BIO 202: Chapter 9 Urinary System

urinary system

plays critical role in maintaining homeostasis and carries out most functions by the kidneys

functions of the urinary system

• filter blood to remove metabolic wastes

• regulate fluid, electrolyte, and acid base balance

• assists liver in detoxifying certain compounds

• produce glucose during starvation

• produce the hormone erythropoietin

kidneys

paired, retroperitoneal, and encased by external connective tissue layers

renal fascia

superficial layer of dense irregular collagenous connective tissue

• anchors the kidneys to the posterior abdominal wall and the peritoneum 

adipose capsule

thick layer of adipose tissue

• cushions and holds the kidneys in place

renal capsule

thin, tough layer of dense irregular collagenous connective tissue

• encases each kidney like plastic wrap

renal hilum

medial indentation where blood vessels and the ureter enter and exit the each kidney 

renal cortex

outermost region of the kidney

• dark brown due to high density of blood vessels

• contains most nephron structures that perform blood filtration

renal medulla

middle region of the kidney

• composed of triangular renal pyramids

renal pyramids

contain looping nephron tubules and associated drainage structures which give them a striped/striated appearance

renal columns

inward extensions of the cortex that separate renal pyramids and contain blood vessels

renal papilla

tip or apex of each renal pyramid

minor calyces

urine from each renal papilla drains into this small space

• merge into major calyces which drain the rest of the renal pelvis

renal pelvis

innermost region of the kidneys that acts as a basin for urine collection

• major calyces drain here 

• exits through the renal hilum and narrows to form the ureter 

renal calculi

aka kidney stones, hard mineral deposits that form in the kidneys when urine becomes too concentrated

• cause severe pain, blood in urine, and difficulty urinating

• best prevented by staying hydrated 

renal arteries

enter through the renal hilum

segmental arteries

located in the region of the renal pelvis

interlobar arteries

positioned between the renal pyramids

arcuate arteries

curve around the base (top) of the renal pyramids

interlobular arteries

radiate outward in the renal cortex

afferent arterioles 

branch from interlobular arteries, each supply a glomerulus 

glomerulus

ball of fenestrated capillaries where blood is filtered

• allow rapid passage of water and small solutes

efferent arterioles

drain the glomerulus and have two options of pathways that it can follow

• peritubular capillaries

• vasa recta

peritubular capillaries 

capillary bed surrounding cortical nephron tubules 

• provide tubules with oxygen and nutrients 

• reabsorb substances from the tubules back into the blood 

vasa recta

long, straight capillaries that run parallel to nephron loops of juxtamedullary nephrons in the medulla

• keep the medulla’s salt and water balance to make concentrated urine

interlobular veins

drain blood from the peritubular capillaries and vasa recta in the cortex

arcuate veins

follow the curve around the base of the pyramids

interlobar veins

pass between the medullary pyramids

renal vein

drains into the inferior vena cava

abdominal aorta → renal arteries → segmental arteries → interlobar arteries → arcuate arteries → interlobular arteries → afferent arterioles → glomerulus → efferent arterioles → peritubular capillaries or vasa recta → interlobular veins → arcuate veins → interlobar veins → renal vein → inferior vena cava

renal blood flow

true

true or false: the kidneys have segmental arteries but NO segmental veins because each segmental artery is an end artery that supplies a distinct renal segment with little to no overlap

filtration

process at the glomerulus in which blood pressure forces water and small solutes into the Bowman’s capsule while keeping large proteins and cells in the blood

reabsorption

process in the tubules in which needed substances move back into the blood

• recovery of substances from filtrate after filtration

secretion

process in which wastes and excess ions are added from the blood into the filtrate for removal 

diffusion

process in which passive movement of molecules from high to low concentration throughout the nephron and capillaries

filtrate

fluid that gets pushed out of the blood in the glomerulus of the kidney and collected in the nephron

• contains water, ions, glucose, amino acids, and wastes

nephron

microscopic functional units of filtration and urine formation in the kidneys

• two parts: renal corpuscle and renal tubule

renal corpuscle 

composed of the glomerulus and glomerular (Bowman’s) capsule

• site of filtration 

glomerular (Bowman’s) capsule

surrounds the glomerulus with two layers

• parietal layer: simple squamous epithelium outer wall

• visceral layer: podocytes with interlocking foot processes that form filtration slits

capsular space

space between the parietal and visceral layers of the glomerular capsule where filtrate collects before entering the renal tubule

filtration membrane

made of glomerular endothelial cells, podocytes, and their shared basal lamina

• blocks large substances while allowing water, electrolytes, glucose, amino acids, and nitrogenous waste to pass

renal tubule

lined with thin layer of simple epithelium ideal for reabsorption and has three main segments where filtrate passes through and is modified into urine

• proximal convoluted tubule

• nephron loop

• distal convoluted tubule

proximal convoluted tubule

has dense covering of microvilli that increase surface area

• reabsorbs about 65-70% of filtrate volume

• reabsorbs water, glucose, amino acids, sodium, potassium, chloride, calcium, magnesium, bicarbonate, and vitamins

• secretes hydrogen ions, drugs, and toxins  

nephron loop

• descending limb: lined with simple squamous epithelium and reabsorbs water

• ascending limb: lined with simple cuboidal epithelium and reabsorbs sodium, potassium, and chloride

distal convoluted tubule

has few microvilli which reflects its reduced role in reabsorption

• fine-tunes reabsorption of sodium, calcium, and chloride under hormonal control

