Renal Function
Terms:
Enuresis: bed wetting
Glomerular filtration rate (GFR)
Loop of Henle (think of paper towel- thin only absorbs water, thick absorbs particles )
Renal Clearance (volume that goes through the kidney in 1 min; cardiac output of kidneys)
Turbidity (too much increase in osmolality)
Functions of the Kidneys
Waste Elimination
- Na & K ions
- Organic ions (Salisalate, penicillin, Uric acid, Bile acids)
- Urea (protein metabolism product)
- Drugs- those not bound to plasma proteins
Endocrine Function
- RAAS
- Erythropoetin (85-95% formed in the kidneys, sent to bone marrow, creates vitamin
D) and stimulated by hypoxia, high altitudes, anemia, cardiac/pulmonary disease
processes, ESRD are often anemic
- Vitamin D- activated by kidneys- activate in calcium absorption, desposition in the
bone ESRD pt cannot activate the vitamin
- Nephron is the functional unit
*Apirin can make gout worse
Renal Blood Supply
Renal Artery
2 Arterioles
- Afferent & Efferent (to/from Glomerulus)
2 Capillary Bed
- Glomerulus- high pressure system
- Peritubular- low pressure
Peritubular Network
Low pressure system from the efferent arteriole
Function
- Convenient for rapid reabsorption of water and solutes (leaving waste behind)
Parts:
- Peritubular capillaries- surround the tubules
- Vasa recta- long, straight, follows Loop of Henle
Glomerulus
High pressure capillary filtration system- between 2 arterioles
2-3 times higher than other capillaries
GFR
Plasma filters into Bowmans space based on:
- Capillary pressure
- Osmotic pressure
- Capillary pressure
Filtrate is similar to plasma
- No large proteins
Approximately 125 ml/minute
- Variable (a few mls to 200 ml/min)
Regulation of Renal Blood Flow
Kindeys receive 20-25% of Cardiac Output
- Goal: maintain GFR to insure waste removal
Mechanism:
- Intrinsic Factor – autoregulation, local hormones
- Extrinsic Factors- SNS, blood-borne hormones
Specific Regulation Mechanism
Neural & Humoral
- Extrinsic
- SNS
- Angiotensin 2
- ADH
Auto- regulation – Juxtaglomerular Complex
- Intrinsic
*Dopamine, prostaglandin cause vasodilation
Juxtaglomerular Complex- to maintain GFR (intrinsic)
Produce and store Renin
Feedback Control
- BP
- Na levels
1. Decrease BP = release of Renin = RAAS stimulation = Angiotension II causes constriction of
EFFERENT Arterioles to maintain GFR.
2. Monitors Na levels in the Macula Densa (to decide how much Renin to release to maintain
BP) –Angiotension II Aldosterone (function in the kidneys) = Na/water retention to maintain
Tubular Function- Urine Production
Glomerulus produces an “unfiltrate of urine”
Tubules:
- Reabsorbs electrolytes, water, nutrients to maintain homeostasis
- Eliminates waste material, excess
Movement Across membrane require:
Active and passive transport
- Na, K, CK, Ca, PO4 ions
- Urate, Glucose, Amino acids
Concentration Gradient
- Water, urea
Review: Endocrine Function of the Kidney
RAAS
- Plays an important part in regulation of BP
Erythropoietin
- Regulates the differentiation of red blood cells in bone marrow
Vitamin D
- Increases calcium absorption from the gastrointestinal tract
- Helps regulate calcium deposition in bone
Drugs that Affect the Kidneys
Diuretics- categories
- Loop- Lasix, demandex
•Furosemedie
- Thiazide- HCTZ, Maxzide;
- Aldosterone agonist- Spironolactone
- Thiazide: first line choice for hypertension even though its not as effective as loop. Its
“last minute” so it effects the DCT so it makes sure anything that got past the PCT
and LOH, it still won’t go into the convolutes tube. Least effective
- Loop: Most effective
Action of Diuretics
Loop Diuretics
- Exert their effect in the hick ascending loop of Henle
Thiazide Diuretics
- Prevent the reabsorption of Nacl in the DCT; potassium wasting; causes decrease
peripheral vascular resistance
The Aldosterone Antagonists
- Reduce sodium reabsorption and decrease potassium secretion in the late DCT and
cortical collecting tubule site regulated by aldosterone
- Potassium-spring diuretics
- Spironolactone less effective than others
Testing Renal Functions
UA (uranalysis)
GFR (best test)
Blood
- BUN
- Creatine
Urinalysis- UA
- Clear, amber-colored fluid
- Approximately 95% water and 5% dissolved solids
- Kidneys norma;;y produce approximately 1.5L of urine eachday
- Contains metabolic wastes and few or no plasma proteins, blood cells, or glucose
molecules
- Urine osmolality – “specific gravity”
•1.005 to 1.025
- pH 4.5 to 8; avererage 6
Urine Abnormals
- specific gravity high- dehydration
- specific gravity low- decrease renal function (can’t concentrate the urine)
- Presence of RBCs, WBCs, bacteria molecules, glucose, ketones, etc.
GFR
Creatine
- Produce of Creatine metabolism in the muscle
- Filtered by the Kidneys BUT NOT reabsorbed
Creatine Clearance used to measure GFR
- 24 hour urine collection
- Two 1-hour urine specimens are collected with blood drawn in between
BUN- Blood Urea Nitrogen
By-product of protein metabolism
Eliminated entirely by the kidneys
Levels may be affected by:
- Protein intake, GI bleed, Dehydration
2/3rd of renal function must be lost before you see an increase in Bun
Normal Range- 8.0 to 20.0mg/dl
Creatine
Reflects GFR
Formation and release is relatively constant
Freely filtered in the glomeruli
- Not absorbed bac into the blood
- Not much secreted back into the tubules
Excellent screening tool for renal function
Normal range: Creatine- 0.6-1.2
- May need to be adjusted for age
- Related to loss of muscle mass and renal function
Strong relationship btw renal function and creatine level
If Creat. Is 2x normal = loss of ½ renal function
3x normal = loss of ¾ of renal function
levels at 10mg/dl or more = loss of 90% of renal function
BUN- Creatine Ratio
BUN not very specific for renal problems
>15:1= pre renal cause of failure- CHF, upper GI bleed
< 10:1 = liver disease, dialysis, low protein diet
Other studies
- Cystoscopy – direct visualization of the internal structures
- Ultrasonography – use of high-frequency sound waves to visualize deep structures
- Radiologic studies - CAT scan, Flat plate films, IVP, MRI, radioactive imaging
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