NEET MDS Lessons
Physiology
The nephron of the kidney is involved in the regulation of water and soluble substances in blood.
A Nephron
A nephron is the basic structural and functional unit of the kidneys that regulates water and soluble substances in the blood by filtering the blood, reabsorbing what is needed, and excreting the rest as urine.
Its function is vital for homeostasis of blood volume, blood pressure, and plasma osmolarity.
It is regulated by the neuroendocrine system by hormones such as antidiuretic hormone, aldosterone, and parathyroid hormone.
The Glomerulus
The glomerulus is a capillary tuft that receives its blood supply from an afferent arteriole of the renal circulation. Here, fluid and solutes are filtered out of the blood and into the space made by Bowman's capsule.
A group of specialized cells known as juxtaglomerular apparatus (JGA) are located around the afferent arteriole where it enters the renal corpuscle. The JGA secretes an enzyme called renin, due to a variety of stimuli, and it is involved in the process of blood volume homeostasis.
The Bowman's capsule surrounds the glomerulus. It is composed of visceral (simple squamous epithelial cells; inner) and parietal (simple squamous epithelial cells; outer) layers.
Red blood cells and large proteins, such as serum albumins, cannot pass through the glomerulus under normal circumstances. However, in some injuries they may be able to pass through and can cause blood and protein content to enter the urine, which is a sign of problems in the kidney.
Proximal Convoluted Tubule
The proximal tubule is the first site of water reabsorption into the bloodstream, and the site where the majority of water and salt reabsorption takes place. Water reabsorption in the proximal convoluted tubule occurs due to both passive diffusion across the basolateral membrane, and active transport from Na+/K+/ATPase pumps that actively transports sodium across the basolateral membrane.
Water and glucose follow sodium through the basolateral membrane via an osmotic gradient, in a process called co-transport. Approximately 2/3rds of water in the nephron and 100% of the glucose in the nephron are reabsorbed by cotransport in the proximal convoluted tubule.
Fluid leaving this tubule generally is unchanged due to the equivalent water and ion reabsorption, with an osmolarity (ion concentration) of 300 mOSm/L, which is the same osmolarity as normal plasma.
The Loop of Henle
The loop of Henle is a U-shaped tube that consists of a descending limb and ascending limb. It transfers fluid from the proximal to the distal tubule. The descending limb is highly permeable to water but completely impermeable to ions, causing a large amount of water to be reabsorbed, which increases fluid osmolarity to about 1200 mOSm/L. In contrast, the ascending limb of Henle's loop is impermeable to water but highly permeable to ions, which causes a large drop in the osmolarity of fluid passing through the loop, from 1200 mOSM/L to 100 mOSm/L.
Distal Convoluted Tubule and Collecting Duct
The distal convoluted tubule and collecting duct is the final site of reabsorption in the nephron. Unlike the other components of the nephron, its permeability to water is variable depending on a hormone stimulus to enable the complex regulation of blood osmolarity, volume, pressure, and pH.
Normally, it is impermeable to water and permeable to ions, driving the osmolarity of fluid even lower. However, anti-diuretic hormone (secreted from the pituitary gland as a part of homeostasis) will act on the distal convoluted tubule to increase the permeability of the tubule to water to increase water reabsorption. This example results in increased blood volume and increased blood pressure. Many other hormones will induce other important changes in the distal convoluted tubule that fulfill the other homeostatic functions of the kidney.
The collecting duct is similar in function to the distal convoluted tubule and generally responds the same way to the same hormone stimuli. It is, however, different in terms of histology. The osmolarity of fluid through the distal tubule and collecting duct is highly variable depending on hormone stimulus. After passage through the collecting duct, the fluid is brought into the ureter, where it leaves the kidney as urine.
The Lymphatic System
Functions of the lymphatic system:
1) to maintain the pressure and volume of the extracellular fluid by returning excess water and dissolved substances from the interstitial fluid to the circulation.
