Human Physiology Week 1 Notes
Human Physiology Week 1 Notes BISC 3122
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This 7 page Class Notes was uploaded by PhenomenalNotetaker on Sunday January 17, 2016. The Class Notes belongs to BISC 3122 at George Washington University taught by Dr. Packer in Fall 2016. Since its upload, it has received 170 views. For similar materials see Human Physiology in Biology at George Washington University.
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Date Created: 01/17/16
Human Physiology Lecture #1 : Homeostasis and Membrane Transport Homeostasis Homeostasis is the state of maintaining relatively, not absolutely, constant internal conditions. It is a dynamic (i.e. varying) steady state. Body temperature, for instance, is not regulated perfectly like some other conditions. It is usually regulated within a range. It varies around an average of about 37⁰C which is called a set point. - Not all vertebrates regulate temperature; only mammals and birds maintain fairly constant body temperatures. - When is there an exception for mammals and birds? When mammals are fighting infections – pyrogens are secreted by bacteria and when introduced into the blood, increase the set point thereby causing a fever. Sometimes birds go into hibernation – deep sleep – and let their body temperature fall to conserve energy. Some hormones show diurnal (daily) rhythms. Melatonin, secreted by the pineal gland and cortisol are examples of such hormones. Blood pressure and heart rate both vary with exercise but each vary within certain limits. Blood sugar levels vary within healthy people – maybe 80 or 90 mg% when fasting up about 160 mg% or so after eating a sweet or a carb-heavy meal. Homeostatic controls keep blood sugar within those limits. Diabetes mellitus – if blood sugar goes higher than about 250 mg% - glucose excreted in urine (urine becomes sweet) – urine volume goes up (frequent urination and thus thirst for water). There exist internal and external environments. The internal environment encompasses temperature, volume and composition of Intracellular and extracellular fluid; the extracellular fluid composition is simply when we eat/drink. Homeostasis is dependent on mass balance and control systems. - Mass balance = existing body load + intake/metabolic production –excretion/metabolic removal. - Control systems include the input signal, the integrating centre which could be the CNS, the output signal and the response. Usually reflexes regulated by neurons of the nervous system. Negative feedback loops are homeostatic but positive feedback loops are not even though some show up in physiological systems such as during birth and blood clotting. - An example of negative feedback is the dramatic increase in blood pressure when we stand: at first, BP decreases, then brain blood flow decreases, these changes are sensed and the sympathetic nervous system takes action, HR, stroke volume and vasoconstriction occur therefore collectively causing an increase in BP. To summarize, something perturbs in a certain direction and its reflex causes the opposite thing to happen. Membrane Transport I. Non Carrier Mediated Transport 1. Through lipid matrix 2. Through protein channels - Polar substances have to pass through protein channels while non-polar materials are able to diffuse through the phospholipid membrane as diffusion depends on inter- molecular collisions. (Note: Net diffusion stops at equilibrium both diffusion in general stops at absolute zero). Ions naturally move down an electrochemical gradient as indicated in the image. - 340 genes in the human genome code for channel proteins - For most channels, the default position is closed and some signal opens them as shown below. Many channels are made of multiple protein subunits that assemble in the membrane. One protein subunit Channel through of channel Channel through center center of of membrane protein membrane protein (viewed from above) Diffusion can occur in the absence of a concentration gradient but there will be no net flux. Net flux is only from high to low concentration – what is the driving force? Simple diffusion is the driving force for most flux that occurs in the membranes but it has a limitation – it is slow over long distances (take for instance, if you put sugar in your coffee and don’t stir, the top part of the coffee will not be as sweet. Actually, it’ll take hours for the sugar to diffuse upwards down the gradient). Fick’s law J net= KpA(C –oC ) i Kp = permeability constant; depends upon 3 important factors: weight, shape and charge. A = area; especially important with channels (Co– C)i= concentration gradient *Net flux = J Surface area determines how fast flux happens i.e. the speed of flux varies directly with the number of open channels. It takes 5 seconds to diffuse 100 μm (most human cells are 100 μm in diameter); RBC is 7.5 μm; WBC is 15 μm. Water diffusion (osmosis) is slow through membranes lacking water channels called aquaporins (another example of a protein channel). A U-tube with a semi-permeable membrane can be used to observe water diffusion as shown below. Osmosis results in the transfer of solvent molecules from a sample of low (or zero) solute concentration (in this case, the pure water) to a sample of higher solute concentration (the solution). Eventually, it stops moving as a result of hydrostatic pressure (gravity). As the level continues to rise, gravity starts to hinder it from rising further by pushing water back through the membrane. II. Carrier Mediated Transport A. Facilitated diffusion: this is for large hydrophilic molecules such as glucose and amino acids. An example is insulin sensitive Glucose transporter – GLUT 4. Characteristics: - Net flux with the concentration gradient - No direct energy required (because it is still diffusion regardless) - Saturable - Competitive inhibition B. Primary active transport: only four enzymes actively transport materials; Na /K ATPase, H ATPase (makes HCl for digestion in the stomach), H /K ATPase and Ca ATPase (controls muscle contraction). Characteristics: - Net flux against gradients of concentration and charge - Direct energy required - Saturable - Competitive inhibition - Only ions C. Secondary active transport: it is the transport of one solute against its concentration gradient by using the energy generated by the gradient of another solute transport. Characteristics: - Net flux against gradients of concentration and charge - Energy supplied by Na gradient in most cases - Saturable - Competitive inhibition III. Filtration: this is driven by hydrostatic pressure across capillary walls. In most tissues, the endothelium cells that make up the capillary have immobile gaps through which materials can flow through. Systolate drives the blood in the direction indicated by the arrow (from artery to vein). As it moves, fluid is transferred into the interstitial space and this is driven by the pressure gradient. Not that bigger materials like red and white blood cells cannot pass through these gaps. IV. Phagocytosis (cell eating): white blood cells are an example; they are macrophages (big eaters). Macrophages are the first line of defense to fight infections as they help to clean up poorly functioning or invaded cells. When an invasion by bacteria for instance occurs, they are first on the scene as they can sense the bacterial cell wall. That is, they know when to act by general chemotaxis – they can sense chemotaxins produced by the bacteria. They then extend their pseudopods, form a vacuole with the bacteria, and fuse enzymes (usually lysozymes) which then break up and kill the bacteria. V. Pinocytosis: This is the engulfing of smaller particles. In this case, there could be a receptor on the membrane of the cell that enhances this process (receptor mediated endocytosis). An appropriate ligand in solution (maybe a protein) will bind to the receptor it recognizes, then invagination occurs leading to the formation of a small vesicle with the ligand in it. This vesicle can then fuse with lysozymes which break down the ligand and the receptor that was in the vesicle is usually recycled back out to the membrane to keep the cell from getting too small. VI. Exocytosis: here, something is produced often out of the Golgi apparatus (e.g. a protein) and the protein migrates to the membrane where it docks at a specific place, forms a vesicle, fuses with the membrane and lets out the protein. An example where this is in action is in the pancreas with the secretion of insulin into the blood. In exocytosis, the empty vesicles are collected/assembled to form a coated pit which is then recycled.
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