Urinary System

components of the urinary system

1. kidneys

2. ureters

3. bladder

4. urethra

5. associated nerves/blood vessels

functions of urinary system

1. regulate blood volume, blood pressure, pH, and concentration of electrolytes

2. reabsorb glucose and excreting wastes

3. release hormones (renin and erythropoitin)

4. activate vitamin D

location of the kidneys (+ which is lower)

between peritoneum and posterior wall of abdomen in retroperitoneal space

• right is slightly lower than left due to position of the liver

cortex of kidney (+ function)

outer layer that filtrates to form urine

medulla of kidney (+ function)

inner layer that collects and excretes urine

renal pyramids (+ function)

8-18 cone-shaped subdivisions that contain kidney’s secreting apparatus and tubules

renal columns (+ function)

lines of blood vessels and fibrous material between pyramids

• anchors cortex

renal papilla

location where renal pyramids empty urine into minor and major calyces

minor calyx

receive urine from each renal pyramid

major calyx

receive urine from 2-4 minor calyces

When does filtrate become urine?

When it enters calyces because no further reabsorption occurs

renal pelvis

a large cavity that receives urine from major and minor calyces and drains into ureters

renal blood flow

renal artery and vein pass into parenchyma at hilum

How does the renal artery branch to become renal vein?

Renal artery → (extensive branching) afferent arteriole → glomerular capillaries → efferent arteriole → peritubular capillaries → (extensive branching) renal vein

nephron

functional unit of kidney composed of blood vessels and tubules that collect filtrate

How many nephrons per kidney?

~ 1 million

filtration in nephron (where it occurs, describe)

• mostly in renal corpuscle

• most solutes of the right size (small enough) enters through the glomerular capillaries into the nephron

reabsorption in nephron (where it occurs, describe)

• mostly proximal convoluted tubule (PCT)

• solutes are reabsorbed into the blood that were taken out that the body still needs

secretion in nephron (where it occurs, describe)

• mostly distal convoluted tubule (DCT)

• unwanted stuff in blood is taken into nephron and added to filtrate

Where does H2O absorption happen?

descending loop of Henle and PCT

visceral layer of Bowman’s capsule

podocytes with pedicels that wrap around the single layer of cells of glomerular capillaries and form inner wall of capsule

podocyte

modified simple squamous epithelial cells with pedicels

parietal layer of Bowman’s capsule

simple squamous outer wall

filtration membrane

fenestrations formed by the two layers of Bowman’s capsule

Types of Nephrons

1. cortical

2. juxtamedullary

What determines the amount of water absorbed out of loop of Henle in a nephron?

length of loop of Henle and the osmotic force as you go deeper into the medulla

cortical nephrons

loop of Henle only goes into medulla a little and, since the loop is shorter, cortical nephrons can only produce dilute urine (most common nephron)

juxtamedullary nephron

loop goes deeper into medulla and, since loop is longer, lots of water is able to be pulled out, making it capable of producing concentrated urine

renal corpuscle

glomerulus and Bowman’s capsule

Which nephrons can you find a thin and thick limb of the loop of Henle in?

juxtamedullary nephrons

components of proximal convoluted tubule specific to it (+ type of epithelium)

has mitochondria and microvilli for reabsorption because active transport requires lots of ATP and the microvilli increase surface area and rate of reabsorption back into blood

• simple cuboidal epithelium

components of distal convoluted tubule specific to it (+ type of epithelium)

sparse microvilli

• simple cuboidal epithelium

components of descending nephron loop specific to it (+ type of epithelium)

also called the “thin segment” in juxtamedullary nephrons because it has high water permability

• simple squamous epithelium

components of ascending nephron loop specific to it (+ type of epithelium)

also called the “thick segment” in juxtamedullary nephrons but it remains thin on some nephrons

• cuboidal or short columnar epithelium

cells of the collecting duct

1. principal cells

2. intercalated cells

principal cells of collecting duct (function, hormones that affect it, appearance)

function: maintain water and Na+ balance

• are affected by ADH and aldosterone

• have sparse short microvilli

intercalated discs of collecting duct (function, type of epithelium, two cell types)

function: maintain acid-base balance in blood

• cuboidal cells with abundant microvilli

• A and B cell types

Where are these cells located?

