Urinalysis: Chapter 4 Renal Function

Nephron

Functional unit of the kidney, with 1-1.5 million nephrons per kidney

divided into cortical and juxtamedullary

Cortical Nephrons

-85% of the nephrons

-located within the kidney cortex

-responsible for waste product removal

-nutrient reabsorption.

Juxtamedullary Nephrons

-Responsible for urine concentration

-longer loops of henle

Renal Functions

Include blood flow, glomerular filtration, tubular reabsorption, and tubular secretion.

What is the pathway of blood flow in the kidney?

Renal artery --> afferent arteriole --> glomerulus --> efferent arteriole --> peritubular capillaries and vasa recta --> renal vein

What is the first vessel that carries blood into the kidney?

Renal artery

Renal Plasma Flow

the amount of plasma that passes through the kidneys in one minute

600-700 mL/min

Where does glomerular filtration occur?

glomerulus which is in the bowman's capsule

What is the approximate nonselective filtration size in glomerular filtration?

<70,000 daltons

What factors influence glomerular filtration?

Cellular structure, hydrostatic and oncotic pressure, and the renin-angiotensin-aldosterone system (RAAS)

Glomerulus

A coil of 8 capillary lobes

in Bowman's capsule

What are the three layers of the glomerulus?

1) Capillary wall with fenestrated endothelium, 2) Basement membrane, 3) Bowman's capsule(innermost layer)

What is the inner layer of Bowman's capsule composed of?

Podocytes and filtration slits

What is the primary protein associated with renal disease?

Albumin

Why is albumin negatively charged in the glomerulus?

Due to the shield of negativity

What is the Shield of Negativity?

A barrier that repels molecules with a positive charge.

How does the Shield of Negativity work?

It repels molecules with a positive charge, even if they are small enough to pass through the three layers of the barrier.

Glomerular Pressure

Regulated by the juxtaglomerular apparatus to maintain consistent glomerular blood pressure

Low systemic blood pressure

-larger afferent and smaller efferent

-prevents decreased glomerular blood flow

High systemic blood pressure

-smaller afferent arteriole

-prevents overfiltration and glomerular damage

Renin

An enzyme secreted by the juxtaglomerular cells when blood pressure decreases

Renin-Angiotensin-Aldosterone System (RAAS)

- regulates blood flow to and within the glomerulus

- responds to blood pressure and plasma sodium changes

juxtaglomerular apparatus

macula densa + juxtaglomerular cells

Macula densa

Controls the efferent arteriole

Juxtaglomerular cells

controls the afferent arteriole

secrete renin

RAAS Process

-Renin is released by kidneys in response to decreased blood volume

-causes angiotensinogen to split & produce angiotensin I

-lungs convert angiotensin I to angiotensin II

-angiotensin II stimulates adrenal gland to release aldosterone

Result of RAAS

-stimulates water and sodium in proximal convoluted tubules

-triggers release of aldosterone

-riggers release of antidiuretic hormone (ADH)

Normal Glomerular Filtration Rate

120-125 ml/min

What is tubular reabsorption?

Process of removing substances from glomerular filtrate and returning them to the blood.

Where does tubular reabsorption start?

Tubular reabsorption starts when the plasma ultrafiltrate enters the proximal convoluted tubule.

What are the 2 types of tubular reabsorption?

active and passive transport

active transport

Energy-requiring process that moves material across a cell membrane against a concentration difference

Active transport of glucose and amino acids takes place where?

proximal convoluted tubule

Active transport of chloride takes place where?

resorbed in the ascending loop of henle

Active transport of sodium takes place where?

resorbed in the distal convoluted tubule

passive transport

movement of molecules across a membrane as a result of differences in their concentration gradients

Where is water NOT reabsorbed?

