Q.4 Write short notes on -
(a) Structure of neurons (2007)
(b) Leucocytes (2007)
(c) Thyroid disease (2007)
(d) Functions of hcl in digestion (2008)
(e) Ultrafilteration (2006, 10, 14, 16, 18)
(f) Thyroid gland (2006, 16)
Or. Thyroid hormone (2018)
(g) Parathyroid hormone (2005, 09)
(h) Chloride shift (2007, 19)
(i) Blood Clotting Factors (2006)
(j) Osmoregulation by kidneys (2008)
(k) Synthesis of urea (2008, 18)
Or. Urine formation (2011)
(l) Hormones of mucosa of alimentary canal (2008)
(m) Saltatory conduction (2009)
Or. Saltatory conduction of nerve impulse. (2017)
Or. Write short note on saltatory conduction of nerve impulse. (2019)
Ans. (a) Structure of Neurons: -
Neurons are the basic building blocks of the nervous system. These specialized cells are the information-processing units of the brain responsible for receiving and transmitting information.
Each part of the neuron plays a role in the communication of information throughout the body.It is divided into following parts -
(1) Dendrites: -
These are treelike extensions at the beginning of a neuron that help increase the surface area of the cell body and are covered with synapses. These tiny protrusions receive information from other neurons and transmit electrical stimulation to the soma.
Dendrite Characteristics: -
· Most neurons have many dendrites.
· Short and highly branched.
· Transmits information to the cell body
(2) The soma is where the signals from the dendrites are joined and passed on. The soma and the nucleus do not play an active role in the transmission of the neural signal. Instead, these two structures serve to maintain the cell and keep the neuron functional.
The support structures of the cell include mitochondria, which provide energy for the cell, and the Golgi apparatus, which packages products created by the cell and secretes them outside the cell wall.
(3) The axon hillock is located and the end of the soma and controls the firing of the neuron. If the total strength of the signal exceeds the threshold limit of the axon hillock, the structure will fire a signal down the axon.
(4) The axon is the elongated fiber that extends from the cell body to the terminal endings and transmits the neural signal. The larger the axon, the faster it transmits information. Some axons are covered with a fatty substance called myelin that acts as an insulator. These myelinated axons transmit information much faster than other neurons.
Axon Characteristics: -
· Most neurons have only one axon.
· Transmit information away from the cell body.
· May or may not have a myelin covering.
(5) The terminal buttons are located at the end of the neuron and are responsible for sending the signal on to other neurons. At the end of the terminal button is a gap known as a synapse. Neurotransmitters are used to carry the signal across the synapse to other neurons.
(b) Leucocytes: -
White blood cells (WBCs), or leukocytes (also spelled “leucocytes”), are cells of the immune system defending the body against both infectious disease and foreign materials. Five different and diverse types of leukocytes exist, but they are all produced and derived from a multipotent cell in the bone marrow known as a hematopoietic stem cell. Leukocytes are found throughout the body, including the blood and lymphatic system.
The number of leukocytes in the blood is often an indicator of disease. There are normally between 4×109 and 1.1×1010 white blood cells in a litre of blood, making up approximately 1% of blood in a healthy adult. In conditions such as leukemia, the number of leukocytes is higher than normal, and in leukopenia, this number is much lower. The physical properties of leukocytes, such as volume, conductivity, and granularity, may change due to activation, the presence of immature cells, or the presence of malignant leukocytes in leukemia.
Types: -
There are several different types of white blood cells. They all have many things in common, but are all different. A major distinguishing feature of some leukocytes is the presence of granules; white blood cells are often characterized as granulocytes or agranulocytes:
(1) Granulocytes (polymorphonuclear leukocytes): -
Leukocytes characterised by the presence of differently staining granules in their cytoplasm when viewed under light microscopy. These granules are membrane-bound enzymes which primarily act in the digestion of endocytosed particles. There are three types of granulocytes: neutrophils, basophils, and eosinophils, which are named according to their staining properties.
