Chapter 24 – The Urinary System

Urinary system functions

• Regulates blood volume and composition

• Removes nitrogenous wastes, toxins, excess ion removal

Kidney anatomy

• Located in posterior abdominal cavity– Right kidney is lower

than the left due to crowding by the liver

• Medial surface has hilum – Depression that serves as

entrance/exit for blood and lymphatic vessels, nerves, ureter

Kidney supportive layers

• Renal capsule – deepest – Transparent membrane directly covering kidneys

• Perirenal fat capsule – Fat the supports/protects kidneys – Renal ptosis – “dropping” of kidneys due to extreme loss of

body fat • Can cause obstruction of ureters

– Hydronephrosis – urine backup

• Renal fascia – most superficial– Dense fibrosis connective tissue that anchors kidneys to

surrounding structures

Kidney internal anatomy • Renal cortex –

superficial – Light, granular

appearance • Renal medulla – deep

– Darker red-brown – Arranged in triangular

renal pyramids • Base oriented toward

cortex; apex/papilla points interiorly

• Separated by renal columns– Extensions of cortex

Kidney internal anatomy cont • Minor calyces– Cuplike projections

surrounding each papillae

– Join together to form 2-3 major calyces• Collect urine and dump

into renal pelvis – Smooth muscle in

calyces and renal pelvis move contents via peristasis

Nephrons • Structural and functional

units of kidneys – Over 1 million per kidney

• Form urine – Transmitted via collecting

ducts to renal pelvis

Nephron structure • Renal corpuscle – Glomerulus – ball of capillaries – Bowman’s capsule – blind end of renal tubule; surrounds

glomerulus • Remainder of renal tubule– Simple epithelium – Proximal convoluted tubule – close to glomerulus – Loop of Henle

• Descending limb and loops to ascending limb – Distal convoluted tubule

• Empties into a collecting duct – One duct receives from multiple nephrons and empties into renal pelvis

Renal corpuscle • Capillaries are fenestrated

(has pores)– Allows water and other small

solutes to enter tubule • Filtrate

• Bowman’s capsule layers – Parietal – simple squamous

epithelium – Visceral

• Podocytes – branching epithelial cells – Foot processes cling to

basement membrane – Filtration slits – openings

between foot processes » Filtrate enters

Capillary beds

• Glomerulus – Afferent arteriole feeds

into bed; efferent arteriole drains it

– Diameter of afferent arteriole is larger than efferent • Causes blood pressure in

bed to be much higher than other beds

Juxtaglomerular apparatus • Located where distal portion

of tubule lies against afferent arteriole

• Granular/juxtaglomerular cells– Surround arteriole – Granules of renin – Mechanoreceptors that detect

blood pressure changes • Macula densa of loop of Henle

– Chemoreceptors that detect sodium chloride concentration

Capillary beds cont

• Peritubular capillaries – Arise from efferent

arterioles – Have close association

with renal tubules• Recapture water and

other molecules – Empty into venules

Nephron classification

• Cortical – 85%– Located mainly in the

cortex, with just a small portion of the loop in the medulla

• Juxtamedullary– Start at cortex/medulla

border and go deep into medulla

Ureters• Carry urine from kidney to bladder – Smooth muscle – peristalsis

• Lined with transitional epithelium • When bladder fills, increase in pressure compresses

and closes ureters• Renal calculi – kidney stones – Excess calcium, magnesium, uric acid crystallizes and

precipitates out – Large stones can block urine drainage

• Can get trapped in ureter – muscle contractions on it cause severe pain

Urinary bladder • Trigone

– Triangular shaped area formed by entrance of 2 ureters and urethra

• Internal walls have rugae that disappear when distended

• Micturition – urination – When ~200ml in bladder,

stretch receptors send impulse to brain • Anuria <50ml per day

– Extremely low blood pressure or kidney failure

Urethra • Carries urine from bladder to external environment • Changes from transitional epithelium →

pseudostratified → stratified squamous • Sphincters – Internal urethral sphincter – involuntary

• Opposite of most – contraction OPENS; relaxation closes – External urethral sphincter – voluntary

• As urethra passes through urogenital diaphragm

• Size in males is larger since it needs to travel through the penis

Kidney physiology

• Forms ~180L of filtrate daily– Over 99% of which gets

reabsorbed • 3 major processes – Glomerular filtration – Tubular reabsorption – Tubular secretion

Glomerular filtration • Passive process • Filtration membrane

exceedingly permeable to water and small solutes

• High pressure in glomerulus forces filtrate out – Water, glucose, amino acids,

ions, nitrogenous wastes • Similar concentration to

plasma concentration – Proteins and other large

molecules prohibited from passage

Regulation of glomerular filtration rate (GFR)

• Intrinsic – renal autoregulation – Myogenic mechanism – Tubuloglomerular feedback mechanism

• Extrinsic – neural and hormonal – Rennin-angiotensin mechanism – Regulates GFR to regulate systemic blood pressure – Overrides intrinsic control in high stress or emergency

