Fluid And Electrolyte Balance

Question 1

The main objectives of fluid and electrolyte balance are to maintain internal

Answer 1

1. Fluid volume
2. overall osmolarity levels of specific ions and
3. pH.
   All of these have consequences on the function of different organs or organ systems

Question 2

What percent of body weight is water and males and females and what percent is where inside the body

Answer 2

50%/30 L of body weight in females, 60%/42 L in males are made up of water. Most inside the cells 66%, 26% in interstitial fluids and 7% in plasma

Question 3

Maintaining proper levels follows the rules of mass balance

Answer 3

Intake and output are balanced

Question 4

We lose water through

Answer 4

Sensible/perceived and insensible/unperceived sweat, feces, lungs through exhalation, and a nearly obligatory amount of 1.5 L per day in urine production. Of these urine volume is the only thing that is able to be regulated

Question 5

In order to maintain proper water levels

Answer 5

We need to eat/drink the equivalent of about 2 L per day some 0.3 L per day is made through metabolic reactions examples cellular reaction and dehydration synthesis

Question 6

We will break this chapter into four parts

Answer 6

1. Water balance,
2. sodium potassium and ECF,
3. behavioral mechanisms,
4. acid base balance

Question 7

Ground rules (5)

Answer 7

1. Enough fluid/blood must maintain positive GFR at all times without over hydrating
2. Kidneys can only conserve water not create it the only input is through ingestion
3. Movement of water can only occur through osmosis not active transport pumps
   * 4. Permeability of water changes between the descending/increasing Permeability and ascending/ decreasing Permeability limbs of the nephron loop.
4. Osmolarity steadily increases from the cortical regions of the kidneys about 300 mOsm to the medullary region about 1200 mOsm, establishing a Gradient of Increasing osmolarity

Question 8

Water is conserved through reabsorption from

Answer 8

The nephrons since anything inside the tubule is technically considered outside the body if it’s not pulled back and you lose it for effort

Question 9

Recall that GFR is always

Answer 9

Positive pushing Isosmotic ~300mOsm of fluid into the tubules 

Question 10

To conserve water the

Answer 10

DCT and collecting duct together called the distal nephron alter their permeability to water and amount of sodium that is reabsorbed

Question 11

Thick regions of the ascending limb and distal nephron

Answer 11

Are normally more or less impermeable to water unless aqua Porins are present. Pores on the apical surface that allow water to enter the cells

Question 12

Thin regions of the descending Limb are comparatively

Answer 12

Permeable to water the increasing osmolarity of the kidney medulla creates the necessary gradient to allow water reabsorption but only when it’s allowed

Question 13

When water intake increases

Answer 13

Nephrons produce dilute urine (as low as 50 mOsm; hypoosmotic to ECF) by maintaining normal basal rates of sodium reabsorption and not reabsorbing as much water, more water is excreted. Basically it traps water in the tubule and doesn’t let it diffuse back in!

Question 14

When water intake decreases

Answer 14

Nephrons need to produce urine that is more concentrated (up to 1200 mOsm hyperosmotic) than the surrounding ECF which is a bit more complicated process. The distal nephron needs to re-absorb water without also reabsorbing more sodium than it needs to. Remember water can only move through osmotic gradients. 

Question 15

Vasopressin a.k.a. antidiuretic hormone ADH/ no Pee

Answer 15

Is a small nine amino acid peptide hormone produced by the posterior pituitary gland that modulate permeability of the distal nephron to water by instructing cells to insert aqua Porins Increasing Permeability. It’s effects graded an increase in vasopressin causes an increase of water reabsorption back into the body 

Question 16

Vasopressin initiates a

Answer 16

GPCR cAMP second messenger pathway that causes exocytosis of intracellular vesicular aquaporins (AQP2) into cells apical surface

Question 17

Stimuli for vasopressin Secretion

Answer 17

1. High plasma osmolarity greater than 280 mOsm: this is the most potent signal, osmo receptor neurons in the hypothalamus shrink in response to hyper osmolarity of ECF causing them to fire.
2. Low blood volume and blood pressure: less potent signal, barrow receptors in carotid artery and aorta detect low BV and BP levels increasing vasopressin secretion

Question 18

Sodium balance an ECF volume are intertwined. In the proximal tubule

Answer 18

 water freely follows sodium reabsorption. However water and sodium are independently regulated in the distal nephron when sodium is reabsorbed vasopressin must be present (aqua Porin ) for water to follow