(2) Agranulocytes (mononuclear leucocytes): -
Leukocytes characterized by the apparent absence of granules in their cytoplasm. Although the name implies a lack of granules these cells do contain non-specific azurophilic granules, which are lysosomes The cells include lymphocytes, monocytes, and macrophages.
Overview Table: -
leukocytes characterized by the apparent absence of granules in their cytoplasm. Although the name implies a lack of granules these cells do contain non-specific azurophilic granules, which are lysosomes The cells include lymphocytes, monocytes, and macrophages.
Overview table -
Neutrophils defend against bacterial or fungal infection and other very small inflammatory processes that are usually first responders to microbial infection; their activity and death in large numbers forms pus. They are commonly referred to as polymorphonuclear (PMN) leukocytes, although technically PMN refers to all granulocytes. They have a multilobed nucleus which may appear like multiple nuclei, hence the name polymorphonuclear leukocyte. The cytoplasm may look transparent because of fine granules that are faintly pink. Neutrophils are very active in phagocytosing bacteria and are present in large amount in the pus of wounds. These cells are not able to renew their lysosomes used in digesting microbes and die after having phagocytosed . Eosinophil.
Eosinophils primarily deal with parasitic infections and an increase in them may indicate such. Eosinophils are also the predominant inflammatory cells in allergic reactions. The most important causes of eosinophilia include allergies such as asthma, hay fever, and hives; and also parasitic infections. Generally their nucleus is bi-lobed. The cytoplasm is full of granules which assume a characteristic pink-orange color with eosin stain.
Basophil: -
Basophils are chiefly responsible for allergic and antigen response by releasing the chemical histamine causing inflammation. The nucleus is bi- or tri-lobed, but it is hard to see because of the number of coarse granules which hide it. They are characterized by their large blue granules.
Lymphocyte: -
Lymphocytes are much more common in the lymphatic system. Lymphocytes are distinguished by having a deeply staining nucleus which may be eccentric in location, and a relatively small amount of cytoplasm. The blood has three types of lymphocytes:
B cells: -
B cells make antibodies that bind to pathogens to enable their destruction. (B cells not only make antibodies that bind to pathogens, but after an attack, some B cells will retain the ability to produce an antibody to serve as a ‘memory’ system.)
T cells: -
CD4+ (helper) T cells co-ordinate the immune response and are important in the defense against intracellular bacteria. In acute HIV infection, these T cells are the main index to identify the individual’s immune system activity
Natural Killer Cells: -
Natural killer cells are able to kill cells of the body which are displaying a signal to kill them, as they have been infected by a virus or have become
cancerous.
Monocyte: -
Monocytes share the “vacuum cleaner” (phagocytosis) function of neutrophils, but are much longer lived as they have an additional role: they present pieces of pathogens to T cells so that the pathogens may be recognized again and killed, or so that an antibody response may be mounted. Monocytes eventually leave the bloodstream to become tissue macrophages which remove dead cell debris as well as attacking microorganisms. Neither of these can be dealt with effectively by the neutrophils. Unlike neutrophils, monocytes are able to replace their lysosomal contents and are thought to have a much longer active life. They have the kidney shaped nucleus and are typically agranulated. They also possess abundant cytoplasm.
Once monocytes move from the bloodstream out into the body tissues, they undergo changes (differentiate) allowing phagocytosis and are then known as macrophages.
Fixed Leukocytes: -
Some leukocytes migrate into the tissues of the body to take up a permanent residence at that location rather than remaining in the blood. Often these cells have specific names depending upon which tissue they settle in, such as fixed macrophages in the liver which become known as Kupffer cells. These cells still serve a role in the immune system.
