Module 4 Section 1 (Intro to Renal PHYG & Water Balance)

Question 1

Identify the major fluid compartments and describe how they are interrelated.

Answer 1

1) Intracellular fluid (ICF):
- Fluid within cells
- Comprises of about 2/3 of total body fluid

2) Extracellular fluid (ECF):
- Fluid surrounding the cells, which includes the plasma, interstitial fluid, lymph and transcellular fluid (ex: cerebrospinal fluid)
- Comprises 1/3 of total body fluid
• The plasma = 1/5 of ECF
• Interstitial fluid = 4/5 of ECF
• Lymph and transcellular fluid are considered negligible

Question 2

**Compare and contrast short-term vs long-term control of the ECF.

Answer 2

Short-Term Control Factors:
1) The baroreceptor reflex:
- Located in the carotid artery and carotid arch (areas within major arteries of the body) which detect changes in arterial BP
- Through the effects of the ANS on the heart and blood vessels, these receptors regulates BP
• When BP falls too low, CO and total peripheral resistance will incr to raise BP
• When BP is higher than normal, both decr to reduce BP
2) Fluid Shifts
- A decr in plasma volume can temporarily be compensated for by a shift of the fluids -> out of the interstitial compartment -> plasma
- The opposite it also true (incr in plasma volume = fluid -> interstitial compartment

Long-Term Control Factors:

1) Fluid Input/Output
- Long-term regulation of BP is a primary function of the kidneys and the thirst mechanism, controlling fluid output and input, respectively.
- Control of urine output via the kidneys is critical for long-term regulation of BP

Question 3

Describe the differences b/w isotonic, hypertonic, and hypotonic, and how cells in each of these solutions would be affected.

Answer 3

Isotonic solutions

• They are solutions having equal osmotic pressures
• They have equal solute conc
• Isotonic environments show no effect on cells

Hypotonic solutions

• They are solution having lower osmotic pressures
• They have a low conc
• Hypotonic environments can cause cells to swell

Hypertonic solution:

• They are solutions having comparatively higher osmotic pressures
• They have a high conc
• Hypertonic environments cause cells to shrink.
• The cellular consequences of hypertonicity generally relate to a decr of normal cell function as the ICF is decreased. The brain is particularly sensitive and shrinking of neurons can cause confusion, delirium, and even coma or death.

Question 4

Describe the pathways involved in the regulation of water balance in terms of intake and output.

Chart on slide 15

Answer 4

1) Within the hypothalamus, near the vasopressin-secreting cells and thirst centre, are hypothalamic osmoreceptors which constantly monitor the osmolarity of the fluid surrounding them.
- As osmolarity incr, both vasopressin secretion and thirst are stimulated.

2) The vasopressin acts on the kidneys to incr water reabsorption, while thirst stimulates the intake of water to the body through drinking.
- This continues until the hypertonicity is relieved.
- If the fluid around the osmoreceptors is hypotonic, then vasopressin secretion and thirst are not stimulated, promoting water loss.

3) Large losses of ECF volume can also impact these pathways.
- Within the left atrium of the heart are what are called left atrial volume receptors, which monitor the pressure of the blood in the left atrium.
• They are activated when there is a greater than 7% loss of ECF volume and blood pressure.
• Once activated, they also stimulate the hypothalamic pathways to stimulate vasopressin release and thirst.

Question 5

What is the importance of the kidneys? (7)

Answer 5

Regulates:

1) electrolytes
2) Acid-base control
3) Blood volume control
4) Regulation of blood pressure

Maintains the stability of:

1) ECF volume
2) electrolyte composition
3) Osmolarity

Question 6

What are the 3 major body compartments for water (fluid)?

Answer 6

1) ICF
2) Plasma
3) Interstitial fluid

*They remain distinct due to their presence of “barriers” b/w them

Question 7

Several barriers separate the body-fluid compartments. This limits the movement of water and solutes b/w the various compartments to differing degrees. What are these barriers?

Answer 7

1) Barriers b/w the plasma and interstitial fluid:
- Separated by blood vessel walls
- At the level of capillaries, water and everything else in plasma (except proteins) can freely exchange w/ the interstitial fluid
• B/c of this, the composition of the plasma and interstitial fluid are essentially identical (except for the plasma proteins)
• Thus, any change in one of these compartments will quickly be reflected in the other compartments.

2) Barriers b/w the ICF and ECF
- This barrier = the plasma membrane that surrounds each cell in the body
- ICF contains proteins that don’t exchanges w/ ECF
- There’s an unequal distribution of ions across this barrier
• Ex: the conc of K+ is > in the ICF and the conc of Na+ is > in the ECF -> this is b/c the barrier doesn’t allow the passive movement of either ICF or ECF constituents across the plasma membrane, thus preventing them from equilibrating through diffusion.

Question 8

True or false: all exchanges of H2O and other constituents between the ICF and the external world are dependent upon the ECF.

Answer 8

True

Even though cells tightly regulate their own ICF, it can be said that overall control of fluid balance is dependent upon regulating the ECF

Question 9

In order to maintain fluid balance in the body, 2 factors are regulated. What are they?