Question 19

The main stimulus for vasopressin (water conservation) Secretion

Answer 19

Is high osmolarity where is the main stimuli for sodium reabsorption are low blood volume and blood pressure

Question 20

Aldosterone is a

Answer 20

• Steroid hormone produced by the adrenal cortex that regulates sodium reabsorption and secondly potassium Secretion.
• by Increasing activity of the sodium potassium pump‘s in the distal nephron
• the specific target are the principal P cells located there

Question 21

Aldosterone mechanism of action (4)

Answer 21

1. After secretion aldosterone diffuses from blood into P cells in distal nephron
2. The receptor ligand interaction acts on the nucleus to increase DNA transcription and synthesis of new sodium, potassium channels and sodium potassium pump‘s and proteins that act on existing ones
3. Sodium and potassium channels on the apical side are stimulated to stay open longer, new channels/pumps are inserted, more sodium is a reabsorbed and from filtrate and more potassium is secreted to filtrate
4. Sodium is pumped out and potassium is pumped and by the sodium potassium pump on the basal lateral side into/from ECF which then diffuses into the blood

Question 22

Stimulation for aldosterone Secretion falls into

Answer 22

Direct or indirect categories

Question 23

Direct stimuli for aldosterone secretion

3

Answer 23

1. Increase in potassium concentration in the extracellular fluid most common of these and an extreme cases namely dehydration
2. Increase osmolarity
3. Decrease sodium concentration
   * when these stimuli are detected they act directly on adrenal cells which secrete aldosterone

Question 24

The indirect stimulus for aldosterone Secretion is

Answer 24

A decrease in blood pressure which initiates the renin-angiotensin system (RAS) also known as the renin angiotensin aldosterone system (RAAS) and there are three ways that it can be initiated through detection of low BP.

Question 25

What are the three ways that the RAAS pathway (indirect) can be initiated through a decrease in BP

Answer 25

1. Directly by granular cells in JGA’s afferent arterioles
2. Indirectly by CVCC increases SNS output to stimulate granular cells
3. Indirectly by macula densa cells in JGA through decrease filtrate flow, sons paragon signals that stimulate granular cells. Conversely during increase filtrate flow macular densa cells will inhibit granular cells
   * However it happens when stimulated granular Cells will secrete the enzyme renin and that’s begin the RAS cascade

Question 26

The RAS cascade

Answer 26

Increase in renin > plasma angiotensin is converted to angiotensin I > converted to angiotensin 2 (by angiotensin converting enzyme ACE)
-ANG2 effects increasing BP (6)

Question 27

Angiotensin two has the following affects which act to increase blood pressure via the RAS cascade (6)

Answer 27

1. Increase secretion of aldosterone > increases osmolarity> increases thirst [&vasopressin secretion?] > increases extracellular fluid/blood volume
2. Increase the secretion of vasopressin> increases water reabsorbed in DCT
3. Increase sodium reabsorption in PCT through increase sodium hydrogen exchanger activity >increase osmolarity
4. Increase vasoconstriction
5. Increase SNS output on CVCC >increase cardiac output
6. Increased thirst

Question 28

While sodium and water reabsorption are handled by aldosterone and RAS the opposite sodium secretion and water loss are regulated by

Answer 28

Natriuretic hormones, Primarily atrial Natriuretic hormone ANP, but also brain natriuretic hormones BNP. Which are peptide hormones released by atrial and ventricular cells respectively. 
* BNP is used in hospital ER is to diagnose difficulty in breathing due to heart failure opposed to other causes and help indicate sudden death from cardiac arrhythmia

Question 29

The main stimulus for ANP secretion is

Answer 29

The stretching of the atrial walls which indicates an increase in blood pressure. ANP then initiate a GPCR CGMP second messenger pathway in target cells.

Question 30

ANP GPCR CGMP second messenger pathway in target cells causes (4)

Answer 30

1. Decrease sodium reabsorption in collecting ducts (direct effect)
2. Increase sodium excretion (indirect effect) by suppressing secretion of vasopressin, renin, and aldosterone
3. Vasodilation of afferent arterial >increases GFR> increases water excretion and decreases plasma volume.
4. Alter CVCC effects in medulla to decrease blood pressure