(c) Thyroid Disease: -
Thyroid disease occurs when the thyroid gland doesn’t supply the proper amount of hormones needed by the body. If the thyroid is overactive, it releases too much thyroid hormone into the bloodstream, resulting in hyperthyroidism. (“Hyper” is from the Greek, meaning “over” or “above.”) Hyperthyroidism causes the body to use up energy more quickly than it should, and chemical activity (like metabolism) in the cells speeds up.
An underactive thyroid produces too little thyroid hormone, resulting in hypothyroidism. (“Hypo” means “under” or “below.”) When the amount of hormone released into the bloodstream is below normal, the body uses up energy more slowly, and chemical activity (metabolism) in the cells slows down.
Although they are two different conditions, in both hypothyroidism and hyperthyroidism the thyroid can become larger than normal. An enlarged thyroid gland is a lump that can be felt under the skin at the front of the neck. When it is large enough to see easily, it’s called a goiter. People who don’t get enough iodine in their diets also can get an enlarged thyroid, but this is rare in the United States because foods here usually supply enough iodine.
Hyperthyroidism: -
Hyperthyroidism can cause nervousness, irritability, increased perspiration, intolerance to heat, fatigue, difficulty sleeping, a fast heartbeat, irregular menstrual periods in girls, and muscle weakness. People with this problem might lose weight even though they’re eating more than usual. The eyes may feel irritated or look like they’re staring. Sometimes the tissues around the eyes become inflamed and swollen, and the eyes appear to bulge out, but this is less common in teens than in adults with hyperthyroidism.
Graves disease, an autoimmune disease, is the most common cause of hyperthyroidism. The condition makes a person’s immune system produce abnormal types of antibodies (normally antibodies help the body fight infection). These abnormal antibodies make the thyroid gland produce more thyroid hormones. Eventually, the thyroid gland enlarges, which can result in a goiter. For reasons that doctors don’t yet understand, autoimmune thyroid diseases like Graves disease are much more common in women and are most likely to occur in teens and young and middle-aged adults.
However, in most cases, the disease doesn’t go away. Some people continue taking medication for months or years to keep Graves disease under control, but it can be a hassle to take medication 1 to 3 times a day for a long period. So many doctors recommend a permanent treatment.
Radioactive iodine (RAI) is the most commonly recommended permanent treatment for teens with Graves disease today. It is usually given at a hospital, but doesn’t require a hospital stay. RAI is considered safe for teens when given in the standard amount. It is taken in capsules or mixed with a glass of water. The thyroid gland quickly absorbs the RAI from the bloodstream and, within a few months, the gland shrinks and symptoms gradually disappear. RAI has been used to treat Graves disease successfully for more than 50 years.
The other permanent treatment for Graves disease is surgery to remove most of the thyroid gland (thyroidectomy). The operation is performed in a hospital under general anesthesia, meaning the person is asleep and feels nothing during the surgery. A small incision (cut) in the lower central part of the neck usually leaves a thin scar. After surgery, there typically is swelling in the area of the incision. People sometimes have a sore throat and some trouble swallowing following surgery, although they should be able to eat and drink normally. These symptoms usually disappear within a few days.
After treatment for hyperthyroidism, hormone production often slows down to hypothyroid (underactive) levels, so the person needs to take a thyroid hormone replacement tablet each day. This treatment is a lot easier to manage than taking pills to control the hyperthyroidism - fewer blood tests, doctor visits, and medication adjustments are necessary.
Hypothyroidism: -
A person with mild hypothyroidism may feel just fine - in fact, the condition might cause no symptoms at all. However, symptoms can become more obvious if hypothyroidism progresses.
People with underactive thyroids might feel depressed and sluggish. They might gain weight, even though they’re not eating more or getting less exercise than usual. Teens with hypothyroidism also might have slow growth in height, slow sexual development, irregular menstrual periods in girls, muscle weakness, dry skin, hair loss, poor memory, and difficulty concentrating.
Hashimoto’s thyroiditis is also an autoimmune disease and is the most common cause of hypothyroidism in teens. In this condition, the body’s immune system attacks the cells in the thyroid gland, preventing it from producing enough thyroid hormone. The thyroid responds by working harder to make enough hormones. This can make it become enlarged and may result in a goiter.
