Chapter 26: The Urinary System Flashcards
describe the major structures of the urinary system and the functions they perform.
functions of the kidneys
-
regulation of blood ionic composition
* Na+, K+, Ca2+, Cl-, and phosphate (HPO4)2- -
regulation of blood pH
* excrete variable amounts of H+ ions and conserve bicarbonate ions (HCO3)- which buffer H+ in the blood, helping regulate blood pH -
regulation of blood volume
* conserving or eliminating water in the urine (total volume of urine 95% water) -
regulation of blood pressure
* by secreting the enzyme renin, which activates the renin- angiotensin-aldosterone pathway. Increased renin = increased BP - the kidneys release renin to raise increase blood pressure - maintenance of blood osmolarity
- by separately regulating water loss and solute loss in the urine
- blood osmolarity remains about 300milliosmoles per liter (300 mOsm/L)
-
production of hormones
* produce calcitriol and erythropoietin -
regulation of blood glucose level
* can use glutamine in gluconeogenesis to create new glucose molecules and then release them into the blood -
excretion of wastes and foreign substances
* form urine, excreting urea, ammonia, bilirubin, creatinine, uric acid as well as excreting drugs and environmental toxins
Uric acid is the breakdown product of nucleic acids and a waste eliminated by the kidneys.
describe the external and internal gross anatomical features of the kidneys.
kidney anatomy, position (retroperitoneal) and histology
casts -
tiny masses of material, hardened in the lumen of the urinary tubule and ard flushed out when filtrate builds up behind them
renal hilum (hilus) – indentation near the center of the concave medial border of each kidney
a. through this, the ureter emerges as well as blood and lymphatic vessels and nerves
renal capsule – deepest layer surrounding each kidney
a. smooth, transparent sheet of dense irregular connective tissue
b. continuous with the outer coat of the ureter
c. barrier against trauma, helps maintain shape of kidney
adipose capsule – middle layer surrounding each kidney
a. mass of fatty tissue surrounding the renal capsule b. also protects kidney from trauma, holds in place
renal fascia – superficial layer of the kidney
a. thin layer of dense irregular connective tissue
b. anchors the kidney to the surrounding structures and abd wall
c. deep to the peritoneum on the anterior surface
renal cortex – superficial light red region of the kidney
a. smooth-textured area extending from the renal capsule to the bases of the renal pyramids and into the spaces between them
b. divided into two zones
- cortical zone – outer zone
- juxtamedullary zone – inner zone
- *renal medulla** – deep, darker reddish-brown inner region (Urea recyling can cause a buildup of urea here)
- *renal pyramids** – several cone shaped structures that constitute the renal medulla
- *renal papilla** – apex of each pyramid
a. points toward the renal hilum
- *renal columns** – the portions of the renal cortex that extend between renal pyramids
- *renal lobe** – consists of a renal pyramid, some of the renal column on either side of the renal pyramid, and the renal cortex at the base of the renal pyramid.
Parenchyma – the functional part of the organ
a. In the kidney, consists of the renal cortex and renal pyramids of the renal medulla
trace the path of blood flow through the kidneys.
Blood flow through the kidneys
vessel that brings blood to glomerulus = afferent arteriole
Filtrate in the glomerulus is recieved by the proximal convoluted tubule
interlobar arteries > arcuate arteries > glomerular capillaries > arcuate veins
Kidneys receive 20 to 25% of the resting cardiac output via the renal arteries. The renal arteries branch to form segmental arteries, which branch to form interlobar arteries (through renal columns) to arcuate arteries (over bases of pyramids) to interlobular arteries. The interlobular arteries branch to form afferent arterioles to each nephron. Afferent arterioles branch to form glomerular capillaries where filtration occurs. Glomerular capillaries merge to form efferent arterioles, which then branch to form peritubular capillaries. Juxtamedullary nephrons also have vasa recta capillaries around them. Peritubular capillaries merge to form peritubular veins and with the vasa recta to form interlobular veins to arcuate veins to interlobar veins. Blood exits the kidney via renal veins.
Order of filtrate flow
glomerular capsule, proximal convoluted tubule (PCT), nephron loop, distal convoluted tubule ( DCT), collecting duct
Path of Urine
- nephron
- papillary duct
- minor calyx
- major calyx
- renal pelvis
Path of urine beginning with the kidneys
renal pelvis, ureters, bladder, urethra
describe the parts of a nephron.
A. Nephrons – functional unit of the kidney
* About 1,000,000 Form filtrate (filtered fluid)
B. papillary ducts – extend through the renal papillae of the pyramids.
- Drains into calyces
Once fluid drains into calyces, it becomes urine and no further reabsorption can occur.
C. minor calyx (plural is calyces) – 8-18 in each kidney
- receives urine from the papillary ducts of one renal papilla and delivers it to a major calyx
D. major calyx – 2-3 in each kidney from here, urine drains into a single large cavity called the renal pelvis
E. renal pelvis – cavity in the center of the kidney formed by the expanded proximal portion of the ureter major calyces open into it
F. renal sinus – cavity within the kidney into which the hilum expands.
- Contains part of the renal pelvis, the calyces, and branches of the renal blood vessels and nerves.
- Position of these structures in the renal sinus stabilized by adipose tissue
G. blood and nerve supply
renal artery – left and right
a. receive roughly 25% of resting cardiac output
b. divide into segmental arteries
- segmental arteries divide into interlobar arteries
- arcuate arteries – where the interlobar arteries arch between the renal medulla and cortex at the bases of the renal pyramids.
