Fluid Balance and Homeostasis

Fluid Homeostasis

• Fluid intake and output are balanced during steady-state conditions, with a continuous exchange of fluids and solutes with the external environment.
• Daily intake of water is approximately 2300 ml, consisting of 2100 ml from liquids and food, and 200 ml from carbohydrate oxidation.

Daily Water Loss

• Insensible water loss occurs through diffusion through the skin (300-400 ml/day) and respiratory tract (300-400 ml/day), totalling around 700 ml/day under normal conditions.
• In cases of extensive burns, the rate of evaporation can increase up to 10-fold, resulting in a loss of 3-5 liters of water per day.
• Fluid loss through sweat depends on physical activity and environmental temperature, ranging from 100 ml/day to 1-2 liters/hour.
• Water loss in feces is approximately 100 ml/day, but can increase to liters in cases of diarrhea, posing a life-threatening risk.
• Water loss through kidneys occurs via urine, with the kidneys adjusting the amount of water and electrolytes (sodium, chloride, and potassium) to secrete based on intake.

Body Fluid Compartments

• The body has two main fluid compartments: extracellular and intracellular.
• Extracellular fluid consists of interstitial fluid, blood plasma, and transcellular fluid (including fluid in synovial, peritoneal, and pericardial spaces, as well as CSF).
• Intracellular fluid is contained within cells, making up 40% of total body weight in an average person, with a volume of approximately 28-42 liters.
• Male bodies have a lower fat percentage and a higher water percentage compared to female bodies, with males having around 42 liters of water.
• Concentrations of fluids and electrolytes are consistent across all cells, even in animals.

Intracellular Fluid Compartment

• The intracellular fluid compartment contains 28-42 liters of water, making up 40% of an average person's total body weight.
• The concentration of substances is the same in every cell, including in animals.

Extracellular Fluid Compartment

• The extracellular fluid compartment makes up 20% of an average person's total body weight, or around 14 liters in a 70 kg person.
• Interstitial fluid accounts for ¾ of the extracellular fluid, with a volume of around 11L.
• Blood plasma accounts for ¼ of the extracellular fluid, with a volume of around 3L.
• Capillary pores are permeable to all substances except proteins.

Hematocrit

• Hematocrit is the percentage of red blood cells in the blood.
• Centrifugation does not pack red blood cells together, leaving around 3-4% of plasma entrapped among the cells.
• True hematocrit is around 96%.
• Normal hematocrit values are 40% for men and 36% for women.
• Anemia is defined as a hematocrit of less than 10%.
• Polycythemia is defined as a hematocrit of more than 65%.

Ionic Composition of Plasma and Interstitial Fluid

• Plasma and interstitial fluid have a similar ionic composition, despite being separated by permeable capillary membranes.
• The main difference is the higher protein concentration in plasma, with only a small amount able to leak into interstitial spaces.
• The Donnan effect explains why cation concentrations are 2% higher in plasma than in interstitial fluid.
• Plasma proteins have a net negative charge, binding cations and holding extra amounts in plasma.
• Interstitial fluid has a higher concentration of anions due to the negative charges of plasma proteins repelling them.

Intracellular Fluid Constituents

• Cell membrane is permeable to water but not to most electrolytes in the body.
• Intracellular fluid contains a small amount of sodium and chloride, but a large amount of potassium, phosphate, magnesium, and sulfate ions.
• These ions have low concentrations in the extracellular fluid.
• Intracellular fluid contains a large amount of protein, approximately 4 times that of plasma.

Measurement of Body Fluid Compartment Volumes

• The indicator-dilution principle is used to measure body fluid compartment volumes.
• An indicator substance is placed in the compartment, dispersed evenly throughout the compartment's fluid.
• The volume of the compartment can be calculated using the formula: VolumeB = (VolumeA) x (ConcentrationA) / (ConcentrationB).

Determination of Volumes of Specific Body Fluid Compartments

Measurement of Total Body Water

• Radioactive water (tritium) and heavy water (deuterium) can be used to measure total body water.
• Antipyrine can also be used, as it is lipid soluble and penetrates cell membranes and extracellular compartments.
• The dilution principle is used to calculate total body water after a few hours of injection.

Measurement of Extracellular Fluid Volume

• Substances that disperse in plasma and interstitial fluid but do not permeate the cell membrane are used.
• Examples of such substances include radioactive sodium, radioactive chloride, radioactive iothalamate, thiosulfate ion, and inulin.
• Measurement takes 30-60 minutes and occurs in the extracellular fluid.
• Note: Radioactive sodium may diffuse slightly into the cells.