Hypothyroidism is usually easily diagnosed with a physical examination and blood tests, and treatment with thyroid hormone replacement pills can restore normal levels of thyroid hormone in the blood. This treatment is pretty simple, but it does require doctor visits once or twice a year for an examination, blood tests, and medication adjustments as needed.
Goiters and Thyroid Nodules: -
It can take months or years for a goiter to develop. In teens, goiters are usually caused by the autoimmune thyroid conditions discussed above, which might show no obvious symptoms until the goiter is visible as a swelling at the front of the neck. People with this problem might have the sensation that food is stuck in the throat, especially when they lie down or sleep on their backs.
Generally, treatment of the thyroid disease causing the goiter will decrease or control the enlargement. If the thyroid continues to get larger despite treatment and becomes large enough to cause discomfort or a lump in the neck, surgery may be required. However, surgery is not necessary for most people.
A thyroid nodule is a lump or enlarged area in the thyroid gland. Sometimes a nodule can appear in a healthy gland. It may feel like a lump in the throat, or there may be tenderness or pain in the front of the neck. If the nodule is large enough, it may be visible at the front of the neck.
Most thyroid nodules are harmless. A nodule may simply be an overgrowth of normal thyroid tissue, a swelling caused by inflammation (such as in autoimmune thyroid diseases) or a collection of fluid called a cyst.
Thyroid Disease, Growth, and Puberty: -
Once puberty starts, the body goes through some very noticeable changes. Because thyroid hormones play an important role in this process, thyroid disease may slow down or interfere with a teen’s physical development. But it’s important to know that not everyone grows or develops at the same age or at the same rate. If your friend seems to grow 4 inches overnight and you haven’t had a growth spurt yet, it doesn’t mean there’s something wrong with you or your thyroid.
A thyroid problem may also cause a girl to have changes in her periods. Girls with thyroid problems may have a decrease or increase in menstrual flow or there may be a shorter or longer time between periods than usual. However, because girls who are just starting to menstruate often have irregular periods for the first year or so, changes in periods are usually nothing to worry about and don’t mean a person has thyroid disease.
People who are concerned that they might have a thyroid problem should visit the doctor. Chances are, the problem is something simpler. And if a person does have thyroid disease, diagnosing and treating it properly - including bringing the blood levels of thyroid hormones back to normal - will usually prevent or correct any problems.
(d) Functions of HCl in digestion: -
Hydrochloric acid is formed by cells in the first portion of the stomach. The chlorine of the acid is believed to come from table salt, the other constituent of which (sodium) is used by the body in maintaining important chemical states in the blood. One action of the hydrochloric acid is the softening of protein substances. As it combines with the proteins water is taken up resulting in a soft spongy or gelatinous mass being formed. In protein foods which remain in contact with hydrochloric acid for ten or twelve hours, digestion proceeds to a certain fractional point and no further. Other acids such as sulphuric, phosphoric, lactic (in sour milk), or citric (in citrus fruits) will act on proteins essentially in the same manner.
The second important function of hydrochloric acid is its activation of pepsin, an enzyme in the gastric juice which is effective only in the presence of an acid. The effect of acid on pepsin is largely independent of the actual concentration of acid present. Pepsin, when activated, dissolves the proteins already softened, and as it continues in contact with these substances, various pro-ducts of digestion and hydrolysis are produced. Proteins are broken into fractional parts in each of which only a few amino acids are present. Rennin, another ferment in the gastric juice, acts in the formation of the curd of milk.
It is believed that certain types of bacteria and amoeba are destroyed by the normal acid of the stomach. Accompanying some forms of parasitic infection in the bowels the absence of hydrochloric acid in the stomach is commonly observed. Some who have investigated the subject claim that virulent organisms such as streptococcus-hemolyticus are destroyed largely in the stomach.