- Cortical radiate arteries – divisions of the arcuate arteries that radiate outward and enter the renal cortex
- afferent arteriole – branches off the cortical radiate arteries
a. blood vessel of a kidney that divides into the capillary network called a glomerulus
1. one afferent arteriole for each glomerulus.
Glomerulus – rounded mass of blood vessels
a. Microscopic tuft of capillaries surrounded by the glomerular capsule of each kidney tubule
efferent arteriole – union of glomerulus capillary mass into single arteriole that carries blood out of the glomerulus
peritubular capillaries – divided efferent arterioles that surround tubular parts of the nephron in the renal cortex
vasa recta – extensions of the efferent arteriole of a juxtamedullary nephron that run alongside the nephron loop in the medullary region of the kidney
- long, loop shaped capillaries that supply tubular portions of the nephron in the renal medulla.
renal vein – single vein that leaves the renal hilum and carries venous blood to the inferior vena cava
- peritubular capillaries reunite to form cortical radiate veins then arcuate veins, then interlobar veins, and finally the renal vein.
renal ganglion – where most renal nerves originate
a. sympathetic division of ANS.
b. Most are vasomotor nerves that regulate flow of blood through kidney by causing vasoconstriction or dilation.
renal plexus – network of nerves around kidneys
nephron – functional unit of the kidneys
number of nephrons is constant from birth.
- Increase in kidney size is due to growth of individual nephrons
- No regeneration once injured or diseased.
- Signs of kidney disease not usually apparent until function is less than 25% because remaining nephrons adapt and handle it.
- Removal of one kidney induces hypertrophy of the other which eventually can filter blood at 80% of the rate of two normal kidneys consists of two parts:
a. renal corpuscle – where blood plasma is filtered
* consists of glomerulus and glomerular capsule 2. lies within the renal cortex
b. renal tubule – filtered fluid passes into
- *glomerulus** – capillary network
- *glomerular capsule or Bowman’s capsule** – **filters blood double walled epithelial cup at the proximal end of a nephron that encloses the glomerular capillaries
a. AKA Bowman’s capsule.
renal tubule – 3 main sections. In the order that fluid passes through:
a. proximal convoluted tubule or PCT *reabsorbs the most substances *lined with cells that have a border of microvilli
1. convoluted = tightly coiled
b. loop of Henle or nephron loop
1. extends into the renal medulla before turning around and returning to renal cortex
descending limb of the loop of Henle – downward extension of nephron loop from renal cortex into the renal medulla ascending limb of the loop of Henle – after hairpin turn, nephron loop extends upward to the renal cortex into the DCT.
- distal convoluted tubule or DCT
- *collecting duct –** receive output from the DCTs of several nephrons
- *papillary duct** – united and converged collecting ducts
- several hundred in each kidney b. drain into the minor calyces
cortical nephron – 80-85% of the nephrons
- the renal corpuscles lie in the outer portion of the renal cortex
- have short nephron loops that lie mainly in the cortex and only penetrate the outer region of the renal medulla
- short nephron loops receive blood supply from peritubular capillaries that arise from efferent arterioles
juxtamedullary nephron – the other 15-20% of nephrons
- renal corpuscles lie deep to the cortex, close to the medulla.
- Have long nephron loops that extend into the deepest region of the medulla
- Long nephron loops receive blood supply from peritubular capillaries and from the vasa recta that arise from efferent arterioles
- Long nephron loops allow the kidneys to excrete very dilute or very concentrated urine
- Ascending limb of the long nephron loop consists of two portions:
- *thin ascending limb of the loop of Henle** – same lumen thickness but thinner epithelium. Only in long loops
- *thick ascending limb of the loop of Henle** – same thickness in all loops.
explain the histology of a nephron and collecting duct.
Histology of the glomerular Nephron and Collecting Duct:
glomerular capsule – consists of visceral and parietal layers
visceral layer consists of modified simple squamous epithelial cells called podocytes
a. podocyte – modified simple squamous epithelial cells of the visceral layers of the glomerular capsule
- have many footlike projections called pedicels, that wrap around the single layer of endothelial cells of the glomerular capillaries and form the inner wall of the capsule.
Parietal layer – consists of simple squamous epithelium and forms the outer wall of the capsule.
capsular space or Bowman’s space – the space between the two layers of the glomerular capsule
- is the lumen of the urinary tube
K. renal tubule and collecting duct – different types of cells form the renal tubule and collecting duct:
L. macula densa – region of crowded together columnar tubule cells at the final part of the ascending limb of the nephron loop where it makes contact with the afferent arteriole serving that renal corpuscle. detect sodium and chloride ions in the filtrate
- juxtaglomerular (JG) cells – modified smooth muscle fibers alongside the macula densa, the wall of the afferent arteriole, and sometimes the efferent arteriole
- juxtaglomerular apparatus or JGA – the macula densa cells and justaglomerular cells combined helps regulate BP in kidneys
In the last part of the DCT and continuing into the collecting ducts, two different types of cells are present:
principal cells – receptors for both ADH and aldosterone
- most cells of the ducts are principal cells
intercalated cells – play a role in homeostasis of blood pH.
identify the three basic functions performed by nephrons and collecting ducts and indicate where each occurs.
renal physiology – 3 functions
glomerular filtration – first step of urine production
- water and most solutes in blood plasma move across the wall of glomerular capillaries and are filtered and move into the glomerular capsule and into the renal tubule
tubular reabsorption – tubule cells reabsorb 99% of the filtered water and many useful solutes.
