Intro to the Kidneys and Body Fluid

Question 1

Provide an overview of the fluid compartments in the body (ie. the percentages in each).

Answer 1

• TOTAL BODY WATER: 60% body weight
  
  • INTRACELLULAR WATER: 40% body weight (28L)
  • EXTRACELLULAR WATER: 20% body weight (14L)
• [Osmosis determines the movement of water between the ICF and the ECF.]

Question 2

What are the two sub-compartments of the ECFV (extracellular fluid volume)?

Answer 2

• plasma (3L)
• interstitial fluid (11L)

Starling’s forces determine fluid and solute movement between the plasma and the interstitial fluid.

[The main difference between plasma and interstitial fluid is that plasma contains more proteins whereasinterstitial fluid contains fewer proteins.]

Question 3

What is osmolarity, and how does it relate to the ICF and ECF?

Answer 3

• Osmolarity is the total concentration of osmotically active solutes present in one litre of a solution.
• Solute composition of the ECF and ICF differs, but osmolarity must be kept the same to avoid excessive shifts of water between the ECF and ICF.
• Sodium is the principle electrolyte of the ECF, and therefore sodium (with the associated anions) is the major determinant of ECF osmolarity.

Question 4

Why is the control of bodily fluids important?

Answer 4

• it’s important for cell structure and function Large shifts between the ECFV and ICFV will disrupt tissue structure and function. - it’s important for tissue perfusion This depends on the balance between circulating volume (plasma) and interstitial volume.

Question 5

What two key processes does salt and water balance depend on?

Answer 5

• OSMOREGULATION: maintains the osmotic equilibrium between the ICFV and ECFV - VOLUME REGULATION: maintains adequate ECFV to support plasma volume

Question 6

Describe osmoregulation.

Answer 6

It is the control of ECFV osmolarity in order to maintain osmotic equilibrium between the ICFV and ECFV. Sodium (with its associated anions) is the principal electrolyte contributing to ECFV osmolarity.

Question 7

How is plasma osmolarity estimated?

Answer 7

It can be estimated by: 2[Na] + 2[K] + [glucose] + [urea] (all units in mmol/L)

Question 8

Describe the mechanism of osmoregulation.

Answer 8

There are two ways to change the osmolarity of a solution: 1) add/ remove solute 2) add/ remove water The body accomplishes osmoregulation by adding or removing water, NOT sodium. - if plasma osmolarity rises, more water is needed (the kidneys will respond by producing a small volume of concentrated urine) - if plasma osmolarity falls, there is too much water (the kidneys will respond by producing a large volume of diluted urine)

Question 9

Describe volume regulation.

Answer 9

• It refers specifically to the control of the circulating (plasma) volume.
• The changes are detected by stretch and pressure receptors in the cardiovascular system.
• A fall in blood volume is opposed by sodium retention (as the water will follow osmotically, restoring volume). [So the kidneys retain sodium and water osmotically follows]

NOTE: although the total AMOUNT of body sodium may be increased, CONCENTRATION (and hence osmolarity) is little changed because the retained sodium brings water with it.

Question 10

Describe the kidneys and the urinary tract.

Answer 10

The function of the kidney is homeostasis. The production of urine is a by-product of kidney function. The urinary tract is important for temporary storage and then to remove urine from the body.

Question 11

List some functions of the kidney.

Answer 11

• osmoregulation - volume regulation - acid-base balance - regulation of electrolyte’s balance (eg. K+, Ca2+, PO4 3-) - removal of metabolic waste products from the blood - removal of foreign chemicals in the blood (eg. drugs) - regulation of RBC production (erythropoietin)

Question 12

Describe the nephron in the kidney.

Answer 12

The nephron consists of special blood vessels and elaborate tubules (tiny tubes). They are microscopic structures, so there are 1.25 million per kidney. It is here when urine production begins. Each nephron consists of several major structures: - blood vessels - the glomerulus - Bowman’s capsule - the renal tubule

Question 13

List the arrangement of blood vessels and blood flow through the kidney and connection wit nephrons.

Answer 13

1) Renal artery 2) Segmental arteries 3) Interlobar arteries 4) Arcuate arteries 5) Interlobular arteries 6) Afferent arterioles 7) NEPHRONS 8) Venules 9) Interlobular veins 10) Arcuate veins 11) Interlobar veins 12) Renal vein

Question 14

List the four basic processes of urine formation.

Answer 14

1) Glomerular Filtration 2) Tubular Reabsorption 3) Tubular Secretion 4) Excretion of water and solutes in the urine

Question 15

Describe the first step of urine formation.

Answer 15

GLOMERULAR FILTRATION

• The glomerular filtration rate:
  
  • is the first step in the production of urine
  • is the amount of filtrate kidneys produce each minute
  • it averages 125 ml/min (approximately 20% of renal plasma flow)
  • is reduced in renal failure
  • plasma creatinine can be used as an index of GFR
• Hydrostatic pressure forces fluids and solutes through the glomerular capillary membrane.
• Small molecules pass readily - large ones (proteins) and cells cannot pass.
• This leads to a plasma ultrafiltrate in the Bowman’s Capsule.

Question 16

Describe the second step of urine formation.

Answer 16

TUBULAR REABSORPTION Many substances are filtered and then reabsorbed from the tubular lumen into the peritubular capillaries.

Question 17

Why is the third step of urine formation important?

Answer 17

Tubular secretion is important for: - disposing of substances not already in the filtrate - eliminating undesirable substances such as urea and uric acid - ridding the body of excess K+ - controlling the blood pH

Question 18

How do we calculate total body water balance?

Answer 18

Balance = INPUT - OUTPUT

Question 19

Describe the physiological responses to water restriction.

Answer 19

• loss of water (sweat, breathing) - thirsty, but no water is available to drink
• plasma osmolality rises - the response is increased secretion of ADH
• This results in:
  
  • decreased urine volume
  • increased urine osmolality

Question 20

Describe the physiological responses to an increase in water intake.

Answer 20

There is an increase in water absorption through the GIT. - plasma osmolality falls - the response is reduced secretion of ADH This results in: - increased urine volume - decreased urine osmolality

Question 21

What are the two major systems of importance in the control of sodium balance?

Answer 21

• sodium retaining systems: the renin-angiotensin-aldosterone system (RAAS) - sodium eliminating pathways: atrial natriuretic peptides (ANP)

Question 22

Describe the hormonal and renal responses to increases in sodium intake.

Answer 22

With an increase in sodium intake, ECFV is increased. This does two things: - increases ANP - decreases RAAS These two combined result in increased sodium excretion from the kidneys.

Question 23

Describe the hormonal and renal responses to decreases in sodium intake.

Answer 23

With a decrease in sodium intake, ECFV is decreased. This does two things: - decreases ANP - increases RAAS These two combined result in decreased sodium excretion.