2) lymph nodes and other lymphoid tissues are the site of clonal production of immunocompetent lymphocytes and macrophages in the specific immune response.
Filtration forces water and dissolved substances from the capillaries into the interstitial fluid. Not all of this water is returned to the blood by osmosis, and excess fluid is picked up by lymph capillaries to become lymph. From lymph capillaries fluid flows into lymph veins (lymphatic vessels) which virtually parallel the circulatory veins and are structurally very similar to them, including the presence of semilunar valves.
The lymphatic veins flow into one of two lymph ducts. The right lymph duct drains the right arm, shoulder area, and the right side of the head and neck. The left lymph duct, or thoracic duct, drains everything else, including the legs, GI tract and other abdominal organs, thoracic organs, and the left side of the head and neck and left arm and shoulder.
These ducts then drain into the subclavian veins on each side where they join the internal jugular veins to form the brachiocephalic veins.
Lymph nodes lie along the lymph veins successively filtering lymph. Afferent lymph veins enter each node, efferent veins lead to the next node becoming afferent veins upon reaching it.
Lymphokinetic motion (flow of the lymph) due to:
1) Lymph flows down the pressure gradient.
2) Muscular and respiratory pumps push lymph forward due to function of the semilunar valves.
Other lymphoid tissue:
1. Lymph nodes: Lymph nodes are small encapsulated organs located along the pathway of lymphatic vessels. They vary from about 1 mm to 1 to 2 cm in diameter and are widely distributed throughout the body, with large concentrations occurring in the areas of convergence of lymph vessels. They serve as filters through which lymph percolates on its way to the blood. Antigen-activated lymphocytes differentiate and proliferate by cloning in the lymph nodes.
2. Diffuse Lymphatic Tissue and Lymphatic nodules: The alimentary canal, respiratory passages, and genitourinary tract are guarded by accumulations of lymphatic tissue that are not enclosed by a capsule (i.e. they are diffuse) and are found in connective tissue beneath the epithelial mucosa. These cells intercept foreign antigens and then travel to lymph nodes to undergo differentiation and proliferation. Local concentrations of lymphocytes in these systems and other areas are called lymphatic nodules. In general these are single and random but are more concentrated in the GI tract in the ileum, appendix, cecum, and tonsils. These are collectively called the Gut Associated Lymphatic Tissue (GALT). MALT (Mucosa Associated Lymphatic Tissue) includes these plus the diffuse lymph tissue in the respiratory tract.
3. The thymus: The thymus is where immature lymphocytes differentiate into T-lymphocytes. The thymus is fully formed and functional at birth. Characteristic features of thymic structure persist until about puberty, when lymphocyte processing and proliferation are dramatically reduced and eventually eliminated and the thymic tissue is largely replaced by adipose tissue. The lymphocytes released by the thymus are carried to lymph nodes, spleen, and other lymphatic tissue where they form colonies. These colonies form the basis of T-lymphocyte proliferation in the specific immune response. T-lymphocytes survive for long periods and recirculate through lymphatic tissues.
The transformation of primitive or immature lymphocytes into T-lymphocytes and their proliferation in the lymph nodes is promoted by a thymic hormone called thymosin. Ocassionally the thymus persists and may become cancerous after puberty and and the continued secretion of thymosin and the production of abnormal T-cells may contribute to some autoimmune disorders. Conversely, lack of thymosin may also allow inadequate immunologic surveillance and thymosin has been used experimentally to stimulate T-lymphocyte proliferation to fight lymphoma and other cancers.
4. The spleen: The spleen filters the blood and reacts immunologically to blood-borne antigens. This is both a morphologic (physical) and physiologic process. In addition to large numbers of lymphocytes the spleen contains specialized vascular spaces, a meshwork of reticular cells and fibers, and a rich supply of macrophages which monitor the blood. Connective tissue forms a capsule and trabeculae which contain myofibroblasts, which are contractile. The human spleen holds relatively little blood compared to other mammals, but it has the capacity for contraction to release this blood into the circulation during anoxic stress. White pulp in the spleen contains lymphocytes and is equivalent to other lymph tissue, while red pulp contains large numbers of red blood cells that it filters and degrades.
