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Renal Physiology

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Title: Renal Physiology


1
Renal Physiology
PART ONE Renal Physiology overview PART
TWO Renal Physiology details
2
Role of the kidney in maintaining water,
electrolytes, and pH balance
  • Plasma leaks out of the capillaries in the
    glomerulus. The kidneys return the nutrients to
    the plasma, while removing the waste products.
    This also maintains the pH balance.
  • Under the direction of aldosterone, they keep the
    balance between electrolytes, especially sodium
    and potassium.
  • This keeps the plasma volume constant to maintain
    BP.

3
Role of Kidneys
  • The kidneys can adjust blood volume, composition,
    and pressure
  • BLOOD VOLUME
  • Adjusts the volume of water lost in urine by
    responding to ADH
  • BLOOD COMPOSITION
  • Releasing erythropoietin (increases RBC
    production)
  • BLOOD PRESSURE
  • Releasing renin (increases blood pressure)

4
Sympathetic Nervous System Effect on Kidneys
  • Changes in glomerular blood flow and pressure
  • The stimulation of renin release from the
    juxtaglomerular apparatus
  • Changes in water and sodium reabsorption by the
    nephron

5
Hypothalamus
  • The hypothalamus monitors the concentration of
    water in the plasma.
  • If the plasma is too concentrated (high osmotic
    pressure), it means there are many electrolytes
    and not enough water inside the blood vessels
    (the person is dehydrated, and blood pressure
    will drop).
  • Since water goes to the area that has the most
    particles (particles SUCK water!), water will be
    drawn out of the nearby cells, which will cause
    them to shrink.
  • If the plasma is too dilute (low osmotic
    pressure), it means there is too much water and
    too few electrolytes inside the blood vessels
    (the person is over-hydrated, and blood pressure
    will rise).
  • Water will be drawn out of the blood vessels to
    enter the nearby cells (causing them to swell) or
    the space between them (interstitial space).

6
Hypothalamus and Adrenal Gland
  • When a person is dehydrated and has low blood
    pressure, the hypothalamus will sense that the
    osmotic pressure of the plasma is too high (above
    homeostatic levels plasma is too concentrated
    too many electrolytes and not enough water is in
    the plasma), it tells the pituitary gland to
    release ADH (antidiuretic hormone) to cause the
    kidneys to retain additional water to dilute the
    plasma. This will make the low blood pressure go
    back up.
  • The adrenal cortex will also release aldosterone,
    which causes sodium ions to be reabsorbed by the
    kidneys, and water will follow. This will also
    increase the plasma volume (which will dilute
    it), and also help the low blood pressure to go
    back up.
  • If the osmotic pressure is too low (plasma is too
    dilute too much water and not enough
    electrolytes in the plasma), ADH and aldosterone
    are not released, and excess water will pass out
    of the body as urine. This will make the high
    blood pressure go back down.

7
Quiz Yourself
  • What does it mean when the osmotic pressure is
    too high? Too low?
  • What are the causes of each of these situations?
  • How does the body compensate for each of these
    situations?
  • What does it mean when the plasma is too dilute?
    Too concentrated?
  • What are the causes of each of these situations?
  • How does the body compensate for each of these
    situations?

8
pH Imbalances
  • Many things can alter the pH of the blood
  • Beverages we drink
  • Acids produced by metabolism
  • Breathing rate
  • Vomiting (loss of acid)
  • Diarrhea (loss of base)
  • pH imbalances are dangerous because many enzymes
    only function within a narrow pH range.

9
Renal Physiology
Basic Mechanisms of Urine Formation
1) Glomerular filtration 2) Tubular reabsorption
3) Tubular secretion 4) Excretion How do we
determine these rates? Master formula
10
Glomerular Filtration
  • The capillaries in the glomerulus contain many
    holes, called fenestrations. As blood passes
    through the glomerulus, the plasma passes through
    the fenestrations. Proteins and other large
    substances do not cross through they stay in the
    bloodstream.
  • The filtered plasma leaves the bloodstream in
    this way, and enters the glomerular capsule, and
    then enters the proximal convoluted tubule.

11
Glomerular Filtration
  • In a sprinkler hose, the higher the water
    pressure, the faster the water squirts through
    its holes. The same process is also true for the
    glomerulus.
  • The blood pressure inside the glomerulus affects
    how fast the fluid can filter through the
    fenestrations. Therefore, blood pressure affects
    the glomerular filtration rate (GFR). The higher
    the blood pressure, the higher the GFR.
  • The pre-capillary sphincters can also control how
    much pressure is in the glomerulus, much like the
    cold water faucet controls the pressure in a hose.

12
Glomerular Filtration
  • GFR is used as a measure of kidney function.
  • Normal GFR is 125 ml per minute for both kidneys
    combined.
  • That means 7.5 liters per hour, or 180 liters per
    day.
  • That is 45 gallons of filtrate produced per day!
  • Of course, most of that is reabsorbed.
  • Average urine output is about 1.2 liters per day.
  • That means you need to drink 1.2 liters of fluid
    per day (remember that caffeine and alcohol are
    diuretics, so you need more than that to
    compensate if you drink those beverages). You
    need to drink more (about 2 liters per day) if
    you are getting a cold or flu.

