Water Balance

What are the major functions of the kidneys?

Maintenance of water and electrolyte balance; regulation of acid-base balance; excretion of waste products such as urea and creatinine; excretion of foreign compounds such as drugs; erythropoietin production; renin secretion; and conversion of vitamin D into its active form.

What is the kidney's role in water and electrolyte balance?

The kidneys regulate the amounts of water and electrolytes excreted so that body fluid composition and volume remain balanced.

Which waste products are specifically listed as being excreted by the kidneys?

Urea and creatinine.

What types of foreign compounds can be excreted by the kidneys?

Foreign compounds such as drugs.

What hormone produced by the kidneys stimulates red blood cell production?

Erythropoietin.

What substance secreted by the kidneys participates in regulation of the renin-angiotensin-aldosterone system?

Renin.

What role do the kidneys have in vitamin D metabolism?

The kidneys convert vitamin D into its active form.

What is homeostasis?

Homeostasis is the process through which bodily equilibrium is maintained.

Why is the human body described as an "open system"?

The body is an open system because it exchanges material and energy with its surroundings.

What is meant by maintaining a constant "internal milieu"?

The body maintains relatively constant internal conditions such as body fluid composition, temperature, blood pressure, and blood glucose level despite exchanges with the environment.

What must happen to output when input changes if homeostasis is to be maintained?

Any change in input must be balanced by a corresponding change in output.

What are the two major sources of input into the body?

Ingestion and metabolic production.

What counts as output from the body?

A substance can be excreted to the outside or used up in a chemical reaction.

What is the definition of balance for a substance in the body?

Total body input of the substance equals total body output.

What is positive balance?

Positive balance occurs when gain is greater than loss.

What is negative balance?

Negative balance occurs when loss is greater than gain.

What are the major routes of water output shown in the water-balance diagram?

Kidneys, lungs, feces, sweat, and skin.

What are the major fluid regions shown between water input and output?

Plasma, interstitial fluid, and intracellular fluid.

What membrane separates plasma from interstitial fluid?

The capillary membrane.

What membrane separates interstitial fluid from intracellular fluid?

The cell membrane.

What is the approximate total daily water input under steady-state conditions?

Approximately 2,300 mL/day.

How much water is obtained daily from drinking water and water contained in food?

Approximately 2,100 mL/day.

How much water is produced daily by metabolism?

Approximately 200 mL/day.

What metabolic process is specifically mentioned as producing water?

Oxidation of carbohydrates.

Why can daily water input vary substantially?

It varies with factors such as climate, habits, and physical activity.

What is insensible water loss?

Water loss of which we are not consciously aware.

What are the two major routes of insensible water loss?

Evaporation from the respiratory tract and evaporation from the skin surface.

How much water is normally lost by evaporation from the respiratory tract?

Approximately 300–400 mL/day.

How much water is normally lost by evaporation from the skin surface, excluding sweat?

Approximately 300–400 mL/day.

Does insensible water loss from the skin include sweat?

No. Evaporation from the skin surface described as insensible loss does not include sweat.

What limits evaporation of water from the skin?

The cholesterol-filled cornified layer acts as a barrier.

How can extensive burns affect water loss through the skin?

Extensive burns can increase water loss to approximately 3–5 L/day because the normal skin barrier is disrupted.

How much fluid is normally lost in sweat per day?

Approximately 100 mL/day.

What factors make sweat loss variable?

Physical activity and environmental temperature.

How high can sweat loss become during heavy exercise and hot weather?

Approximately 1–2 L/hour.

How much fluid is normally lost in feces each day?

Approximately 100 mL/day.

How can diarrhea affect fecal water loss?

Diarrhea can increase fecal water loss to several liters per day.

Why is urinary water loss especially important in water balance?

Urine is the most important means by which the body adjusts output to maintain balance between water intake and water loss.

What range of daily water excretion can the kidneys produce according to the lecture?

Approximately 0.5–20 L/day.

What range of electrolyte excretion can the kidneys produce according to the lecture?

Approximately 20 mEq/day to 300–500 mEq/day.

What is the typical daily input-output relationship during steady-state water balance?

Water input equals water output, with both totaling approximately 2,300 mL/day.

In the lecture's summary, how much daily water input comes from ingested fluid?

2,100 mL/day.

In the lecture's summary, how much daily water input comes from metabolism?

200 mL/day.

In the lecture's summary, what is the total daily water input?

2,300 mL/day.

In the lecture's summary, how much water is lost insensibly through the skin?

350 mL/day.

In the lecture's summary, how much water is lost insensibly through the lungs?

350 mL/day.

In the lecture's summary, how much water is lost through sweat?

100 mL/day.

In the lecture's summary, how much water is lost through feces?

100 mL/day.

In the lecture's summary, how much water is lost through urine?

1,400 mL/day.

In the lecture's summary, what is the total daily water output?

2,300 mL/day.

Using the lecture's typical values, what percentage of the 2,300 mL daily water output is urine?

About 61% because 1,400 mL of the 2,300 mL total output is urine.

