NEET MDS Lessons
Physiology
Red blood cell cycle:
RBCs enter the blood at a rate of about 2 million cells per second. The stimulus for erythropoiesis is the hormone erythropoietin, secreted mostly by the kidney. RBCs require Vitamin B12, folic acid, and iron. The lifespan of RBC averages 120 days. Aged and damaged red cells are disposed of in the spleen and liver by macrophages. The globin is digested and the amino acids released into the blood for protein manufacture; the heme is toxic and cannot be reused, so it is made into bilirubin and removed from the blood by the liver to be excreted in the bile. The red bile pigment bilirubin oxidizes into the green pigment biliverdin and together they give bile and feces their characteristic color. Iron is picked up by a globulin protein (apotransferrin) to be transported as transferrin and then stored, mostly in the liver, as hemosiderin or ferritin. Ferritin is short term iron storage in constant equilibrium with plasma iron carried by transferrin. Hemosiderin is long term iron storage, forming dense granules visible in liver and other cells which are difficult for the body to mobilize.
Some iron is lost from the blood due to hemorrhage, menstruation, etc. and must be replaced from the diet. On average men need to replace about 1 mg of iron per day, women need 2 mg. Apotransferrin (transferrin without the iron) is present in GI lining cells and is also released in the bile. It picks up iron from the GI tract and stimulates receptors on the lining cells which absorb it by pinocytosis. Once through the mucosal cell iron is carried in blood as transferrin to the liver and marrow. Iron leaves the transferrin molecule to bind to ferritin in these tissues. Most excess iron will not be absorbed due to saturation of ferritin, reduction of apotransferrin, and an inhibitory process in the lining tissue.
Erythropoietin Mechanism:
Myeloid (blood producing) tissue is found in the red bone marrow located in the spongy bone. As a person ages much of this marrow becomes fatty and ceases production. But it retains stem cells and can be called on to regenerate and produce blood cells later in an emergency. RBCs enter the blood at a rate of about 2 million cells per second. The stimulus for erythropoiesis is the hormone erythropoietin, secreted mostly by the kidney. This hormone triggers more of the pleuripotential stem cells (hemocytoblasts) to follow the pathway to red blood cells and to divide more rapidly.
It takes from 3 to 5 days for development of a reticulocyte from a hemocytoblast. Reticulocytes, immature rbc, move into the circulation and develop over a 1 to 2 day period into mature erythrocytes. About 1 to 2 % of rbc in the circulation are reticulocytes, and the exact percentage is a measure of the rate of erythropoiesis.
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
The Adrenal Glands
The adrenal glands are two small structures situated one at top each kidney. Both in anatomy and in function, they consist of two distinct regions:
- an outer layer, the adrenal cortex, which surrounds
- the adrenal medulla.
The Adrenal Cortex
cells of the adrenal cortex secrete a variety of steroid hormones.
- glucocorticoids (e.g., cortisol)
- mineralocorticoids (e.g., aldosterone)
- androgens (e.g., testosterone)
- Production of all three classes is triggered by the secretion of ACTH from the anterior lobe of the pituitary.
Glucocorticoids
They Effect by raising the level of blood sugar (glucose). One way they do this is by stimulating gluconeogenesis in the liver: the conversion of fat and protein into intermediate metabolites that are ultimately converted into glucose.
The most abundant glucocorticoid is cortisol (also called hydrocortisone).
Cortisol and the other glucocorticoids also have a potent anti-inflammatory effect on the body. They depress the immune response, especially cell-mediated immune responses.
Mineralocorticoids
The most important of them is the steroid aldosterone. Aldosterone acts on the kidney promoting the reabsorption of sodium ions (Na+) into the blood. Water follows the salt and this helps maintain normal blood pressure.
Aldosterone also
- acts on sweat glands to reduce the loss of sodium in perspiration;
- acts on taste cells to increase the sensitivity of the taste buds to sources of sodium.
