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.
Oxygen Transport in Blood: Hemoglobin
A. Association & Dissociation of Oxygen + Hemoglobin
1. oxyhemoglobin (HbO2) - oxygen molecule bound
2. deoxyhemoglobin (HHb) - oxygen unbound
H-Hb + O2 <= === => HbO2 + H+
3. binding gets more efficient as each O2 binds
4. release gets easier as each O2 is released
5. Several factors regulate AFFINITY of O2
a. Partial Pressure of O2
b. temperature
c. blood pH (acidity)
d. concentration of “diphosphoglycerate” (DPG)
B. Effects of Partial Pressure of O2
1. oxygen-hemoglobin dissociation curve
a. 104 mm (lungs) - 100% saturation (20 ml/100 ml)
b. 40 mm (tissues) - 75% saturation (15 ml/100 ml)
c. right shift - Decreased Affinity, more O2 unloaded
d. left shift- Increased Affinity, less O2 unloaded
C. Effects of Temperature
1. HIGHER Temperature --> Decreased Affinity (right)
2. LOWER Temperature --> Increased Affinity (left)
D. Effects of pH (Acidity)
1. HIGHER pH --> Increased Affinity (left)
2. LOWER pH --> Decreased Affinity (right) "Bohr Effect"
a. more Carbon Dioxide, lower pH (more H+), more O2 release
E. Effects of Diphosphoglycerate (DPG)
1. DPG - produced by anaerobic processes in RBCs
2. HIGHER DPG > Decreased Affinity (right)
3. thyroxine, testosterone, epinephrine, NE - increase RBC metabolism and DPG production, cause RIGHT shift
F. Oxygen Transport Problems
1. hypoxia - below normal delivery of Oxygen
a. anemic hypoxia - low RBC or hemoglobin
b. stagnant hypoxia - impaired/blocked blood flow
c. hypoxemic hypoxia - poor lung gas exchange
2. carbon monoxide poisoning - CO has greater Affinity than Oxygen or Carbon Dioxide
Cardiac Control: The Cardiac Center in the medulla.
Outputs:
The cardioacceleratory center sends impulses through the sympathetic nervous system in the cardiac nerves. These fibers innervate the SA node and AV node and the ventricular myocardium. Effects on the SA and AV nodes are an increase in depolarization rate by reducing the resting membrane polarization. Effect on the myocardium is to increase contractility thus increasing force and therefore volume of contraction. Sympathetic stimulation increases both rate and volume of the heart.
The cardioinhibitory center sends impulses through the parasympathetic division, the vagus nerve, to the SA and AV nodes, but only sparingly to the atrial myocardium, and not at all to ventricular myocardium. Its effect is to slow the rate of depolarization by increasing the resting potential, i.e. hyperpolarization.
The parasympathetic division controls the heart at rest, keeping its rhythm slow and regular. This is referred to as normal vagal tone. Parasympathetic effects are inhibited and the sympathetic division exerts its effects during stress, i.e. exercise, emotions, "fight or flight" response, and temperature.
Inputs to the Cardiac Center:
Baroreceptors in the aortic and carotid sinuses. The baroreceptor reflex is responsible for the moment to moment maintenance of normal blood pressure.
Higher brain (hypothalamus): stimulates the center in response to exercise, emotions, "fight or flight", temperature.
Intrinsic Controls of the Heart:
Right Heart Reflex - Pressoreceptors (stretch receptors) in the right atrium respond to stretch due to increased venous return. The reflex acts through a short neural circuit to stimulate the sympathetic nervous system resulting in increased rate and force of contraction. This regulates output to input
The Frank-Starling Law - (Starling's Law of the Heart) - Like skeletal muscle the myocardium has a length tension curve which results in an optimum level of stretch producing the maximum force of contraction. A healthy heart normally operates at a stretch less than this optimum level and when exercise causes increased venous return and increased stretch of the myocardium, the result is increased force of contraction to automatically pump the increased volume out of the heart. I.e. the heart automatically compensates its output to its input.
An important relationship in cardiac output is this one:
Blood Flow = D Pressure / Resistance to Blood Flow
A small fraction of cardiac muscle fibers have myogenicity and autorhythmicity.
