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Biology: The Urinary System Cheat Sheet by

• Explains the functions of the urinary system • Identify the structures of the urinary system • Describes the general process filtration, reabsorption, and secretion. • Discuss the regulation of the urinary system

Terms - Alphab­etical

Aquaporins – A membrane protein that allows the transport of water across a cell membrane
Calcitriol – Active form of Vitamin D
Defecation – Removal of feces
Detrusor Muscle – Smooth muscle in the wall of the bladder
Internal Urethral Sphincter – A ring of muscle that controls the flow of urine
Juxtag­lom­erular Cells – Cells in the kidney that synthe­size, store, and secretes renin when blood pressure decreases
Erythr­opo­ietin – Stimulates bone marrow to produce red blood cells
Excretion – Remove urine from body
External Urethral Sphincter -A ring of muscle that can be volunt­arily controlled to contract and prevent urination
Macula Densa – An area of cells that line the upper portion of the ascending loop of henle that detects sodium levels and regulated blood vessel diameter to regulate blood pressure
Nephron – The functional unit of the kidney
Peritu­bular Capill­aries – Small blood vessels that surround the nephron
Renal Capsule – Outermost connective tissue layer of the kidney
Rugae – Folds in the stomach lining – is collapsed and stretched out as bladder fills
Transi­tional Epithelium – Cells that can change shape


• Regulates blood water and ion compos­ition
• Excretes waste products of metabolism
• The kidneys are the most important regulator of
blood pressure.
• Produces renin – renin-­ang­iot­ensin system
• Regulates blood pH by excreting excess H+ ions

• The kidneys also release two hormones:
Calcitriol – tells small intestine to make calcium
Increase in salt -> increases water -> Increases blood volume -> increases blood pressure



2 Kidneys – Bean shaped, where urine is formed
•Regulates blood volume and blood pressure
•Produce hormones
•Excrete wastes
2 Ureters – Muscular tubes that carry urine from kidneys to bladder
1 Bladder – Expandable organ that stores urine until released from the body
1 Urethra – Tube that carries urine from bladder to outside of the body
Note: Highli­ghted in the picture above.


•Primary organ of excretion
•Covered by renal capsule
•Adipose tissue on outside
Renal Medulla – Middle Layer
Renal Cortex – Outer Layer
Renal Pelvis – Forms ureter
  •Renal Calyces

Layers of the Bladder Wall

Stretchy as it fills with urine
Inner mucosal layer – Cubed shaped cells stretch and flatten
•Includes transi­tional epithelium
•Forms folds or rugae
Middle muscular layer
•Three layers of smooth muscle
•Called detrusor muscle
Outer layer
• Superior surface – serous membrane of peritoneum
• Remaining surface – covered by fibrous coating


Internal Urethral Sphincter – ring of muscle that opens and closes
•Smooth muscle surrou­nding the opening to the urethra
•Opening and closing is under involu­ntary control
External Urethral Sphincter
•Skeletal muscle surrou­nding the urethra below the internal urethral sphincter
•Under voluntary control


Mostly of nitrog­enous wastes (metabolic wastes):
•Urea made by the break down of amino acids
•Uric acid made by the break down of nucleo­tides
•Creat­inine made by muscle cells from the breakdown of creatine phosphate


• Are the structural and functional units of the kidney. Each kidney has over 1 million of these units.
• Produces urine
• Has blood supply with 2 capillary areas (glome­rular and peritu­bular)
• Each nephron consists of a glomerulus, which is a knot of capill­aries and a renal tubule.
• The tubule forms a cup shape around the glomerulus – called the glomerular capsule or Bowman’s capsule

Glomerular capsul­e/B­owman’s capsule is different from glomer­ulus.

Anatomy of a Nephron

Anatomy of a Nephron

Glomerulus – a knot of capill­aries inside the glomerular capsule where pores produce a blood filtrate – located in the renal cortex.
Proximal Convoluted Tubule – Epithelial layer with a brush border of microvilli to allow reabso­rption of filtrate compon­ents.
Loop of Henle – U-shaped structure that has a descending limb to allow water to leave and an ascending limb that pushes out salt.
Distal Convoluted Tubule – Made of epithelial cells rich in mitoch­ondria and thus is important for movement of molecules from the blood to the tubule (tubular secret­ion). Waste products are actively transp­orted out of blood capill­aries and into the tubule.
Collecting Ducts – Several nephrons share a collecting duct which serves to carry urine to the renal pelvis. Water continues to leave through the collecting ducts to be reabsorbed into the blood if ADH is present.

Functions of the Nephron

Filtration – Fluid from the glomerular capill­aries into Bowman’s capsule.
Reabso­rption - Fluid moves from nephron tubule back into the blood stream.
Secretion - Fluid moves from peritu­bular capill­aries into the nephron tubule.
Excretion - Fluid moves from the nephron tubules to bladder to outside of the body.
Amount filtered – Amount reabsorbed + Amount secreted
= Amount of substance excreted.

Formation of Urine

Formation of Urine
•3 Processes:
•Glome­rular Filtration
•Tubular Reabso­rption
•Tubular Secretion

Formation of Urine I

Glomerular Filtra­tion:
• Filtration occurs in the renal corpuscles
• Water and small molecules move from the glomerulus to the Bowman’s capsule while large molecules and blood cells remain in the glomerular blood.
- Afferent and efferent arteriole
• Substances are filtered out due to high blood pressure
• This pushes all plasma molecules (Water, Na+, Cl, glucose, nutrients) out of the capill­aries & into the capsule. This is Filtrate.