- Water and solutes return to the blood as it flows through the peritubular capillaries and vasa recta
- Tubular reabsorption returns a substance to the blood
tubular secretion – renal tubule and duct cells secrete materials such as wastes, drugs, and excess ions into the fluid
- tubular secretion removes a substance from the blood
describe the filtration membrane.
glomerular filtration
glomerular filtrate – the fluid that enters the glomerular capsular space
- is the fluid produced when blood is filtered by the filtration membrane in the glomeruli of the kidneys
filtration fraction – the fraction of blood plasma in the afferent arterioles of the kidneys that becomes glomerular filtrate
- typically 0.16-0.20 (16-20%) but the value varies with both health and disease
- On average: daily volume of glomerular filtrate in females = 150 L, males = 180 L
filtration membrane – leaky barrier formed by both the glomerular capillaries and the podocytes
- sandwich like assembly permits filtration of water and small solutes but prevents filtration of most plasma proteins, blood cells, and platelets.
- Substances filtered from the blood cross 3 filtration barriers:
- A glomerular endothelial cell – see fenestrations below
- The basal lamina – layer of acellular material between the podocytes and endothelium.
- Consists of minute collagen fibers and proteoglycans in a glycoprotein matrix.
- Negative charges in the matrix prevent filtration of larger negatively charged plasma proteins
- A filtration slit formed by a podocyte – see below
Fenestrations – large pores in glomerular endothelial cells that measure 0.07-0.1 micrometers.
- This size permits all solutes in blood plasma to exit glomerular capillaries but prevents filtration of blood cells and platelets.
- Mesangial cells – contractile cells located among the glomerular capillaries and in the cleft between afferent and efferent arterioles that help regular glomerular filtration
Pedicels – 1000’s of footlike structures extending from each podocyte
a. Wrap around glomerular capillaries leaving small slits
- *filtration slits** – the spaces between pedicels
- *slit membrane** – a thin membrane that extends across each filtration slit
a. permits the passage of molecules with diameter less than 0.007 micrometers, including water, glucose, amino acids, vitamins, very small plasma proteins, ammonia, urea, and ions
b. less than 1% of albumin passes the slit membrane because it is slightly too large at 0.007 micrometers
principle of filtration – the use of pressure to force fluids and solutes through a membrane
a. same in glomerular capillaries as in other body capillaries (Starling’s Law Ch 21.2)
b. the volume of fluid filtered by the renal corpuscle is much larger than other body blood capillaries for 3 reasons:
1. large surface area for filtration – glomerular capillaries present a large surface area for filtration because they are long and extensive
* mesangial cells regular how much surface area is available
2. thin and porous filtration membrane – several layers but only 0.1mm thick
* glomerular capillaries are about 50x leakier than other blood capillaries due to the large fenestrations
3. high glomerular capillary blood pressure – efferent arteriole diameter is smaller than afferent arteriole
* resistance to outflow of blood in glomerulus is high, therefore BP in glomerular capillaries is higher than elsewhere in the body.
discuss the pressures that promote and oppose glomerular filtration.
net filtration pressure (NFP) – determined by 3 main pressures: GBHP, CHP, BCOP
glomerular blood hydrostatic pressure (GBHP) – the blood pressure in glomerular capillaries.
a. Generally about 55mmHg
b. Promotes filtration by forcing water and solutes in blood plasma through the filtration membrane
capsular hydrostatic pressure (CHP) – the hydrostatic pressure exerted against the filtration membrane by fluid already in the capsular space and renal tubule
a. opposes filtration
b. represents a “back pressure” of about 15mmHg
blood colloid osmotic pressure (BCOP) – due to the presence of proteins such as albumin, globulins, and fibrinogen in blood plasma
a. opposes filtration b. averages 30mmHg
NFP = GBHP -CHP-BCOP
- = 55mmHg – 15mmHg – 30mmHg
- = 10mmHg
- Pressure of only 10mmHg causes a normal amount of blood plasma (excluding proteins) to filter from the glomerulus into the capsular space.
B. glomerular filtration rate (GFR) – the amount of filtrate formed in all renal corpuscles of both kidneys each minute. Average adults 125mL/min in males, 105mL/min in females
regulation of GFR – GFR is directly related to the net filtration pressure. Any change in NFP will affect GFR
- filtration stops if NFP drops below 45mmHg because opposing pressures = 45mmHg
- mechanisms that regulate GFR operate in 2 main ways:
- by adjusting blood flow into and out of the glomerulus
- GFR increases when blood flow into glomerular capillaries increases
- Control of both afferent and efferent arterioles controls blood flow into and out of the glomerulus
- by altering the glomerular capillary surface area available for filtration
* Mesangial cells contract or relax changing the amount of surface area available for filtration
c. 3 mechanisms that control GFR: renal autoregulation, neural regulation, and hormonal regulation
renal autoregulation of GFR – capability of the kidneys themselves the help maintain a constant renal blood flow and GFR despite normal everyday changes in blood pressure
a. consists of 2 mechanisms: myogenic mechanism and tubuloglomerular feedback:
myogenic mechanism – occurs when stretching triggers contraction of smooth muscle cells in the walls of afferent arterioles. Normalizes renal blood flow and GFR within seconds after a change in body BP.
- Increased BP increases GFR, smooth muscle cells contract, narrowing the afferent arteriole’s lumen, decreasing blood flow and returning GFR to normal.
