URINARY SYSTEM
§
Consists of 2 kidneys, 2 ureters, urinary
bladder, and the urethra.
FUNCTIONS
1. Excretion
2. Regulation of blood volume + pressure.
3. Regulation of the concentration of solutes in
the blood.
4. Regulation of RBC synthesis.
5. Regulation of vit. D synthesis.
ANATOMY OF THE KIDNEYS
Kidneys – bean-shaped organs; size of a tightly clenched
fist
Renal capsule – CT that surround each kidney
Hilum – where the renal artery and nerves enter
Renal sinus – cavity that contains blood vessels
Cortex (outer) Medulla (inner) – surround the renal
sinus
Renal pyramids – located bet. the cortex & medulla
Calyx – funnel-shaped structure that surround the tip of
each pyramid
Renal pelvis – formed by calyces into a larger funnel
Ureter – exits the kidney and connects to the urinary
bladder
Nephron – functional unit of the kidney
v Renal corpuscle
v Proximal convoluted tubule
v Loop of Henle
v Distal convoluted tubule
Collecting duct – carries the fluid from the cortex
through the medulla
Papillary duct – empty their contents into a calyx
Juxtamedullary nephrons (15%) – have loops of Henle
that extend deep into the medulla
Cortical nephrons (85%) – have loops of Henle that do
not extend deep into the medulla
Bowman’s capsule – consists of the enlarged end of the
nephron
Filtrate – the fluid that passes across the filtration
membrane
Arteries and Veins
Renal arteries – branch off the abdominal aorta and
enter the kidneys
Interlobar arteries – pass bet. the renal pyramids
Arcuate arteries – arch bet. the cortex and the medulla
Interlobular arteries – branch off the arcuate arteries
and project into the cortex
Afferent arterioles – arise form branches of the
interlobular arteries and extend to the glomerular
capillaries
Efferent arterioles – extend from the glomerular
capillaries
Peritubular capillaries – surround the proximal
convoluted and distal convoluted tubules and the loops
of Henle
Vasa recta – specialized portions of the peritubular
capillaries that extend deep into the medulla
Juxtaglomerular apparatus – formed where the distal
convoluted tubule comes in contact with the afferent
arteriole next to BC
URINE FORMATION
1. FILTRATION
§ Movement of water, ions, and small molecules
through the FM into the BC
Filtration pressure – forces fluid from the glomerular
capillary across the FM into the BC
Glomerular capillary pressure – BP in the glomerular
capillary
Capsular pressure – pressure of filtrate already inside
the BC
Colloid osmotic pressure – pressure of filtrate within
the glomerular capillary
Regulation of Filtration
Cardiovascular shock – the filtration pressure and
filtrate formation fall dramatically
2.
§
TUBULAR REABSORPTION
Movement of substances from the filtrate across
the wall of the nephrons back into the blood of
the peritubular capillaries
3.
§
TUBULAR SECRETION
Active transport of solutes across the nephron
walls into the filtrate
Glomerulus – tuft of capillaries that resembles a ball of
yarn; lies within the indentation of Bowman’s capsule
Podocytes – inner layer of Bowman’s capsule that
consists of specialized cells
Filtration membrane – formed by glomerular
capillaries, basement membrane, and podocytes of BC
A x 2 .
REGULATION OF URINE
CONCENTRATION AND VOLUME
Hormonal Mechanism
Renin-Angiotensin-Aldosterone-Mechanism
§ Renin is secreted from the kidney when blood
pressure decreases
§ Renin converts angiotensinogen to angiotensin
I; then converted to angiotensinogen II; which
stimulates aldosterone
§ Aldosterone
increases
Na+
and
Cl+
reabsorption from the nephron
REGULATION
OF
EXTRACELLULAR
FLUID
COMPOSITION
Thirst Regulation
§ Blood concentration increases = thirst center
initiates sensation of thirst
§ When water is consumed, blood concentration
increases = thirst sensation decreases
§ Blood pressure decreases = thirst sensation is
triggered
§ Consumption of water = increases blood volume
+ allows blood pressure to increase
Antidiuretic Hormone Mechanism
§ Secreted from the posterior pituitary when
blood concentration increases / blood pressure
decreases
§ Increases the permeability to water of the distal
convoluted tubules and collecting ducts
§ Increases water reabsorption by the kidney
Thirst center – neurons in the hypothalamus that control
water intake
Atrial Natriuretic Hormone
§ Secreted from the RA in response to increased
blood pressure
§ Acts on the kidney to increase Na+ & water loss
in the urine
URINE MOVEMENT
Anatomy and Histology of the Ureters, Urinary
Bladder, and Urethra
Ureters – small tubes that carry urine
Urinary bladder – hollow muscular container that stores
urine
Urethra – tube that carries urine form the urinary
bladder to the outside of the body
Internal urinary sphincter (males) – smooth muscle at
the junction of the urinary bladder and urethra
External urinary sphincter (males + females) – skeletal
muscle that surrounds the urethra
Micturition Reflex
§ Activated by stretch of the urinary bladder wall
BODY FLUID COMPARTMENTS
Intracellular fluid compartment – fluid inside all the
cells in the body
Extracellular fluid compartment – fluid outside all the
cells in the body
Thirst – one of the important means of regulating ECF
volume and concentration
Ion Concentration Regulation
§ Sodium ions are dominant extracellular ions
Ø Aldosterone
increases
Na+
reabsorption from filtrate
Ø ADH increases water reabsorption
from the nephron
Ø ANH increases Na+ loss in urine
§
Aldosterone increases K+ secretion in urine
Ø Increased blood levels of K+ stimulate
Ø Decreased blood levels of K+ inhibit
§
PTH increases extracellular Ca2+ levels
Ø Causes bone resorption
Ø Increases Ca2+ uptake in the kidney
§
PTH increases vit. D synthesis
§
Calcitonin (thyroid gland)
Ø Inhibits bone resorption
Ø Lowers blood Ca2+ levels
§
Phosphate + Sulfate Ions
Ø When levels are low in the filtrate = ions
are reabsorbed
Ø When levels are high = excess is lost in
the urine
REGULATION OF ACID-BASE BALANCE
Buffers
§ Resist changes in the pH;
§ Proteins, phosphate buffer system, bicarbonate
buffer system
Extracellular fluid – contains more Na, Ca, Cl, HCO3
Respiratory System
§ Increase respiratory rate raises pH = rate of
CO2 elimination is increased
§ Reduced respiratory rate = reduces pH (CO2
elimination is reduced)
Exchange bet. BFC
§ Water moves continually bet. compartments I
response to hydrostatic pressure differences
and osmotic differences bet. the compartments
Kidneys
§ Excrete H+ in response to decreasing blood pH
§ Reabsorb H+ in response to an increasing
blood pH
Composition of the Fluid in the BFC
Intracellular fluid – contains more K, Mg, PO4, SO4,
protein,
A x 2 .
Acidosis and Alkalosis
Acidosis – occurs when blood pH falls below 7.35
v Respiratory acidosis – respiratory is unable to
eliminate adequate amounts of CO2
v Metabolic acidosis – excess production of acidic
substances (lactic acid + ketone bodies)
Alkalosis – occurs when blood pH increases above 7.45
v Respiratory
alkalosis
–
results
from
hyperventilation in response to stress
v Metabolic alkalosis – results from rapid
elimination of H+ from body
A x 2 .