(e) Ultrafiltration: -
Blood flows from the afferent arteriole into the glomerulus, a tuft of fenestrated capillaries enclosed in the Bowman’s capsule. Here 15 to 25 percent of the plasma’s water and solutes are filtered through a single-cell layer of the capillary walls, through a basement membrane, and into the lumen of the Bowman’s capsule. The filtrate then flows into the renal tubule, to undergo tubular reabsorption.
The rate of glomerular filtration depends on three factors: the hydrostatic pressure difference between the capillaries and the Bowman’s capsule (due to blood pressure), the colloid osmotic pressure, which opposes filtration, and the hydraulic permeability of the three-layered tissue separating the capillaries and the lumen of the Bowman’s capsule. Overall, blood pressure in the body has a major effect on the glomerular filtration rate, because the amount of blood passing through the glomerulus determines how much and how fast the fluid can be filtered. Among the dangers of very low blood pressure, therefore, is the loss of kidney function. This is a primary reason for the use of inflatable “shock suits” on the lower body in cases of extreme blood loss from trauma. By reducing blood flow to the legs, blood pressure in the trunk is kept higher, in an effort to maintain kidney function.
The glomerular filtration rate can be regulated by the body through endocrine responses. In the case of autoregulation, increased blood pressure stretches walls of the afferent arteriole, which responds by contracting - thereby reducing fluctuation of blood pressure in the glomerulus. A drop in blood pressure brings about a decrease in the glomerular filtration rate, which, in turn, results in a decrease in sodium ions in the filtrate. (The filtrate moves through the nephron more slowly, allowing more sodium to be reabsorbed along the way.). This lower sodium level in the filtrate is detected by the macula densa, modified cells of the wall of the distal convoluted tubule that lie adjacent to the afferent and efferent arterioles of the glomerulus. In response to the low sodium, cells of the juxtaglomerular apparatus (JGA) release renin. This triggers a series of biochemical reactions which bring about an increase of blood pressure, and thereby an increase in GFR. This series of reactions includes an increase in angiotensin II, which helps bring blood pressure back up by (1) causing vasoconstriction in arterioles throughout much of the body, and (2) promoting increased synthesis of antidiiuretic hormone (ADH), which increases resorption of water in the collecting ducts of the kidney, thereby increasing blood volume. Angiotensin II also promotes the release of aldosterone from adrenal cortex, which promotes retention of both sodium and water, thereby helping to bring blood pressure back up.
The glomerular filtration rate can also be regulated by sympathetic nerve responses, which can result in the constriction or dilation of the afferent arterioles in times of great bodily stress. Vasoconstriction increases blood pressure, and therefore GFR, whereas vasodilation decreases blood pressure and GFR.
biological terms, ultrafiltration occurs at the barrier between the blood and the filtrate in the renal corpuscle or Bowman’s capsule in the kidneys. The Bowman’s capsule contains a dense capillary network called the glomerulus. Blood flows into these capillaries through a wide afferent arteriole and leaves through a narrower efferent arteriole. The blood pressure inside these capillaries is high because:
The renal artery contains blood at very high pressure which enters the glomerulus via the short afferent arteriole.
The efferent arteriole has a smaller diameter than the afferent arteriole.
The high pressure forces small molecules such as water, glucose, amino acids, sodium chloride and urea through the filter, from the blood in the glomerular capsule across the basement membrane of the Bowman’s capsule and into the nephron. This type of high pressure filtration is ultrafiltration. The fluid formed in this way is called glomerular filtrate.
Glomerular pressure is about 75 millimeters of mercury (10 kPa). It is opposed by osmotic pressure(30 mmHg, 4.0 kPa) and hydrostatic pressure (20 mmHg, 2.7 kPa) of solutes present in capsular space. This difference in pressure is called effective pressure(25 mmHg)(3.3 kPa).
It is also used in hemodialysis to clean whole blood while keeping its composition intact.
