Module 4 Section 1

Question 1

explain the kidneys

Answer 1

involved in many homeostatic functions including regulating electrolytes, acid-base control, blood volume control, and regulation of blood pressure

Question 2

water balance

Answer 2

• every cell in the body requires a well-regulated environment in order to survive and function
• body water plays a large role in creating this environment, which is actually comprised of two major compartments

Question 3

two major components of body water

Answer 3

1. intracellular fluid (ICF)
2. extracellular fluid (ECF)

Question 4

intracellular fluid (ICF)

Answer 4

the fluid within cells and comprises about two thirds of total body fluid

Question 5

extracellular fluid (ECF)

Answer 5

• this is the fluid surrounding the cells which includes the plasma, the interstitial fluid, lymph, and transcellular fluid (like cerebrospinal fluid) and comprises one third of total body fluid
• the plasma is about one fifth of the ECF, interstitial fluid is about four fifths of the ECF, and lymph and transcellular fluid are considered negligible

Question 6

barriers between body-fluid compartments

Answer 6

• several barriers separate the body-fluid compartments
• this limits the movement of water and solutes between the various compartments to differing degrees

Question 7

barriers between the plasma and the interstitial fluid

Answer 7

• plasma and interstitial fluid are separated by the blood vessel walls
• at the level of capillaries, water and everything else in the plasma (except proteins) can freely exchange with the interstitial fluid
• the composition of the plasma and the interstitial fluid are essentially identical, expect for the plasma proteins
• any changes in one of these compartments is quickly reflected in the other compartment

Question 8

barriers between the intracellular fluid and the extracellular fluid

Answer 8

• this barrier is the plasma membrane that surrounds each cell in the body
• the ICF contains proteins that do not exchange with the ECF
• there is an unequal distribution of ions across this barrier

Question 9

example of unequal distribution of ions across ICF/ECF barrier

Answer 9

• the concentration of K+ is greater in the ICF while the concentration of Na+ is greater in the ECF
• this is because the barrier does not allow the passive movement of either ICF or ECF constituents across the plasma membrane, preventing them from equilibrating through the process of diffusion

Question 10

ECF volume and osmolarity

Answer 10

• all exchanges of water and other constituents between the ICF and the external world are dependent upon the ECF
• even though cells tightly regulate their own ICF, overall control of fluid balance is dependent upon regulating the ECF

Question 11

two factors regulated to maintain fluid balance in the body

Answer 11

1. ECF volume - closely regulated to maintain blood pressure. the maintenance of salt balance is important in the long-term regulation of ECF volume
2. ECF osmolarity - closely regulated to prevent the swelling or shrinkage of cells

Question 12

effect of ECF volume on arterial blood pressure

Answer 12

• ECF volume directly influences blood pressure by changing plasma volume
• increasing ECF volume will increase plasma volume, and increase arterial blood pressure
• there are mechanisms in place to adjust blood pressure until the ECF volume is returned to normal values

Question 13

short-term control factors

Answer 13

1. baroreceptor reflex
2. fluid shifts

Question 14

long-term control factors

Answer 14

fluid input/output
- the short-term control mechanisms are only temporary and can only compensate for fairly minor changes in ECF volumes
- long-term regulation of blood pressure is a primary function of the kidneys and the thirst mechanism, controlling fluid output and input
- control of urine output by the kidneys is critical for long-term regulation of blood pressure

Question 15

baroreceptor reflex

Answer 15

• baroreceptors are mechanoreceptors that are located in the carotid artery and the aortic arch (areas within major arteries of the body), and they detect changes in arterial blood pressure
• through the effects of the autonomic nervous system on the heart and blood vessels, the baroreceptor reflex regulates blood pressure
• when pressure falls too low, CO and TPR will increase to raise blood pressure
• when blood pressure rises above normal, both decrease to reduce blood pressure

Question 16

fluid shifts

Answer 16

• a decrease in plasma volume can temporarily be compensated for by a shift of the fluids out of the interstitial compartment to the plasma
• the opposite is also true, an increase in plasma volume can cause fluid to shift to the interstitial compartment

Question 17

salt input

Answer 17

• regulation of salt input is poor as it is dependent upon dietary salt
• it is necessary on a daily basis to replace the salt lost in the feces and sweat, which is about 0.5g/day for normal activity levels (higher activity levels will increase the amount lost in sweat)
• average input for Canadians is around 3.5g/day

Question 18

control of salt

Answer 18

• 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
• by controlling salt levels, ECF volume is being controlled
• to maintain salt levels, salt input must equal salt output

Question 19

salt output

Answer 19

• the excess salt must be eliminated, which occurs in the kidneys
• the 3 pathways to eliminate salt are in the feces, sweat, and kidneys
• the kidneys have the greatest role in salt elimination

Question 20

control of ECF osmolarity

Answer 20

• osmolarity: a measure of the concentration of a particular solute in solution
• high osmolarity: more solute, less water in solution
• water moves down its concentration gradient until the osmotic pressure across a membrane is equalized
• regulating osmolarity is very important in preventing changes in cell volume

Question 21

decrease in water in the ECF

Answer 21

• causes osmolarity to increase and ECF is hypertonic
• 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

Question 22

increase in water in the ECF

Answer 22

• the osmolarity would decrease and the ECF would become hypotonic (lower osmotic pressure than the surrounding cells)
• 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 ECF were very hypotonic, the amount of water moving into cells would cause them to burst

Question 23

hypotonicity

Answer 23

• hypotonicity of ECF is usually associated with over-hydration, or excess free H2O
• has 3 major causes

Question 24

3 major causes of hypotonicity

Answer 24

1. renal failure - individuals are not able to produce a concentrated urine
2. rapid water ingestion - can occur in healthy individuals 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 25

causes for hypertonicity

Answer 25

- insufficient water intake, or not drinking enough - diabetes insipidus, which involves a deficiency in vasopressin - excessive water loss due to heavy sweating during extreme exercise, prolonged bouts of vomiting, or diarrhoea

Question 26

isotonic

Answer 26

an isotonic solution has an equal osmolarity to that or normal body fluids

Question 27

into what body water compartment the saline solution being injected

Answer 27

injected into the blood plasma within the veins, which is part of the ECF compartment and makes up about one-fifth of the ECF

Question 28

when isotonic fluid is injected into the ECF

Answer 28

- ECF volume increases - solute concentration stays the same (remains isotonic) - osmolarity doesn't change, ECF and ICF stay in osmotic equilibrium - no net fluid shift occurs between compartments and cells neither shrinks nor swells

Question 29

impact of isotonic fluid loss on cells

Answer 29

- fluid loss is restricted to the ECF - there is no osmotic gradient created - no fluid shifts occur between ECF and ICF - cells remain unchanged in size

Question 30

regulation of water balance step 1

Answer 30

- 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 increases, both vasopressin secretion and thirst are stimulated

Question 31

regulation of water balance step 2

Answer 31

- the vasopressin acts on the kidneys to increase 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

Question 32

regulation of water balance step 3

Answer 32

- large losses of ECF volume can also impact these pathways - within the eft atrium of the heart are 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