NEET MDS Lessons
Physiology
The pituitary gland is pea-sized structure located at the base of the brain. In humans, it consists of two lobes:
- the Anterior Lobe and
- the Posterior Lobe
The Anterior Lobe
The anterior lobe contains six types of secretory cells All of them secrete their hormone in response to hormones reaching them from the hypothalamus of the brain.
Thyroid Stimulating Hormone (TSH)
TSH (also known as thyrotropin) is a glycoprotein The secretion of TSH is
- stimulated by the arrival of thyrotropin releasing hormone (TRH) from the hypothalamus.
- inhibited by the arrival of somatostatin from the hypothalamus.
TSH stimulates the thyroid gland to secrete its hormone thyroxine (T4).
Some develop antibodies against their own TSH receptors making more T4 causing hyperthyroidism. The condition is called thyrotoxicosis or Graves' disease.
Hormone deficiencies
A deficiency of TSH causes hypothyroidism: inadequate levels of T4 (and thus of T3 )..
Follicle-Stimulating Hormone (FSH)
FSH is a heterodimeric glycoprotein Synthesis and release of FSH is triggered by the arrival from the hypothalamus of gonadotropin-releasing hormone (GnRH).
FSH in females :In sexually-mature females, FSH (assisted by LH) acts on the follicle to stimulate it to release estrogens.
FSH in males :In mature males, FSH acts on spermatogonia stimulating (with the aid of testosterone) the production of sperm.
Luteinizing Hormone (LH)
LH is synthesized within the same pituitary cells as FSH and under the same stimulus (GnRH). It is also a heterodimeric glycoprotein
LH in females
In sexually-mature females, LH
- stimulates the follicle to secrete estrogen in the first half of the menstrual cycle
- a surge of LH triggers the completion of meiosis I of the egg and its release (ovulation) in the middle of the cycle
- stimulates the now-empty follicle to develop into the corpus luteum, which secretes progesterone during the latter half of the menstrual cycle.
LH in males
LH acts on the interstitial cells (also known as Leydig cells) of the testes stimulating them to synthesize and secrete the male sex hormone, testosterone.
LH in males is also known as interstitial cell stimulating hormone (ICSH).
Prolactin (PRL)
Prolactin is a protein of 198 amino acids. During pregnancy it helps in the preparation of the breasts for future milk production. After birth, prolactin promotes the synthesis of milk.
Prolactin secretion is
- stimulated by TRH
- repressed by estrogens and dopamine.
Growth Hormone (GH)
- Human growth hormone (also called somatotropin) is a protein
- The GH-secreting cells are stimulated to synthesize and release GH by the intermittent arrival of growth hormone releasing hormone (GHRH) from the hypothalamus. GH promotes body growth
In Child
- hyposecretion of GH produces dwarfism
- hypersecretion leads to gigantism
In adults, a hypersecretion of GH leads to acromegaly.
ACTH — the adrenocorticotropic hormone
ACTH acts on the cells of the adrenal cortex, stimulating them to produce
- glucocorticoids, like cortisol
- mineralocorticoids, like aldosterone
- androgens (male sex hormones, like testosterone
Hypersecretion of ACTH cause of Cushing's disease.
Proteins:
- about 50 - 60% of the dry mass of a typical cell
- subunit is the amino acid & amino acids are linked by peptide bonds
- 2 functional categories = structural (proteins part of the structure of a cell like those in the cell membrane) & enzymes
Enzymes are catalysts. Enzymes bind temporarily to one or more of the reactants of the reaction they catalyze. In doing so, they lower the amount of activation energy needed and thus speed up the reaction
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.
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
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.
1 - Passive processes - require no expenditure of energy by a cell:
- Simple diffusion = net movement of a substance from an area of high concentration to an area of low concentration. The rate of diffusion is influenced by:
- concentration gradient
- cross-sectional area through which diffusion occurs
- temperature
- molecular weight of a substance
- distance through which diffusion occurs
- Osmosis = diffusion of water across a semi permeable membrane (like a cell membrane) from an area of low solute concentration to an area of high solute concentration
- Facilitated diffusion = movement of a substance across a cell membrane from an area of high concentration to an area of low concentration. This process requires the use of 'carriers' (membrane proteins). In the example below, a ligand molecule (e.g., acetylcholine) binds to the membrane protein. This causes a conformational change or, in other words, an 'opening' in the protein through which a substance (e.g., sodium ions) can pass.
2 - Active processes - require the expenditure of energy by cells:
- Active transport = movement of a substance across a cell membrane from an area of low concentration to an area of high concentration using a carrier molecule
- Endo- & exocytosis - moving material into (endo-) or out of (exo-) cell in bulk form
An anti-diruetic is a substance that decreases urine volume, and ADH is the primary example of it within the body. ADH is a hormone secreted from the posterior pituitary gland in response to increased plasma osmolarity (i.e., increased ion concentration in the blood), which is generally due to an increased concentration of ions relative to the volume of plasma, or decreased plasma volume.
The increased plasma osmolarity is sensed by osmoreceptors in the hypothalamus, which will stimulate the posterior pituitary gland to release ADH. ADH will then act on the nephrons of the kidneys to cause a decrease in plasma osmolarity and an increase in urine osmolarity.
ADH increases the permeability to water of the distal convoluted tubule and collecting duct, which are normally impermeable to water. This effect causes increased water reabsorption and retention and decreases the volume of urine produced relative to its ion content.
After ADH acts on the nephron to decrease plasma osmolarity (and leads to increased blood volume) and increase urine osmolarity, the osmoreceptors in the hypothalamus will inactivate, and ADH secretion will end. Due to this response, ADH secretion is considered to be a form of negative feedback.