BIO127 – Chapter 14 Excretory/Urinary System
What you should learn
By the end of this study, you should be able to:
Name the four main parts of the urinary system and tell one job each performs.
Describe how urine is made, sent out, and removed from the body, step by step.
Learn and understand medical words using their basic parts: roots, combining forms, prefixes, and suffixes.
Explain what “excretion” means and list all body parts that get rid of waste.
List the body’s main nitrogen wastes and where they come from.
Explain eight other jobs kidneys do besides making urine (like controlling blood pressure, balancing acids and bases, and making hormones).
Describe the kidney’s outside and inside parts, the tiny nephron parts, and how blood and filtered liquid flow through a nephron.
Compare filtering, taking back, and secreting – what they involve, which way things move, and what makes them happen.
Talk about how water pills (diuretics) change how much urine is made.
Describe the key parts of the ureters, bladder, male and female urethras, and the brain’s reflex that controls peeing.
Summarize changes that happen with age, common tests, and sicknesses of the system that gets rid of waste.
Medical Words – Basic Parts
azot/o – nitrogen (like in "azotemia" = too much nitrogen waste in blood).
cyst/o – bladder ("cystitis" = bladder inflammation).
glomerul/o – tiny filtering part of the kidney (glomerulus).
nephr/o & ren/o – kidney (like in "nephron", "renal artery").
pyel/o – funnel-shaped part of the kidney where urine collects (renal pelvis).
ur/o – urine / path urine travels ("urology" = study of urinary system).
ureter/o – tube from kidney to bladder (ureter).
urethr/o – tube from bladder to outside (urethra).
HYPOX - LACK OF OXYGEN
EMIA - low oxygen in the blood
OLYGO - NOT ENOUGH
Excretion – What it means
Excretion = getting rid of waste products made by the body’s chemical reactions.
Body parts that excrete waste:
Skin – through sweat: salts, lactic acid, urea.
Lungs – through breathing out humid air: CO_2 (carbon dioxide).
Liver – puts wastes/toxins into bile (like bilirubin).
Kidneys – make urine; get rid of nitrogen wastes, extra minerals, bilirubin, extra H^+ (acids).
Overall system jobs:
Keep fluids & electrolytes (salts) in balance.
Keep acid-base balance (pH) in check.
Control blood volume & overall blood pressure.
Removal of metabolic waste
Nitrogen Wastes
Ammonia (NH3)
Very toxic; comes from breaking down amino acids.
Liver cells change it into urea (less poisonous, dissolves in water).
Urea
Main nitrogen waste in humans; comes from breaking down proteins.
Uric Acid
Comes from breaking down DNA/RNA. (nucleic acid)
High levels can lead to gout or kidney stones.
Creatinine
Made when creatine phosphate (a muscle energy store) breaks down.
Doctors use it to guess how well kidneys are filtering (GFR) because it’s filtered out and barely taken back or secreted.
Kidney – Big Picture Anatomy
Blood Urea Nitrogen (BUN) - measurement of amount of urea nitrogen in the blood
Urea nitrogen is a waste product of protein breakdown
Two bean-shaped organs, located behind the abdomen “retroperitoneally” (T11–L3 level).
Three layers for protection and function:
Renal Capsule – a tough outer covering surrounding kidney.
Renal Cortex – the outer grainy area between the capsule and the inner part.
Renal Medulla – the inner striped pyramids where most of the work happens.
Inside collecting parts: pyramids are cone shaped renal tissue
Renal pyramids → papillae (tips of pyramids) → small cups (minor calyces) → larger cups (major calyces) → renal pelvis (main collecting area) → ureter.
Renal sinus holds the kidney’s artery, vein, nerves, lymph vessels, pelvis, and fat.
Kidney Blood Flow & Tiny Nephron Parts
Over 1 million tiny nephrons in each kidney – these are the working units that make urine.
Nephron parts
Renal Corpuscle (the "filter in a cup")
Glomerulus – a ball of tiny blood vessels that filter blood.
Bowman’s (glomerular) capsule – a double-layered cup that collects the filtered liquid.