- Decreased BP induces the afferent arteriole smooth muscle cells to relax, dilating the arteriole, increasing blood flow, increasing GFR.
tubuloglomerular feedback – so named because the macula densa provides feedback to the glomerulus
- Increased BP = faster flow of filtered fluid = less reabsorption of ions such as Na+ and Cl- and also water. Macula densa cells detect the increased delivery of Na+, Cl- and water and inhibit release of NO from cells in the juxtaglomerular apparatus. NO causes vasodilation so afferent arterioles constrict in declining level of NO. Therefore, less blood flow into the glomerular capillaries and GFR decreases
- When BP falls, opposite sequence of events occurs, though to a lesser degree.
- Tubuloglomerular feedback occurs slower than myogenic mechanism
neural regulation of GFR – kidney blood vessels supplied by sympathetic ANS fibers that release norepinephrine.
- Norepinephrine causes vasoconstriction through activation of alpha-1 receptors, which are particularly plentiful in the smooth muscle fibers of afferent arterioles.
- At rest, sympathetic stimulation is low, afferent and efferent arterioles are dilated, renal autoregulation of GFR prevails.
- With moderate sympathetic stimulation, both afferent and efferent arterioles constrict to the same degree. Blood flow in and out of the glomerulus is restricted equally, decreases GFR only slightly.
- With greater sympathetic stimulation, vasoconstriction of the afferent arterioles predominates.
- Ex: exercise or hemorrhage.
- As a result, blood flow into glomerular capillaries is greatly decreased and GFR drops
- Results in decreased urine output, conserving blood volume, and greater blood flow to other body tissues
hormonal regulation of GFR – 2 hormones contribute to regulation of GFR: angiotensin II and ANP
angiotensin II – very potent vasoconstrictor that narrows both afferent and efferent arterioles and reduces renal blood flow, decreasing GFR.
atrial natriuretic peptide (ANP) – secreted by cells of the atria of the heart. Secretion is stimulated by stretching of the atria, as occurs when blood volume increases. ANP causes relaxation of the glomerular mesangial cells which increases capillary surface area for filtration, increasing GFR
outline the routes and mechanisms of tubular reabsorption and secretion.
tubular reabsorption and tubular secretion
principles of reabsorption and secretion – reabsorption = return of filtered water and solutes to the bloodstream. Carried out by epithelial cells all along the renal tubule and duct but largest contribution is by the proximal convoluted tubule.
- Solutes reabsorbed by active and passive processes include glucose, amino acids, urea, and ions such as Na+, K+, Ca2+, Cl-, HCO3 -, and HPO4 2-.
- Cells more distal to the proximal convoluted tubule fine- tune reabsorption to maintain homeostasis of water and ions
- Most small proteins and peptides are reabsorbed by pinocytosis
- Tubular secretion is the transfer of materials from blood and tubule cells into the glomerular filtrate.
- Substances secreted include ions: H+, NH4 +, creatinine, certain drugs such as penicillin.
- Two important outcomes: secretion of H+ helps control blood pH and secretion of other substances helps eliminate them from the body
- Tubular secretion allows certain substances to be detected in a urinalysis: anabolic steroids, plasma expanders, erythropoietin, hCG, hGH, amphetamines, and also alcohol or other drugs.
reabsorption routes – two routes before entering a peritubular capillary: between adjacent tubule cells or through an individual tubule cell.
a. Paracellular reabsorption – fluid can leak between the cells in a passive process process involves water and solutes in tubular fluid returning to the bloodstream by moving between tubule cells
- Even though the epithelial cells are connected by tight junctions, the tight junctions between cells in the proximal convoluted tubule are “leaky” and permit some reabsorbed substances to pass between cells into the peritubular capillaries.
- In certain parts of the renal tubule, accounts for up to 50% of reabsorption of certain ions and water that accompanies them via osmosis.
b. Transcellular reabsorption – a substance passes from the fluid in the tubular lumen through the apical membrane of a tubule cell, across the cytosol, and out into the interstitial fluid through the basolateral membrane
transport mechanisms – renal cells transport solutes in or out of tubular fluid, but most specific substances in one direction only.
- Different types of transport proteins are present in the apical and basolateral membranes
- Tight junctions between cells form a barrier that prevents mixing of proteins in the apical and basolateral membrane compartments
primary and secondary active transport
- Primary active transport – energy from hydrolysis of ATP is used to pump a substance across a membrane – the sodium-potassium pump is an example
- Secondary active transport – the energy stored in an ions electrochemical gradient drives another substance across a membrane.
- Couples movement of an ion down its electrochemical gradient to the uphill movement of a second substance against its electrochemical gradient.
- Symporters – membrane proteins that move 2 or more substances in the same direction across a membrane
- Antiporters – move two substances in opposite directions across a membrane
transport maximum (Tm) – the upper limit on how fast a transporter can work.
a. Measured in mg/minute
describe how specific segments of the renal tubule and collecting duct reabsorb water and solutes.
Reabsorbtion - return of substances into blood from filtrate
Secretion - substance in blood entering the already formed filtrate
A. reabsorption and secretion in the proximal convoluted tubule – largest amount of solute and water reabsorption from filtered fluid occurs here.
Reabsorb 65% filtered water, Na+, and K+, 100% filtered organic solutes such as glucose and amino acids, 50% filtered Cl-, 80-90% filtered HCO3 -, 50% filtered urea, and variable amounts of filtered Ca2+, Mg2+, and HPO4 2-. Secrete variable amount of H+, ammonium ions NH4 +, and urea.