The spleen functions in both immune and hematopoietic systems. Immune functions include: proliferation of lymphocytes, production of antibodies, removal of antigens from the blood. Hematopoietic functions include: formation of blood cells during fetal life, removal and destruction of aged, damaged and abnormal red cells and platelets, retrieval of iron from hemoglobin degradation, storage of red blood cells.
The Cardiac Cycle: the sequence of events in one heartbeat.
systole - the contraction phase; unless otherwise specified refers to left ventricle, but each chamber has its own systole.
diastole - the relaxation phase; unless otherwise specified refers to left ventricle, but each chamber has its own diastole.
1) quiescent period - period when all chambers are at rest and filling. 70% of ventricular filling occurs during this period. The AV valves are open, the semilunar valves are closed.
2) atrial systole - pushes the last 30% of blood into the ventricle.
3) atrial diastole - atria begin filling.
4) ventricular systole - First the AV valves close causing the first heart sound, then after the isovolumetric contraction phase the semilunar valves open permitting ventricular ejection of blood into the arteries.
5) ventricular diastole - As the ventricles relax the semilunar valves close first producing the second heart sound, then after the isovolumetric relaxation phase the AV valves open allowing ventricular filling.
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
As the contents of the stomach become thoroughly liquefied, they pass into the duodenum, the first segment of the small intestine. The duodenum is the first 10" of the small intestine
Two ducts enter the duodenum:
- one draining the gall bladder and hence the liver
- the other draining the exocrine portion of the pancreas.
From the intestinal mucosal cells, and from the liver and gallbladder. Secretions from the pancreas and bile from the gallbladder enter the duodenum through the hepatopancreatic ampulla and the sphincter of Oddi. These lie where the pancreatic duct and common bile duct join before entering the duodenum. The presence of fatty chyme in the duodenum causes release of the hormone CCK into the bloodstream. CCK is one of the enterogastrones and its main function, besides inhibiting the stomach, is to stimulate the release of enzymes by the pancreas, and the contraction of the gallbladder to release bile. It also stimulates the liver to produce bile. Consumption of excess fat results in excessive bile production by the liver, and this can lead to the formation of gallstones from precipitation of the bile salts.
The acid in the chyme stimulates the release of secretin which causes the pancreas to release bicarbonate which neutralizes the acidity
Proteinuria—Protein content in urine, often due to leaky or damaged glomeruli.
Oliguria—An abnormally small amount of urine, often due to shock or kidney damage.
Polyuria—An abnormally large amount of urine, often caused by diabetes.
Dysuria—Painful or uncomfortable urination, often from urinary tract infections.
Hematuria—Red blood cells in urine, from infection or injury.
Glycosuria—Glucose in urine, due to excess plasma glucose in diabetes, beyond the amount able to be reabsorbed in the proximal convoluted tubule.
Each hormone in the body is unique. Each one is different in it's chemical composition, structure, and action. With respect to their chemical structure, hormones may be classified into three groups: amines, proteins, and steroids.
Amines- these simple hormones are structural variation of the amino acid tyrosine. This group includes thyroxine from the thyroid gland and epinephrine and norepinephrine from the adrenal medulla.
Proteins- these hormones are chains of amino acids. Insulin from the pancreas, growth hormone from the anterior pituitary gland, and calcitonin from the thyroid gland are all proteins. Short chains of amino acids are called peptides. Antidiuretic hormone and oxytocin, synthesized by the hypothalamus, are peptide hormones.
Steroids- cholesterol is the precursor for the steroid hormones, which include cortisol and aldosterone from the adrenal cortex, estrogen and progesterone from the ovaries, and testosterone from the testes.