proximal convoluted tubule

Where are these cells located?

descending loop of Henle

Where are these cells located?

distal convoluted tubule

Where are these cells located?

collecting duct

peritubular capillaries (location, function)

surround cortical nephrons

pick up water reabsorbed from filtrate and return it to blood

vasa recta (+ type of nephrons, function)

long vessels that run parallel to long nephron loops

• in juxtamedullary nephrons

• involved in formation of concentrated urine and picks up water reabsorbed from filtrate and returns it to blood

juxtaglomerular complex (+ function, type of cells)

made from modified portions of the most distal part of ascending limb of loop of Henle and the afferent arteriole

function: regulate rate of filtrate formation and blood pressure

types of cells: macula densa and granular cells

macula densa cells of juxtaglomerular complex (+ function)

group of tall, closely packed cells of ascending loop of Henle

• act as chemoreceptors that monitor NaCl content of filtrate entering DCT

granular cells of juxtaglomerular complex (+ function)

large, smooth muscle cells surrounding afferent arterioles

• act as mechanoreceptors that monitor BP

• secretory granules contain renin (enzyme)

outward pressure (+ function)

hydrostatic pressure in glomerular capillaries

• promote filtration

inward pressures (+ functions)

1. hydrostatic pressure in the capsular space

2. colloid osmotic pressure in glomerular capillaries

• inhibit filtration

hydrostatic pressure in glomerular capillaries (+ value)

the force pushing water and small solutes out of capillaries

• HPgc = 55 mm Hg

Hydrostatic pressure in the capsular space (+ value)

due to fluid pressure of filtrate in confined space with narrow outlet

• Hpcs = 15 mm Hg

Colloid osmotic pressure in glomerular capillaries (+ value)

“pull” of plasma proteins

• OPgc = 30 mm Hg

Net filtration pressure (+ equation, what it controls)

outward pressure - inward pressure

55 mm Hg (outward) - 45 mm Hg (inward) = NFP of 10 mm Hg

• controls glomerular filtration rate

glomerular filtration rate

volume of filtrate formed by both kidneys per minute (mL/min)

myogenic mechanism

smooth muscle contracts when stretched to help maintain normal glomerular filtration rate despite regular fluctuations in BP

How does the myogenic mechanism adapt to a rise in BP?

stretches smooth muscle of afferent arteriole, causing it to contract and constrict blood flow into glomerulus (protecting it from high BP)

How does the myogenic mechanism adapt to a fall in BP?

causes afferent arteriole to relax and vasodilate

How does tubuloglomerular feedback mechanism adapt to rise in glomerular filtration rate?

increased flow causes less time to reabsorb NaCl

• macula densa cells respond to elevated NaCl levels by releasing vasoconstrictor chemicals

• constriction of afferent arterioles lowers GFR back to normal, allowing adequate time for NaCl reabsorption

How does tubuloglomerular feedback mechanism adapt to a fall in glomerular filtration rate?

decreased flow causes too much time to reabsorb NaCl

• macula densa cells respond to low NaCl levels by releasing vasodilator chemicals

• dilation of afferent arteriole raises GFR back to normal, allowing adequate time for NaCl reabsorption

Sympathetic nervous system controls at rest vs. in ECF crisis

at rest: SNS activity is low, afferent arterioles dilated

in ECF crisis: SNS fibers release lots of norepinephrine, causing vasoconstriction throughout body

• vasoconstriction causes GFR to reduce urine output, increase blood volume, and increase blood pressure

tubular reabsorption (+ types)

quickly reclaims most tubular contents and returns them to blood

1. transcellular

2. paracellular

transcellular tubular reabsorption

substances move directly through tubule cell via active transport, facilitated transport, diffusion, etc.

paracellular tubular reabsorption

substances pass between tubular cells

obligatory water reabsorption

occurs in the PCT and descending loop of Henle where aquaporins are already present and water moves based on osmotic force (water follows ions)

facultative water reabsorption

occurs in collecting ducts where concentration of aquaporins depends on ADH

• as ADH increases, aquaporins increase (more water reabsorbed)

transport maximum

reflects the number of transport proteins available to move that substance

• when transporters are saturated with their solute, the rest is excreted in urine

hormones involved with the urinary system

1. aldosterone

2. ADH

3. RAAS

4. ANP

aldosterone functions

• causes Na+ to be reabsorbed into blood by creating more Na+ channels in collecting duct

