Key Concepts for the Urinary System
Chapter 25 (pg. 997-1029)
Urinary System
Purpose
Regulate volume and chemistry of blood.
Remove waste and fluids
Regulate red blood cell counts-EPO
Gluconeogenesis- makes glucose out of non-carbohydrates. This is normally a
Liver function, this is caused from prolonged fasting.
Organs
Kidneys
Ureters
Urinary bladder
Urethra
Location and External Anatomy
Retroperitoneal-T12-L3, surrounded by three membranes:
Renal capsule-fibrous/tough, protects kidneys from infection in surrounding
tissue
Adipose capsule-fatty mass cushions and protects. Also attaches the kidney
to the posterior wall.
Renal fascia-outer membrane attaches the kidney to adrenal gland to surrounding
structures.
Internal Anatomy
Regions
Cortex-lighter
Medulla-darker, pyramids
Pelvis-flat, funnel shaped. Joins with ureter at hilus
Calyces-extensions of pelvis
Calyces major and Calyces minor surround the papillae
Blood and Nerve Supply (Illustration pg. 999)
Renal arteries deliver ¼ of the total cardiac output to kidneys each minute
Perpendicular branches of abdominal aorta, the right artery is longer than left
Each artery branches into 5 segmental arteries just outside hilus of kidney
They then branch into lobar arteries inside kidney
They then divide to form several interlobular arteries
At the base of the medulary pyramid they then branch into the arcuate arteries
Renal Veins
Interlobular
Arcuate
Interlobar
Renal veins
There are no lobar or segmental veins
Renal Plexus (ANS)
Nephron
The structural and functional units of the kidneys
Each nephron consists of:
Glomerulus
Bowman’s capsule
Renal tubule
Glomerulus-fenestrated capillaries
Forms filtrate
Solution is a protein free, solute rich fluid, which passes from blood
Bowman’s capsule
Particles of (CHON) filtered out
Bowman’s capsule
Parietal Layer-simple squamous, no filtration function
Visceral layer- by glomerulous, has branching epithelial cells called
Podocytes (foot cells). They terminate in foot processes, which
intertwine as they cling to the basement membrane of the
glomerulus.
The clefts or openings between the foot processes called filtration
slits, allow filtrate to enter the capsular space.
Renal tubule
3 parts
Proximal convoluted tubule, loop on Henle and distal
convoluted tubule
Collecting ducts- receives filtrate from many nephrons
Run through the medullary pyramids and give them a striped
appearance.
They come together at the papilla and form papillary ducts
which deliver urine to the minor calyces.
Principle cells-no microvilli
Intercalated cells-microvilli
Proximal Convoluted Tubule
Simple cuboidal epithelial cells with brush border
absorb water from filtrate and secrete substances into it
Loop of Henle
Descending-proximal part-same as proximal convoluted tubule
Distal part-thin segment-simple squamous-permeable to water
Ascending-cuboidal/columnar-thick segments
Distal Convoluted Tubule
Cuboidal, but thinner than proximal convoluted tubule, reduced
microvilli/brush border
Cortical Nephrons represent 85% of the nephrons in the kidneys and are
found mostly in the cortex
Juxtamedullary nephrons represent 15% in the kidneys, these are
important in urine concentration, they have more extensive thin
segments, and their loops of Henle invade deeply the medulla
Capillary Beds of Nephrons
Glomerulus
Fed and drained by arterioles, the afferent arteriole and the
efferent arteriole
2 reasons why glomerular pressure is high
1)Arterioles are high-resistance vessels
2) Afferent arteriole has a larger diameter than efferent
Peritubular capillaries
Arise from efferent arterioles, tend not to break up into
peritubular capillaries, instead they form venules
They absorb water and solute removed from nephron
99% is reclaimed
Vasa Recta (Straight-vessel)
Thin walls
Extend deep into the medulla paralleling the longest loops of
Henle
Vascular resistance in microcirculation
High pressure in the efferent arterioles reinforces the high
glomerular pressure
Juxtaglomerular Apparatus
Controls filtration rate
Mechanoreceptors=pressure
Chemo receptors=solute content
Juxtaglomerular cells-Afferent arteriole wall pressure
Macula Densa-Distal convoluted tubule wall chemistry
Filtration Membrane
Porous, between blood and capsule has 3 layers
1)fenestrated epithelium (tunica intima)
2) gel like basement membrane (basal lamina of other layers)
negatively charged glycoproteins repel many
macromolecules, others stopped at filtration slits
3)visceral membrane of glomerular capsule (podocytes)Net Filtration Pressure (NFP)
NFP=HP (hydrostatic P=glomerulous) ¾(OP osmotic pressure +HP
capsular) (pg. 1007 left column, upper paragraph for better looking
formula)
Glomerular Filtration Rate (GFR)
FR is directly proportional to NFP
3 factors governing filtration rate at capillary beds
1)surface area-not easily changed
2)permeability-not easily changed
3)NFP
Try to keep these constant, why is this important?
