The Urinary System

By the end of this class you should understand:

• The major functions of the urinary system• The major parts of the nephron and the function

of each• The purpose of reabsorption and what materials

are always or sometimes reabsorbed• The major components of urine and how this

reflects the state of the body• How major defects of the urinary system put the

body at risk

The Urinary System

• Also known as the renal system and excretory system

• Consists primarily of the kidneys which remove waste from the blood

• The waste becomes urine which collects in the bladder and is excreted by urination (peeing)

Three Components of Urine

• Water– If the body is dehydrated, a

minimal amount of water is still excreted

• Excess salts and any nutrients that are way above normal levels– Diabetes was originally diagnosed

by testing urine for sweetness• Anything in the blood that the

body does not recognize as useful (including drug metabolites)

Urine Drug Test

• Why must we pee in a cup for a job?

• Drugs are not recognized by the body as being nutrients and so are not reabsorbed back into the blood when they are filtered

• The urine essentially reflects the state of the blood, including drugs and metabolites (broken-down drugs after processing by the liver)

Kidneys• The kidneys are wrapped

in layers of connective tissue– Up to a quarter of all blood

flow can pass through kidneys, so bruising and laceration can be lethal

• The outer layer is the cortex

• The inner core is the medulla– The medulla is very salty

Kidneys

• The kidneys produce filtrate when they filter blood plasma

• The filtrate is then processed– The end result is urine

• A huge amount of blood flows through the kidneys every day (up to ¼ of all blood flow)– The kidneys can excrete excess

water at an impressive rate if hyperhydrated

Nephron

• The kidney is composed mostly of a set of tubules, called a nephron– The kidney may have up

to a million nephrons• The nephron is mostly a

tube of epithelial tissue called a tubule– The tubule contains

filtrate, which becomes urine

Nephron Diagram

Filtration• Blood enters a special leaky

artery called the glomerulus– Plasma leaks out of the

glomerulus into the glomerular capsule (AKA Bowman’s capsule)

• RBCs and proteins are too large to enter the capsule

• Plasma, including salts, nutrients, and wastes, pass through the glomerulus– Referred to as glomerular

filtration

Renal Circulation• The blood flow through the

kidneys is called renal circulation

• After blood enters the renal artery, it becomes tiny afferent arterioles

• These become the glomeruli (leaky arteries), which then become efferent arterioles

• The efferent arterioles branch to become the capillaries

Tubule Function• The tubule’s primary function is

to reabsorb nutrients– Primarily glucose, amino acids

and vitamins• This is performed by active

transport and can only be performed at a certain rate– The concentration of nutrients

also pulls water out via osmosis– Excess nutrients will end up in

the urine• The nutrients are put back into

the peritubular capillaries that follow the nephron

Additional Reabsorption

• In addition to reabsorbing nutrients and salts, certain nutrients are reabsorbed only if they are in demand

• Primary example is calcium– Reabsorbed from the kidney only when

PTH is in the blood, which is the signal for low calcium

– What gland releases PTH?• Additional example is sodium– Reabsorbed more strongly when

aldosterone is in the blood, which is the signal for low sodium

– What gland releases aldosterone?

Tubular Secretion• Some waste products and

drugs are also actively transported into filtrate by tubular secretion– Also requires active transport

• Creatinine (waste product from anaerobic cell activity) is reliably filtered and secreted by the kidney– Blood tests in the hospital for

creatinine levels test kidney function

Loop of Henle

• The loop of Henle is the middle part of the tubule– The loop of Henle takes advantage

of the medulla’s saltiness• The descending limb allows

water to leave the tubule through osmosis

• The ascending limb is impermeable to water but has ion channels so that salts diffuse out of the tubule

Loop of Henle function• The Loop of Henle is designed

to reclaim almost all the water and salt from the filtrate

• The capillaries of the medulla are called vasa recta, which reclaim water and salt for the blood– If the Loop of Henle did not

reclaim the water and salt, we would pee as fast as we filtered

– Instead, we can concentrate our urine, allowing us to not pee all day

Collecting Duct

• The tubules begin fusing to become collecting ducts– The collecting duct still contains

dilute urine (with a lot of water)• There are pores that allow water

to leave the collecting ducts and enter the medulla

• These pores are only opened by the action of a hormone called ADH, to conserve water– Where is ADH secreted from?

Release of Urine• When filtrate leaves the

collecting duct, it goes through the ureter to the bladder, where it is now officially urine

• The release of urine (urination, or micturition) is under parasympathetic control– Which is why for some people it’s

hard to go when you’re being watched

– There are sympathetic nerve fibers in the smooth muscle of the bladder, which is why panic can cause some to wet themselves

Urination

• The urethra is the tube that carries urine from the bladder to the outside of the body– In men the urethra meets with the

vas deferens, so urine exits the same opening of the penis that semen exits

– In women the urethra has a separate opening from the vagina

• The urethra has sphincter muscles that can hold it closed, but the brain must be trained to control them (hence diapers for babies)

Kidney Homeostasis

• Kidney function is vital to life– Many nitrogenous wastes that are eliminated by

the kidney are toxic to our cells– The body can survive with one kidney, but that

kidney can become overworked and fail

• Kidney injuries are very hard to survive– So much blood flow through the kidney that being

stabbed in the kidney can result in death from loss of blood in seconds, similar to the heart

Dialysis• If both kidneys fail through

nonviolent means, the patient may require dialysis, which is essentially artificial kidney function– There are several options, all of

which are inferior to having actual working kidneys but all of which are superior to death

– The most effective is hemodialysis, where blood is pumped through a dialysis machine that cleanses the blood

Other Kidney Problems

• Kidney Stones– Crystals form in the kidney or

bladder, caused by a combination of factors including high calcium, alkaline blood, and not drinking enough water

• Urinary Tract Infection– Bacteria can invade the urethra

and potentially damage the body– Infected bladder is worse,

infected kidney can be fatal

That’s the end of class!

• For those of you disappointed in the lack of naughty bits, Wednesday’s lesson is all about naughty bits!

• For those of you who thought this was too much naughty bits already, you may want to reconsider coming on Wednesday…