Body Fluids and Fluid Compartments PDF 20-Dec-21

20-Dec-21 Body Fluids and Fluid Compartments CODE: CBF-103 By Dr. Sally Anwar Sayed Assistant Prof. of Medical Physiology, Faculty of Medicine, Assiut University...

20-Dec-21 Body Fluids and Fluid Compartments CODE: CBF-103 By Dr. Sally Anwar Sayed Assistant Prof. of Medical Physiology, Faculty of Medicine, Assiut University Prof. Eman Sayed Hassan Prof. of Medical Physiology, Faculty of Medicine, Assiut University Objectives: 1-Explain the importance of water in the body 2-Identify the body’s main fluid compartments 3-Contrast the composition of the intracellular fluid with that of the extracellular fluid 4- Identify the ions most important to the function of the body 5- Forces influence fluid movement across the capillary wall 6- Demonstrate causes of edema and safety factors against it 1  20-Dec-21 Body Water Content -Human beings are mostly water, Factors which determine the overall water weight of a human being include: 1. Sex (men contain more fluid than women; adult man 60%, adult woman 50%). This is because women typically have less skeletal muscle and more fat than males. 2. Age (the younger you are the greater fluid you have; infant 75%, old person 45%) This is because infants have low bone mass and low body fat than old person 3. Body fat percentage (the more the fat, the less the water content; fatty obese people have lower water content). -Adipose (fat) tissue is the least hydrated (10%) tissue in the body -Brain and kidneys have the highest proportions of water. 2  20-Dec-21 Water balance To achieve water balance fluid intake should =fluid output Daily Intake of Water Daily Loss of Body Water 1- Drinking of liquids 1-Insensible Water Loss: there is a water in the food, (2 liters) continuous loss of water by evaporation from 2- water is synthesized in the body as a result the respiratory tract and diffusion through of oxidation of carbohydrates, adding about the skin, 200-300 ml/day. 2-Fluid Loss in Sweat. The amount of water lost by sweating is depending on physical activity and environmental temperature. 3-Water Loss in Feces. Only a small amount of water 4-Water Loss by the Kidneys. The remaining water loss from the body occurs in the urine excreted by the kidneys This provides a total water intake of about This provides a total water output about 2300 ml/day. 2300 ml/day. Fluid Compartments There are two main fluid compartments in the body: 1. The intracellular fluid compartment (ICF) (2/3; 28L):The intracellular fluid (ICF) compartment is the system that includes all fluid enclosed in cells by their plasma membranes. 2. The extracellular fluid compartment (ECF) (1/3; 14L): The ECF is the body’s internal environment and the cells external environment; ECF surrounds all cells in the body. ECF divided into: 1- The fluid component of the blood (called plasma; the fluid in which blood cells and platelets circulate) and 2- The interstitial fluid (IF) that surrounds all cells (not in the blood) 3- lymph and transcellular fluid. Water moves freely between compartment, so the Osmolarity of the body fluids is 290 mOsm/L 3  20-Dec-21  Lymph is the fluid that circulates throughout the lymphatics. -It is formed when the interstitial fluid is collected through lymph capillaries. -It is then transported through larger lymph vessels to lymph nodes, where it is cleaned by lymphocytes. - Lymphatics return excess fluid and protein content to the general circulation by coalescing into the lymphatic ducts, which in turn empty into to the subclavian veins.  Transcellular fluid is the fluid that fills up the spaces of chambers that are created or formed from the linings of epithelial cells. Examples of transcellular fluid are: Aqueous humour, vitreous humour, Cerebrospinal fluid, Cerumen (earwax), Chyle, Chyme, Endolymph, perilymph, Gastric juice, Pericardial fluid, Peritoneal fluid, Pleural fluid and Synovial fluid. 4  20-Dec-21 Blood Volume:  Blood contains both ECF (the fluid in plasma) and ICF (the fluid in the red blood cells).  The blood volume is especially important in the control of cardiovascular dynamics.  The average blood volume of adults is about 5 liters.  About 60% of the blood is plasma and 40 % is red blood cells, Measurement of Blood Volume: Total blood volume = Plasma volume/(1- Hematocrit) Hematocrit is the percentage ratio of packed cell volume to total blood volume [(volume of cells/volume of blood)*100] e.g. if plasma volume is 3 liters and hematocrit is 0.40, total blood volume would be calculated as: Total blood volume = 3 liters/1-0.4= 5 L Ionic Composition of ECF and ICF The ECF, including the plasma and the interstitial fluid, contains:  Large amounts of sodium, chloride ions, and bicarbonate ions,  Only small quantities of potassium, calcium, magnesium, phosphate, and organic acid ions.  -The most important difference between these two compartments is the higher concentration of protein in the plasma. The intracellular fluid contains  Small quantities of sodium and chloride ions and  Large amounts of potassium and phosphate ions  Moderate quantities of magnesium and sulfate ions,  Also, cells contain large amounts of protein, almost four times as much as in the plasma. 5  20-Dec-21 Distribution of K+ in the body: 1. Intracellular: 150 mmol/L 2. Extracellular: 4-5 mol/L Distribution of Na+ in the body 1. Intracellular: 10 mEq/L 2. Extracellular: 145 mEq/L Protein: ICF>plasma>interstitial fluid Roles of Electrolytes: Sodium (it is the main cation in ECF): 1. Maintenance of blood volume and arterial blood pressure. 