13
Altering GFR
  • Several different mechanisms can change the
    diameter of the afferent and efferent arterioles
    to alter the GFR
  • Hormonal (hormones)
  • Autonomic (nervous system)
  • Autoregulation or local (smooth muscle sphincters
    around the arterioles or capillaries near the
    glomerulus)

14
Remember the route the fluid takes Glomerulus ?
Proximal convoluted tubule (PCT) ? Descending
limb of LOH ? Ascending limb of LOH ?
Convoluted tubule ? Collecting duct
15
Tubular Reabsorption
  • This is the process by which substance in the
    renal tubules are transferred back into the
    bloodstream. Reabsorption is the removal of
    water and solute molecules from filtrate after it
    enters the renal tubules.
  • Fluid goes from the glomerulus to the proximal
    convoluted tubule (PCT), down the loop of Henle
    and back up, then into the distal convoluted
    tubule (DCT), and into the collecting duct.
  • In the PCT, the nutrients are reabsorbed. If
    there are more nutrients than can be reabsorbed
    (such as excess sugar), it will be excreted in
    the urine.
  • When the nutrients in the PCT are reabsorbed, the
    inside of the tubule will have more water and
    less nutrients. Since water goes to the area that
    has a higher concentration of particles
    (osmosis), water will also leave the tubules
    this occurs in the DCT.
  • By the time the fluid has reached the collecting
    duct, nothing but waste products are left, such
    as urea, ammonia, and bilirubin.

16
Tubular Reabsorption
  • Capillaries follow the renal tubules and wrap
    around them.
  • The straight capillaries that travel
    longitudinally next to the tubules are called
    vasa recta, and the capillaries that wrap around
    the tubule are called peritubular capillaries.
  • There is a space between the capillaries and the
    tube, called the peritubular space.

17
Tubular Reabsorption
Filtrate arriving from Bowmans Capsule
Tubular Cells
Lumen of Tubule
Peritubular Capillaries
  • The peritubular capillaries are nearby, and the
    particle concentration is low inside of them.
    Therefore, the particles in the peritubular space
    (high concentration of particles) will leave that
    space and enter into the peritubular capillaries
    by osmosis.
  • That is how the nutrients are reabsorbed from the
    tubules back into the bloodstream.

18
Tubular Reabsorption
  • The ascending limb of the Loop of Henle and the
    DCT are impermeable unless hormones cause
    substances to be moved through their walls.
  • If the blood is low in sodium, (after excessive
    sweating), aldosterone (from the adrenal cortex)
    will cause more sodium to be pumped out of the
    tubule and into the peritubular space. The sodium
    will then enter the capillaries.
  • Since water follows where salt goes, whenever the
    body needs more water (such as dehydration), ADH
    is released (from the neurohypophysis posterior
    pituitary). ADH is also called vasopressin.
  • Aldosterone and ADH will increase blood volume,
    increasing blood pressure.
  • These two hormones begin their action in the
    ascending limb and continue to work in the DCT.

19
Tubular Secretion
  • Some substances are unable to filter through the
    glomerulus, but are not wanted by the body.
  • Examples are pollutants like pesticides, and many
    drugs, such as penicillin and non-steroidal
    anti-inflammatory drugs (NSAIDs).
  • As blood passes through the peritubular
    capillaries, those substances are moved from the
    capillaries directly into the PCT and DCT.
  • This is called tubular secretion.

20
Juxtaglomerular Apparatus
  • The distal end of the renal tubule passes next to
    the glomerulus to form the juxtaglomerular
    apparatus (juxta means next to).

21
Juxtaglomerular Apparatus Alters BP and GFR
  • Macula densa
  • Juxtaglomerular cells

22
Juxtaglomerular Apparatus
  • If blood pressure is too low, the macula densa
    releases adenosine, which causes
    vasoconstriction of the afferent arteriole. This
    will slow the GFR, so less water is lost, and
    blood pressure increases.

23
Juxtaglomerular Apparatus
  • If blood pressure is too high, the macula densa
    stops releasing adenosine, which allows the
    sphincters to relax.
  • This will increase GFR so more water is lost, and
    blood pressure decreases.

24
Juxtaglomerular Apparatus
  • Juxtaglomerular cells secrete renin if the blood
    pressure is still too low after adenosine has
    caused vasoconstriction.
  • Renin causes more sodium to be reabsorbed, and
    water follows, so blood volume increases, so
    blood pressure increases.

