Urinalysis chapter 4

What are the two types of nephrons?

• cortical (85%)

• juxtamedullary (15%)

What are cortical nephrons responsible for?

removal of waste products and reabsorption of nutrients

What are juxtamedullary nephrons responsible for?

The concentration of urine

What renal functions control the kidney’s ability to clear waste products and maintain the body’s electrolyte and fluid balance?

• renal blood flow

• glomerular filtration

• tubular reabsorption

• tubular secretion

What is the order of blood flow through the renal capillaries?

renal artery, afferent arteriole, glomerulus, efferent arteriole, peritubular capillaries, vasa recta, renal vein

What allows the arterioles to create the hydrostatic pressure differential?

Size of the arterioles, afferent bigger than efferent

What does the hydrostatic pressure differential of the glomerulus maintain?

• glomerular filtration

• glomerular capillary pressure

• renal blood flow

Where does the immediate reabsorption of essential substances take place?

proximal convoluted tubule

Where does the final adjustment to urinary composition take place?

distal convoluted tubule

Where do the major exchanges of water and salt take place?

the loops of Henle

What is the average renal blood flow?

1200 ml/min

What is the average total renal plasma flow?

600 to 700 ml/min

What do normal values for renal blood flow and renal function depend on and must be adjusted for?

Body size

What does the glomerular filtration barrier serve as a non selective filter for?

plasma substances with molecular weights less than 70,000

What other factors influence the actual filtration process of the glomerulus?

• cellular structure of the capillary walls and bowmen capsule

• hydrostatic pressure

• oncotic pressure

• RAAS

What is the fenestrated endothelium?

Pores in the glomerular endothelium that increase the capillary permeability but do no allow large molecules or blood through

What is the shield of negativity?

A negative charge on the glomerular barrier that repels negativity charged molecules

What is usually repelled by the shield of negativity?

albumin

What maintains glomerular blood pressure?

Juxtaglomerular apparatus

How does the juxtaglomerular apparatus maintain glomerular blood pressure regardless of systematic blood pressure?

Dilation and constriction of the afferent and efferent arterioles

What does RAAS regulate?

Flow of blood to and within the glomerulus

What does RAAS stand for?

Renin-angiotensin-aldosterone system

What does the RASS respond to?

Changes in blood pressure and plasma sodium content

What happens with low plasma sodium?

Decreases water retention in circulatory system leading to decreased blood volume and low blood pressure

What is renin and when is it secreted?

An enzyme produced by the juxtaglomerular cells and is secreted when the macula densa sense low blood pressure/plasma sodium

What does renin react with?

angiotensinogen

What hormone is made from the reaction of renin and angiotensinogen?

Angiotensin I

How is angiotensin I converted into angiotensin II?

In the alveoli of the lungs through the actions of ACE (angiotensin converting enzyme)

How does angiotensin II correct renal blood flow?

• dilates afferent arterioles and constricts efferent arterioles

• stimulating reabsorption of sodium and water in the proximal convoluting tubules

• triggers release of aldosterone by the adrenal cortex and ADH from the hypothalamus

What does aldosterone do?

Acts on the distal convoluted tubule to reabsorb sodium

What is the difference between renal filtrate and plasma?

Filtrate contains no plasma proteins, protein bound substances and cells

What is the specific gravity of filtrate as it leaves the glomerulus?

1.010

What are the cellular mechanism involved in tubular reabsorption?

• active transport

• passive transport

What must happen for active transport to occur?

substance to be reabsorbed must combine with a carrier protein

What is active transport used to reabsorb?

• glucose, amino acids and salts in the proximal convoluted tubule

• chloride in the ascending loop of henle

• sodium in the distal convoluted loop

What is passive transport the result of?

The movement of molecules across a membrane as a result of differences in their concentration or electrical potential on opposite sides of the membrane

Where does passive reabsorption of water take place?

All parts of the nephron except the ascending loop of Henle

Where does passive reabsorption of urea take place?

the proximal convoluted tubule and ascending loop of Henle

Where does the passive reabsorption of sodium take place and what is it accompanied by?

the ascending loop of henle and it accompanies the active transport of chloride

Like passive transport, what can active transport also be influenced by?

Concentration of a substance

How does concentration affect active transport?

When the concentration of a substance is abnormally high it exceeds the maximal resorptive capacity and the substance begins to appear in the urine

What is the renal threshold?

The plasma concentration as which active transport stops

What is the renal threshold for glucose?

160-180 mg/dl

Where does renal concentration begin?

the descending and ascending loops of Henle

What is in the renal medulla that helps with concentration?

A high osmotic gradient

What is the selective reabsorption process in the loops of henle called?

countercurrent mechanism

Where is water reabsorbed and where is sodium and chloride reabsorbed in the loop of henle?

water is reabsorbed in the descending and sodium in the ascending loops

What prevents excessive reabsorption of water as the filtrate passes through the highly concetrated medulla?

The impermeability of the ascending loop to water

What does the countercurrent mechanism help to maintain?

the osmotic gradient of the medulla

Where does the final concentration of filtrate begin?

Starts in the late distal convoluted tubule and continues in the collecting duct

What does reabsorption of water in the collecting duct depend on?

the osmotic gradient in the medulla and the hormone vasopressin(ADH)

What cause the release of ADH by the posterior pituitary gland?

