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PHGY 209 Mammalian Physiology 1 - Body Fluids

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McGill University
PHGY 209
Ann Wechsler

Introduction & Body Fluids Physiology  Physiology, derived from 2 words: “Physis” and “Logos”: “Inquiry into Nature”  Hippocrates (~460 - ~370 B.C.E.) – Father of medicine and antiquities o Referred to Physiology: “The Healing Power of Nature”  Medical perspective nowadays, world of envt in promoting good health and healing  A lot of time invested in finding out the conditions, which exist in nature to promote good health – physiology  Jean Fernel (1552) o Turned physiologoy to be associated to medicine itself o Referred physiology as “The Natural Part of Medicine”, which referred to the structure of the body, which discipline we call anatomy today o Thus, physiology was really anatomy  18 century: The term “physiology” and the discipline as such came to a fundamental aspect of medical institution o The term acquired a more restricted definition, which is still used nowadays: Physiology is “The study of the normal functional activities in the healthy living organism.”  Physiology may be studied at many different levels o Cell physiology o Organ physiology o Systems physiology o Organismal physiology o Molecular physiology, the way in which molecules interact, they govern function and regulation of function.  Very little distinction between the various disciplines, tremendous overlap in terms of research  We don’t distinguish as much as we used to in different disciplines  Of all the disciplines, physiology remains the integrative science, has a prospective of putting the information from the molecules in cells and organs in the whole functioning individual.  Basic cell functions o All cells must obtain nutrients and oxygen in order to survive, by obtaining from envt surrounding the individual cells o They utilize the nutrients and O2 to perform reactions, yielding energy, energy to be used in all activities that the cells carry o At some point, they eliminate CO2 and waste, release to the envt o Thus, they obtain from the envt and give back to the envt, in different forms.  Unicellular Organism o Easy for a unicellular organism (amoeba) to obtain nutrients and O2 from the envt surrounding it (water) o The can get rid of their wastes and CO2 directly in the aqueous envt  Multicellular Organisms o Majority of cells are far removed from the actual envt surrounding the organism as a whole (air, water) o How do the individual cells in our body obtain what they need?  Claude Bernard (1813 - 1878) – Father of modern physiology  Envt surrounding the individual cell inside the body is vastly different from the envt surrounding the living organism (external envt)  Internal envt remains relatively constant under conditions of health  Envt surrounding the individual cells: “Milieu intérieur”  All kinds of changes which can exert on the external envt and what allows us to be able to move from envt to envt is the fact that the internal envt remains essentially cst – that relative cstancy which has led to so many different forms of life Homeostasis  Homeostasis – Relative constancy of the “Milieu Intérieur” o Walter Cannon (1871 – 1945) – “Homeostasis”: “a state of dynamic constancy” (Think dynamic constancy of a container of water, adding and removing water at the same rate)  Fundamental Principle in Physiology o REMEMBER At all levels of organization (organ systems, molecules), functional activities are directed at maintaining optimal and relatively constant INTERNAL CONDITIONS (“Milieu Interieur” – the internal envt surrounding individual cells) o Organ systems maintain homeostasis, which is essential for the viability of the individual cells, and the cells make up the organ systems.  The dynamic constancy: must have a system of sensors/detectors, which can determine any subtle change in the internal envt, to give rise to any compensatory adjustments so that the status quo is achieved, in order to restore the relative constancy in the “Milieu intérieur”.  Failure to maintain homeostasis disrupts normal function, leading to DISEASE o The goal of medicine is to try to restore function to the normal level o Physiology is the scientific basis of clinical medicine – have to find out what disrupted the function and have to readjust things so as to restore the function o ** Required reading: Homeostasis – Chapter 1 in Vander’s (as well as the system of feedback – positive and negative feedback)  “Milieu Intérieur” comprises the various body fluids  Aspects of body fluids o Volume – total volume of fluids in the body o Distribution of body fluids and different compartments in the body o Characteristics/properties/composition of the body fluids o Functions of the fluids Water  Water is by far the most abundant single constituent of the body o Accounts for 45% to 75% of total body mass (~ ½ up to ¾ of body mass is H2O)  Body Water is the medium in which: o All solutes are dissolved: inorganic ions, organic materials, carbohydrates, sugars, amino acids, large proteins o Metabolic reactions take place: maintain the viability of the individual cells and ultimately the body as a whole  45% to 75% of body mass is water - why is there a variation? o Have to look at the water content in the various tissues in the body – not much variation  Skin……………………………………..