25
Autoregulation
  • The nephron can alter the blood pressure and flow
    into the glomerulus by autoregulation.
  • The JGA senses the blood pressure going into the
    glomerulus and the flow rate of the fluid going
    through the renal tubule. If the GFR is too low,
    the JGA will cause the pre-capillary sphincters
    on the nearby arterioles to relax, increasing
    blood pressure, like turning up the faucet on a
    hose.
  • If that restores the desired filtration rate and
    flow, no further action is needed. If not, the
    kidneys produce the enzyme renin, which converts
    angiotensinogen into A-1. That makes the lungs
    produce angiotensin converting enzyme (ACE),
    which turns A1 into A2, which constricts blood
    vessels, and also causes the release of
    aldosterone, raising the blood pressure.

26
Hormonal Regulation
  • If a person sweats from activity, eats very salty
    food, or has diarrhea, it changes the sodium and
    water content of the plasma.
  • Two hormones that affect the ascending limb of
    the Loop of Henle are aldosterone and
    antidiuretic hormone (ADH).
  • Adosterone is produced by the adrenal cortex and
    causes additional sodium ions to be pumped our of
    the tubule and into the bloodstream. Water comes
    with it by osmosis, and the blood pressure
    increases.
  • ADH is produced by the posterior pituitary gland
    and causes retention of additional water from the
    DCT and collecting ducts. Sodium is not included
    in this process, so the result is to dilute the
    plasma during dehydration, when the plasma is
    becoming to concentrated with particles.

27
Erythropoietin
  • The kidneys also monitor the oxygen content of
    the blood.
  • If O2 levels are low, the JGA releases
    erythropoietin to stimulate the bone marrow to
    produce more red blood cells.

28
Neural Regulation
  • The kidneys receive about 22 of the blood pumped
    out of the heart, so that is a substantial
    quantity passing through the kidneys at any given
    time.
  • If there is a stressor and the sympathetic
    nervous system causes us to go into fight or
    flight mode, the skeletal muscles need to have a
    maximum amount of blood flow.
  • Neurons from the sympathetic nervous system
    innervate the kidneys to decrease renal blood
    flow during critical situations.

29
Urine
  • Urine contains ions such as sodium, chlorine, and
    potassium, as well as suspended solids, known as
    sediments, such as cells, mineral crystals, mucus
    threads, and sometimes bacteria.
  • The pH of urine is normally 4.6-8
  • A urinalysis can identify abnormal processes
    occurring in the body.
  • Because urine is a waste product, its contents
    are influenced by the foods and drinks we ingest.
  • We may lose fluid elsewhere, such as through
    sweating or diarrhea, which causes the urine to
    become more concentrated.
  • Acids produced through metabolism can also change
    the pH of our urine. Even changes in breathing
    rate can change the urine pH as excess acids or
    bases are excreted to maintain normal plasma pH.

30
Abnormal Urinalysis
  • These substances should not be in the urine. When
    they are, it is abnormal.
  • Glucose
  • Blood
  • Protein
  • Pus
  • Bilirubin
  • Ketones

31
Causes of abnormal UA
  • Glucose diabetes mellitus
  • Blood bleeding in urinary tract from infection
    or kidney stone
  • Protein kidney disease, hypertension, excessive
    exercise, pregnancy
  • Pus bacterial infection in urinary tract
  • Bilirubin liver malfunction
  • Ketones excessive breakdown of lipids

32
Micturition
  • Urination is technically known as micturition.
  • Once the volume in the urinary bladder exceeds
    200 ml stretch receptors in its walls send
    impulses to the brain, indicating the need to
    eliminate.
  • When you make the decision to urinate, the
    parasympathetic nervous system stimulates the
    smooth muscle in the urinary bladders internal
    sphincter to contract.
  • Remember, the internal sphincter is smooth muscle
    (involuntary) and the external sphincter is
    skeletal muscle (voluntary). Both must relax for
    urine to exit.

33
Diuretics for hypertension and congestive heart
failure
  • Diuretics decrease plasma volume. This group of
    drugs are thiazide diuretics (such as Lasix). The
    inhibit the reabsorption of sodium and potassium
    from the renal tubule, causing more water to pass
    out as urine.
  • Compared to sodium, the homeostatic range of
    potassium is quite narrow.
  • Lasix (Furosemide) inhibits reabsorption of
    potassium more than other diuretics. Low blood
    levels of potassium are called hypokalemia. It is
    important for someone on Lasix to take potassium
    supplements or eat fruits or vegetables that have
    a lot of potassium (such as cantaloupe).
  • However, too much potassium from excessive
    supplements can have fatal side effects.

34
Diuretics
  • Furosemide (Lasix)
  • Mannitol
  • Spironolactone
  • Amiloride

35
Homeostasis
  • Maintaining the proper concentration of sodium
    and water is critical.
  • If the plasma is too concentrated with particles,
    nearby cells can shrink and lose their function.
  • If the plasma is too dilute, water can enter the
    nearby cells and cause them to expand, also
    decreasing their function.
  • This is especially dangerous in the brain.
  • Studies have shown a close link between obesity,
    diabetes, and kidney disease. Exercise helps
    maintain normal kidney function by increasing
    blood flow, and it decreases the incidence of
    high blood pressure. People receiving dialysis
    and those who have had kidney transplants
    especially need to exercise.
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