• Shunts blood to vital organs

Myogenic mechanism

• When stretched, smooth muscle tends to contract

• When blood pressure increases, smooth muscle of afferent arteriole contracts – Reduces pressure difference in the glomerulus • Otherwise, too much filtrate is formed

• When blood pressure decreases, afferent arteriole dilates to keep pressure difference

Tubuloglomerular feedback mechanism

• Controlled by macula densa cells in loop (NaCl levels)• When NaCl levels in filtrate are too high:– Afferent arteriole constricts – reduction of pressure

difference • Filtrate travels more slowly through tubule, allowing for better

reabsorption

• When NaCl levels in filtrate are too low:– Afferent arteriole dilates – increases pressure difference

• Filtrate travels quickly through tubule; NaCl doesn’t get reabsorbed (stays in filtrate)

Renin-angiotensin mechanism

• Granular/juxtaglomerular cells release renin– Due to decline in blood pressure or decline in osmotic

concentration of tubular fluid at macula dense • Angiotensinogen – Plasma protein produced by liver

– In the presence of renin, converts to angiotensin I

– Angiotensin I gets converted to angiotensin II by angiotensin converting enzyme (ACE)• Located in capillary endothelium, especially in lungs

Angiotensin II• Regulates blood pressure by:– Vascontriction of arterioles throughout body

• Increases blood pressure – Stimulates reabsorption of NaCl

• Causes water to be reabsorbed as well; increase in blood volume increases blood pressure

– Stimulates hypothalamus • Produce ADH and stimulates thirst center

– Increases blood volume

– Decreases pressure in peritubular capillaries• Allows more fluid to enter

– Causes contraction of glomerular mesangial cells• Located between glomerular capillaries • Decreases surface area for filtration

Tubular reabsorption

• Most of filtrate needs to be reabsorbed and returned to bloodstream

• Reabsorption of water and ions adjusted to regulate blood composition

• Passive transport – no ATP required; active transport – ATP is required

Sodium reabsorption

• Most abundant cation in filtrate • Active transport – Pumped into tubule cells; pumped out into

interstitial fluid by sodium-potassium pump• From interstitial fluid, sodium enters peritubular

capillaries

Reabsorption of water, ions, nutrients

• Sodium ions in capillaries cause an electrical gradient that attracts anions (Cl- and HCO3-)

• Sodium ions create an osmotic gradient that attracts water – Causes filtrate to become more concentated

• Other solutes diffuse down concentration gradient

• Secondary active transport – Glucose, amino acids, vitamins, cations – Sodium carrier protein can co-transport other solutes

• Each solute has a different, specific carrier protein – Transport maximum

• Finite number of carrier proteins for each solute – If saturated, excess will be excreted

Reabsorption cont

• Fat-soluble substances – Do not require carrier proteins since they can travel

directly through plasma membrane • Non-reabsorbed substances – No carriers for the specific molecule – Not fat-soluble – Too large to pass through tight junctions of tubular cells – Nitrogenous wastes from protein and nucleic acid

metabolism • Urea, creatinine, uric acid

Tubular secretion

• Secretes substances back into tubules:– Substances bound to proteins were too large to be

filtered through glomerulus – Wastes that were reabsorbed due to passive transport – Excess K+ removal

• Virtually all potassium is originally absorbed in the PCT– Blood pH homeostasis

• If too low – H+ ions are secreted; bicarbonate ions are retained

• If too high – bicarbonate ions remain in filtrate (don’t enter the bloodstream)

Formation of urine

• Dilute – DCT and collecting ducts are impermeable to water • Water can not be reabsorbed

• Concentrated– ADH causes aquaporins to be placed in cells of DCT

and collecting ducts – Diuretics increase water output by:• Inhibition of ADH (alcohol)• Interferes with sodium absorption

Urine

• Color– Pale to dark yellow due to pigment urochrome– Certain foods and drugs can change color – Pink – presence of red blood cells (infection or

trauma)– Cloudy – presence of bacteria

• pH– Usually acidic (~6) … can range from 4.5 – 8– Acidity inhibits bacterial growth

Abnormal urinary constituents • Glycosuria – glucose – nonpathological – recent intake of excessive sugary foods– Pathological – diabetes mellitus

• Proteinuria or albuminuria – protein/albumin – Nonpathological – pregnancy, high protein diet, excessive

physical exertion – Pathological – severe hypertension, liver failure, renal disease,

glomerulonephritis

• Ketonuria – ketone bodies – Starvation, uncontrolled diabetes mellitus

Abnormal urinary constituents cont • Hemoglobinuria – hemoglobin – Transfusion reaction, hemolytic anemia, severe burns

• Bilirubinuria - bile pigments – Liver disease, blockage of liver or gallbladder ducts

• Hematuria – RBCs– Urinary tract bleeding – infection, trauma, stones, cancer

• Pyruria - pus/WBCs– Infection