A decrease in the amount of water in the body

What does ADH act on?

the walls of the distal convoluted tubule and the collecting duct

What does a high level of ADH do?

Increases the permeability of the collecting duct and distal tubule to water

What happens to ADH levels and urine volume when the body is dehydrated?

ADH levels increase and urine volume decreases

What does tubular secretion involve?

passage of substances from the blood in the peritubular capillaries to the tubular filtrate

What two major functions does tubular secretion serve?

• elimination of waste products not filtered by the glomerulus

• regulating the acid-base balance in the body through the secretion of hydrogen ions

What does the buffering capacity of blood depend on?

Bicarbonate ions that are filtered by the glomerulus and must be returned to the blood

What prevents the excretion of bicarbonate in the filtrate?

The secretion of hydrogen ions by renal tubular cells, mostly in the proximal convoluted tubule

What are the two primary methods of hydrogen ion excretion?

• combining with filtered phosphate instead of bicarbonate

• by combining with ammonia

What can the distal convoluted tubule and collecting duct to if there is an additional need to eliminate hydrogen ions?

produce ammonium ions

What can a disruption of the secretory functions of kidneys result in?

metabolic acidosis or renal tubular acidosis

What are the standard tests used to measure the filtering capacity of the golmeruli?

Clearance test

What do clearance tests measure?

The rate in mL/min which kidneys are able to remove a filterable substance from the blood

To ensure accurate measurement of the GFR the substance being used must?

Be neither secreted nor reabsorbed by the tubules

What substances are used in clearance tests?

• creatinine

• beta2-microglobulin (B2M)

• radioisotopes

What is an exogenous procedure?

A test that requires an infused substance

What is an endogenous procedure?

When the test substance is already present in the body

What is creatinine?

A waste product of muscle metabolism

Why is creatinine often used to measure glomerular function?

It is normally found in relatively constant levels in the blood

What are some disadvanatages of using creatinine?

• some is secreted to tubules as blood levels rise

• medication can inhibit secretion of creatinine causing falsely low serum levels

• bacteria can break it down if stored improperly

• meat heavy diets can raise creatinine levels

• not reliable for those suffering from muscle-wasting diseases or performing heavy exercise

• must be corrected for body surface area

What is the greatest source of error in clearance procedures?

improperly times urine specimens

What is the standard formula for milliliters of plasma cleared in a minute?

C=(UV)/P

Do estimated glomerular filtration rates (eGFR) require a timed urine specimen?

No

What is cystatin C?

A small protein produced by all nucleated cells

Why is cystatin C used to measure GFR?

It is filtered, reabsorbed and broken down by the renal tubular cells so serum concentration can be used to directly measure GFR

What is beta2-microglobulin?

A protein that found on human leukocyte antigens that regularly dissociates from the cells

What is B2M used for?

To distinguish kidney disorders as either glomerular or tubular

How does the concentration of B2M distinguish between different disorders of the kidney?

If serum levels are normal but urine levels increased than a tubular disorder is indicated.

If serum levels are increased but urine levels remain low than a glomeruli disorder is indicated

Where does the value of a GFR test lie?

In the determination of the extent of nephron damage in renal disease, to monitor the effectiveness of treatment to prevent further damage and to determine the feasibility of administering medication

GFR is determined by both the?

• number of functioning nephrons

• the functional capacity of those nephrons

What is used to determine the functionality of the tubules?

concertation tests

What is urine concertation largely determined by?

Hydration levels

What does a urine-to-serum ratio or a urine osmolality indicate?

Normal tubular reabsorption

What does osmolality measure?

the number of particles in a solution

What does specific gravity measure?

Both the number and density of the particles in a solution

Why is osmolality more accurate for evaluation of renal concentrating ability?

Renal concentration involves small particles so larger particles such as glucose and urea do not contribute

What makes the conversion from temperature to milliosmoles in freezing point osmometers?

The fact that 1 mol of a nonionizing substance dissolved in 1kg of water lowers the freezing point

What is the dew point?

Temperature water vapor condenses to a liquid

What do solutes do to the vapor pressure?

Decrease it

What factors influence osmolarity?

• lipemic serum (displacement of water by insoluble lipids)

• lactic acid

• volatile substances (do not effect vapor pressure but do effect freezing point)

What are the major clinical uses of osmolarity ?

• evaluating renal concentrating ability

• monitoring renal disease

• monitoring fluid and electrolyte therapy

• differential diagnosis of hyper and hyp-natremia

• evaluating the secretion and renal response to ADH

Why are reference values for urine osmolality hard to establish?

Because of factors like fluid intake and exercise that can influence urine concertation

What is free water clearance?

A test used to determine the ability of the kidney to respond to the state of body hydration

How is free water clearance calculated?

By first determining the osmolar clearance and subtracting the urine volume

What is the equation for osmolar clearance?

Cosm= (Uosm x V)/ P osm

What does the osmolar clearance indicate?

How much water must be cleared each minute to produce a urine with the same osmolality as the plasma

What test is most commonly associated with tubular secretion and renal blood flow?

p-aminohippuric acid (PAH) test

What is the criteria to measure blood flow?

Substance must be removed from the blood primarily in the peritubular capillaries rather than the by the glomerulus

What does the ability of kidneys to produce an acid urine depend on?

the tubular secretion of hydrogen ions as well as the production and secretion of ammonia by the distal convoluted tubule