Using the lecture's typical values, what is the combined daily insensible water loss from skin and lungs?

700 mL/day: 350 mL through the skin + 350 mL through the lungs.

What is one major role of the kidneys in matching water and electrolyte intake?

The kidneys adjust the excretion rate of water and electrolytes to match precisely the intake of these substances.

How do the kidneys respond to excessive losses of fluids and electrolytes?

They compensate by adjusting excretion to conserve fluids and electrolytes.

What equation summarizes normal water balance?

Water input = water output.

What is intracellular fluid (ICF)?

Fluid located inside all cells.

What is extracellular fluid (ECF)?

Fluid located outside the cells.

What are the two major components of extracellular fluid?

Plasma and interstitial fluid.

What percentage of body weight is normally total body water (TBW)?

Approximately 50–70% of body weight, with an average of about 60%.

What is the average total body water percentage used in the lecture?

60% of body weight.

How much total body water is expected in the lecture's average 70-kg person?

42 L.

How is total body water calculated for a 70-kg person using the lecture's average value?

0.60 × 70 kg = 42 kg of water, approximately equivalent to 42 L.

How much intracellular fluid is present in the lecture's 70-kg example?

28 L.

How much extracellular fluid is present in the lecture's 70-kg example?

14 L.

How much plasma is present in the lecture's body-fluid compartment example?

3 L.

How much interstitial fluid is present in the lecture's body-fluid compartment example?

11 L.

How do plasma and interstitial fluid volumes combine to form ECF volume in the example?

3 L plasma + 11 L interstitial fluid = 14 L extracellular fluid.

How do ECF and ICF volumes combine to give total body water in the example?

14 L ECF + 28 L ICF = 42 L total body water.

What fraction of total body water is intracellular fluid in the 42-L example?

28/42, or approximately two-thirds of total body water.

What fraction of total body water is extracellular fluid in the 42-L example?

14/42, or approximately one-third of total body water.

What proportion of ECF is interstitial fluid in the lecture's diagram?

Approximately 80% of ECF.

What proportion of ECF is circulating plasma in the lecture's diagram?

Approximately 20% of ECF.

How similar are the ionic compositions of plasma and interstitial fluid?

Their compositions are very similar.

How does ECF composition compare with ICF composition?

ECF composition differs greatly from ICF composition.

Are cells permeable to water?

Yes. The lecture states that all cells are permeable to H2O.

What governs water movement between ICF and ECF?

Osmosis.

What governs water movement between plasma and interstitial fluid?

Imbalances between capillary blood pressure, or hydrostatic pressure, and colloid osmotic pressure, or oncotic pressure due to protein.

What is capillary hydrostatic pressure in the context of fluid movement?

It is capillary blood pressure and is one of the forces governing water movement between plasma and interstitial fluid.

What is colloid osmotic pressure?

It is oncotic pressure generated by proteins and is one of the forces governing water movement between plasma and interstitial fluid.

What is osmosis?

Osmosis is the diffusion of water molecules across a selectively permeable membrane.

In which direction does water move during osmosis?

Water moves from an area of diluted solution toward an area of concentrated solution until the two sides reach osmotic balance.

What type of membrane is required for osmosis as defined in the lecture?

A selectively permeable membrane.

What is osmotic equilibrium?

The balanced state reached when osmotic water movement has equalized the relevant osmotic difference between the two sides of a selectively permeable membrane.

What is osmotic pressure?

The precise amount of pressure required to prevent osmosis.

What determines the osmotic pressure of a solution?

Osmotic pressure is proportional to the solution's osmolarity.

What does osmolarity reflect?

The concentration of osmotically active solute particles.

Why does one molecule of NaCl produce two osmotically active particles?

Because NaCl dissociates into Na+ and Cl−.

How many osmotically active particles are produced from one molecule of NaCl according to the lecture?

Two: Na+ and Cl−.

How many osmotically active particles are produced from one molecule of CaCl2 according to the lecture?

Three.

What is osmolarity?

The amount of osmotically active material, in moles, dissolved in one liter of solution.

What is osmolality?

The amount of osmotically active material, in moles, dissolved per kilogram of solvent.

What is the key difference between osmolarity and osmolality?

Osmolarity is expressed relative to a liter of solution, whereas osmolality is expressed relative to a kilogram of solvent.

Why must ECF volume be closely regulated?

To maintain arterial blood pressure.

What provides short-term regulation of blood pressure according to the lecture?

Baroreceptors and temporary fluid shifts between plasma and interstitial fluid.

What organ mainly regulates ECF volume?

The kidneys.

Why must ECF osmolarity be closely regulated?

To prevent cells from swelling or shrinking.

Which ions contribute approximately 90% of ECF osmolarity?

Na+ and its attendant anions.

What percentage of ECF osmolarity is attributed to Na+ and its attendant anions in the lecture?

Approximately 90%.

What happens to water when Na+ concentration or amount changes in the ECF?

Water follows Na+ to maintain osmotic equilibrium.

What relationship does the lecture give among salt, osmolarity, and water?

More salt → higher osmolarity → more water.