Myogenicity is the property of spontaneous impulse generation. The slow sodium channels are leaky and cause the polarity to spontaneously rise to threshold for action potential generation. The fastest of these cells, those in the SA node, set the pace for the heartbeat.
Autorhythmicity - the natural rhythm of spontaneous depolarization. Those with the fastest autorhythmicity act as the 1. heart's pacemaker.
Contractility - like skeletal muscle, most cardiac muscle cells respond to stimuli by contracting. The autorhythmic cells have very little contractility however. Contractility in the other cells can be varied by the effect of neurotransmitters.
Inotropic effects - factors which affect the force or energy of muscular contractions. Digoxin, epinephrine, norepinephrine, and dopamine have positive inotropic effects. Betal blockers and calcium channel blockers have negative inotropic effects
Sequence of events in cardiac conduction: The electrical events in the cardiac cycle.
1) SA node depolarizes and the impulse spreads across the atrial myocardium and through the internodal fibers to the AV node. The atrial myocardium depolarizes resulting in atrial contraction, a physical event.
2) AV node picks up the impulse and transfers it to the AV Bundle (Bundle of His). This produces the major portion of the delay seen in the cardiac cycle. It takes approximately .03 sec from SA node depolarization to the impulse reaching the AV node, and .13 seconds for the impulse to get through the AV node and reach the Bundle of His. Also during this period the atria repolarize.
3) From the AV node the impulse travels through the bundle branches and through the Purkinje fibers to the ventricular myocardium, causing ventricular depolarization and ventricular contraction, a physical event.
4) Ventricular repolarization occurs.
GENERAL VISCERAL AFFERENT (GVA) PATHWAYS
Pain and Pressure Sensation via the Spinal Cord
Visceral pain receptors are located in peritoneal surfaces, pleural membranes, the dura mater, walls of arteries, and the walls of the GI tube.
Nociceptors in the walls of the GI tube are particularly sensitive to stretch and overdistension.
General visceral nociceptors conduct signals into the spinal cord over the monopolar neurons of the posterior root ganglia. They terminate in laminae III and IV of the posterior horn as do the pain and temperature pathways of the GSA system , their peripheral processes reach the visceral receptors via the gray rami communicantes and ganglia of the sympathetic chain
Second-order neurons from the posterior horn cross in the anterior white commissure and ascend to the thalamus in the anterior and lateral spinothalamic tracts,
Projections from the VPL of the thalamus relay signals to the sensory cortex.
The localization of visceral pain is relatively poor, making it difficult to tell the exact source of the stimuli.
Blood Pressure, Blood Chemistry, and Alveolar Stretch Detection
The walls of the aorta and the carotid sinuses contain special baroreceptors (pressure receptors) which respond to changes in blood pressure. These mechanoreceptors are the peripheral endings of GVA fibers of the glossopharyngeal (IX) and vagus (X) nerves
The GVA fibers from the carotid sinus baroreceptors enter the solitary tract of the brainstem and terminate in the vasomotor center of the medulla (Fig-14). This is the CNS control center for cardiovascular activity.
Stretch receptors in the alveoli of the lungs conduct information concerning rhythmic alveolar inflation and deflation over GVA X fibers to the solitary tract and then to the respiratory center of the brainstem. This route is an important link in the Hering-Breuer reflex, which helps to regulate respiration.
Carotid body chemoreceptors, sensitive to changes in blood PO2 and, to a lesser extent, PCO2 and pH, conduct signals to both the vasomotor and respiratory centers over GVA IX nerve fibers
GVA X fibers conduct similar information from the aortic chemoreceptors to both centers
Blood is a liquid tissue. Suspended in the watery plasma are seven types of cells and cell fragments.
- red blood cells (RBCs) or erythrocytes
- platelets or thrombocytes
- five kinds of white blood cells (WBCs) or leukocytes
- Three kinds of granulocytes
- neutrophils
- eosinophils
- basophils
- Two kinds of leukocytes without granules in their cytoplasm
- lymphocytes
- monocytes
- Three kinds of granulocytes
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