• secretes potassium, hydrogen ions, and drugs

collecting duct

regulated by antidiuretic hormone for water reabsorption 

• reabsorbs urea to maintain medullary concentration gradient 

• adjusts urine concentration based on hydration 

• NOT part of the nephron

cortical collecting ducts

collecting ducts in the renal cortex

medullary collecting ducts

collecting ducts in the renal medulla

papillary ducts

collecting ducts near the renal papilla that empty into minor calyces

• formed by medullary collecting ducts

juxtaglomerular apparatus

controls the flow of filtrate through the nephron and blood pressure within the glomerulus

• made up of three cell types: macula densa cells, juxtaglomerular cells, mesangial cells

macula densa cells

tightly packed sensory cells located where the ascending limb of the nephron loop meets the distal convoluted tubule 

• monitor concentration of sodium chloride (NaCl) in the filtrate

• act as chemical sensors that signal when salt levels are too high or low 

juxtaglomerular cells

specialized smooth muscles cells found on the afferent arteriole

• store and secrete the enzyme renin which starts chain reaction to increase blood pressure and maintain filtration

mesangial cells

cells located within the glomerulus and between the afferent and effferent arterioles

• give structural support and contract or relax to change surface area available for filtration

• indirectly affect the glomerular filtration rate

glomerular filtration rate

volume of filtrate produced by the kidneys each minute

• regulated by the juxtaglomerular apparatus

urine

fluid containing water, salts, and metabolic waste products

• completes its transformation once it leaves the papillary ducts and enters the minor calyces

urinary tract

made up of several organs of the urinary system

• ureters, urinary bladder, urethra

ureters

tubes that carry urine from the kidneys to the urinary bladder

• lined with transitional epithelium for stretch and smooth muscle for peristalsis

• connect to posteroinferior wall of urinary bladder at ureteral orifices

ureteral orifices

openings in the posteroinferior wall of the urinary bladder where each ureter connects 

urinary bladder

muscular, hollow organ that stores urine until it is ready to be expelled

• lined with transitional epithelium for stretch

• contains detrusor muscle and urinary bladder rugae 

detrusor muscle

smooth muscle in the urinary bladder that contracts during urination

urinary bladder rugae

inner wall folds of the urinary bladder that flatten out as the bladder fills

trigone

smooth, triangular shaped region at the base of the bladder

• has three openings for the two ureters and one urethra

internal urethral orifice

opening at the bottom of the trigone that marks the start of the urethra and the point where urine leaves the bladder

urethra

final tube through which urine leaves the body

• surrounded by two sphincters

internal urethral sphincter

involuntary control, made of smooth muscle

external urethral sphincter

voluntary control, made of skeletal muscle

• part of the levator ani muscle group (pelvis urogenital diaphragm)

external urethral orifice

urine passes through this during micturition/urination when both sphincters relax

filtrate forms in nephron → renal tubule → collecting duct → papillary ducts → minor calyces → major calyces → renal pelvis → ureters → ureteral orifices → urinary bladder → trigone → internal urethral orifice → internal urethral sphincter → external urethral sphincter → urethra → external urethral orifice → urine exits body 

path of urine flow

female urethra

short at about 4 cm

• more prone to urinary tract infections

male urethra

much longer at about 20 cm

• divided into three regions

prostatic urethra

part of male urethra that passes through the prostate gland

membranous urethra

short segment of male urethra that passes through the pelvic diaphragm

spongy or penile urethra

part of male urethra that passes through the penis within erectile tissue

urinalysis

quick and noninvasive laboratory examination of urine properties

• physical: color, clarity, and odor

• chemical: pH, protein, glucose, ketones, or blood 

• microscopic: cells, crystals, bacteria, or casts

urochrome

pigment that gives urine its normal, pale yellow color

dialysis

treatment that takes over the kidney’s role of cleaning the blood when the kidneys can no longer do so effectively

• removes waste products, excess fluids, and toxins

• helps keep electrolytes in balance 

hemodialysis

type of dialysis in which blood is pumped through a machine with a special filter (dialyzer) and then returned to the body

peritoneal dialysis

type of dialysis in which cleansing fluid is placed in the abdominal cavity and the peritoneum acts as a natural filter before the fluid is drained

high creatinine in blood

above 5-10 mg/dL

• kidney function decline causes creatinine to build up in blood as kidneys cannot filter it into urine

high blood urea nitrogen

poor filtration as kidneys cannot remove urea nitrogen through urine

proteinuria

protein in urine which may be due to kidney damage

hematuria

blood in urine which may be due to kidney disease, infection, or injury

mucosa of ureter

innermost layer consisting of transitional epithelium (stratified) and thin layer of loose connective tissue

• apical cells: dome shaped or flat and can stretch or flatten to allow expansion of the urinary bladder

• basal cells: cuboidal and provide support for upper layers

submucosa of ureter 

middle layer made of loose connective tissue

• contains many glands that secrete mucus to act as a protective barrier

muscularis of ureter 

layer of smooth muscle that contracts to propel urine through the urinary tract by peristalsis 

adventitia of ureter

outermost layer composed of dense irregular collagenous connective tissue

• anchors the ureters and bladder in place