The secretion of aldosterone is stimulated by:
- a drop in the level of sodium ions in the blood;
- a rise in the level of potassium ions in the blood;
- angiotensin II
- ACTH (as is that of cortisol)
Androgens
The adrenal cortex secretes precursors to androgens such as testosterone.
Excessive production of adrenal androgens can cause premature puberty in young boys.
In females, the adrenal cortex is a major source of androgens. Their hypersecretion may produce a masculine pattern of body hair and cessation of menstruation.
Addison's Disease: Hyposecretion of the adrenal cortices
Addison's disease has many causes, such as
- destruction of the adrenal glands by infection;
- their destruction by an autoimmune attack;
- an inherited mutation in the ACTH receptor on adrenal cells.
Cushing's Syndrome: Excessive levels of glucocorticoids
In Cushing's syndrome, the level of adrenal hormones, especially of the glucocorticoids, is too high.It can be caused by:
- excessive production of ACTH by the anterior lobe of the pituitary;
- excessive production of adrenal hormones themselves (e.g., because of a tumor), or (quite commonly)
- as a result of glucocorticoid therapy for some other disorder such as
- rheumatoid arthritis or
- preventing the rejection of an organ transplant.
The Adrenal Medulla
The adrenal medulla consists of masses of neurons that are part of the sympathetic branch of the autonomic nervous system. Instead of releasing their neurotransmitters at a synapse, these neurons release them into the blood. Thus, although part of the nervous system, the adrenal medulla functions as an endocrine gland.The adrenal medulla releases:
- adrenaline (also called epinephrine) and
- noradrenaline (also called norepinephrine)
Both are derived from the amino acid tyrosine.
Release of adrenaline and noradrenaline is triggered by nervous stimulation in response to physical or mental stress. The hormones bind to adrenergic receptors transmembrane proteins in the plasma membrane of many cell types.
Some of the effects are:
- increase in the rate and strength of the heartbeat resulting in increased blood pressure;
- blood shunted from the skin and viscera to the skeletal muscles, coronary arteries, liver, and brain;
- rise in blood sugar;
- increased metabolic rate;
- bronchi dilate;
- pupils dilate;
- hair stands on end (gooseflesh in humans);
- clotting time of the blood is reduced;
- increased ACTH secretion from the anterior lobe of the pituitary.
All of these effects prepare the body to take immediate and vigorous action.
Function of Blood
- transport through the body of
- oxygen and carbon dioxide
- food molecules (glucose, lipids, amino acids)
- ions (e.g., Na+, Ca2+, HCO3−)
- wastes (e.g., urea)
- hormones
- heat
- defense of the body against infections and other foreign materials. All the WBCs participate in these defenses
Normal Chemical Composition of Urine
Urine is an aqueous solution of greater than 95% water, with a minimum of these remaining constituents, in order of decreasing concentration:
Urea 9.3 g/L.
Chloride 1.87 g/L.
Sodium 1.17 g/L.
Potassium 0.750 g/L.
Creatinine 0.670 g/L .
Other dissolved ions, inorganic and organic compounds (proteins, hormones, metabolites).
Urine is sterile until it reaches the urethra, where epithelial cells lining the urethra are colonized by facultatively anaerobic gram-negative rods and cocci. Urea is essentially a processed form of ammonia that is non-toxic to mammals, unlike ammonia, which can be highly toxic. It is processed from ammonia and carbon dioxide in the liver.
Exchange of gases takes place in Lungs
- A person with an average ventilation rate of 7.5 L/min will breathe in and out 10,800 liters of gas each day
- From this gas the person will take in about 420 liters of oxygen (19 moles/day) and will give out about 340 liters of carbon dioxide (15 moles/day)
- The ratio of CO2 expired/O2 inspired is called the respiratory quotient (RQ)
- RQ = CO2 out/O2 in = 340/420 = 0.81
- In cellular respiration of glucose CO2 out = O2 in; RQ = 1
- The overall RQ is less than 1 because our diet is a mixture of carbohydrates and fat; the RQ for metabolizing fat is only 0.7
- All of the exchange of gas takes place in the lungs
- The lungs also give off large amounts of heat and water vapor
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