Review Sheet 2 for Test #3
Biology 2213
Dr. James Adams
THE URINARY SYSTEM
-- Chapter 25; on test 3
Two Major Functions:
1. Osmoregulation --
Regulating water volume, ions, and acid-base balance
2. Excretion of
Metabolic (Nitrogenous) Wastes (internally produced)
Auxiliary Functions:
1. Release of renin from juxtaglomerular cells of afferent arteriole (regulation of BP)
2. Release of erythropoietin
3. Converting vitamin D to it's active form
4. Gluconeogenesis
Urinary System Anatomy -- refer to your laboratory practical AStructures to Know@ sheet,
and remember, as always, you are responsible for knowing the epithelial linings/muscle
in the walls of the various organs
Gross Anatomy
1. Kidney location, Renal capsule,
Fat (Adipose) capsule, Renal fascia
2. Cortex and Medulla (with renal pyramids/columns), Pelvis (with calyces), renal hilus
3. Renal Artery and Vein, Interlobar arteries and veins, Arcuate arteries and veins
4. Ureters, Bladder (with trigone; special
tri-layered detrusor muscle), urethra, internal and
external urethral sphincters
Microscopic Anatomy -- of the Nephron (Renal Tubule System); >1,000,000 per kidney
1. Renal Corpuscle = Glomerulus +
Glomerular (Bowman=s) Capsule (with
podocytes/
filtration memb)
2. Proximal/Distal Convoluted Tubules
(PCT/DCT)
3. Nephron Loop (of Henle)m,
with descending/ascending limbs/arms; thin/thick segments
4. Collecting/Papillary Ducts (form renal pyramids)
5. Blood flow (a portal system)
-- Afferent/efferent arterioles (for glomerulus); peritubular
capillaries; form vasa recta around Loop of Henle in JM
nephrons*
6. Juxtaglomerular (JG) Apparatus -- involves the JG
(granular) cells in afferent arteriole
and macula densa cells in DCT
Types of nephrons: Cortical (85%); *Juxtamedullary (15%) with long
Nephron Loop
--
only
juxtamedullary important for concentrating urine
Physiology
Urine Formation -- three processes involved: filtration, tubular reabsorption and secretion.
About one fifth of plasma of the blood is filtered from the glomerulus (125ml/min; 180 L/day;
or
entire plasma volume 60X a day!).
Of this, less than 1% (1.5 liters/day) leaves the body as
urine because you reabsorb the other 99% back into the bloodstream, which
requires a LOT
of energy.
The two kidneys (<1% of entire body mass) use 20-25% of O2 (energy)
at rest.
I. Glomerular Filtration is passive; through the
filtration membrane
Filtration
membrane -- fenestrated (glomerular) capillaries, basement membrane, slits
between podocyte processes
Small molecules (H20, glucose, amino acids) pass freely; molecules larger than 5 nm do
not enter filtrate; large proteins keep water in bloodstream.
Blood Pressure higher in glomerulus than other caps.; generates lots of filtrate.
Vascular
resistance in microcirculation around nephron: two arterioles (afferent & efferent)
greatly control blood pressure in the glomerulus and peritubular capillaries; efferent
narrower than afferent which reinforces
higher BP
in glomerulus than most capillary beds
in the systemic circulation
--
necessary for filtration.
Need to know concept of Net Filtration Pressure (NFP) and Glomerular Filtration Rate
(GFR)
and their effects on filtrate formation
NFP ends up being a modest 10 mm Hg
Controls of GFR:
1. Intrinsic (autoregulation)
-- maintains GFR in face of widely fluctuating systemic BP
a. Myogenic mechanism
-- reduced stretch also stimulates renin release (see below)
b. Tubuloglomerular feedback mechanism
-- involves Juxtaglomerular Apparatus
Macula densa cells respond to filtrate flow rate/osmotic concentration
When flow rate slow/osmotic concentration low, promote vasodilation of afferent
arterioles; also sets renin-angiontensin mechanism (see below) into motion
When flow rate high/concentration high, promote JG cells of afferent arterioles
to generate vasoconstriction
2. Extrinsic controls -- neural and
hormonal
a. Sympathetic Nervous System Controls
-- overall effects discussed previously,
Extreme stress results in great decline of GFR due to significant constriction
of
afferent arterioles; directly stimulates JG cells to release renin (see
directly below)
b. Renin-Angiotensin-Aldosterone Mechanism
(obviously tied in with other influences)
Influences systemic BP (and therefore indirectly GFR as well)
Remember, angiotensin potent vasoconstrictor
-- main thrust is to raise
systemic BP by constriction of systemic arterioles
Angiotensin has various effects on GFR, besides rise in systemic BP, by
stimulating
release of aldosterone from adrenal cortex, ADH from hypothalamus (increases
reabsorption of both NaCl and water; which reduces flow in DCT)
Take home message: nearly
constant NFP and therefore GFR can be maintained even
as systemic BP varies between 80 and 180 (at rest)
II. Tubular Reabsorption
180 liters of filtrate produced daily, 1.5 liters of urine released. Approx. 99% of all
filtrate must
be reabsorbed from filtrate into peritubular capillary blood. Virtually all
organics reabsorbed; ions
(therefore water) under specific controls.