Formation of Urine II

Formation of Urine II

Tubular Reabso­rption:
• Glomerular Capsule then becomes the Proximal Convoluted Tubule.
• Surrounded by peritu­bular capillary network
• Microvilli line lumen tubules to increase surface area
• 60-70% of reabso­rption occurs in the proximal tubule for reabso­rption
- Water, nutrients, salt, are reabsorbed
- Most metabolic wastes, some salt and some water are not reabsorbed
• Reabso­rption can occur by diffusion, active transport or facili­tated transport
Loop of Henle:
In the Renal Medulla
• U-Shaped tubule
• Descending and ascending limb
• Mainly water and salt movement
• In the descending limb it’s highly permeable to water due to aquaporins
• Water moves out of the tubule via osmosis
• In the ascending limb, the pores are designed so water cannot move through it
- Salt and K+ is actively transp­orted out.
Note: Amino acids and Glucose are 100% reabsorbed
Glucose is not reabsorbed with high blood sugars or diabetes

Formation of Urine III

Tubular Secretion:
•Distal Convoluted Tubule
•Hormonal regulation affects the remaining reabso­rption of ions and water.
•Potassium, H+ ions, penici­llin, creatinine (waste product) are actively transp­orted out of the blood capill­aries and into the tubule.
•Uric acid is also secreted into the tubules to be excreted in the urine. An excess of uric acid can cause gout or kidney stones
Collecting Duct:
•The distal tubule joins with the other distal tubules to form a collecting duct
•Water continues to leave through the collecting duct to be reabsorbed into the blood if ADH is present

Antidi­uretic Hormone

• Water absorption is regulated by antidi­uretic hormone (ADH) which opens up more aquaporins in the collecting duct
• ADH is secreted by the pituitary gland when our fluid intake is low – at night, in the summer

Juxtag­lom­erular Cells

• In the arteriole wall the granular cells – Juxtag­lom­erular cells (JG) – are enlarged smooth muscle cells that have secretory granules that contain the hormone renin – part of the renin-­ang­iot­ensin system (RAS) – It increases Blood Pressure (increase salt and water)
• The JG cells are mechan­ore­ceptors (they sense BP) in the afferent arteriole – responds to changes in blood pressure.
• The macula densa is a group of tall, closely packed cells located adjacent to the JG cells.
• The macula densa cells are chemor­ece­ptors that respond to changes in NaCl content of the filtrate
• These cells work in tandem and are critical regulators of the blood pressure
- Measures sodium content
- Responds to changes in salt concen­tration

Maintain acid-base Balance of Blood

•Homeo­static mechanism
•The kidneys maintain balance by excreting hydrogen ions and reabso­rbing the bicarb­onate ions
•This acid-base balance helps maintain a blood pH of 7.4.
•Narrow Range -> 7.35 – 7.45
How is it Mainta­ined:
•Buffers are chemical or a combin­ation of chemicals that can take up excess H+ (more acidic in blood) or excess OH-(more basic in blood)
•The respir­atory centre in the brain can increase breathing rate if the buffers cannot maintain pH. The kidneys are respon­sible for mainta­ining blood pH.

Kidney Disease

•Chronic Kidney Disease (CKD) is the most common disease and leads to kidney failure
•Primary causes are hypert­ension and diabetes.
• CKD is charac­terized by a gradual loss of kidney function
• Kidney function can be determined by the glomerular filtration rate (GFR). As kidney disease progre­sses, GFR decreases.
• Kidney function can also be determined by testing for urine albumin
• If urine contains protein, kidney damage is present
• Once kidney failure occurs, the only treatment options are dialysis or a kidney transplant.

Kidney Stones

• Kidney Stones affect 10% of the population
• Most common cause is excess uric acid or calciu­m-o­xalate precip­ita­tion• Most common cause is excess uric acid or calciu­m-o­xalate precip­itation
• Small stones often pass without treatment. Large kidney stones may need to be removed with surgery

Overall Process

• Glomerular capsule holds the knot of capill­aries called glomer­ulus.
• The capill­aries allow fluid, waste product, ions, etc., to pass from the blood into the capsule. This is called filtrate. It’s sent along the long renal tubule. – Glomerular Filtration
• The filtrate flows through the proximal convoluted tubule (Renal Cortex)that is made up of epithelial layer with a border of microvilli to allow reabso­rption of filtrate compon­ents.
• Filtrate continues down to the Loop of henle (dips into the Renal Medulla then back to the cortex) where the descending limb pushes water out and the ascending limb pushes salt out. – Tubular Reabso­rption
• Remaining filtrate goes to the distal convoluted tubule and collecting ducts where the filtrate is now officially urine. - Tubular Secretion

Extra terms

Aldost­erone - A steroid hormone made by the adrenal cortex (the outer layer of the adrenal gland). It helps control the balance of water and salts in the kidney by keeping sodium in and releasing potassium from the body. Too much aldost­erone can cause high blood pressure and a build-up of fluid in body tissues.
Angiot­ensin II - Causes the muscular walls of small arteries (arter­ioles) to constrict (narrow), increasing blood pressure. Angiot­ensin II also triggers your adrenal glands to release aldost­erone and your pituitary gland to release antidi­uretic hormone


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