Most solute reabsorption in the PCT involves Na+.
- Na+ transport occurs via symport and antiport mechanisms in the PCT.
B. secretion of NH3 and NH4+ in the proximal convoluted tubule
- ammonia (NH3) is a poisonous waste product derived from deamination of various amino acids – this reaction occurs mainly in hepatocytes.
- Hepatocytes convert most ammonia to urea, a less toxic compound
- Most excretion of these nitrogen containing waste products occurs via the urine
- Urea and ammonia are both filtered at the glomerulus and secreted by proximal convoluted tubule cells into the tubular fluid.
- PCT cells can produce additional NH3 by deaminating the amino acid glutamate in a reaction that also generates HCO3 -. The NH3 quickly binds H+ to become an ammonium ion NH4 +, which can substitute for H+ aboard Na+-H+ antiporters in the apical membrane and be secreted into tubular fluid.
- The HCO3 – generated in this reaction moves through the basolateral membrane and diffuses into the bloodstream, providing additional buffers in blood plasma.
C. reabsorption in the loop of Henle
- fluid enters the nephron loop at a rate of 40-45mL/min
- the tubular fluid chemical composition is quite different from the glomerular filtrate because glucose, amino acids, and other nutrients are no longer present.
- The osmolarity of tubular fluid is still close to the osmolarity of blood because reabsorption of water by osmosis keeps pace with reabsorption of solutes all along the PCT.
- The nephron loops reabsorb 15% of the filtered water, 20-40% Na+ and K+, 35% of Cl-, 10-20% of the HCO3 -, and a variable amount of Ca2+ and Mg2+.
Reabsorption of water via osmosis is NOT automatically coupled to reabsorption of filtered solutes because part of the nephron loop is relatively impermeable to water.
15% of filtered water is reabsorbed in the descending limb of the nephron loop but little or no water is reabsorbed in the ascending limb.
- In this segment of the tubule, the apical membranes are virtually impermeable to water.
- Because ions but not water molecules are reabsorbed, the osmolarity of the tubular fluid decreases progressively as fluid flows toward the end of the ascending limb.
- The nephron loop sets the stage for independent regulation of both the volume and osmolarity of body fluids.
- Na+ - K+ - 2Cl- symporters in the apical membrane of cells in the thick ascending limb of the nephron loop reabsorb Na+ and Cl- but the K+ ions leak back out leakage channels in the apical membrane back into the tubular fluid.
D. reabsorption in the early distal convoluted tubule
- fluid enters the DCT at about 25mL/min
- The early part of the DCT reabsorbs about 10-15% of the water, 5% Na+, 5% Cl-
- Reabsorption of Na+ and Cl- occurs by Na+ - Cl- symporters in the apical membranes
- Sodium potassium pumps and Cl- leakage channels in the basolateral membrane permit reabsorption of Na+ and Cl- into the peritubular capillaries
- The DCT is also a major site where parathyroid hormone (PTH) stimulates reabsorption of Ca2+.
- The amount of Ca2+ reabsorption in the early DCT depends on the body’s needs.
E. reabsorption and secretion in the late distal convoluted tubule and collecting duct
Two types of cells are present in the late or terminal parts of the DCT and throughout the collecting duct: principal cells and intercalated cells
Principal cells reabsorb Na+ and secrete K+
Intercalated cells reabsorb K+ and HCO3 -, and secrete H+.
In the late DCT and collecting ducts, the amount of water and solute reabsorption depends on the body’s needs.
Na+ passes through the apical membrane of principal cells via Na+ leakage channels
- Then sodium-potassium pumps actively transport Na+ across the basolateral membranes
- Then Na+ passively diffuses into the peritubular capillaries from the interstitial spaces around the tubule cells.
- K+ is mainly secreted into the urine by diffusing down its concentration gradient from the apical membrane into the tubular fluid where the K+ concentration is very low
- Basolateral sodium potassium pumps continually bring K+ into the principal cells and K+ leakage channels in all membranes allow K+ ions to leak out.
F. hormonal regulation of tubular reabsorption and tubular secretion
absence of angiotensin converting enxyme will lead to decreased blood pressure
5 hormones affect the extent of Na+, Cl-, Ca2+, and water reabsorption as well as K+ secretion by the renal tubules: angiotensin II, aldosterone, ADH, ANP, and PTH
- renin angiotensin aldosterone system – produces angiotensin II, the active form of angiotensinogen
- renin – secreted by juxtaglomerular cells when blood volume and pressure decreases, causing afferent arteriole walls to be less stretched.
Sympathetic stimulation also directly stimulates release of renin from juxtaglomerular cells
Renin clips off a 10-amino acid peptide called angiotensin I from angiotensinogen which is synthesized by hepatocytes.
I. angiotensin II – the active form of the hormone; ACE converts angiotensin I to angiotensin II by clipping off 2 amino acids.
Angiotensin II affects renal physiology in 3 main ways:
- Decreases GFR by causing vasoconstriction of afferent arterioles
- Enhances reabsorption of Na+, Cl-, and water in the PCT by stimulating the activity of Na+ - H+ antiporters
- It stimulates the adrenal cortex to release aldosterone
J. Aldosterone – a hormone that stimulates the principal cells in the collecting ducts to reabsorb more Na+ and Cl- and secrete more K+.