You will need to know the following concepts (most of which you=ve already been
exposed to at
an earlier date): Active transport (mostly of Na+), co-(sym-)transport,
passive reabsorption, solvent drag,
osmosis using aquaporins
Transport maximum B each substance that is actively or cotransported can only be
transported
at a rate which is based on the flow rate and number of transport proteins;
for virtually every substance there is a maximum amount that
can be reabsorbed.
Beyond that point, solutes in the filtrate will be excreted.
Reabsorption in different parts of the Nephron:
1. PCT (with microvilli)
-- majority of reabsorption from here. All organics, approx
65% of Na+ (and therefore water), and selected portions of other ions.
Also almost all uric acid and some urea (!)
Any organics (glucose, amino acids) left in the filtrate past the PCT will be
lost in the urine (think of diabetes mellitus and glucose)
2. Loops of Henle
-- descending limb permeable to water; ascending limb is not
(important for concentrating urine, described below); solute transport not
coupled to osmosis (water does not follow the solutes directly in this case)
By end of Loop of
Henle, 25% more of Na+, 10% of water, 35% of Cl- reabsorbed
3. DCT and Collecting Duct
-- 25 % of volume, and 10% of salt remain
Nearly all of both the water and salt can be reabsorbed as needed
Permeability to water completely dependent on ADH presence to
insert
aquaporins (facultative water reabsorption); lack
of ADH means little water
reabsorption from these regions,
and release of dilute urine
Reabsorption of Na+ (and secretion of K+) dependent on aldosterone release
Remember also that ANF (or ANP) released from the atria when venous return is
high inhibits aldosterone release, and also increases GFR
Reabsorption of Ca+2 from DCT dependent on PTH
[Nice summary diagrams: pgs.
992 - 998]
III. Tubular Secretion
This is the ability to put materials (in some cases replace materials) from peritubular
capillary blood into the filtrate in the nephron. This includes uric acid and some urea
that was reabsorbed,
manipulation of K+ and/or H+, and Cl- and/or
HCO3-
(involved in acid base balance)
Controlling Urine Concentration
When copious amounts of water drunk, need to release dilute urine
However, we are terrestrial, and much more frequently we face the need to conserve
water (concentrate urine) than get rid of
excess water.
Two nephron parts vital to concentrating urine:
Nephron Loops and Collecting Ducts
Remember salts pumped from ascending limb (which draws some water from
descending limb);
this makes the tissue of the medulla very salty (high osmotic
concentration); in other words, as
you descend into the medulla, there is an
increasing salt gradient. As fluid passes through the collecting ducts,
thereby
descending through the medulla, increasing amounts of water can be drawn
out (assuming ADH is present). Additionally, the collecting ducts are permeable
to urea and some urea diffuses out, adding to the medullary solute concentration
gradient. In other words, we retain some urea, which sounds weird, but it helps
with the
ability to concentrate the urine.
Renal Clearance: A measure of the amount of substance released compared to the
amount of the same substance filtered; gives an idea of the reabsorption/secretion
capabilities of the nephrons
Urine Characteristics:
Color/transparency; odor; pH (varies anywhere from 4 to 8)
Chemical Composition -- most abundant components: water, urea, sodium in that order
many other ions
Any organics in the urine (red blood cells, proteins, etc.) indicate some pathology
Rest of Urinary System on lab "Structures to know" sheet -- Ureters, Bladder, Urethra
Know about expansion capabilities of Bladder, and Micturition (Urination;
involving
involuntary internal urethral sphincter [at bladder] and
voluntary external urethral
sphincter [at abdominal wall in both sexes])
FLUID, ELECTROLYTE, and ACID-BASE BALANCE -- Chap.
26; on Test 4
Almost entire chapter is review of
information from previous chapters; very little new
information, just put together in a different fashion than previously.
