Renal Function

functional unit of the kidney

nephron

how many nephrons are found in each kidney

1 to 1.5 million

functions of the kidneys

selectively clear waste products from the blood and simultaneously maintain the body’s water and electrolyte balances

what four processes control the functions of the kidneys

1. renal blood flow

2. glomerular filtration

3. tubular reabsorption

4. tubular secretion

why is renal blood flow important

it supplies blood to the kidneys

why is glomerular filtration important

the glomerulus serves as a nonselective filter of certain plasma substances

what system controls the regulation of blood flow to the kidneys

the renin-angiotension-aldosterone system

what activates the renin-angiontension-aldosterone system

responds to changes in blood pressure and plasma sodium content

how does the renin-angiotension-aldosterone system work

1. renin (kidney enzyme) + angiotensinogen (inactive substance in blood) → angiontensin I (inactive hormone)

2. angiotensin I + lungs → angiotensin II (active hormone)

3. angiotensin II corrects renal blood flow

four ways angiotensin II corrects renal blood flow

1. causes vasodilation of the afferent arterioles and vasoconstriction of the renal arterioles

2. stimulates reabsorption of sodium in the proximal convoluted tubules

3. triggers the release of aldosterone (Na-retaining hormone) by the renal cortex to cause reabsorption of Na and excretion of K

4. triggers the release of ADH by the hypothalamus to stimulate water reabsorption in the collecting duct

importance of tubular reabsorption

the body cannot lose 120mL of water containing essential substances every minute

the nephrons reabsorb essential substances and water when plasma ultrafiltrate enters the proximal convoluted tubules

cellular mechanisms involved in tubular reabsorption

1. active transport

2. passive transport

active transport

energy is created when the substance to be reabsorbed combines with a carrier protein in the cell membrane which transfers the substance across the cell membranes and back into the bloodstream

substances reabsorbed via active transport

1. proximal convoluted tubule

   1. glucose

   2. amino acids

   3. salts

2. ascending loop of Henle: chloride

3. distal convoluted tubule: sodium

passive transport

the movement of molecules across a membrane as the result of differences in their concentration or electrical potentional on opposite sides of the membranes (gradients)

where does passive reabsorption of water take place

in all parts of the nephron except the ascending loop of Henle bc it’s walls are impermeable to water

substances reabsorbed via passive transport

1. urea (proximal convoluted tubules)

2. sodium (accompanies chloride in the ascending loop of Henle)

renal threshold

plasma concentration at which active transport stops

what occurs when renal threshold of a substance is reached

the substance spills over into the urine

why does renal concentration begin in the loops of Henle

water is removed by osmosis in the descending loop while Na and Cl are reabsorbed in the ascending loop

excessive water reabsorption is prevented by the water impermeable walls of the ascending loop

countercurrent mechanism

selective reabsorption process of the loops of Henle which serves to maintain the osmotic gradient → essential for the final concentrate of the filtrate when it reaches the collecting duct

function of anti-diuretic hormone

affects the permeability of the walls of the distal convoluted tubules and collecting ducts

consequence of high ADH

high ADH → increased permeability → increased water reabsorption → concentrated urine

consequence of low ADH

low ADH → idecreased permeability → decreased water reabsorption → dilute urine

importance of tubular secretion

1. eliminates waste products not filtered by the glomerulus

2. regulates the acid-base balance through the secretion of H+

test used to evaluate glomerular filtration

clearance tests

clearance test

measures the rate at which the kidneys are able to remove a filterable substance from the blood that:

• is neither reabsorbed nor secreted by the tubules

• stable in urine during 24hr collection period

• consistent plasma level

• available in the body

substance measured in routine glomerular filtration rate

creatinine (waste product of muscle metabolism) bc it is found at a constant level in the blood → reliable for screening

equation used for CrCl

CrCl = urine Cr (mg/dL) x urine volume (mL/min) / plasma Cr (mg/dl) * 1.73/ body surface area (m2)

CrCl clinical significance

determined by the number of functioning nephrons and the functional capacity of the nephrons

estimated glomerular filtration rate (eGFR) significance

used to monitor pts diagnosed with renal disease or at risk

when prescribing medications to a renal impaired pt

test used to evaluate tubular reabsorption

osmolarity test

osmolarity test

measures a colligative property that is related to the number of particles in the solution and compares this with the value obtained from the pure solvent

measures amount of a substance in urine, blood, etc

four colligative properties affected by solute dissolved in solvent

1. lower freezing point

2. higher boiling point

3. increased osmotic pressure

4. lower vapor pressure

five clinical uses of osmolarity

1. evaluating renal concentration ability

2. monitor renal disease

3. monitor fluid/lyte therapy

4. dx hyper or hyponatermia

5. evalute response of ADH

normal serum osmolarity value

275-300 mOsm

normal urine osmolarity

difficult to establish bc fluid intake and exercise can influence urine concentration

use of urine to serum osmolarity ratio

differentiates whether diabetes is caused by decreased ADH production or lack of ADH-response of the renal tubules

free water clearance test

determines the ability of the kidney to respond to the state of water hydration

test used to evaluate tubular secretion and renal blood flow

p-aminohippuric acid test (PAH)

PAH test

measures the exact amount of blood flowing through the kidney by utilizing the same principles as CrCl

significance of titratable acidity and urinary ammonia

the ability to produce an acid urine depends on the tubular secretion of H+ and ammonia cells of the DCT

renal tubular acidosis

the inability to produce an acid urine in the presence of metabolic acidosis → impaired tubular secretion of H+ (PCT) or defects in ammonia secretion (DCT)

other tests used to determine defective function of tubular secretion and renal blood flow

urine pH, titratable acidity, and urinary ammonia