An Introduction to the Kidneys and Body Fluids

Question 1

What does osmolarity/osmolality refer to?

Answer 1

Refer to the same thing - the total concentration of osmotically active particles in a solution

Question 2

How can osmolarity be measured/calculated?

Answer 2

Osmolarity can be measured directly in a lab - has the units mOsmol/Kg pure water
Osmolarity can be calculated from a blood test by adding the [ ] of main solutes - has the units mOsmol/L of solution

Question 3

Why do osmolarity/osmolality values differ slightly?

Answer 3

Values expressed as osmolality or osmolarity will slightly differ as one L of solution has slightly less water than one L of pure water

Question 4

Give an example to show that in electrolytes, anions and cations contribute to molarity independently

Answer 4

E.g. 100 mmol/L NaCl = 100 mmol/L
Na+ + 100 mmol/L Cl- = 200 mmol/L

Question 5

Give an example to show that non electrolyte solutes contribute to osmolarity the same as molar concentration

Answer 5

E.g. 5 mmol/L of glucose gives 5 mOsmol/L

Question 6

Give an example of how the principle of electroneutrality allows us to not need to count anions when we have cation values

Answer 6

E.g. 140 Na+ = 140 of an anion (A-) = 280 for anion and cation

Question 7

What is the body composition of water in males and females?

Answer 7

Body composition is approximately 60% water in males and 50% in females

Question 8

How is body water distributed in the main compartments?

Answer 8

This water is distributed in two main fluid compartments - ICF and ECF - 2/3rds of body water is intracellular and 1/3rd is extracellular

Question 9

Explain why ICF and ECF must be in osmotic equilibrium

Answer 9

• The cell membrane is semipermeable
• It is permeable to water via aquaporins but it impermeable to most solutes
• Change in solute concentration in either ICF or ECF will create an osmotic gradient which shifts water between compartments
• The ICF has K+ with associated anions as its main electrolyte while ECF has Na+ with associated anions as its main electrolyte

Question 10

Why must large shifts in water between ICF and ECF be avoided?

Answer 10

To prevent changes in cell volume and the most serious complications can be neurological

Question 11

What is osmoregulation

Answer 11

A physiological process that maintains constant ECF osmolarity

Question 12

Why is regulation of the ECF volume important?

Answer 12

To ensure effective circulating volume (adequate blood volume and pressure for perfusion of tissues)

Question 13

What is the ECF compartment subdivided into?

Answer 13

• Interstitial (extravascular) compartment (75% of ECF)
• Plasma/vascular compartment (25% of ECF)

Question 14

What is volume regulation and how is it maintained?

Answer 14

Volume regulation is control of the ECF volume to ensure appropriate plasma volume (maintained by balance of starling forces between extravascular and vascular)

Question 15

Summarise by defining osmoregulation and volume regulation

Answer 15

• Osmoregulation is control of salt concentration by adjusting the amount of pure water in the body
• Volume regulation - control of the amount of salt and water in the ECF and hence ECF volume
• Many organs and systems involved but kidney is central to both processes

Question 16

State the central and byproduct functions of the kidneys

Answer 16

• Homeostasis is the central function of the kidney
• The production of urine is a by product of this central function

Question 17

State the renal functions

Answer 17

• Osmoregulation
• Volume regulation
• Acid base balance
• Regulation of electrolyte balance e.g. potassium, calcium, phosphate
• Removal of metabolic waste products from blood
• Removal of foreign chemicals in the blood e.g. drugs
• Regulation of red blood cell production (erythropoietin)

Question 18

Describe the structural organisation of the nephron

Answer 18

Renal tubule -
- Bowman’s capsule
- Proximal convoluted tubule
- Loop of Henle
- Distal convoluted tubule
- Collecting duct - drains into ureter

Blood vessels -
- Afferent arteriole
- Glomerulus
- Efferent arteriole
- Peritubular capillaries - surround proximal and distal tubules
- Vasa recta - surrounds the loop of Henle

Question 19

State the 4 basic processes of renal function

Answer 19

1. Glomerular filtration
2. Tubular reabsorption
3. Tubular secretion
4. Excretion of water and solutes in the urine

Question 20

Describe glomerular filtration (what drives it, what it forms)

Answer 20

• Balance of starling forces drive water and solute across the capillary membrane
• In the glomerulus there is hydrostatic pressure due to blood pressure which is opposed by oncotic pressure and hydrostatic pressure of bowman’s capsule and interstitial space - net hydrostatic pressure favours filtration
• Small molecules pass readily - large ones and cells cannot pass
• This leads to a plasma ultrafiltrate in the bowman’s capsule the first step in the production of urine

Question 21

Define glomerular filtration rate and why it is an important measure

Answer 21

• The amount of filtrate produced by the kidneys each minute
• Averages 125 ml/min
• Very important clinical indicator (reduced in renal failure)

Question 22

What can be used as an indicator of GFR?