Body Fluids -- Water
Total body water represents $50% of body mass; most in infants, least in elderly
Adult males avg. 60%; females 50% (fat is the least hydrated tissue)
Fluid Compartments: (numbers for a 70 kg man)
1. Intracellular Fluid Compartment (ICF) -- 25 liters
2. Extracellular Fluid Compartment (ECF) -- 15 liters:
subdivided into plasma (3 liters) and interstitial fluid (several types: 12 liters)
Main differences between compartments are the solutes; grouped into electrolytes
(ions) and non-electrolytes (includes many organics, such as protein)
Understand concept of milliequivalents for electrolytes (ionic compounds)
You should also know main solutes in all compartments, and therefore the differences
between them; for example, the main extracellular cation is . . .? . . . the main intracellular
anion is . . .? (Figure 26.2, pg. 1014)
Fluid Movements Between Compartments
-- SEE Fig. 26.3
The main concept to remember here is that all compartments will more or less remain
isotonic to one another; any momentary fluctuation in the solute content in one
compartment is offset by osmotic movements of water
-- thus, volume of ICF is
determined by solute concentration of ECF
Exchanges:
Between plasma & interstitial fluid -- across capillary walls (remember
capillary
dynamics)
Between interstitial fluid and ICF -- selectively across cell membranes
Only plasma circulates, so it is the ultimate link in exchanges between compartments
I. Water Balance -- Input must equal output; see
Fig. 26.4, pg. 1016
Inputs -- drink, food, cellular respiration (and other metabolic processes)
Outputs -- urine, sweat, feces, insensible losses (respiratory surfaces, skin)
Regulation of Intake -- the thirst mechanism
Involves the hypothalamus -- remember, the BBB is leaky here, so that the
hypothalamal receptors can sample the plasma contents; thirst triggered when
osmotic content of plasma elevated
(water content lower), which causes
hypothalamal osmoreceptors to lose water, which depolarizes them
Salivary glands reduce water output
-- dry mouth
Thirst quenched almost immediately -- moistening of upper GI tract mucosae and
stretch receptors in stomach involved. Prevents overhydration
Regulation of Output
We have obligatory water losses (insensible, fecal
and a certain minimal urine loss)
Renal concentrating mechanisms are the main way to conserve water.
This involves
release of ADH from the hypothalamus/posterior pituitary at the same time as
the sensation of thirst is generated
As you will
see, water balance very much tied to Na+ balance as well (where
solutes go, water follows).
Disorders B know about dehydration and hypotonic hydration
II. Electrolyte Balance -- will examine Na+, K+, and Ca+2 mainly (though this will
involve Cl- and HPO4-2)
A. Central Role of Na+
-- Most abundant extracellular cation, and only one exerting
significant osmotic pressure, which means it is central in moving water around
between compartments
(Don=t forget, Na+ also plays a major role in electrical gradients)
Regulation of Na+ Balance
-- interestingly, virtually no chemoreceptors that respond
specifically to Na+ have been found yet in the body
1. Aldosterone is, of course, a key player; stimulators of aldosterone release include
i. Angiotensin
ii. Elevated K+ levels
(Addison=s disease/pica)
2. ANP
3. Other Hormones
-- Female Sex Hormones
4. Baroreceptors in the Circulatory System
-- in essence, these are Na+ receptors,
since blood volume is dependent on solute volume
Remember, besides pulling water around, Na+ typically also pulls Cl- around
B. Potassium Balance -- very much tied to acid base balance, as shifts in
H+ often offset by opposite direction shifts in K+. With acidosis, ECF K+ goes up;
with alkalosis, K+ re-enters cells (H+ exits)
You should know
the reasons why this happens! (See acid-base balance below)
Regulation of K+ Balance
-- main site: DCT and cortical collecting ducts
Main thrust of renal mechanisms is to excrete K; faced with shortages the kidneys
have a very limited ability to conserve K+, and K+ may be lost even when needed.
Factors involved: K+ levels directly; aldosterone
C. Calcium (and Phosphate) Balance -- 99% of body=s calcium is in bones (as calcium
phosphate salts), yet ionic calcium necessary for many physiological events
Regulation of Ca+2 Balance
-- two hormones, PTH and calcitonin, of which PTH is far
more crucial (you already know what these hormones do!)
Calcitonin
(from thyroid) targets mainly bones (osteoblasts)
PTH targets: bones (osteoclasts), small intestine (vitamin D) and kidneys (reabsorp.
from DCT, conversion of inactive to active vitamin D)
Osteoclasts/-blasts working on bones will also release/deposit phosphate ions
from/to
bones, so PTH and calcitonin are majorly involved in phosphate balance as well
As a rule, 75% of filtered phosphate ions reabsorbed (regulated by its transport
maximum)
D. Regulation of Anions -- know about Cl- and HPO4-2 (phosphate); be aware that,
like potassium, chloride ions are also involved in acid-base balance by being shifted
with bicarbonate (HCO3-)
III. Acid-Base Balance -- pH balance so crucial to normal metabolic functioning
ICF typically at pH 7; plasma varies but typically slightly alkaline
Most H+ generated as metabolic by-products
Regulation of H+ Balance: three main mechanisms
1. Chemical Buffer Systems -- involves
weak acids and associated weak bases
a. Bicarbonate Buffer System
-- the only important ECF buffer system (though there
is some protein buffering in plasma
b. Phosphate Buffer System
-- only important in ICF, though protein buffering also
important in ICF
c. Protein Buffer Systems
-- amine (base) and carboxyl (acid) ends; also important
in ICF
2. Respiratory Regulation --
two times buffering capabilities of all chemical buffers
Clearly, this is directly tied to the bicarbonate buffering system
Changes in AVR in a healthy individual can go well beyond compensating for most
pH fluctuations
3. Renal Regulation -- only kidneys can actually remove metabolically produced acids
Mainly this involves regulating bicarbonate ion levels (conserving and generating new
ones; understand concept presented in Fig. 26.13, pg. 1030)