NEET MDS Lessons
Physiology
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.
CNS PROTECTION
- Bones of the Skull Frontal, Temporal, Parietal, Sphenoid, Occipital
- Cranial Meninges Dura mater, Arachnoid Space, Pia mater
- Cerebrospinal Fluid
Secreted by Chroid Plexi in Ventricles
Circulation through ventricles and central canal
Lateral and Median apertures from the 4th ventricle into the subarachnoid space
Arachnoid villi of the superior sagittal sinus return CSF to the venous circulation
Hydrocephalic Condition, blockage of the mesencephalic aqueduct, backup of CSF, Insertion of a shunt to drain the excess CSF
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
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.
Events in Muscle Contraction - the sequence of events in crossbridge formation:
1) In response to Ca2+ release into the sarcoplasm, the troponin-tropomyosin complex removes its block from actin, and the myosin heads immediately bind to active sites.
2) The myosin heads then swivel, the Working Stroke, pulling the Z-lines closer together and shortening the sarcomeres. As this occurs the products of ATP hydrolysis, ADP and Pi, are released.
3) ATP is taken up by the myosin heads as the crossbridges detach. If ATP is unavailable at this point the crossbridges cannot detach and release. Such a condition occurs in rigor mortis, the tensing seen in muscles after death, and in extreme forms of contracture in which muscle metabolism can no longer provide ATP.
4) ATP is hydrolyzed and the energy transferred to the myosin heads as they cock and reset for the next stimulus.
Excitation-Contraction Coupling: the Neuromuscular Junction
Each muscle cell is stimulated by a motor neuron axon. The point where the axon terminus contacts the sarcolemma is at a synapse called the neuromuscular junction. The terminus of the axon at the sarcolemma is called the motor end plate. The sarcolemma is polarized, in part due to the unequal distribution of ions due to the Sodium/Potassium Pump.
1) Impulse arrives at the motor end plate (axon terminus) causing Ca2+ to enter the axon.
2) Ca2+ binds to ACh vesicles causing them to release the ACh (acetylcholine) into the synapse by exocytosis.
3) ACH diffuses across the synapse to bind to receptors on the sarcolemma. Binding of ACH to the receptors opens chemically-gated ion channels causing Na+ to enter the cell producing depolarization.
4) When threshold depolarization occurs, a new impulse (action potential) is produced that will move along the sarcolemma. (This occurs because voltage-gated ion channels open as a result of the depolarization -
5) The sarcolemma repolarizes:
a) K+ leaves cell (potassium channels open as sodium channels close) returning positive ions to the outside of the sarcolemma. (More K+ actually leaves than necessary and the membrane is hyperpolarized briefly. This causes the relative refractory period) (b) Na+/K+ pump eventually restores resting ion distribution. The Na+/K+ pump is very slow compared to the movement of ions through the ion gates. But a muscle can be stimulated thousands of times before the ion distribution is substantially affected.
6) ACH broken down by ACH-E (a.k.a. ACHase, cholinesterase). This permits the receptors to respond to another stimulus.
Excitation-Contraction Coupling:
1) The impulse (action potential) travels along the sarcolemma. At each point the voltaged-gated Na+ channels open to cause depolarization, and then the K+ channels open to produce repolarization.
2) The impulse enters the cell through the T-tublules, located at each Z-disk, and reach the sarcoplasmic reticulum (SR), stimulating it.
3) The SR releases Ca2+ into the sarcoplasm, triggering the muscle contraction as previously discussed.
4) Ca2+ is pumped out of the sarcoplasm by the SR and another stimulus will be required to continue the muscle contraction.
Acute Obstructive Disorders
1. Heimlich maneuver
2. Bypass, tracheostomy w/catheter to suck up secretion
Bleeding Disorders
A deficiency of a clotting factor can lead to uncontrolled bleeding.
The deficiency may arise because
- not enough of the factor is produced or
- a mutant version of the factor fails to perform properly.
Examples:
- von Willebrand disease (the most common)
- hemophilia A for factor 8 deficiency
- hemophilia B for factor 9 deficiency.
- hemophilia C for factor 11 deficiency
In some cases of von Willebrand disease, either a deficient level or a mutant version of the factor eliminates its protective effect on factor 8. The resulting low level of factor 8 mimics hemophilia A.