Answer 22

Plasma creatinine can be used as an index of GFR

Question 23

Describe tubular reabsorption and where it occurs

Answer 23

• Many substances are filtered and then reabsorbed from the tubular lumen into the peritubular capillaries
• About 70% of filtered salt and water reabsorbed from proximal tubule
• 20-25% is from the loop of Henle
• Variable fraction of remaining 5-10% is reabsorbed from distal tubule and collecting duct

Question 24

Describe tubular secretion - what it does and what it is important in

Answer 24

• This is important for disposing substances beyond their level in the filtrate
• Eliminating toxins and metabolic by products
• Important in potassium balance - excess K+ secreted in distal tubule and collecting duct and in acid base balance relies on H+ secretion in distal tubule and collecting duct

Question 25

Describe the excretion of water/solutes in the urine (how it can be calculated and what it can be used to estimate)

Answer 25

• The tubular fluid remaining after filtration, reabsorption and secretion is excreted as urine
• The amount excreted = (the amount filtered - amount reabsorbed) + amount secreted
• If none is reabsorbed or secreted then amount excreted = amount filtered
• This can be used to estimate GFR

Question 26

Describe what happens when water is added to the ECF

Answer 26

• If water is added to the ECF then this dilutes it (changes osmolarity ie the concentration) and it expands
• As it is diluted the water potential is higher so water moves down the gradient and into the ICF
• This then causes the ICF to expand so both have expanded

Question 27

Describe the steps that lead to homeostasis following changes in water intake/excretion

Answer 27

• If there is a change in water intake or excretion there is a change in body fluid osmolality
• This is detected by sensors (osmoreceptors in the anterior hypothalamus)
• This then sends signals to the effector- vasopressin (ADH) secreted by posterior pituitary gland is changed
• This then changes the renal water excretion which changes the output to match the intake

Question 28

Describe the physiological response to water restriction

Answer 28

• Loss of water (skin, lungs)
• Plasma osmolality rises

Response is -
- Increased thirst
- Increased secretion of hormone, ADH

ADH increases renal water reabsorption -
- Decreased urine volume
- Increased urine osmolality

Question 29

Describe the physiological response to water intake increasing

Answer 29

• Increase in water absorption through GI tract -
• Plasma osmolality falls

Response is -
- Decreased thirst
- Reduced secretion of ADH

This results in:
- Urine volume increases
- Urine osmolality decreases

Question 30

What happens in extremes of water intake?

Answer 30

• It may be that a person has not consumed enough water and so produces a small volume of very concentrated urine but continues not to drink
• We cannot avoid losing less than 0.5L a day of urine minimum so the kidneys ability to change absorption of water can no longer accommodate and osmolarity changes
• It may also be that a person that has consumed too much water and so is producing large volumes of very dilute urine already consumes more and the kidneys ability to produce urine is outdone so it cannot accommodate and osmolarity changes
• Changes in osmolarity then cause fluid to shift between ECF and ICF

Question 31

How is volume of ECF regulated

Answer 31

• ECF volume is determined by the amount of sodium in this compartment
• Sodium intake and excretion must be balanced to maintain constant ECF volume
• ECF has Na+ and associated anions as its main electrolyte that determines its osmolarity
• When NaCl and water are added to ECF it does not change osmolarity as concentration has not changed so the salt and water are retained in the ECF
• There is no osmotic gradient so only the ECF is expanded
• If there is a change in the amount of sodium in the ECF there is a change in ECF volume
• This is detected by sensors (volume/pressure sensors in the CVS)
• This then signals to hormonal systems that change the renal sodium and water excretion to match output with intake

Question 32

Describe the hormonal system involved in sodium balance

Answer 32

Renin angiotensin aldosterone system (RAAS)
- Increases renal Na reabsorption
- Increases ECF volume

Cardiac natriuretic peptides (ANP) - secreted as response to stretch of the atria due to increased venous return which is interpreted as an increase in blood volume
- Decreases renal Na reabsorption
- Decreases ECF volume