Body Fluid Compartments PDF

Cell Structure & Function Physiology Body fluid compar tments W ater forms about 2/3 body weight (about 60%) or 42 Liters in adult males. The remaining (40%) are: proteins, fats & minerals. Factors affecting body water content: 1-...

Cell Structure & Function Physiology Body fluid compar tments W ater forms about 2/3 body weight (about 60%) or 42 Liters in adult males. The remaining (40%) are: proteins, fats & minerals. Factors affecting body water content: 1- Age: Water in newly born infants represent 80% of total body weight but in old age, water represents only about 45%. 2- Sex: In females water represent 50% only of total body weight due to high fat content. 3- Fat content or the Body mass index (BMI): in obese persons water represents 50% only of total body weight. Total body water is either: Intracellular fluid (ICF): inside the body & representing 2/3 of total body water or 40% of total body weight or 28 Liters. Extracellular fluid (ECF): outside the cell & represent 1/3 of total body water or 20 % of total body weight or 14 Liters). ✓ Intravascular: inside blood vessels (plasma) (1/4 of ECF or 5% of total body weight or 3 L). ✓ Interstitial: bathing the body cells and called internal environment (3/4 of ECF or 15% of total body weight or 10.5 L). ✓ Transcellular fluid: small compartment (about 0.5 Liter). It includes fluid in the synovial, peritoneal, pericardial, and intraocular spaces, as well as the cerebrospinal fluid; it is usually considered to be a specialized type of ECF, although in some cases its composition may differ markedly from ECF. ICF & ECF are separated by cell membranes which keeps the composition of the 2 fluids different. The intravascular and interstitial fluids are separated by the capillary wall which is highly permeable and allow for exchange between the two compartments but, it keeps the protein content of intravascular fluid is more than that in interstitial fluid due to the large size of proteins that cannot pass through the pores of the capillary membrane. 4 Cell Structure & Function Physiology T he ECF contains large amounts of sodium (Na +) & Ca 2+ as cations & Cl- & HCO3- as anions with small amounts of K + in comparison with the ICF. The ICF contains large amounts of potassium (K+) as cation & phosphates, sulphates and proteins as anions with small amounts of Na+, Mg2+. The cells take up nutrients such as glucose, amino acids & fats from interstitial fluid and give out CO 2 and other waste products to be excreted by the kidneys, lungs or skin. Determination of water volume in different body compartments According to indicator dilution method or Fick's principle Mass = volume X concentration Measuring the intravascular volume: If a known amount of indicator substance is injected in the blood it will be diluted with plasma only if its molecules can't cross the capillary wall and by measuring the degree of dilution, the plasma volume can be calculated. 5 Cell Structure & Function Physiology The substances used are for measuring plasma volume: Evan՚s blue dye. Radioactive iodine bound to albumin 131I or 132I. Radiolabeled fibrinogen or radiolabeled albumin. The substances used are for measuring RBCs volume: RBCs containing radioactive chromium 51Cr, phosphorus 32P or iron 55Fe. Blood volume = plasma volume + RBC s volume. Or after measuring the hematocrit value (HV) 𝟏𝟎𝟎 Blood volume = plasma volume × 𝟏𝟎𝟎−𝑯𝑽 Measuring the ECF volume: If the substance used can cross the capillary wall but can't cross the cell membrane it will be diluted with all ECF as: Saccharides: as inulin, sucrose or mannitol. Ions: Na thiocyanate, thiosulphate or radioactive sodium. Measuring the total body water volume: If it can cross both the capillary wall and cell membrane it will be diluted by the total body water as deuterium oxide (D 2O) or heavy water (deuterium is hydrogen isotope) or the antipyrine (rarely used). The greater the fluid volume the less will be the concentration after dilution. The body fluid volume compartment that holds the substance is called the distribution volume. Initial volume (V1) X Initial concentration of substance (C1) = body fluid volume compartment (V2) X final concentration of the substance after dilution (C2) 𝐕𝟏 𝐗 𝐂𝟏 V2 = 𝐂𝟐 Characters of the substance used: Inert – nontoxic – not utilized by tissues – not rapidly excreted – distributed uniformly in the compartment to be measured. Some body fluid compartments can't be measured directly by dilution method, so it is calculated by extraction. These are: ICF volume = total body water – ECF volume Interstitial fluid volume = ECF volume – intravascular fluid volume 6 Cell Structure & Function Physiology Tissue fluid (interstitial fluid) Definition: it is the medium in which the cells are bathed. Functions: it is the medium for exchange of various substances between cells and capillaries (O2 and nutrients diffuse from arterial end of the capillaries to reach the tissue fluid then to the cells while CO 2 and waste products from cells reach the tissue fluid then diffuse through the venous end of the capillaries). Formation of tissue fluid: Through: 1- Filtration at the arterial end of the capillary. 