Increased secretion of aldosterone would result in an increase of blood sodium.
The consequence of reabsorbing more Na+ and Cl- is that more water is reabsorbed, causing an increase in blood volume and BP.
K. antidiuretic hormone (ADH) or vasopressin – released by the posterior pituitary
consumption of salty food will cause an increase in ADH
regulates facultative water reabsorption by increasing the water permeability of principal cells in the last part of the DCT and throughout the collecting duct.
- In the absence of ADH, the apical membranes of principal cells have a very low permeability to water.
- Within principal cells are tiny vesicles containing many copies of a water channel protein called aquaporin-2.
- ADH stimulates insertion of aquaporin-2 containing vesicles into the apical membranes via exocytosis, increasing water permeability reabsorbing more water.
- When ADH level is high, kidneys can produce 400-500mL concentrated urine. When ADH level is low, the kidneys produce a large volume of dilute urine.
- Negative feedback involving ADH regulates facultative water reabsorption.
- When osmolarity or osmotic pressure of plasma and interstitial fluid increases (water concentration decreases) by as little as 1%, osmoreceptors in the hypothalamus detect the change
- Their nerve impulses stimulate secretion of more ADH into the blood
- Principal cells become more permeable to water, plasma osmolarity decreases to normal
f. A second powerful stimulus for ADH secretion is a decrease in blood volume. Ex. Hemorrhaging or severe dehydration
g. Diabetes insipidus – pathological absence of ADH activity
1. A person may excrete up to 20 liters of very dilute urine daily.
L. atrial natriuretic peptide (ANP) – a large increase in blood volume promotes release of ANP from the heart can inhibit reabsorption of Na+ and water in the PCT and collecting duct.
Also suppresses the secretion of aldosterone and ADH
These effects increase the excretion of Na+ in urine and increase urine output, which decreases blood volume and BP.
M. parathyroid hormone (PTH) – hormone that regulates ionic composition
- lower than normal level of Ca2+ in the blood stimulates the parathyroid glands to release PTH.
PTH in turn stimulates cells in the early DCT to reabsorb more Ca2+ into the blood. - PTH also inhibits HPO4 – (phosphate) reabsorption in the PCTs, thereby promoting phosphate excretion
All hormones od renal control of BP / Blood presssure
aldosterone
angiotensin
renin
(NOT CALCITIOL)
explain how specific segments of the renal tubule and collecting duct secrete solutes into the urine.
production of dilute and concentrated urine
- fluid intake is variable but body fluid volume remains stable kidneys regulate the rate of water loss in urine
- ADH controls whether dilute or concentrated urine is formed.
- In the absence of ADH, urine is very dilute however a high level of ADH stimulates reabsorption of more water into blood, producing concentrated urine.
formation of dilute urine
- glomerular filtrate has the same ratio of water and solute particles as blood, its osmolarity is about 300mOsm/liter
- Fluid leaving the PCT is still isotonic to plasma
- When dilute urine is being formed, the osmolarity of the fluid in the tubular lumen increases as it flows down the descending limb of the nephron loop, decreases as it flows up the ascending loop, and decreases still more as it flows through the rest of the nephron and collecting duct.
a. These changes in osmolarity result from the following conditions along the path of tubular fluid:
- Because the osmolarity of the interstitial fluid of the renal medulla becomes progressively greater, more and more water is reabsorbed by osmosis as tubular fluid flows down the descending limb toward the tip of the nephron loop
- Cells lining the thick ascending limb of the loop have symporters that actively reabsorb Na+, K+, and Cl- from the tubular fluid. The ions pass from the tubular fluid into thick ascending limb cells, then into interstitial fluid, and finally some diffuse into the blood inside the vasa recta
- Although solutes are being reabsorbed in the thick ascending limb, this portion of the nephron always has low water permeability, so water does not follow by osmosis, therefore the osmolarity of the tubular fluid drops to about 150mOsm/liter as solutes but not water molecules are leaving the tubular fluid. The fluid entering the DCT is thus more dilute than plasma
- While fluid flows along the DCT, additional solutes but only a few water molecules are reabsorbed. The early DCT cells are not very permeable to water and are not regulated by ADH
- The principal cells of the late DCT and collecting ducts are impermeable to water when ADH level is low. Thus, tubular fluid becomes progressively more dilute as it flows onward.
- By the time tubular fluid drains into the renal pelvis, its concentration can be as low as 65-70 mOsm/liter (4xmore dilute than blood plasma and glomerular filtrate)
C. formation of concentrated urine – when water intake is low or water loss is high (as in heavy sweating), the kidneys must conserve water while still eliminating wastes and excess ions.
Under influence of ADH, kidneys produce a small volume of high concentrated urine
Urine can be 4x more concentrated (1200mOsm/liter) than blood plasma or glomerular filtrate
The ability of ADK to cause excretion of concentrated urine depends on the presence of an osmotic gradient of solutes in the interstitial fluid of the renal medulla.
D. osmotic gradient – solutes in the interstitial fluid of the renal medulla. Increases to about 1200mOsm/liter deep in the renal medulla.
The 3 major solutes that contribute to this high osmolarity are Na+, Cl-, and urea.
Two main factors contribute to building and maintaining this osmotic gradient:
- Differences in solute and water permeability and reabsorption in different section of the long nephron loops and the collecting ducts
- The countercurrent flow of fluid through tube-shaped structures in the renal medulla
E. countercurrent mechanism—general
- countercurrent flow refers to the flow of fluid in opposite directions.