…….70%  Muscle……………………………………….75%  Heart, Liver, Brain, Kidney……. 70-80%  Bone*..………………………………………..25%  Fat (adipose tissue)*……………………10%  ** The difference in water content in individuals lies ultimately in the water content in bone and fat (primarily fat, showing a lot of variation from one individual to another)  Water as a proportion of body mass: the absolute amount of water in an individual does not vary significantly but the proportion of water relative to fat and solids vary because one may have more fat than another. o The more fat in the body, the less water expressed as a percent of body mass  If the water content is computed as a fraction of lean body mass (i.e. excluding the fat), the differences between individuals become insignificant (Absolute amount of water does not change between the individuals in the image)  Standard values: 21 yr-old, white, male, 70kg weight. Must make adjustments for: age, gender, weight, ethnic origin (genetically determined physiological values, varies)  Variations in body with age and gender o Newborn: ~75% of body mass as water, little subcutaneous fat o Puberty (variation between genders):  Male: ~60% of body mass as water  Female: ~50% of body mass as water, because female acquire more subcutaneous fat (poor in water) at puberty o Aged people:  Male: ~50% of body mass as water, muscle atrophies with age and muscle is rich in water but replaced by fibrous tissue and fat  Female: ~45% of body mass as water o Thus, as we grow older, the percentage water as a proportion of body mass decreases (“we dry”), and women have more fat than men, thus lower water percentage. Notice the trends as we age, and the difference between male and female at puberty, changes in age between male and female o The final volume of body water is crucial in terms of water-soluble medication because the difference in the solubility changes the dosage. The same dose administered to both gender varies in the body because women have less total body water than men. In some precise clinical situation, not only the weight is taken into consideration but also the total body water in which the medication is diluted.  Body water is remarkably dynamically constant in health, only minute variations.  Total body water in the individual is a dynamic steady state (overall result) resulting from exchanges between the individual and external envt; we take in fluid, we release fluid. Even vaster exchange that happens internally between the different compartments  Water balance: individual and exchanges with external envt Intake o We take in water from our diet (fluid and food intake, varies) o Production of water as the result of metabolic activity in the body. Final steps in the oxidative process of carbohydrates, proteins, and fats, is the production of carbon dioxide, energy, and water. o For every 100 cal. we take in, we produce 14 mL of water Output – Obligatory losses o We lose water by evaporation: from evaporation from the skin, and from the air we exhale. We are not usually aware of these evaporative losses – insensible losses. We only become aware of the loss of water from the lungs in the water – the “cloud” forming in front of the mouth as we exhale. Bringing air into lungs, air comes into contact with the moist lining of lungs, picks up the water and exhale it. Moist air is condensed as it is exhaled. Obligatory loss from the kidneys – we produce whether we ingest or not, the kidney must get rid of the waste produced by metabolic activity in the body and releases them in watery medium and stool. We also release, although not obligatorily, in the form or urine, some of the water that we drink (facultative, because the volume is related to intake of fluid). o Obligatory vs Facultative Losses o Obligatory losses: evaporation, urine & stool – still need to get rid of about 1.5L daily no matter what Facultative losses: vary with the intake. The kidney (major homeostatic organ for regulating water balance) produces and releases urine so as to maintain a steady body water content/% of body mass, in order to keep the individual in balance. o Insensible Perspiration vs Sweating  Insensible perspiration: the physical process of evaporation of pure water from surface of the skin  Pure water  Passive evaporation (affected by surrounding temperature and relative humidity)  Takes place on the entire skin surface  Goes on continuously  Sweating: perspiration
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