(f) Thyroid gland: -
A gland that makes and stores hormones that help regulate the heart rate, blood pressure, body temperature, and the rate at which food is converted into energy. Thyroid hormones are essential for the function of every cell in the body. They help regulate growth and the rate of chemical reactions (metabolism) in the body. Thyroid hormones also help children grow and develop.
The thyroid gland is located in the lower part of the neck, below the Adam’s apple, wrapped around the trachea (windpipe). It has the shape of a butterfly: two wings (lobes) attached to one another by a middle part.
The thyroid uses iodine, a mineral found in some foods and in iodized salt, to make its hormones. The two most important thyroid hormones are thyroxine (T4) and triiodothyronine (T3). Thyroid stimulating hormone (TSH), which is produced by the pituitary gland, acts to stimulate hormone production by the thyroid gland. The thyroid gland also makes the hormone calcitonin, which is involved in calcium metabolism and stimulating bone cells to add calcium to bone.
(g) Parathyroid Hormone: -
Parathyroid hormone (PTH), or parathormone, is secreted by the parathyroid glands as a polypeptide containing 84 amino acids. It acts to increase the concentration of calcium (Ca2+) in the blood, whereas calcitonin (a hormone produced by the parafollicular cells (C cells) of the thyroid gland) acts to decrease calcium concentration. PTH acts to increase the concentration of calcium in the blood by acting upon parathyroid hormone receptor in three parts of the body: PTH half-life is approximately 4 minutes.
Function: -
Regulation of serum calcium: -
Parathyroid hormone regulates serum calcium levels through its effects on the following tissues:
PTH was one of the first hormones to be shown to use the G-protein, adenylyl cyclase second messenger system. Normal total plasma calcium level ranges from 8.5 to 10.2 mg/dL (2.12 mmol/L to 2.55 mmol/L).
Regulation of Serum Phosphate: -
PTH reduces the reabsorption of phosphate from the proximal tubule of the kidney which means more phosphate is excreted through the urine.
However, PTH enhances the uptake of phosphate from the intestine and bones into the blood. In the bone, slightly more calcium than phosphate is released from the breakdown of bone. In the intestines, which is mediated by an increase in activated vitamin D, the absorption of phosphate is not as dependent on vitamin D as is that of calcium. The end result is a small net drop in the serum concentration of phosphate.
Vitamin D Synthesis: -
PTH increases the activity of 1-รก-hydroxylase enzyme, which converts 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol, the active form of vitamin D.
Regulation of PTH Secretion: -
Secretion of parathyroid hormone is chiefly controlled by serum [Ca2+] through negative feedback, which is achieved by the activation of calcium-sensing receptors located on parathyroid cells The second messenger of parathyroid chief cells responsible for PTH secretion is cAMP.
Stimulators: -
· Decreased serum [Ca2+].
· Mild decreases in serum [Mg2+].
Inhibitors: -
· Increased serum [Ca2+].
· Severe decreases in serum [Mg2+], which also produces symptoms of hypoparathyroidism (such as hypocalcemia)
Clinical Significance: -
A high level of PTH in the blood is known as hyperparathyroidism.
If the cause is in the parathyroid gland it is called primary hyperparathyroidism. The causes are parathyroid adenoma, parathyroid hyperplasia and parathyroid cancer.
If the cause is outside the gland, it is known as secondary hyperparathyroidism. This can occur in chronic renal failure. In secondary hyperparathyroidism, serum Calcium levels are decreased, which causes the hypersecretion of PTH from the parathyroid glands. PTH acts on the proximal tubules in the kidney to decrease reabsorption of Phosphate (increasing its excretion in urine, decreasing its serum concentration).
Note: -
However, in chronic renal failure, because the kidneys are failing they are unable to excrete phosphate in the urine, so in this case of secondary hyperparathyroidism, serum calcium will be decreased, but serum phosphate will be increased.