Answer 9

ECF Volume:
- This is closely regulated to maintain BP by changing plasma volume.
• Increasing ECF volume will incr plasma volume, this increasing arterial BP
- The maintenance of salt balance is important in the long-term regulation of ECF volume.

ECF Osmolarity:
- This is closely regulated to prevent the swelling or shrinkage of cells.

Question 10

How is salt balance maintained through salt input and output.

Answer 10

Sodium, and the anions that are associated with it (mainly chloride), account for more than 90% of the ECF’s solutes. Whenever salt is transported across a membrane, water follows due to osmosis. Therefore, by controlling salt levels, ECF volume is being controlled.

1) Salt input:
- Regulation is dependent upon dietary salt intake.
- It’s necessary to replace the salt lost in feces and sweat daily (about 0.5g/day for normal activity levels; higher activity levels will incr the amount lost in sweat, thus more salt should be consumed.

2) Salt output:
- The excess salt must be eliminated, which occurs in the kidneys.
- There are 3 pathways to eliminate salt: through the feces, sweat and the kidneys

Question 11

What is osmolarity?

Answer 11

Osmolarity is defined as a measure of the conc of a particular solute in solution.

Question 12

What does a high osmolarity mean?

Answer 12

A high osmolarity means that there is more solute, and therefore less water, in solution.

Water moves down its concentration gradient until the osmotic pressure across a membrane is equalized. Because of this, regulating osmolarity is very important in preventing changes in cell volume.

Question 13

Define the term hypertonic.

Answer 13

A hypertonic solution is one in which the concentration of solutes within that solution is greater than that of another solution that is separated by a membrane.

Question 14

Take a situation where there is a decr in water in the ECF. This causes the osmolarity to incr and the ECF is what we call hypertonic.

Since there is normally a balance in the osmolarity of the ICF and ECF, hypertonicity of the ECF causes water to move out of the cells into the ECF until the osmotic pressure is equalized. This loss of water from cells can cause them to shrink.

Using this knowledge, what do you think would occur if there was an increase in water in the ECF?

Answer 14

If there were an incr in water in the ECF, the osmolarity would decrease and the ECF would become hypotonic, meaning it would have a lower osmotic pressure than the surrounding cells.

This would result in water moving into the cells until the osmotic pressures were equalized.
- This movement of water into the cells would cause them to expand.

If the ECF were very hypotonic, the amount of water moving into cells would cause them to burst.

As already mentioned, the cellular consequence of hypotonic ECF is the swelling, which impairs cellular function.

Question 15

Hypertonicity of the ECF, the excessive concentration of ECF solutes, is usually associated with dehydration and has three major causes.

Using what you have learned thus far in the course as well as past experience, come up with a few causes for hypertonicity. (3)

Answer 15

1) Insufficient water intake, or not drinking enough.
2) Diabetes insipidus, which involves a deficiency in vasopressin.
3) Excessive water loss due to heavy sweating during extreme exercise, prolonged bouts of vomiting, or diarrhea.

Question 16

Hypotonicity of the ECF is usually associated with overhydration, or excess free H2O, and has three major causes. What are they?

Answer 16

1) Renal Failure: these ppl aren’t able to produce a concentrated urine.
2) Rapid Water Ingestion: this can occur in healthy ppl if they drink volumes of water in excess of what the kidneys can deal with in a timely manner.
3) Over Secretion of Vasopressin: vasopressin promotes water retention

Question 17

Suppose that a patient with a normal osmotic balance is given a therapeutic IV administration of an isotonic solution.

Using what you have learned thus far, answer the following questions:

1) What do you think is meant by isotonic?
2) Into what body water compartment is the solution being injected?
3) What will be the effect on the volume and conc of solutes in this compartment, and how do you predict this would impact cells?
4) How do you think isotonic fluid loss, such as haemorrhage, would impact cells?

Answer 17

1) An isotonic solution has an equal osmolarity to that of normal body fluids.
2) In this case, the saline solution is being injected into the blood plasma within the veins, which makes up approximately 1/5 of the ECF.
3) When isotonic fluid is injected into the ECF compartment, ECF volume incr, but the conc of ECF solutes remain unchanged; the ECF remains isotonic. Since the ECF osmolarity has not changed, the ECF and ICF are still in osmotic equilibrium, and there is no net fluid shift b/w the 2 compartments. Cells would neither shrink nor swell, illustrating the need for intravenous fluid therapy to be isotonic in order to prevent fluctuations of intracellular volume.
4) Similarly, in isotonic fluid loss, the loss is confined to the ECF (with no corresponding loss of fluid from the ICF). No osmotic gradient is created that would result in net fluid shifts.

Question 18

Select whether the following measures act in the short term and/or long term to adjust BP through control of the ECF:

• Baroreceptor reflex
• The kidneys’ control of urine output
• Fluid shifts in/out of the interstitial compartments
• The body’s control of the thirst mechanism
• A change in the cardiac output and total peripheral resistance

Answer 18

• Baroreceptor reflex = short-term control
• The kidneys’ control of urine output = long-term control
• Fluid shifts in/out of the interstitial compartments = short-term control
• The body’s control of the thirst mechanism = long-term control
• A change in the cardiac output and total peripheral resistance = short-term control