Reading Part 4: The Urinary System Concepts: Chapter 25 Tortora: Chapter 26 Coloring Book:...

ReadingPart 4: The Urinary System

Concepts: Chapter 25Tortora: Chapter 26Coloring Book: pp146-150

The Urinary System

Urinary system contains:Kidneys (2)Ureters (2)BladderUrethra

The Urinary System

Most of the work in the urinary system is done by the kidneys. They are responsible for: Regulating

ionic composition of blood (Na+, K+, Ca++, Cl-, HPO4--)

blood pH (excrete H+ & conserve HCO3-) blood volume (via H2O) blood pressure (via H2O & the hormone renin) blood glucose level (by using the aa glutamine to

make glucose—called gluconeogenesis) blood osomolarity (total # of solutes/liter)

The Urinary System

Production of hormones (calcitriol, the active form of vit. D & erythropoietin which stimulates RBC production)

Excretion of wastes & foreign substances (ammonia & urea from aa breakdown, bilirubin from hemoglobin breakdown, uric acid from nucleic acid breakdown, drugs & toxins)

The rest of the urinary system is responsible for urine transport & storage.

The Urinary System--anatomy

Kidneys are reddish kidney-bean shaped organs.

Size of a bar of soap.Btwn peritoneum & posterior wall of

abdomen.At level of T12-L3 (some protection from

ribs 11-12).Concave side faces vertebral column.Blood vessels, nerve & lymph enter kidney

at renal hilum.


Smooth outer region is called renal cortex.

Inner region is called renal medulla.Within medulla are regions called renal

pyramids.Base of pyramid faces cortex, apex (called

renal papilla) faces hilum.Within medulla are the functional units of

the kidney, called nephrons.


Urine formed in nephron drains into papillary duct, then minor calyxmajor calyx renal pelvis ureter bladder urethra & out.


Kidneys receive 25% of cardiac output via right & left renal arteries.

These arteries branch several times until they form afferent arterioles.

Each afferent arteriole ends as a tangled ball of capillaries called a glomerulus at the nephron.


Glomerular capillaries reunite to form efferent arterioles that carry blood out of glomerulus.

Efferent arterioles branch to form peritubular capillaries which surround some of the tubular parts of the nephron.

Some of these vessels form long loops called vasa recta.

Eventually all these vessels reunite to form renal vein.


Each nephron has 2 parts:Renal corpuscle

Blood plasma filtered hereConsists of glomerulus & Bowman’s capsule


Renal tubuleFiltered fluid passes into hereConsists of:

Proximal convoluted tubule (PCT) Loop of Henle (descending limb, thin & thick

ascending limbs) Distal convoluted tubule (DCT)

Distal convoluted tubules from several nephrons empty into a single collecting duct. Many of these will unite to form papillary ducts.

The Urinary System--physiology

To produce urine, nephrons & collecting ducts do 3 basic things:

1. Glomerular filtration—water & most blood solutes move across glomerular wall into Bowman’s capsule.

2. Tubular reabsorption—Most water & solutes are moved back into blood stream from the tubules.

3. Tubular secretion—Some materials (water, drugs, wastes) from blood are moved back into the tubules.


By filtering, reabsorbing & secreting, nephrons help to maintain homeostasis of blood volume & composition.

1 million nephrons/kidneyFilters 150-180 liters/day 99% of that returns to bloodstream1-2 liters excreted as urine


The cells that line the glomerular capillaries are “leaky.” They form a filtration membrane.Large solutes like large proteins & blood

cells can’t pass thru.Small molecules do pass thru the capillaries

& into Bowman’s capsule:Water, glucose, vitamins, amino acids ammonia,

urea, ions & some small proteins.


In addition to the permeability of the filtration membrane, glomerular blood pressure plays a role.

The hydrostatic pressure of blood here is high & helps to force fluids & solutes thru the filtration membrane like pushing material thru a sieve.

The fluid that collects in Bowman’s capsule is called filtrate.

Amount of filtrate formed each minute determines the glomerular filtration rate (GFR).


Kidneys have mechanisms to adjust GFR: Renal autoregulation: if BP rises or falls, afferent

vessels to nephrons adjust to keep GFR constant. Neural regulation: CNS responds to things like

exercise or hemorrhage to ↓ GFR (less urine & ↑ blood volume to tissues)

Hormonal regulation: Angiotensin II constricts vessels to nephron when blood volume decreases.