• water follows Na+ out, increasing blood volume and BP

• causes K+ to be secreted back into filtrate

ADH functions

• facilitated H2O reabsorption

• direct vasoconstriction

RAAS functions

• angiotensin II is direct vasoconstrictor

• aldosterone causes reabsorption of Na+ (and therefore H2O) to increase BP

ANP (atrial natriuretic peptide) functions

• direct vasocontriction

• secretion of Na+ and H2O into urine to decrease BP

tubular secretions

• follows same steps as reabsorption but in reverse

• selected substances such as K+, H+, NH4+, and creatinine from nearby peritubular capillaries are transported from IF into filtrate

types of countercurrent mechanisms

1. countercurrent multiplier

2. countercurrent exchanger

countercurrent multiplier (+ what it depends on)

interaction of filtrate flow in ascending and descending limbs of long nephron loop

depends on:

• flow in opposite directions through adjacent parallel limbs

• descending limb is permeable to water but not solutes

• ascending limb is impermeable to water but pumps out solutes

countercurrent exchanger

blood flow in ascending/descending limbs of vasa recta provides passage exchange with IF

What is the function of both types of countercurrent mechanisms?

produce and maintain medullary osmotic gradient (300 mOsm in cortex and 1200 mOsm at tip of medulla)

How does the vasa recta maintain the medullary osmotic gradient?

by preventing rapid removal of solutes from IF and removing reabsorbed water

What is the osmotic concentration in the cortex vs the inner medulla?

cortex: 300 mOsm

medulla: 1200 mOsm

How does urea contribute to the medullary osmotic gradient?

1. enters filtrate in ascending limb of loop of Henle

2. cortical collecting ducts reabsorb water, concentrating urea left behind

3. in deep medulla, some of the (now more concentrated) urea leaves the collecting ducts and either enters the IF or is excreted in urine

origin or urea

NH3 is toxic to humans so the liver converts it to urea

What is the purpose of a urinalysis

used to help diagnose diseases and detect the presence of illegal substances in the body

What is used to analyze renal function?

urinalysis and blood test

composition of urine

95% water and 5% solutes

types of nitrogenous wastes in urine

urea, uric acid, creatinine

uric acid

nucleic acid metabolism

creatinine

creatine phosphate metabolism

What are the normal solutes in urine?

Na+, K+, PO4³-, SO4²-, Ca²+, Mg²+, HCO3^-

What does glucose in urine indicate?

diabetes mellitus

What does high levels of proteins in the urine indicate?

nonpathological: pregnancy or excessive exertion

pathological: hypertension, heart failure, renal disease

What does high levels of ketone bodies in the urine indicate?

starvation or uncontrolled diabetes mellitus

What does high levels of hemoglobin in the urine indicate?

transfusion reaction, hemolytic anemia, severe burns

What does high levels of bile pigments in the urine indicate?

liver disease or obstruction of bile duct from liver to gallbladder

What does high levels of erythrocytes in the urine indicate?

bleeding urinary tract due to trauma, kidney stones, infection, cancer

What does high levels of leukocytes in the urine indicate?

UTI

normal color of urine

clear or pale to deep yellow

abnormal colors of urine and potential causes

pink, brown, smoky

• may result from certain foods or meds or presence of blood or bile pigments in urine

odor of urine

slightly aromatic when fresh (depends on diet) and develops an ammonia scent upon standing as bacteria metabolizes urea

What can cause abnormal odor of urine?

drugs, vegetables, disease

pH of urine (+ what can change it)

slightly acidic (~6)

acidic diet: lots of protein and whole wheat lowers the pH

alkaline diet: vegetarian diet can raise the pH

ureter (function, length, how does it perform its function)

transports urine from kidneys to bladder

• 25 cm in length

• uses peristaltic waves, gravity, and hydrostatic pressure

How does the urine not back-flow from the bladder to the ureter?

as bladder pressure increases, distal ends of ureters close