A low level of PTH in the blood is known as hypoparathyroidism. Causes include surgical misadventure (eg inadvertent removal during routine thyroid surgery), autoimmune disorder, and inborn errors of metabolism.
(h) Chloride Shift: -
Chloride shift (also known as the Hamburger shift) is a process which occurs in a cardiovascular system and refers to the exchange of bicarbonate (HCO3-) and chloride (Cl-) across the membrane of red blood cells. Carbon dioxide (CO2) generated in tissues enters the blood and dissolves in water in the red blood cells to form carbonic acid (H2CO3), which then dissociates to form bicarbonate (HCO-3) and a hydrogen ion (H+). When carbon dioxide levels fall as the blood passes through the lungs, bicarbonate levels fall in the serum and bicarbonate moves out of the red blood cells. To balance the charges when bicarbonate exits the cell, a chloride anion from the plasma enters the red blood cell when the bicarbonate anion leaves. Reverse changes occur in the lungs when carbon dioxide is eliminated from the blood. Here, the exchange of bicarbonate for chloride in red blood cells flushes the bicarbonate from the blood and increases the rate of gas exchange. This chloride shift may also regulate the affinity of hemoglobin for oxygen through the chloride ion acting as an allosteric effector.
Reaction (as it occurs in the lung)
PLASMA RBC
HCO3- —> —> —> HCO3-
Na+ K+
Cl- <— <— <— <— Cl-
Bicarbonate in the red blood cell (RBC) exchanging with chloride from plasma
(h) Blood Clotting Factors: -
(i) Osmoregulation by Kidneys: -
Kidneys are vertebrate osmoregulatory organs in which blood pressure forces fluid to filter through the walls of blood capillaries into tubules that process the filtrate into urine. Each human kidney has about 1.2 million tiny balls of capillaries called glomeruli, where the blood pressure is very high. A filtrate of the blood plasma, free of cells and protein, seeps from these capillaries into a hollow ball called a glomerular (Bowman) capsule. From there, it flows into a series of tubules that remove most of the salt and water along with useful material such as glucose and vitamins, while secreting hydrogen and potassium ions, urea, and drugs (for example, penicillin and aspirin) into the tubular fluid. A final tube in the pathway, called the collecting duct, adjusts the salinity of the urine by reabsorbing variable amounts of water, before the urine leaves the kidney for storage in the urinary bladder and eventual elimination from the body.
Two hormones, aldosterone and antidiuretic hormone, regulate the amounts of salt and water reabsorbed, enabling the human kidney to adjust water loss or retention to the body’s state of hydration. Human blood plasma and tissue fluid normally has an osmolarity of 300 milliosmoles per liter (mOsm/L); that is, 0.3 mole of dissolved particles per liter of solution. Human urine can be as dilute (hypoosmotic) as 50 mOsm/L when the body is voiding excess water, or as concentrated (hyperosmotic) as 1,200 mOsm/L when conserving water.
(j) Synthesis of Urea: -
.
Urea is the major end product of nitrogen metabolism in humans and mammals. Ammonia, the product of oxidative deamination reactions, is toxic in even small amounts and must be removed from the body. The urea cycle or the ornithine cycle describes the conversion reactions of ammonia into urea. Since these reactions occur in the liver, the urea is then transported to the kidneys where it is excreted. The overall urea formation reaction is:
2 Ammonia + carbon dioxide + 3ATP —> urea + water + 3 ADP
One amine group comes from oxidative deamination of glutamic acid while the other amine group comes from aspartic acid. Aspartic acid is regenerated from fumaric acid produced by the urea cycle. The fumaric acid first undergoes reactions through a portion of the citric acid cycle to produce oxaloacetic acid which is then changed by transamination into aspartic acid.