Reabsorption is the next important task of the nephron.

Almost everything that enters the kidney is reabsorbed or put back into the blood stream.

Filtered fluid becomes tubular fluid once it enters the PCT.

Composition of tubular fluid changes as it passes thru nephron.

Fluid that drains from papillary duct at the end is called urine.


Substances can be reabsorbed via active & passive transport or pinocytosis.

Be reabsorbing (bringing substances back into the blood stream) & secreting (putting substances from the blood into the tubular fluid) the kidney is able to maintain constant fluid volume in your body even though your intake can vary.


When fluid intake is low, kidney can conserve water & produce small amounts of concentrated urine.

When fluid intake is high, urine is more dilute & in larger quantities.


The concentration of solutes in the interstitial fluid surrounding the nephron will determine how much water is reabsorbed—i.e. urine concentration.

Remember the rules for osmosis!


See AP section of website for tutorial. Material entering Bowman’s capsule is called

filtrate. Filtrate = H2O, NaCl, HCO3-, H+, urea,

glucose, aa, some drugs. #1--PCT—this area actively transports NaCl

from filtrate. H2O will follow. These 2 things are picked up by the blood vessels surrounding each nephron & eventually end up in renal vein.

Skip to:

http://www.sumanasinc.com/webcontent/animations/content/kidney.html


#3--Ascending limb also actively reabsorbs NaCl, but it’s NOT permeable to H2O. NaCl conc. increases in the surrounding tissue.

#2--Descending loop is NOT permeable to NaCl, but IS permeable to H2O. High NaCl conc. in tissue makes H2O diffuse out of descending loop. As a result, fluid in tube gets concentrated.

Because a lot of solute leaves in #3, the fluid in


#4--(DCT) ends up being less concentrated than tissue fluid. This causes more H2O to leave the DCT osmotically. By now, urea is the primary solute in the filtrate.

#5--Collecting duct--This part is permeable to H2O but nothing else. As tube descends thru tissue w/ increasing conc., H2O is reabsorbed via osmosis. At the end of collecting duct, concentrated filtrate is called urine.

The reabsorption of fluids can be affected by hormones and drugs.


Hormonal regulationRenin-Angiotensin-Aldosterone System

(RAAS)Antidiuretic Hormone (ADH)Atrial Natriuretic Peptide (ANP)


Renin-Angiotensin-Aldosterone System (RAAS)When blood vol. & pressure decrease

(blood loss, diarrhea) walls of afferent arterioles are not stretched as much.

Renin is releasedRenin is an enzyme that changes inactive

angiotensin I into active angiotensin II.


Causes afferent arterioles to constrict (↓ GFR)↑ reabsorption of Na+, Cl- & H2O in PCT.Stimulates adrenal gland to release the hormone

aldosterone. This causes ↑ reabsorption of Na+, Cl-, plus secretion of K+. Result = H2O excretion ↓’s.

Blood volume increases.


ADH (aka vasopressin)When blood volume decreases

(dehydration, bleeding) pituitary gland releases ADH.

Causes DCT & collecting duct to be more permeable to water.

Alcohol & caffeine inhibit ADH


ANPAn increase in blood vol. promotes its

release. Inhibits reabsorption of Na+ & water in PCT &

collecting duct.Suppresses secretion of ADH & aldosterone.

Diuresis


UrinalysisLooking at chemical, physical & microscopic

properties of urine can show problems in other parts of body.

Traces of substances not normally found in urine can indicate metabolic problems.

Constituent Abnormal amts may indicate:

Albumin High BP or bacterial infection

Glucose Diabetes

RBC’s Kidney stones, trauma, kidney disease

Urobilirubin Anemia, liver disease, mono

Microbes Bacterial infection

The Urinary System—storage & elimination

From collecting ducts urine passes to the calyces, renal pelvis, ureter, bladder & urethra.

Ureters from each kidney contract to push urine into bladder. Gravity assists.

Bladder is a hollow, distensible, muscular organ.

Micturition=release of urine from bladder.


Urethra is terminal portion of urinary system.4 cm in females, 20 cm in males.

The End

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