- This occurs when fluid flowing in one tube runs counter to fluid flowing in a nearby parallel tube
- Ex. Flow of tubular fluid through the descending and ascending limbs of the nephron loop and the flow of blood through the ascending and descending parts of the vasa recta.
two types of countercurrent mechanisms exist in the kidneys: countercurrent multiplication and countercurrent exchange. Countercurrent multiplication – the process by which a progressively increasing osmotic gradient is formed in the interstitial fluid of the renal medulla as a result of countercurrent flow.
- Involves the long nephron loops of juxtamedullary nephrons.
- The long nephron loop is said to function as a countercurrent multiplier.
- The kidneys use this osmotic gradient to excrete concentrated urine.
Countercurrent Exchange – the process by which solutes and water are passively exchanged between blood of the vasa recta and interstitial fluid of the renal medulla as a result of countercurrent flow.
a. The vasa recta is said to function as a countercurrent exchanger because countercurrent flow between the descending and ascending limbs of the vasa recta allows for exchange of solutes and water between the blood and interstitial fluid of the renal medulla.
A. evaluation of kidney function – routine assessment of kidney function involves both the quantity and quality of urine and the levels of wastes in the blood.
Volume of urine eliminated per day is 1-2L in adults.
normal specific gravity range of urine in humans - 1.001 – 1.035
- Fluid intake, BP, blood osmolarity, diet, body temp, diuretics, mental state, and general health influence urine volume.
- Urine is 95% water by volume and 5% electrolytes, solutes, and exogenous substances such as drugs.
- Normal urine is virtually protein free.
- Typical solutes: filtered and secreted electrolytes that are
not reabsorbed, urea, creatinine, uric acid, urobilinogen, and smaller quantities of other substances ex. Fatty acids, pigments, enzymes, hormones.
Formation of new glucose molecule - gluconeogenesis
define urinalysis and describe its importance.
Urinalysis – analysis of the volume and physical, chemical, and microscopic properties of urine
blood tests – two blood-screening tests can provide info re kidney function:
a. blood urea nitrogen (BUN) – measures the blood nitrogen that is part of the urea resulting from catabolism and deamination of amino acids.
it measures deamination of amino acids.
the levels can be lowered by consuming less protein.
it can be caused by renal obstruction.
(not it rises as glomerular filtration rate rises.)
- When GFR decreases severely (as may occur with renal disease or obstruction of the urinary tract) BUN rises steeply
b. Plasma creatinine – measurement which results from catabolism of creatine phosphate in skeletal muscle. - Normally blood creatinine level remains steady because the rate of creatinine excretion in the urine equals discharge from muscles
- Creatinine level above 135mmol/liter is usually indicative of poor renal function.
define renal plasma clearance and describe its importance.
A. renal plasma clearance – the volume of blood that is “cleaned” or cleared of a substance per unit of time, usually expressed in units of milliliters per minute.
- more useful than BUN and blood creatinine values in the diagnosis of kidney problems.
- An evaluation of how effectively the kidneys are removing a given substance from blood plasma.
- High renal plasma clearance indicates efficient excretion of a substance in the urine; low clearance indicates inefficient excretion.
- The clearance of a solute depends on the 3 basic processes of a nephron, glomerular filtration, tubular reabsorption, and tubular secretion.
a. A substance filtered but neither reabsorbed nor secreted has a clearance equal to the glomerular filtration rate because all molecules that pass the filtration membrane appear in the urine.
describe the anatomy, histology, and physiology of the ureters, urinary bladder, and urethra.
urine transportation, storage and elimination – from collecting ducts to minor calyces to major calyces to renal pelvis to ureters to urinary bladder through to urethra.
ureters (including structure) – tube that connects kidney with urinary bladder.
serve as passageways and/or temporary storage areas
- Transports urine from renal pelvis to urinary bladder.
- Peristaltic contractions of the muscular walls of the ureters push urine toward the urinary bladder, but hydrostatic pressure and gravity also contribute a. Waves occur 1-5x/min depending on volume of urine produced
- Ureters are 25-30cm long, thick walled narrow tubes that vary from 1mm to 10mm along their course.
- Retroperitoneal.
- At the base of the urinary bladder, the ureters curve medially and pass obliquely through the wall of the posterior aspect of the urinary bladder.
- Physiological “valve” is effective at preventing urinary backflow: as bladder fills with urine, pressure within it compresses the oblique openings into the ureters, and prevents backflow of urine.
Ureters have 3 layers of tissue that form the walls: mucosa (deep), muscularis, Adventitia
a. Mucosa – deepest coat – mucous membrane with transitional epithelium and an underlying lamina propria of areolar connective tissue with considerable collagen, elastic fibers, and lymphatic tissue (LAMINA PROPORIA)
- The transitional epithelium is able to stretch, an advantage for an organ that must accommodate a varying volume of fluid.
- Mucous secreted by goblet cells of the mucosa prevents the cells from coming into contact with urine (the solute concentration and pH of which may differ drastically from the cytosol of cells that form the walls of the ureters)
b. Muscularis – intermediate coat throughout most of the length of the ureter.
- Composed of inner longitudinal and outer circular layers of smooth muscle fibers.
