The Urinary System Part 1: Overview

Function of… The urinary system gets rid of nitrogen containing waste in the body, regulates the amount of water, salts, and maintains the proper pH of the blood

Function (Con’t) The kidneys filter gallons of fluid from your blood stream each day This eliminates waste but also keeps your blood volume in check The kidneys produce an enzyme that regulates your BP and a hormone that stimulates RBC production And they produce urine

The Kidneys Located against the dorsal body wall just under your 11th to 12th rib - protection They are about the size of a large bar of soap The medial side has the renal hilum or spot where the ureters, blood vessels, and nerves enter and exit

Kidney Anatomy The kidney has 3 main regions The outer region is the renal cortex The middle layer is the renal medulla, which contain the renal pyramids (the tip of the pyramids point deeper into the kidney) The inner layer is the renal pelvis, which is a cavity or basin that collects urine and passes it to the calyces – both are extensions of the ureter

Blood Supply 25% of our blood is passing through our kidneys each minute! This blood passes into the kidney via the renal artery So “dirty blood” enters the kidney via the renal a. and “clean blood” leaves the kidney via the renal vein

Where the Magic Happens! Nephrons are the functional unit of the kidney – they make urine The players to learn here are the glomerulus, renal tubule, Bowman’s capsule, proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), afferent & efferent arterioles, and collecting ducts.

Nephron Anatomy

The Urinary System Part 2: Making Urine

The Big Picture Urine formation is complicated but can be broken down into 3 basic steps Glomerular filtration – filter your blood Tubular reabsorption – bring back “good” stuff that got filtered into the urine Tubular secretion – getting the last of the “bad” stuff out of the blood

Complicated???

Step 1: Glomerular Filtration The glomerulus is a filter Fluid leaves the blood and enters Bowman’s capsule This fluid is just blood plasma without the cells and other large proteins (too big to leave the blood) It’s this fluid that will eventually become pee (urine) This step is all driven by blood pressure

Blood in the Urine Not a good thing! Blood cells should be too big to get through the capillary cells So if blood is in the urine, something is wrong with the glomerulus – boxers often have blood in their urine from blows taken by their kidneys

Step 2: Tubular Reabsorption The filtrate has a lot of good stuff in it and we don’t want to pee it out. Like what? Stuff like water, glucose, amino acids, and useful ions – this gets reabsorbed by the blood stream Bad stuff like nitrogenous waste (urea), uric acid, and creatinine stay behind Note – most of this step takes place in the PCT – proximal convoluted tubule

Step 3: Tubular Secretion This step makes sure we get rid of all the bad stuff Certain drugs and ions are passed into the urine here This occurs in both the PDT and the DCT or distal convoluted tubule

Loop of Henle & the K-Rat This part of the nephron recovers water and salt before it leaves the body as urine It is also the area that makes urine more concentrated then the blood in our blood stream – so we don’t need to drink too much water to survive The Kangaroo rat loop of Henle is so long, and efficient that the K-rat of the desert never needs to drink a single drop of water in their entire life!

Urine Despite up to 180 liters of blood plasma passing into the glomerulus, only 1 to 2 liters of urine are produced per day Urine contains nitrogen waste and other substances not needed by the body Color: clear to dark yellow (caused by the pigment urochrome, a breakdown product of hemoglobin It is sterile, and picks up an ammonia odor over time The pH is slightly acidic (pH of 6) Its specific gravity is greater than 1.0 (1.001 – 1,035) You should not find blood cells, blood proteins, sugar, glucose, hemoglobin, and white blood cells

The Urinary System Part 3: The Three U’s

Ureters The passageway for urine to leave the kidney and enter the bladder Smooth muscle drives the urine down toward the bladder using peristalsis Valves prevent the urine from going back up the ureter

Urinary Bladder A collapsible bag that stores urine Your bladder holds about 500 ml or pee but can actually hold up to 1 liter The bladder has three openings – 2 ureters and 1 urethra

Urethra The urethra carries your pee to the outside of your body You have two valves or sphincter muscles – one is involuntary (smooth muscle) the other is voluntary (skeletal muscle) In females it is short (1.5” long) but much longer in males (8” long)

Micturition The fancy name for peeing – micturition After you get past 200 ml of pee, contractions of the bladder send pee past the first valve and up against the 2nd valve – you control that one If you “hold it in” you can collect another 200 ml before the contractions start again

Incontinence/Retention If you can’t control the 2nd valve, you are incontinent – common among younger children If you can’t pee you have urinary retention and may need a catheter or drainage tube inserted