Renal Tubule -
Proximal Convoluted Tubule (PCT) – where most useful things are reabsorbed (taken back).
Nephron Loop / Loop of Henle – a loop that creates a salt gradient, helping to control water.
Distal Convoluted Tubule (DCT) – takes back and secretes things based on hormone signals.
Collecting Duct (CD) – collects liquid from many nephrons; makes final adjustments to water and salts.
Blood flow path
Renal artery → smaller arteries (segmental, interlobar, arcuate, interlobular) → afferent arteriole (brings blood to filter) → glomerulus → efferent arteriole (takes blood away) → peritubular capillaries (tiny vessels around tubules, and vasa recta in some nephrons) → tiny veins (venules) → renal vein.
Juxtaglomerular Apparatus (JGA)
Juxtaglomerular (granular) cells in the afferent arteriole wall make renin (a hormone).
Macula densa – special cells in the DCT that check salt (Na+) levels and flow in the tubule.
Jobs: controls how fast blood is filtered (GFR), keeps blood pressure steady, and balances Na^+ (sodium).
Urine (Filtered Liquid) Flow Path
Order: Glomerular capsule → PCT → Loop of Henle → DCT → Collecting duct → Minor calyx → Major calyx → Renal pelvis → Ureter → Urinary bladder → Urethra → Then comes out of the body.
How Urine is Made (Physiology)
Filtration (passive, driven by pressure)
Happens in the renal corpuscle.
Blood in the glomerulus is filtered
Blood pressure pushes plasma (liquid part of blood, without big proteins or cells) through a filter.
Normal filtering rate (GFR) is about 125\;mL/min.
Kidneys control this by changing the width of the afferent/efferent arterioles; low body BP means slower filtering and less urine.
Too high of blood pressure can damage the kidney and surrounding vessels
water, amino acids, some nitrogenous waste, glucose, mineral salts are what get filtered out
Reabsorption (mostly active in PCT; water follows passively)
Body takes back useful things: all glucose & amino acids, about 99% of water, and some Na^+ (sodium), Ca^{2+} (calcium), Cl^-.
Ways it happens; special transporters, moving with Na^+ (co-transport), simple movement (diffusion/osmosis).
Secretion (active removal from blood around tubules back into the tubule)
Gets rid of leftover wastes (urea, creatinine), extra K^+ (potassium), H^+ (acid), and drugs.
Very important for balancing acids and bases by adjusting H^+ and HCO_3^- (bicarbonate) levels.
Occurs along the renal tubule
The nephron removes the rest of the waste in blood
Urine amount formula
ext{Urine Excretion} = ext{Amount Filtered} - ext{Amount Reabsorbed} + ext{Amount Secreted}
Acid-Base Balance & Kidney Help
Normal blood pH: 7.35\text{–}7.45 (target is about 7.40 ).
Acidosis (blood too acidic, < 7.35)
Respiratory: breathing too slowly, holding in CO₂ (e.g., lung disease).
Metabolic: kidneys can’t get rid of enough hydrogen H^+ or body makes too much acid.
Kidneys help: breathe faster (get rid of CO₂); kidneys send out more H^+ and take back more HCO₃⁻.
Alkalosis (blood too basic, > 7.45)
Respiratory: breathing too fast (panic attack). or hyperventilation
Metabolic: losing acid (lots of vomiting).
Kidneys help: breathe slower; kidneys get rid of HCO₃⁻ and keep H^+ (can't make new H^+).
Daily Water Balance
Water in (about 2.5 L/day)
200\,mL from body’s own processes (metabolic water).
700\,mL from food.
1,600\,mL from drinks.
Water out (about 2.5 L/day)
Feces 200\,mL.
Breathing out 300\,mL.
Skin evaporation 400\,mL + sweat 100\,mL.
Urine 1,500\,mL (this amount can change the most).
Hormones & Nerves Controlling Water
Antidiuretic Hormone (ADH / vasopressin)
Made in the brain (hypothalamus); stored in pituitary gland.
Triggers: blood too salty (high Na^+), low blood volume/pressure.