- This arrangement is opposite to the GI tract (which has inner circular and outer longitudinal layers)
- The muscularis of the distal third of the ureters also contains an outer layer of longitudinal muscle fibers. (therefore the distal third of the ureter is inner longitudinal, middle circular, outer longitudinal)
- Major function of the muscularis is peristalsis.
c. Adventitia – superficial coat of the ureters – a layer of areolar connective tissue containing blood vessels, lymphatic vessels, and nerves that serve the muscularis and mucosa.
* Adventitia blends in with surrounding connective tissue and anchors the ureters in place.
C. urinary bladder
anatomy and histology – hollow, distensible muscular organ situated in the pelvic cavity posterior to the pubic symphysis.
- Males – directly anterior to the rectum
- Females – anterior to the vagina and inferior to the uterus
- Folds of the peritoneum hold the urinary bladder in position.
- Spherical when distended due to accumulation of urine, collapsed when empty.
- Bladder capacity averages 700-800mL, smaller in women as uterus occupies superior space.
Trigone – small triangular area in the floor of the urinary bladder lies in the anterior corner of the trigone - Internal urethral orifice
a. The two posterior corners of the triangle are the two ureteral openings
b. The anterior corner is the opening into the urethra.
internal urethral orifice – the opening into the urethra from the bladder.
3 coats that make up the wall of the urinary bladder:
Mucosa – deepest coat of urinary bladder, two layers similar to ureters:
- transitional epithelium – permits stretching.
1. Rugae (folds in the mucosa) are also present to permit expansion - b. lamina propria
muscularis – middle layer surrounding the mucosa
a. AKA the detrusor muscle
b. Consists of 3 layers of smooth muscle fibers: inner longitudinal, middle circular, outer longitudinal
- *internal urethral sphincter** – circular fibers around the opening to the urethra in the urinary bladder
- *external urethral sphincter** – inferior to the internal urethral sphincter
a. composed of skeletal muscle
b. is a modification of the deep muscles of the perineum
adventitia – most superficial coat of the urinary bladder on the posterior and inferior surfaces – a layer of areolar connective tissue continuous with that of the ureter.
Serosa – over the superior surface of the urinary ladder – a layer of visceral peritoneum.
D. micturition reflex – sequence of events resulting in urination
- when volume of urine in bladder exceeds 200-400mL, pressure within increases considerably.
- Stretch receptors in bladder wall transmit nerve impulses into the spinal cord where they propagate to the micturition center and trigger a spinal reflex called the micturition reflex.
- Parasympathetic impulses from the micturition center propagate to the urinary bladder wall and internal urethral sphincter.
- The nerve impulses cause contraction of the detrusor muscle and relaxation of the internal urethral sphincter.
- Simultaneously, the micturition center inhibits somatic motor neurons that innervate skeletal muscle in the external urethral sphincter.
- On contraction of the urinary bladder wall and relaxation of the sphincters, urination takes place.
- Conscious desire to urinate occurs before the micturition reflex actually occurs
- Although it’s a reflex, people learn to initiate and stop it voluntarily in early childhood through learned control of the external urethral sphincter and certain muscles of the pelvic floor.
E. micturition center – located in the sacral spinal cord segments S2 and S3
F. urethra – duct from internal urethral orifice in the floor of the urinary bladder to the exterior of the body
- conveys urine in females
- conveys urine and semen in males
- the terminal portion of the urinary system
G. external urethral orifice – the opening of the urethra to the exterior in females, located between clitoris and vaginal opening
H. female urethra – runs through perineal floor of pelvis to exit between labia minora the wall of the female urethra consists of a deep mucosa and superficial muscularis.
a. Mucosa – mucous membrane composed of epithelium and lamina propria
- Near the urinary bladder the mucosa contains transitional epithelium that is continuous with the urinary bladder
- Near the external urethral orifice, the epithelium is nonkeratinized stratified squamous epithelium
- Between these areas, the mucosa contains stratified columnar or pseudostratified columnar epithelium.
b. Muscularis – consists of circularly arranged smooth muscle fibers and is continuous with that of the urinary bladder.
I. male urethra – longer in males, passing through the prostate, then deep muscles of the perineum, and finally through the penis, roughly 20cm total. Also consists of a deep mucosa and superficial muscularis 3 anatomical regions of the male urethra:
J. prostatic urethra – passes through the prostate
K. membranous urethra – AKA intermediate urethra – the shortest portion, passes through the deep muscles of the perineum
L. spongy urethra – the longest portion – passes through the penis
describe the ways that body wastes are handled.
normal pH range of urine: 4.6 – 8.0
waste management in other body systems
- besides the kidneys, several other tissues, organs, and processes contribute to the temporary confinement of wastes, transport of waste materials for disposal, the recycling of materials, and the excretion of excess or toxic substances in the body
- Waste management systems:
Body buffers – buffers in body fluids bind excess hydrogen ions, thereby preventing an increase in the acidity of body fluids. Buffers have limits, eventually the H+ must be eliminated from the body by excretion
The tissue, organ, or process that contributes to waste management by binding excess hydrogen ions, thereby preventing an increase in the acidity of body fluids
Blood – the bloodstream provides pick up and deliver for the transport of wastes
Liver – primary site for metabolic recycling, as occurs in the conversion of amino acids into glucose or of glucose into fatty acids. Also converts toxic substances into less toxic ones, such as ammonia into urea
Lungs – excrete CO2 and expel heat and a little water vapor Sweat (sudoriferous) glands – help eliminate excess heat, water, and CO2, plus small quantities of salts and urea also
GI tract – through defecation, the GI tract excretes solid, undigested foods, wastes, some CO2, water, salts, and heat.
