Elements of Renal Function, Measurements - Lecture 26

Chapter Links and Quizzes

Renal Function

Body Fluids

Control of Fluid

Renal Structure

Function

Audio - Lecture -

Elements of Renal Function

Important Theme - regulation of internal environment; homeostasis

Most important substances in body :

water

glucose

sodium

calcium } electrolytes

potassium

H⁺ ion

Kidneys - are responsible for regulation -- controlled via excretion

Body fluids - total water 42 L or 60% bodyweight (70 k person)

1)Extracellular fluid - outside of cell 33% - 14 l

a. intercellular - around cells; also interstitial fluid - 80% -11 l

b. plasma 20% - 3 l (5 l of blood with 40% hematocrit)

2) intracellular - fluid inside cells 67% -28 l

intracellular -- normally serves as reservoir or sink for water and ions; if too dramatic change - body alters input or output

input -

1. ingestion (drink and food)

2. product of metabolism - only H₂O, H⁺

output -

1. excretion

2. consumption

Na⁺, K⁺ - only ingestion

Water:

Input :

Drink -                              1200 ml/day

food i.e. steak -                1000 ml/day

metabolically produced   350 ml/day

total                                 2550 ml/day

output:

Insensible (skin + lung) 900 ml/day - evaporation

Sweat                         50 ml/day

in feces                          100 ml/day

urine                          1500 ml/day

total                          2550 ml/day

If exercise hard - reduced urine output is obvious - but how is this controlled

Input = Output

What are the precise regulatory mechanisms?

Thirst and Urinary Loss....

Is thirst really controlled?

-not in all cases, water intake can be more habit and sociological factor - drink fluid while talking -i.e. coffee

Thus urinary output single most important mechanism to control fluid - kidney

In addition,

-Kidney important in balancing ions

Audio - Lecture -

-Kidney important in removing wastes i.e.

1. CO₂ removed by lungs some via kidneys -- i.e. H⁺

2. protein - urea - kidney

urea- toxic to the brain - same for uric acid and creatinine

3. nucleic acids - uric acids - kidney

4. creatine (muscle) - creatinine - kidney

foreign chemicals: pesticides, drugs, etc.

Additional function -- list of functions in book

Structure

-paired organs - Mash - Frank Burns was going to remove a kidney - man only had one - remove second - die; cannot clean out blood or control water or electrolytes then }dialysis

1-2 million nephrons/kidney - filtering component

blood vessels

nerves

connective tissue

Nephron - glomerulus and tubules and blood vessels

microscopic structure

Audio - Lecture -

Basic Renal Functions

A) Glomerular Filtration

1. high pressure in glomerular capillaries - fluid moves out

due to efferent arterioles after glomerular capillaries

Pressure in capillaries = 55 mm Hg ---- normal capillary pressure is 35 - 15 mm Hg

high pressure favors filtration -- Starlings Law for filtration

m = KA [(Pc - Pt) - (pp - pt)]

m - fluid movement

K - filtration coefficient - tight in muscle, loose in intestine, p - osmotic forces due to interstitial fluid proteins

m = (55 -15) -(30 -0) = +10 mm Hg - net filtration pressure

Pt = 15 = pressure in Bowman's capsule

pp = 30 = osmotic pressure in capillaries due to proteins

pt = 0 = no proteins in Bowman's capsule

2. thin membranes - low molecular (small) substances; diffuse out, therefore, same concentration in plasma and Bowman's capsule

but no plasma proteins (too big)

filtrate = water, protein free, electrolytes, same concentration as plasma (equilibrium)

diseased kidney - protein in urine, maybe after heavy exercise get blood

fluid filtered 180 l/day, 45 gallons, 450 lbs.

what was net across all other capillaries - 3 l/day

total volume of plasma = 3 l

5 l blood x 60% (plasma) = 3 l

therefore, entire plasma volume is filtered by the kidney 60 times/day - if no re-absorption drains blood in 30 minutes

Urine output is clearly not so large (1.5-2.0 L/day) -- what is happening?

The final urine is very different from this glomerular filtrate

-composition is altered as it flows through tubules

B) Tubular Reabsorption

-tubule to peritubular capillary (water, glucose, Na⁺, Ca⁺⁺, AA)

C) Tubular Secretion

capillary to tubule (H⁺, uric acid, potassium, penicillin (dose every 4 hours))

some substances can be secreted but not reabsorbed - others reabsorbed but not secreted

each substance is handled in a unique manner

many variables i.e. rate of filtration, absorption and secretion that determines final concentration

REVIEW QUESTIONS

1. The amount of fluid found in the extracellular compartments of a 50 Kg individual is:
   1. 20 L
   2. 17 L
   3. 14 L
   4. 10 L
   5. 7 L
2. Which of the following is incorrect about the kidneys and waste removal?
   1. proteins are removed as uric acid
   2. creatinine is removed as creatine
   3. CO₂ is removed as H⁺
   4. urea must be removed via the kidneys because it is toxic to the brain
   5. pesticides and drugs can be removed via the kidneys
3. Which of the following is not part of the kidney?
   1. Bowman's space
   2. Seminiferous tubules
   3. The glomerulus
   4. The nephron.
   5. Collecting tubules
4. The amount of urine excreted is much less than the amount filtered because water is ________ by the kidney.
   1. produced
   2. reabsorbed
   3. consumed
   4. secreted
   5. all of the above
5. The kidneys filter ________ of fluid.
   1. 0.5 gallons or 2 liters/day.
   2. 2 gallons or 4 liters/day
   3. 5 gallons or 20 liters/day.
   4. 45 gallons or 180 liters/day.
6. Large volumes of fluid are filtered by the kidney because:
   1. the capillary pressure is very low.
   2. the tubules are very long.
   3. the blood flow is very slow.
   4. the capillary wall is very thin and porous.
   5. all of the above.
7. Glomerular filtration rate
   1. increases when blood pressure decreases.
   2. decreases when osmotic pressure in the plasma increases.
   3. increases when pressure in the proximal tubules increases.
   4. increases when area decreases.
8. Which of the following substances is not filtered by the kidney?
   1. glucose
   2. amino acids
   3. H⁺
   4. albumin
   5. penicillin
9. Which of the following is secreted?
   1. H⁺
   2. uric acid
   3. potassium
   4. penicillin
   5. All of the above are secreted.
10. Which of the following is not part of the kidney?
    1. Bowman's space.
    2. Seminiferous tubules.
    3. The glomerulus.
    4. The nephron.
    5. Collecting tubules

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