ascending loop of henle

Urochrome

yellow pigment in urine

renal threshold

plasma level causing active transport to stop

maximal capacity

Glucose threshold

160-180 mg/dL

tubular concentration

1)descending loop of henle-- passive reabsorption of water

2)ascending loop of henle--chloride actively reabsorbed & sodium passively reabsorbed

3)countercurrent mechanism--maintains concentration in the medulla

4)aldosterone--controlled Na reabsorption if needed by body

final filtrate concentration

-Water reabsorption controlled by ADH in response to body hydration

-Osmotic gradient in the medulla

-Vasopressin

ADH

antidiuretic hormone (vasopressin)

when amount of water decreases

increase body hydration

decrease ADH, increase urine volume

decrease body hydration

increase ADH, decrease urine volume

tubular secretion

selectively moves substances from blood to filtrate in renal tubules and collecting ducts

2 major functions of tubular secretion?

1. elimination of waste products not filtered by the glomerulus

2. Regulation of the acid-base balance in the body through secretion of hydrogen ions in the form of NH4 and H2PO4

acid-base balance

the equilibrium in the body between acid and base concentrations

What is normal blood pH?

between 7.35 and 7.45

clearance test

standard test used to measure the filtering capacity of the glomeruli (GFR)

What are the primary substances used in a clearance test?

1) creatine

2) beta 2 microglobin

3) cystatin C

4) radioisotopes

endogenous procedure

substance is already present in the body

-method of choice

exogenous procedure

a test that requires a substance to be infused into the body

Creatine Clearance Test

test done to measure the total amount of creatinine excreted in the urine

creatine

waste product of muscle metabolism

Clearance formula

C = U x V / P

urine conc. x urine flow / plasma conc.

normal values for clearance

Men: 107-139 mL/min

female: 87-107 mL/min

Estimated GFR (eGFR)

based on results of serum creatinine, cystatin C, or B2M values

What does MDRD stand for?

Modification of Diet in Renal Disease

What is the purpose of MDRD?

To identify patients at risk for complications from kidney disease

What variables are included in MDRD?

Race, age, and gender

Beta-2-microglobulin (B2M)

-small protein

-rapidly removed from plasma by the kidney

-used to identify end-shape renal disease and early rejection of kidney transplant

Cystatin C

a small protein produced by nucleated cells which is filtered by glomerulus, reabsorbed and broken down by the renal tubule cells thus decreasing plasma levels

tubular reabsorption test

This is a test to determine the ability of the tubules to reabsorb the essential salts and water that have been non-selectively filtered by the glomerulus.

what is specific gravity of tubular reabsorption?

1.010

Osmolarity

the concentration of a solution expressed as the total number of solute particles per liter.

-more practical

Osmolality

the concentration of a solution expressed as the total number of solute particles per kilogram.

-weight does not change

-more accurate

Freezing point osmometers

-primary urine method

-measured sample is supercooled to form crystals

What does a vapor pressure osmometer measure?

Dew point (temperature at which water vapor condenses to a liquid)

Why does using a vapor pressure osmometer require careful technique?

Due to microsamples, commonly used for serum samples

lipemic serum

-Affects both instruments (freezing and vapor osmometers)

-Insoluble lipids displace serum water

lactic acid

-elevates reading both instruments

-separate or refrigerate with 20 mins

How does the volatility of a substance affect osmometer results?

Volatile substances can elevate results in freezing point osmometers.

Why would you prefer to use a vapor osmometer instead of a freezing point osmometer for volatile substances?

Using a vapor osmometer prevents the volatile substance from affecting the instrument's readings.

Free water clearance

a test to determine the ability of the kidney to respond to the state of body hydration

P-aminohippuric acid (PAH) clearance

test most commonly associated with tubular secretion and renal blood flow

metabolic acidosis

a serious condition that occurs when there's too much acid in the body or when the kidneys can't remove enough acid.

Secretion of H+ is done by?

proximal convoluted tubules

Secretion of NH3 is done by?

distal convoluted tubules

ammonia=

total acidity - titratable acidity