Job: puts special water channels (aquaporins) into DCT & CD → body takes back more water only → less urine, more concentrated urine.
Low ADH (e.g., from alcohol) → lots of diluted urine.
causes increase in water reabsorption
Aldosterone (hormone from adrenal gland) (mineralocorticoid)
Activated by the Renin-Angiotensin System (RAS) when BP is low, Na^+ is low, or K^+ is high.
Helps actively take back Na^+ (sodium) and secrete K^+ (potassium) in DCT/CD; water follows sodium → less urine, stable BP.
causes Na to be actively transported from tubule to peritubular capillaries
Atrial Natriuretic Hormone (ANH)
Released by heart (right atrium) when heart stretch/BP is high.
Effects: widens vessels going into the filter and narrows those leaving → faster filtering (↑GFR); stops renin, aldosterone, ADH; stops tubules from taking back Na^+ → lots of sodium and water loss in urine → lowers blood volume & BP.
increases urine production
Nerve control – Strong nerve signals (sympathetic) can narrow the vessels going into the filter during shock to save fluid.
Water Pills & Everyday Stuff
Alcohol – stops ADH from being released.
Caffeine – increases blood flow to kidneys and filtering, reduces kidney’s ability to take back Na^+ (sodium).
Prescription water pills (thiazides, loop diuretics, K-sparing) – usually block specific salt transporters, leading to more sodium and water leaving the body; used for high blood pressure, swelling, heart failure.
Ureters, Bladder, Urethra & Peeing
Ureters: thin tubes behind the abdomen that carry urine from the kidney pelvis to the lower back part of the bladder; muscle waves push urine along.
Urinary Bladder: a muscular bag (detrusor muscle) with folds and a special triangular area (trigone); stores urine temporarily (about 500 mL comfortably, up to 800 mL).
Urethra
Female: about 4 cm long; only for urine.
Male: has prostatic, membranous, penile parts; used for both urine and reproduction.
Sphincters (control valves)
Internal (smooth muscle) – automatic, opens when parasympathetic nerves signal.
External (skeletal muscle, in pelvic floor) – voluntary control.
Micturition Reflex (Peeing Reflex)
Bladder stretch signals → spinal cord → nervous system tells detrusor muscle to contract and internal sphincter to relax.
Brain (pons, cerebrum) can help or stop this by controlling the external sphincter.
Aging Effects on Urinary System
Males: Enlarged prostate (BPH) narrows the urethra → weak stream, difficulty starting.
Females: Stress incontinence (leaking with cough/sneeze) is common due to weaker pelvic floor muscles.
Overall kidney size and filtering ability decrease; harder for body to clear drugs.
Tests for Diagnosis
Cystoscopy – using a scope to look inside the urethra, bladder, and prostate.
Intravenous Pyelography (IVP) – X-ray study of kidneys/ureters/bladder using special dye injected into a vein.
Urinalysis – checking urine’s physical traits (color, clearness, density), chemical levels (with a dipstick), and under a microscope (cells, crystals).
Common Urinary Problems
Urinary Tract Infection (UTI)
Urethritis – infection of the urethra (often from gut bacteria).
Cystitis – inflammation of the bladder; more common in women; can move up to pyelitis (renal pelvis) or pyelonephritis (kidney cortex).
Hydronephrosis – urine backing up in the kidney due to a blockage (stones, clots, growths, narrow ureter).
Polycystic Kidney Disease (PKD) – genetic disease; many fluid-filled cysts grow and destroy kidney tissue.
Kidney Stones (Nephrolithiasis)
Formed from calcium oxalate or uric acid; can block the ureter; treated by drinking water, sound waves (lithotripsy).
Glomerulonephritis
Acute (sudden): after strep throat; leads to protein and blood in urine.
Chronic (long-term): kidney’s filter progressively thickens → can lead to kidney failure.
Main Kidney Jobs (Summary)
Get rid of nitrogen wastes & toxins.
Control water, salts, and pH balance.
Make hormones: renin, erythropoietin, activate vitamin D.
Help make sugar (glucose) during fasting.
Keep blood volume & pressure steady using complex self-regulation and hormone signals.