As stated previously, high ammonia levels are toxic to humans. A complete block of any step in the urea cycle is fatal since there is no known alternative pathway for the synthesis of urea. Inherited disorders from defective enzymes may cause a partial block in some of the reactions and results in hyperammonemia which can lead to mental retardation. Extensive ammonia accumulation leads to extensive liver damage and death. Liver cirrhosis caused by alcoholism creates an interference in the enzymes which produce carbamyl phosphate in the first step on the cycle.
(k) Hormones of Mucosa of Alimentary Canal: - (2008)
Numerous irregularly distributed cells of the mucous membrane of gastro-intestinal mucosa secrete peptide hormones which regulate the motility and secretory functions of stomach, intestine, liver, pancreas, gall bladder etc. These are called gastro intestinal hormones. The following three are most important hormones-
(1) Gastrin: -
As the food reaches into the stomach ,its proteins as well as the stretching of gastric wall ,stimulate certain cells of pyloric glands and perhaps certain cells of duodenal mucosa also to secrete an inactive compound named progastrin. In presence of Hcl , progastrin converts into active gastrin hormone which diffuses into the blood. When circulating in the blood it reaches back into the gastric wall ,it stimulates growth of gastric mucosa, gastric motility, secretion of pepsinogen and Hcl by gastric glands, and possibly contraction of cardiac,but relaxation of pyloric sphineter muscle.
(2) Secretin: -
When the chime reaches into the intestine from stomach ,the Hcl present in it stimulates certain endocrine cells of mucous membranes of duodenum and jejunum,to secrete a hormone named secretin .This hormone stimulates the pancreas and possibly also the liver for secreting a watery fluid rich in bicarbonate .It also stimulates the gastric glands to secrete pepsin but inhibits the secretion of Hcl by these glands.Possibly it also retards intestinal peristalsis.
(3) Cholecystokinin-Pancreozymin (CCK-PZ): -
When stimulated by the proteins and lipids and their digestion products present in the chime ,numerous endocrine cells of mucosa of duodenum and jejunum secrete this hormone.The main function of this hormone are to contract the gall bladder and stimulate pancreas for secretion of its digestive enzymes.Besides it also promotes the growth of pancreas and retards the rate at which the chime passes from stomach into duodenum.
Some other hormones of gastro –intestinal mucosa have also been discovered. One of these namd enterogastrone probably inhibits gastric peristalsis and secretion of Hcl by gastric glands. Others are gastric inhibitory peptide(gip), vasoactive intestinal polypeptide(VIP), somatostatin, motilin and neurotensisn. These hormones are probably mild inhibitors of gastric secretion. Besides this GIP retards gastric peristalsis and promotes insulin-secretion. Somatostain retards secretion of the other gastro-intestinal hormones and pancreatic juice, contraction of gall bladder and absorption of digested nutrients. Motilin controls intestinal persitalsis. Neurotensin inhibits the persitalsis.
(l) Types of Neurons: -
(m) Saltatory Conduction: -
Saltatory conduction (from the Latin saltare, to hop or leap) is the propagation of action potentials along myelinated axons from one node of Ranvier to the next node, increasing the conduction velocity of action potential without needing to increase the diameter of an axon.
Mechanism: -
Because the cytoplasm of the axon is electrically conductive, and because the myelin inhibits charge leakage through the membrane, depolarization at one node of Ranvier is sufficient to elevate the voltage at a neighboring node to the threshold for action potential initiation. Thus in myelinated axons, action potentials do not propagate as waves, but recur at successive nodes and in effect “hop” along the axon, by which process they travel faster than they would otherwise. In summary, the charge will passively depolarize the adjacent node of Ranvier to threshold, triggering an action potential in this region and subsequently depolarizing the next node, and so on. This phenomenon was discovered by Ichiji Tasaki[1][2] and Andrew Huxley[3] and their colleagues.
Other Advantages: -
Apart from increasing the speed of the nerve impulse, the myelin sheath helps in reducing energy expenditure as the area of depolarization and hence the amount of sodium/potassium ions that need to be pumped to bring the concentration back to normal, is decreased.