2- Reabsorption at the venous end of the capillary. Arterial end Venous end Filtration force (capillary hydrostatic pressure) 35 mmHg 17 mmHg Reabsorbing force (plasma colloidal osmotic pressure or 25 mmHg 25 mmHg oncotic pressure) of plasma proteins NET pressure +10 -8 Net result Filtration Reabsorption The fluid filtered from the arterial end of the capillaries is drained by mainly by the veins but some of this fluid is drained by lymphatics. 7 Cell Structure & Function Physiology Oedema Definition: presence of excess fluid in interstitial space. Causes: 1- Increased capillary hydrostatic pressure: This increases the filtration of fluid from blood to tissue spaces. This may occur as a result of: a) venous obstruction by thrombosis b) compression on the veins from outside by the uterus during pregnancy c) right sided heart failure. 2- Decreased colloidal osmotic pressure of the plasma as in Hypoproteinemia: Causes: 1- Decrease synthesis of plasma proteins in Liver cell failure or Undernutrition. 2- Increase loss of plasma proteins as in Severe kidney disease. 3- Increased capillary permeability due to Bacterial & chemical toxins or Allergic conditions. 4- Lymphatic obstruction: (non-pitting oedema) The accumulated fluid has a relatively high protein concentration. Lymphatic obstruction occurs in: filariasis (elephantiasis) or Cancer invasion. 8 Cell Structure & Function Physiology Homeostasis and homeostatic control systems H omeostasis: is keeping the internal body environment constant. Whether its physical or chemical composition to be optimum for cellular function. The internal body environment refers to the fluids that bathes the cells which is the ECF and the blood. disturbances of homeostasis lead to cellular dysfunction or even death. Homeostasis is an active process, and all body systems contribute in homeostasis. Examples: Oxygen and nutrients are continuously taken up by tissues from surrounding interstitial fluid and if this not compensated, the levels of oxygen and nutrients will drop (disturbance of homeostasis) but lungs continuously provide new oxygen and the gastrointestinal tract (GIT) continuously provide new nutrients. At the same time, CO2 and waste products are continuously formed by cell metabolism and added to surrounding interstitial fluid and if this not compensated, the CO 2 and waste products will accumulate 9 Cell Structure & Function Physiology (disturbance of homeostasis) but lungs continuously remove excess CO2 and the kidneys and other excretory organs continuously remove waste products. The cells use oxygen to oxidize nutrients to give energy. This energy is used to do a certain work, but part of this energy is converted into heat. This heat if not lost, the body temperature will rise (disturbance of homeostasis) but our body temperature is kept constant at 37 ◦C by heat regulating systems to preserve enzymatic activity and cellular function. The waste products produced by cellular metabolism includes acidic metabolites which if accumulated the blood pH will drop disturbing cellular function, but the buffer systems, respiration and kidney keep the blood pH constant. The ionic composition and osmolarity of interstitial fluid and blood are kept constant at 290-300 mosmol/L (isotonic). The locomotor system contributes by moving the person to the site of food. The endocrine and nervous system regulate various body functions including temperature regulation by heat regulating center in hypothalamus or ionic composition of the body fluids by hormonal secretions as aldosterone and parathormone. Homeostatic Control Systems These are Network of body components that operate to maintain a given factor (variable or parameter) in the internal environment relatively constant around an optimal level. The control system must include: 1. Sensors (detectors or receptors): that detect in change of the variable (which is termed an error). These sensors send the information detected through an afferent to a control center. 2. Control center: which recognize the error and integrate multiple information to find an appropriate response. Then send corrective signals (orders) through an efferent to effectors. 3. Effectors: these are the organs that respond to the orders of the control center to correct the error. Control systems operate by two mechanisms: Negative feedback (main). Positive feedback. 10 Cell Structure & Function Physiology Negative Feedback Refers to the mechanism by which the response of the control system is in the opposite direction of the change (error). For example: 1- Regulation of body temperature 2- Regulation of blood glucose level 3- Regulation of RBCs number When body temperature increases (error), this error is detected by the hypothalamic thermoreceptors (detectors). They send this information to the heat regulating center (the control center) that sends corrective signals to the sweat gland (effectors) to increase sweating that increases the heat loss from the body until body temperature returns to normal. 11 Cell Structure & Function Physiology Positive Feedback Mechanisms Refers to the mechanism by which the response of the control system is in the same direction of the change (error). So, the controlled variable moves in one direction, Continues increasing or decreasing to a certain level. Examples: 1- Uterine contractions at the time of labor. 2- The process of blood coagulation and platelet aggregation. 12 Cell Structure & Function Physiology 13