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Lecture

1. Body Fluids.pdf

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Department
Physiology
Course
PHGY 209
Professor
Erik Cook
Semester
Fall

Description
Naveen Sooknanan McGill Fall 2011 Body Fluids: The milieu interieur of the body comprises various body fluids, which each have important characteristics  Volume, the proportions of various fluids  Distribution within the body  Characterization, types of fluid and composition  Functions within the organism These body fluids help maintain homeostatic control within the body in a cyclic manner:  Organ system mantains homeostasis  Homeostasis required by cells  Cells make up organ systems Water is the universal solvent in the body, and is usually the most abundant compound found in the body.  Depending on various factors, body water makes up beteen 45% and 75% of total body mass  The polarity and electronegativy of the water molecule make it very biologically significant Body water is the medium in which  Solutes such as ions, oxygen and small organic compounds are dissolved  Metobic reactions take place o Hydrolysis uses water to break down larger macromolecules into smaller components o Dehydration releases water by building larger molecules from smaller ones Water is also able to travel through the compartments of the cell by a process called osmosis, which will be discussed later.  Any significant variation in water level could cause disturbances in homeostasis Water is found in all tissue and organic material in the body, but only body fat (stored in adipose tissue) causes variations in body water level between individuals.  Skin contains 70% water  Muscle contains 75% water o Muslces make up the most body mass  Heart, liver, kidneys, etc. contain 80% water o In fact, all organs contain around this much water  Bone contaisn only 25% water So far, all of these aspects remain relatively constant between individuals, so what accounts for the large water content variation between individuals.  There are relatively similar skin, bone, organ, muscle and bone masses between individuals 1Naveen Sooknanan McGill Fall 2011  Fat stored in adipose tissue contains only 10% water and can vary greatly between individuals In fact, the solid mass and water mass in the body remain relatively constant between indiviuals, it is the percentage of total body mass which changes due to variations in fat content.  In a “normal” individual weighing about 70kg, water makes up about 60% of body mass  In an “obese” individual weighing over 100kg, the total body mass increased because of fat, lowering the water percentage to about 45%  In a “skinny” individual weighing perhaps 60kg, there is less fat in the body, raising body water percentage to about 75% In all cases, solid body mass % is very small  The water and solid masses do not change between individuals, but fat does Therefore, body water content calculated from a “lean body mass” (i.e. body mass excluding fat) we could find body water % to be relatively constant between individuals. For future reference, the “standard physiological male” is a 21 year old white male weighing 70kg. This is used as a reference point as other factors influencing body water percentage including age, gender, weight, ethnic origin, etc. As people age, their body water percentage tends to decrease.  Newborn babies (less than 1 week old) have very little body fat and have a body water percentage of 75%  By puberty, body water level decreases to about 60% for boys and 50% for girls (difference explained later) because of increased fat content and bone development  By 70, body water levels drop to 50% for men and 45% for women because organs tend to “dry up” and muscle turns into fat After puberty, women tend to have more body fat than men because of a higher body fat content in localized regions (breasts and buttocks) and therefore have a lower body water level. To calculate the body mass of a standard physiological male: In a 60kg female, however, This is useful for medicine administered as an aqueous solution because the proper concentration is necessary for proper dosage. If 100mg of medication is administered to the male described above 2Naveen Sooknanan McGill Fall 2011 If the same dosage is administered to the female described above This medicinal concentration may be enough to cause too high a concentration in the body resulting in an overdose. Due to homeostasis, body water level is kept at a remarkably constant level. Water moves throughout the body in the following two ways:  It is consumed/excreted from the body in which case the rate of excretion is equal to the rate of consumption  It moved between compartments of the body in which case the rate of entering a compartment is equal to the rate of water leaving this compartment Water intake can be made up from various sources, take for example:  Oral fluids and food can contain around 2.3L in this example (can vary greatly) o This comes from ingestion  The metabolism or ingested food also produce approximately 0.4L of water in the example above o C H6O 12 6 +O 2 6C0 + 62 O + 2nergy Output of water can be divided into obligatory and facultative losses. Continuing with the example above:  Obligatory losses account of a mandatory 1.5L from the body which come from: o Insensible losses such as evaporation of water from the skin (not sweating) and exhaled water vapours from condensation in the lungs which account for approximately 1L of water o 500mL of using produced by the kidney is mandatory to remove waste from the body  The kidney is a major homeostatic organ in the body o Small amounts of water are present in the stool (feces) which account for approximately 500mL, produced in the GI tract  Facultative losses can vary with intake and in this example count for 1.2L o Kidneys produce and extra 1.2L or urine to deal with the extra water consumption  Kidneys are incredibly efficient And can deal with as much as 18L of water per day  Therefore, the 2.7L of water lost equals the 2.7L of water consumed It is important to differentiate between insensible perspiration and sweating. Firstly, they are controlled by completely different mechanisms in the body.  Insensible perspiration is 3Naveen Sooknanan McGill Fall 2011 o An insensible loss that is not felt o Pure water o Passive evaporation  Affect by ambient temperature and humidity o Takes place over the entire skin surface  Present in individuals lacking sweat glands o Continuous  Sweating, on the other hand, is o Sensible, you know when you’re sweating o An electrolyte solution o Secreted only by sweat glands  Only in certain body parts o Activated by high energy work like exercise  Not continuous o Releases heat to lower body temperature  Note that sweating can cause a sudden loss of as much as 3L of water and must be replaced to prevent dehydration A normal adult’s water turnover per day is 3-4% of their body mass. A baby’s turnover, however, can be as much as 10% of body mass  This is why it is easier for infants to become seriously dehydrated The constancy of body water level in the body also helps maintain  Normal solute concentrations  Normal blood volume and pressure o Allows for adequate supply of oxygen and nutrients to tissues A negative water balance can cause dehydration if serious. This can be caused by  Reduced intake  Excessive loss from gut (diarrhea)  Excessive sweating (exercise)  Excessive loss in expired air o Higher altitudes with lower air pressure influencing stronger evaporation  Excessive in loss urine (diabetes or kidney disease A positive water balance can cause water intoxication if serious. This can happen by  Excessive intake o Rare, due to efficiency of kidneys  Renal failure o Diseased kidney As stated before, the body water content is divided into 2 major compartments: Extracellular fluid (ECF) and Intracellular fluid (ICF):  The ECF can be further subdivided into 4 subcompartments, 2 major and 2 minor o Major:  Plasma 4Naveen Sooknanan McGill Fall 2011  Interstitial fluid (ISF) o Minor:  Lymph  Transcellular Fluid Each of these compartments and subcompartments differ in size, composition and
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