Calculation of Intracellular Volume

• Intracellular volume is calculated by subtracting extracellular volume from total body water

Measurement of Plasma Volume

• Uses serum albumin labeled with radioactive iodine (125I-albumin) or Evans blue dye (T-1824) that binds to plasma proteins

Calculation of Interstitial Fluid Volume

• Interstitial fluid volume is calculated by subtracting plasma volume from extracellular fluid volume

Measurement of Blood Volume

• Total blood volume is calculated by dividing plasma volume by 1 minus hematocrit
• Example: plasma volume = 3 liters, hematocrit = 0.4, total blood volume = 5 liters

Causes of Hyponatremia

• Excess water or loss of sodium
• Diarrhea and vomiting can cause hyponatremia due to loss of sodium
• Overuse of diuretics can also lead to hyponatremia

Effects of Hyponatremia

• Causes cell edema, especially in the brain
• Rapid fall in plasma sodium concentration below 115-120 mmol/L causes brain swelling

Causes of Hypernatremia

• Water loss or excess sodium
• Increased plasma sodium concentration can cause hypernatremia and dehydration
• Loss of water can lead to hypernatremia

Effects of Hypernatremia

• Causes cell shrinkage
• Stimulates secretion of ADH to protect plasma and extracellular fluid
• Treatment involves administering hypoosmotic sodium chloride or dextrose solution

Edema

• Excess fluid in the tissues caused by:
  • Hyponatremia
  • Depression of metabolic systems
  • Lack of adequate nutrition to cells
• Decreased blood flow to tissues reduces oxygen and nutrient supply

Extracellular Edema

• Caused by:
  • Abnormal leakage of fluid from plasma to interstitial spaces across capillaries
  • Failure of lymphatics to return fluid from interstitium back into the blood (lymphedema)

Factors That Increase Capillary Filtration

• Multiple conditions can lead to fluid accumulation in interstitial spaces due to abnormal capillary fluid leakage or impaired lymphatic return of fluid from the interstitium to the circulation.
• These conditions can cause extracellular edema through two types of abnormalities:

  Abnormal Capillary Fluid Leakage

  • Increase in fluid leakage from capillaries into interstitial spaces

  Impaired Lymphatic Return

  • Prevention of lymphatics from returning fluid from the interstitium back to the circulation

Safety Factors Against Edema

• The safety factor caused by low tissue compliance in the negative pressure range is about 3 mm Hg.
• The safety factor caused by increased lymph flow is about 7 mm Hg.
• The safety factor caused by washdown of proteins from the interstitial spaces is about 7 mm Hg.
• The total safety factor against edema is about 17 mm Hg, which means capillary pressure can increase by 17 mm Hg or approximately double the normal value before marked edema occurs.

Potential Spaces

• Examples of potential spaces include pleural, pericardial, peritoneal, and synovial cavities, including joint cavities and bursae.
• These spaces have surfaces that almost touch each other, with a thin layer of fluid in between, and the surfaces slide over each other.
• A viscous proteinaceous fluid lubricates the surfaces to facilitate sliding.

Fluid Exchange Between Capillaries and Potential Spaces

• The surface membrane of a potential space does not offer significant resistance to the passage of fluids, electrolytes, or proteins.
• Fluids, electrolytes, and proteins move back and forth between the space and interstitial fluid in the surrounding tissue with relative ease.
• Each potential space is essentially a large tissue space.
• Fluid in capillaries adjacent to the potential space diffuses into both the interstitial fluid and the potential space.

Lymphatic Vessels and Protein Removal

• Proteins collect in potential spaces due to leakage from capillaries, similar to protein collection in interstitial spaces throughout the body.
• Proteins must be removed through lymphatics or other channels and returned to the circulation.
• Each potential space is directly or indirectly connected with lymph vessels.
• In some cases, large lymph vessels arise directly from the cavity itself (e.g., pleural cavity and peritoneal cavity).

Edema Fluid and Effusion

• Edema fluid in potential spaces is called effusion.
• Edema in subcutaneous tissues adjacent to potential spaces can cause fluid collection in the potential space, resulting in effusion.

Causes of Effusion

• Lymph blockage can cause effusion.
• Abnormalities that cause excessive capillary filtration can cause effusion.

Ascites

• Effusion fluid collected in the abdominal cavity is called ascites.
• In serious cases, 20 liters or more of ascitic fluid can accumulate.

Other Potential Spaces

• Pleural cavity, pericardial cavity, and joint spaces can become seriously swollen when generalized edema is present.
• Injury or local infection in any of these cavities can block lymph drainage, causing isolated swelling.

Normal Fluid Pressure

• Normal fluid pressure in most potential spaces is negative (subatmospheric).
• Examples of normal interstitial fluid hydrostatic pressure: • Pleural cavity: −7 to −8 mm Hg. • Joint spaces: −3 to −5 mm Hg. • Pericardial cavity: −5 to −6 mm Hg.