine if started early enough.
- myxedema: hypothyroidism in adults leads to lowered metabolic rate and vigor. Corrected by giving thyroxine.
- goiter: enlargement of the thyroid gland. Can be caused by:
- inadequate iodine in the diet with resulting low levels of T4 and T3;
- an autoimmune attack against components of the thyroid gland (called Hashimoto's thyroiditis).
2. hyperthyroid diseases; caused by excessive secretion of thyroid hormones
Graves´ disease. Autoantibodies against the TSH receptor bind to the receptor mimicking the effect of TSH binding. Result: excessive production of thyroid hormones. Graves´ disease is an example of an autoimmune disease.
Osteoporosis. High levels of thyroid hormones suppress the production of TSH through the negative-feedback mechanism mentioned above. The resulting low level of TSH causes an increase in the numbers of bone-reabsorbing osteoclasts resulting in osteoporosis.
Calcitonin
Calcitonin is a polypeptide of 32 amino acids. The thyroid cells in which it is synthesized have receptors that bind calcium ions (Ca2+) circulating in the blood. These cells monitor the level of circulating Ca2+. A rise in its level stimulates the cells to release calcitonin.
- bone cells respond by removing Ca2+ from the blood and storing it in the bone
- kidney cells respond by increasing the excretion of Ca2+
Both types of cells have surface receptors for calcitonin.
Because it promotes the transfer of Ca2+ to bones, calcitonin has been examined as a possible treatment for osteoporosis
Exchange of gases:
- External respiration:
- exchange of O2 & CO2 between external environment & the cells of the body
- efficient because alveoli and capillaries have very thin walls & are very abundant (your lungs have about 300 million alveoli with a total surface area of about 75 square meters)
- Internal respiration - intracellular use of O2 to make ATP
- occurs by simple diffusion along partial pressure gradients
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.
Tubular secretion:
Involves transfer of substances from peritubular capillaries into the tubular lumen. It involves transepithelial transport in a direction opposite to that of tubular absorption.
Renal tubules can selectively add some substances that have not been filtered to the substances that already have been filtered via tubular secretion.
Tubular secretion mostly function to eliminate foreign organic ions, hydrogen ions ( as a contribution to acid base balance ), potassium ions ( as a contribution to maintaining optimal plasma K+ level to assure normal proceeding of neural and muscular functions), and urea.
Here we will focus on K+ secretion and will later discuss H+ secretion in acid base balance, while urea secretion will be discussed in water balance.
K+ is filtered in glomerular capillaries and then reabsorbed in proximal convoluted tubules as well as in thick ascending limb of loop of Henley ( Na-2Cl-K symporter)
K+ secretion takes place in collecting tubules (distal nephron) . There are two types of cells in distal nephron:
- Principal cells that reabsorb sodium and secrete K+ .
- Intercalated cells that reabsorb K+ in exchange with H+.
Mechanism of secretion of K+ in principal cells : Two steps
- K+ enters tubular cells by Na/K ATPase on the basolateral membrane.
- K+ leaves the tubular cells via K+ channels in apical membrane.
Aldosterone is a necessary regulatory factor.
If there is increased level of K+ in plasma,excessive K+ is secreted , some of which is reabsorbed back to the plasma in exchange with H+ via the intercalated cells.
Respiration occurs in three steps :
1- Mechanical ventilation : inhaling and exhaling of air between lungs and atmosphere.
2- Gas exchange : between pulmonary alveoli and pulmonary capillaries.
3- Transport of gases from the lung to the peripheral tissues , and from the peripheral tissues back to blood .
These steps are well regulated by neural and chemical regulation.
Respiratory tract is subdivided into upper and lower respiratory tract. The upper respiratory tract involves , nose , oropharynx and nasopharynx , while the lower respiratory tract involves larynx , trachea , bronchi ,and lungs .
Nose fulfills three important functions which are :
1. warming of inhaled air .
b. filtration of air .
c. humidification of air .
Pharynx is a muscular tube , which forms a passageway for air and food .During swallowing the epiglottis closes the larynx and the bolus of food falls in the esophagus .
Larynx is a respiratory organ that connects pharynx with trachea . It is composed of many cartilages and muscles and
vocal cords . Its role in respiration is limited to being a conductive passageway for air .
Trachea is a tube composed of C shaped cartilage rings from anterior side, and of muscle (trachealis muscle ) from its posterior side.The rings prevent trachea from collapsing during the inspiration.
From the trachea the bronchi are branched into right and left bronchus ( primary bronchi) , which enter the lung .Then they repeatedly branch into secondary and tertiary bronchi and then into terminal and respiratory broncholes.There are about 23 branching levels from the right and left bronchi to the respiratory bronchioles , the first upper 17 branching are considered as a part of the conductive zones , while the lower 6 are considered to be respiratory zone.
The cartilaginous component decreases gradually from the trachea to the bronchioles . Bronchioles are totally composed of smooth muscles ( no cartilage) . With each branching the diameter of bronchi get smaller , the smallest diameter of respiratory passageways is that of respiratory bronchiole.
Lungs are evolved by pleura . Pleura is composed of two layers : visceral and parietal .
Between the two layers of pleura , there is a pleural cavity , filled with a fluid that decrease the friction between the visceral and parietal pleura.
Respiratory muscles : There are two group of respiratory muscles:
1. Inspiratory muscles : diaphragm and external intercostal muscle ( contract during quiet breathing ) , and accessory inspiratory muscles : scaleni , sternocleidomastoid , internal pectoral muscle , and others( contract during forceful inspiration).
2. Expiratory muscles : internal intercostal muscles , and abdominal muscles ( contract during forceful expiration)
The Posterior Lobe
The posterior lobe of the pituitary releases two hormones, both synthesized in the hypothalamus, into the circulation.
- Antidiuretic Hormone (ADH).
ADH is a peptide of 9 amino acids. It is also known as arginine vasopressin. ADH acts on the collecting ducts of the kidney to facilitate the reabsorption of water into the blood.- A deficiency of ADH
- leads to excessive loss of urine, a condition known as diabetes nsipidus.
- A deficiency of ADH
- Oxytocin
Oxytocin is a peptide of 9 amino acids. Its principal actions are:- stimulating contractions of the uterus at the time of birth
- stimulating release of milk when the baby begins to suckle
Asthma = Reversible Bronchioconstruction 4%-5% of population
Extrinsic / Atopic = Allergic, inherited (familia), chromosome 11
IgE, Chemical Mediators of inflammation
a. Intrinsic = Negative for Allergy, Normal IgE, Negative Allergic Tests
Nucleotide Imbalance cAMP/cGMP: cAMP = Inhibits mediator release, cGMP = Facilitates mediator release
b. Intolerance to Asprin (Triad Asthma)
c. Nasal Polyps & Asthma
d. Treatment cause, Symptoms in Acute Asthma
1. Bronchial dilators
2. steroids edema from Inflamation
3. Bronchiohygene to prevent Secondary Infection, (Remove Excess Mucus)
4. Education