Too fast and too much=poor absorption
Too slow=everything reabsorbed, even waste we should eliminate
Renal Auto regulation
Maintain constant GFR, 2 methods
1)Myogenic mechanism-tunica media vasoconstrict increasing blood
pressure and stretches wall. Tunica media vasoconstricst blood pressure
goes down causes vasodilatation
2)Tubuloglomerular feedback mechanism- when pressure and osmolality
goes down, this caused vasodilatation of afferent arterioles. When flow,
pressure and olsmolality goes up vasoconstriction of afferent arterioles
Sympathetic Nervous System
Stress or emergency overcomes renal auto regulation
Blood to vital organs goes away from kidneys
Renin-aniotensin mechanism
Release of renin (JG cells) causes a chemical cascade
Angiotensin I = II
Angiotensin II
1) Smooth muscle vasoconstrictor, blood pressure goes up
2) Decrease GFR
3) Stimulates aldosterone released by adrenal cortex, kidneys reabsorb more Na+ and H2O, blood volume and blood pressure goes up
When to start renin-angiotensin
When systemic blood pressure is below 80mm Hg
Low filtrate flow rate/osmolarity
Tubular Reabsorption
Our total blood volume filters into the renal tubules about every 45 mins. 99% is
reabsorbed
Reabsorption rate may be passive (no ATP required) or active (at least on of its
steps is driven by ATP directly or indirectly)
Active reabsorption-Na+ is very common in filtrate, 80% of energy from active
reabsorption
2 sodium and 3 potassium, pumps ATP, keeps tubule cells low in salt this
creates electrochemical gradient
Passive tubular reabsorption-obligatory water absorption, and obliged to follow
sodium
Secondary active transport-glucose, amino acids, vitamins are substances
reabsorbed by a carrier molecule. Lets sodium follow gradient, and carries
something else along
Non-reabsorbed substances
Substances include-urea, uric acid and creatinine
No carriers, not lipid soluble, too big for pores, tight junctions and nitrogenous
Absorptive capabilities of different regions
Proximal convoluted tubule-most active area of reabsorption, reabsorbs anything
Loop on Henle-selective reabsorption
Descending- water volume
Ascending- Na, K,Cl (active),Ca+2 and Mg+2
Key Concepts for Fluid and Electrolyte Balance
Chapter 26 (pg. 1034-1059)
Body water content
Water content for a health man is 60% and 50% for a health woman, about 40 L
Fluid compartments
Intracellular fluid compartment (ICF)-in a healthy male of average size ICF
Accounts for 25 L of the 40 L of body water. This consists of trillions of
compartments: the cells
Extracellular fluid (ECF)- internal environment of each cell, there are 2
subcompartments- 1) plasma and 2)interstitial fluid, the fluid in a microscopic
spaces between tissue cells.
Composition of fluids
Electrolyte- osmolarity concentration, ionic bonds; ionize or dissolve in water; dissolve easier and more important for osmolarity
Electrolyte concentrations are expressed in milliequivalents per liter (mEq/L)
mEq/L= ionic concentration (mg/L) ´ no. of electrical charges
atomic weight of ion on one ion
Intra and extracellular fluid (pg. 1036 illustration)
Na+ / K+- sodium potassium pumps
Movement of fluid from compartments
Anything that changes the solute concentration in any compartment leads to
net water flows or movement
Plasma circulates throughout the body and links the external and internal
environments (lungs, GI, kidneys) exchanges occur continuously
Water balance and extracellular fluid osmolality
Input=ingest=food, drink and metabolic water
Output-urine (60%), perspiration (8%), insensible (expired air, 28%), feces
(4%)
Thirst-hypothalamus-salivary glands obtain the water they require from the blood,
So when blood volume is down, increase in thirst-about 10%
Osmoreceptors lose water by osmosis to the hypertonic ECF, or excited by
baroreceptors inputs, these events cause thirst-2-3%
Regulation of water output
Obligatory water losses=500ml of urine a day
Kidneys regulate water and sodium solutes
Disorders of water balance
Dehydration-output is greater than input; fever, confusion, kidney pain and
hypovolemic shock
Hypotonic hydration-overhydration-nausia, cramping, edema, coma, death
can be corrected by manitol IV which makes osmolaity go up
Edema-swelling-accumulation of fluid in interstitial space. Increased blood
pressure and congestive heart failure are factors that accelerate fluid loss from
blood
Electrolyte balance
Salt balance in body-90% of solutes in ECF
Major osmotic pressure, water follows salt, we reabsorb and reuse
GI disorders can lead to large salt losses in feces or vomit
Sodium regulation
No sodium receptors!
60-65% reabsorbed at PCT
25% at loop of Henle
Influence of aldosterone
Causes sodium and water reabsorption at distal convoluted tubules and collecting
ducts
When aldosterone release in inhibited, no sodium reabsorption occurs beyond
the distal tubule.
Juxtaglomerular apparatus- responds to sympathetic stimulation, decreased
osmolality or decreased stretch, the JG cells release renin. Renin catalyzes the
series of reactions that produce angiotensin II that prompts aldosterone release.
Adrenal cortical cells also directly stimulated to release aldosterone by elevated
K+ levels. Aldosterone brings effects slowly, over hours or even days