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
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