Download Excretion part2b Spring 2016

Urinary System Notes, part 2
D:\840966910.doc
Dr. Shaffer Human A&P II
Page 1
C. Tubular secretion = removal of substances from the blood to the tubular fluid. It helps
control blood pH (secretion of H+) and eliminate substances from the body (eg. K+, NH4,
drugs).
1. Secretion of H+. Blood ought to be acidic, since most metabolic RXNS have
acids as end products. The renal tubule (distal) can lower pH three ways: by
secreting H+ into tubular fluid, by reabsorbing HCO3- ions, and by producing more
HCO3-.
The Collecting Duct also secretes H+, but here it is by primary active transport
(using ATP and a transport protein)
2. Secretion of K+. Most filtered K+ is absorbed, but secretion of K+ is used to
maintain balance of K+ in body fluids, controlled by
a. Aldosterone. Increased aldosterone leads to increased K+ secretion.
b. K+ conc. in plasma. High K+ in plasma leads to high secretion.
c. Na+ conc. in DCT. High Na+ leads to high Na+ absorption which
means higher K+ secretion.
D. Dilute and concentrated urine.
Homeostasis of body fluid depends on the ability of the kidneys to control the rate
of water loss through urine. ADH is the hormone that controls this. There’s a lot
to know about this but some things are pretty easy to explain: If blood volume
and/or blood pressure drop, the Pituitary gland releases Anti Diuretic Hormone
(ADH). Think about the name ANTI diuretic. It means NOT diuretic. If you
don’t produce a lot of urine where does the water go? Into your blood - increasing
blood volume and blood pressure. ADH works by making the walls of the
collecting tubules and the DCT more permeable to water. This allows water to
come out of the tubule and into the blood.
E. Diuretics = Chemicals that enhance urinary output.
- Osmotic Diuretics - substances that are filtered but not reabsorbed will draw
water toward urine. Example: Urea in a high protein diet.
- Alcohol inhibits ADH
- Caffeine promotes vasodilation, increasing GFR
- diuretic medications inhibit Na+ reabsorption
F. Renal clearance. Assessing kidney function is important. Urinalysis is one means of
determining whether the kidneys are functioning properly – examine the contents and
look for things that don’t belong (or for abnormal concentrations of substances), we
covered this in lab. Determining Renal Clearance rates is another important method for
evaluating kidney function. RC is a rate: volume per unit time. It refers to a volume of
plasma that can be “cleared” of a substance in 1 minute. High renal clearance of a
substance would indicate efficient excretion of the substance. Low RC indicates that the
kidneys are not excreting that substance efficiently. It is important to keep in mind that
RC is substance specific, thus we want to observe a low RC for glucose, for example,
because the kidneys are not supposed to clear the blood of glucose.
Urinary System Notes, part 2
D:\840966910.doc
Dr. Shaffer Human A&P II
Page 2
Clearance of a substance will depend on the three processes of the nephron- glomerular
filtration, tubular reabsorption, and tubular secretion.
RC substance x = Concentration substance x in Urine x Rate of Urine Production 
Concentration substance x in Plasma
RC substance x = (mg substance x in the urine/ ml urine) x (ml urine produced/ minute) 
(mg substance x in plasma/ ml of plasma)
If you go through and cancel the units
RC = ml/min
If a substance is filtered, but not reabsorbed or secreted, RC = filtration rate. Inulin is a molecule
which is known to be filtered but not reabsorbed and not secreted. If we plug in some real values
for inulin (from a hypothetical/typical patient):
Urine concentration = 125 mg/ml (measured directly)
Rate of urine produce = 1 ml/min (measured directly)
Plasma concentration = 1 mg/ml (calculated and administered by person performing test)
We get RC = 125 ml/min. In the case of inulin, we know that RC = GFR (because filtration is
the only thing happening to inulin). A clearance value for a substance that is less than that of
inulin means that that particular substance is partially reabsorbed. Ex:
RCurea = 70 ml/min
We know that we’re filtering 125 ml/min so 55 ml/min aren’t getting cleared of urea. The urea
in that 55 ml is reabsorbed. Another ex:
RCglucose = 0 ml/min
We filter 125ml of plasma per minute, but we don’t excrete any glucose - it’s all absorbed.
Knowing the clearance rates for particular drugs is important in calculating drug doses.
Another ex:
RCBactrim = 140 ml/min
We clear 140 ml of plasma of Bactrim in one minute. That’s more than we filter. That’s because
of secretion of Bactrim added to the filtration of Bactrim.
Urinary System Notes, part 2
D:\840966910.doc
Dr. Shaffer Human A&P II
Page 3
Acute Renal Failure (ARF)
1. Prerenal AFR - decreased blood flow to kidneys leads to ischemic nephrons.
a. Caused by blood loss, dehydration, septicemia, cardiogenic shock (what is cardiogenic
shock?)
b. Other causes include: ACE inhibitors, NSAIDs, other drugs
2. Intrarenal ARF – Damage to the renal tissues
a. Ischemic causes include: Renal artery stenosis, glomerulonephritis
b. Nephrotoxic causes : radiocontrast media, heavy metals, cyclosporine, antibiotics,
chemotherapy
3. Postrenal ARF – blocked urine flow backs up to kidneys
a. Prostatic hypertrophy
b. Bilateral ureter obstruction
c. Bladder outlet obstruction
4. Systemic Effects – various body systems involved
a. Renal most common sign is oliguria (what’s that?)
i. High BUN may be from other causes so not good indicator
ii. Elevated creatinine is pretty good sign, but trends in creatinine most reliable as
elderly atrophied pts produce less
iii. Electrolyte imbalances may also suggest ARF
b. Cardiovascular - mostly result of fluid build up
i. Hypertension, pulmonary edema, peripheral edema
ii. Hyperkalemia (what’s that?) can really mess up heart, cause loss of muscle tone,
muscle weakness, tingling lips& fingertips. Monitor K levels – some pts are
asymptomatic
c. Respiratory - dyspnea from pulmonary edema
d. Neurologic - altered mental status from metabolic waste accumulation
e. Hematologic – Anemia is main effect.
i. Impared RBC production results from decreased erythropoietin
ii. RBC survival is only ~60 days
iii. Gastrointestinal Anorexia, nausea, vomiting
iv. Increased risk of GI bleed
v. Breath may smell of urine
This article ends with the following statement: Glomerular Filtration Rate (GFR) . . . is the driving force of
urine production. Normally 120 – 125ml/min, GFR decreases when renal insult occurs. Increased blood
urea nitrogen and creatinine levels are indirect indicators of reduced GFR.