2. Tissue excitability (important in the generation of action potential) 3. Helps glucose transport in the intestine and kidney (secondary active transport) 4. Formation of ECF buffers (acid base balance) 5. Concentration of urine as NaCl. 6  20-Dec-21 Roles of Electrolytes: Potassium (it is the main cation in ICF): 1. The maintenance of the resting membrane potential of the excitable cell membrane which is needed for the normal contraction of smooth, cardiac and skeletal muscle. 2. Potassium can cause endothelium-dependent vasodilation. 3. The maintenance of the arterial blood pressure within normal range. 4. Enhancement of insulin secretion in response to glucose load. 5. Potassium and other minerals such as sodium, calcium and magnesium are essential for normal intestinal peristalsis (rhythmic intestinal contractions that are responsible for propelling the food along the GIT). 6. Maintains an optimal environment for cellular enzymes. 7. Important for cell growth (protein and DNA synthesis) 8. Acid base balance. Forces influence fluid movement across the capillary wall Fluid flux across the capillary is governed by the 2 fundamental forces that cause water flow: Hydrostatic force, which is simply the pressure of the fluid Osmotic (oncotic) force, which represents the osmotic force created by solutes that do not cross the membrane Each force exists on both sides of the membrane. Filtration is the movement of fluid from the plasma into the interstitium, Absorption is movement of fluid from the interstitium into the plasma. 7  20-Dec-21 Forces influence fluid movement across the capillary wall Forces for filtration 1. PC = hydrostatic pressure (blood pressure) in the capillary It is directly related to blood flow (regulated at the arteriole); venous pressure; and blood volume. 2. πIF = oncotic (osmotic) force in the interstitium It is determined by the concentration of protein in the interstitial fluid. Normally the small amount of protein that leaks to the interstitium is minor and is removed by the lymphatics. Under most conditions, this is not an important factor influencing the exchange of fluid. Forces influence fluid movement across the capillary wall Forces for absorption 1. πC = oncotic (osmotic) pressure of plasma it is the oncotic pressure of plasma solutes that cannot diffuse across the capillary membrane, i.e., the plasma proteins. Albumin, synthesized in the liver, is the most abundant plasma protein and thus the biggest contributor to this force. 2. PIF = hydrostatic pressure in the interstitium It is difficult to determine. In most cases it is close to zero or negative (subatmospheric) and is not a significant factor affecting filtration versus reabsorption. It can become significant if edema is present. 8  20-Dec-21 Forces influence fluid movement across the capillary wall Starling Equation There are 4 forces that influence fluid exchange across the capillary wall (2 forces favor filtration and 2 forces oppose it). The formula for fluid exchange is the following: Qf = k [(Pc + πIF) − (PIF + πC)] Qf: fluid movement, K: filtration coefficient that depends upon a number of factors, of which the most important factor is membrane permeability. A positive value of Qf indicates net filtration; a negative value indicates net absorption. Safety Factors That Normally Prevent Edema Abnormality must be severe before serious edema develops. The reason for this is that three major safety factors prevent excessive fluid accumulation in the interstitial spaces: (1) Low compliance of the interstitium The interstitial fluid hydrostatic pressure is averaging about –3 mm Hg. This slight suction in the tissues helps hold the tissues together. (2) The ability of lymph flow to increase 10- to 50-folds/ Increased lymph flow Lymphatics play a pivotal role in maintaining a low interstitial fluid volume and protein content. Lymphatics return excess fluid and protein content to the general circulation. The safety factor from this effect has been calculated to be about 7 mm Hg. (3) Wash down of interstitial fluid protein concentration/ Lymphatic washout of protein This reduces interstitial fluid colloid osmotic pressure as capillary filtration increases. The safety factor from this effect has been calculated to be about 7 mm Hg. 9  20-Dec-21 Safety Factors That Normally Prevent Edema Summary: The safety factor caused by: 1. Low tissue compliance is about 3 mm Hg. 2. Increased lymph flow is about 7 mm Hg. 3. Wash down of proteins from the interstitial spaces is about 7 mm Hg. Putting together all the safety factors against edema, we find that the total safety factor against edema is about 17 mm Hg. This means that the capillary pressure in a peripheral tissue could theoretically rise by 17 mm Hg before marked edema would occur. Edema Definition: It is an accumulation of excess fluid in interstitial space. Causes: I. Increased capillary hydrostatic pressure (increased filtration forces): 1- Localized: e.g. venous thrombosis 2- Generalized: e.g. Heart Failure II. Hypoproteinemia (decreased plasma colloid osmotic pressure, reduced absorption) Causes: 1- Under-nutrition 2- Severe burns 3-Renal edema 4- Repeated haemorrhage III. Increased capillary permeability 1- Bacterial and chemical toxins 2- Allergic reaction IV. Blockage of lymph return (Inadequate lymph drainage) 1. Cancer 2. Parasitic Infections (e.g., filaria nematodes) V. Salt and water retention: Renal faliure increases the blood volume and the capilary pressure 10  20-Dec-21 References 1- Elsevier integrated physiology 2- Oxford handbook of medical physiology 3-Medical Physiology. Guyton &Hall Kaplan Lecture notes (Physiology) 2021 11