Lecture Urinary System Part 2
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What is tubular reabsorption?
- A selective transepithelial process
- -all organic nutrients are absorbed
- -water and ion reabsoprtion are hormonally regulated
- -begins as soon as filtrate enters the PCT
Includes both active and passive processes
- Two routes:
- 1. Transcellular
- 2. Paracellular
What is the transcellular route of tubular reabsorption?
filtrate moves through...
- -luminal membranes of the tubule cells
- -cytosol of tubule cells
- -basolateral membranes of tubule cells
- -endothelium of peritubular capillaries
What is the paracellular route of tubular reabsorption?
Paracellular route is
- -between cells
- -limited to water movement and reabsorption of Ca2+, Mg2+, K+, and some Na+ in the PCT where tight junctions are leaky
How does sodium reabsorption take place?
- Na+ is the most abundant filtrate!
- Takes place via the transcellular route
-Primarily active transport out of the tubule cell by Na+
ATPase in the basolateral membrane
passes in through the luminal membrane by secondary active transport or facilitated diffusion membranes
Low hydrostatic pressure and high osmotic pressure in the peritubular capillaries promotes bulk flow of water and other solutes
How are nutrients, water, and ions absorbed?
reabsorption provides the energy and the means for reabsorpting most other substances, including water
- Organic nutrients are reabsorbed by secondary active transport
- -Transport Maximum (TM) measured in mg/min reflects the number of carriers in the renal tubule
- -When carriers are saturated, excess of that substance is excreted
Water is reabsorbed by osmosis (passive obligatory water reabsorption), aided by water filled pores called aquaporins which create a channel
- Cations and fat soluble substances follow by diffusion
Describe Absorption in the PCT
*Site of most absorption!!!
- -65% Na+ and water
- - all nutrients
- -small protiens
- -all glucose, lactate & amino acids
- -bulk of electrolytes
Describe absorption in the Loop of Henle
O (no solute reabsorption)
O because no aquaporins)
Describe reabsorption in the DCT and collecting ducts
*Reabsorption is hormonally regulated
ADH (absorption of more water)
Aldosterone and ANP
What are the mechanisms of adolsterone concerning reabsoption capabilities?
tragets collecting ducts (principle cells) and distal DCT
promotes synthesis of luminal Na+ and K+ channels
Promotes synthesis of basolateral Na+-K+ ATPases
* as a results, little or no Na+ leaves the urine
What is tubular secretion?
Reabsorption in reverse
K+, H+, NH4+, creatine, ad organic acids move from peritubular capillaries or tubule cells into filtrate
Urine contains both filtered and secreted substances
What is the importance of tubular secretion?
Tubular secretion is important for...
1. Disposes of substances that are bound to plasma proteins
2. Eliminates undersirable substances that have been passively reabsorbed (e.g. urea and uric acid)
3. Rids the body of excess K+
4. Controls blood pH by altering amounts of H+ or HCO3- in the urine
How does osmolality contribute to the regulation of urine concentration and volume?
- Osmolality: the number of solute particles in 1kg of H20
- -Reflects ability to cause osmosis
- Osmolality of body fluids
- -expresed in milliosmols (mOsm) (mOsm/kg)
- -the kidneys maintain osmolity of plasma at ~300mOsm, using countercurrent mechanisms
What is the countercurrent mechanism?
- Occurs when fluid flows in opposite directions in two adjacent segments of the same tube
- -filtrate flow in the loop of Henle (countrcurrent multiplier)
- -blood flow in the vasa recta (countercurrent exchanger)
What is the role of the countercurrent mechanism?
Establish and maintain and osmotic gradient (300mOsm to 1200 mOsm) from renal cortex through medulla
- allow the kdneys to vary in urnie concentration
How does the countercurrent multiplier work?
Countercurrent multiplier is the loop of Henle
- Functions because of these two factors:
- 1. Descending Limb
- -freely permeable to H2O, which passes out of the filtrate into the hyperosmtic medullary interstitial fluid
- -filtrate osmolality increases to 1200 mOsm
- 2. Ascending Limb
- -Impermeable to H2O
- -Selectively permeable to solutes
- *Na+ and Cl- are passively reabsorbed in the thin segement, actively reabsorbd in the thick segment
- -filtrate osmolality decreases to 100 mOsm
- *positive feedback system
How is urea recycled?
- urea moves between the collecting cuts and the loop of henle
- -secreted into filtrate by facilitated diffusion in the ascending thin segment
- -reabsorbed by facilitated diffusion in the collecting ducts deep in the medulla
*contributes to the high osmolality in the medula and the concentration of urine
What is the counter current exchanger?
- The vassa recta
- -maintains the osmostic gradient
- -delivers blood to the medullary tissues
- -protect the medullary osmotic gradient by preventing rapid removal of salt, and by removing reabsorbed H2O
How is dilute urine formed?
filtrate is diluted in the ascending loop of henle
In the absenc of ADH, dilute filtrate contines into the renal pelvis as dilute urine
- Na+ and other ions may be selectively removed in the DCT and collecting duct, decreasing osmolality to as low as 50 mOsm
How is concentrated urine formed?
depends on the medullary osmotic gradient and ADH
ADH trigger the reabsorption of H2
O in the collecting ducts
- facultative water reabsoprtion (Water reasbsorption that depends on the prescence of ADH) occurs so that 99% of H2O in the filtrate is reabsorbed
What are diuretics and what do they do?
Diuretics are checmical that enhance the urinary output
osmotic diuretics: substances not reabsorbed (like high glucose in a diabetic patient)
ADH inhibitors such as alcohol (can lead to dehydration)
substances that inhibit Na+ reabsoprtion and obligatory H2O reabsorption such as caffeine and many drugs
What is renal clearance?
The volume of plasma cleared of a particular substance in a give time (usually 1 min)
- Renal clearance tests are used to
- -determine GFR
- -detect glomerular damage
- -follow the progress of renal disease
How do you measure renal clearance?
- RC: Renal clearance rate (ml/min)
- U: concentration (mg/ml) of the stubstance in urine
- V: flow rate of urine formation (ml/min)
- P: concentration of same substance in plasma
- For any substance freely filtered and neither reabsorbed nor secreteed by the kidneys (like insulin, which is used as the standard to determine GFR)
- RC= GFR= 125ml/min
- -if RC < 125 ml/min, the remaining substance is reabsorbed
- -if RC= 0, the substance is completely reabsorbed
- -if RC > 125 ml/mi, the substance is secreted (most drug metabolites)
What should the color and transparency of urine be?
clear, pale to deep yello (due ot urochrome)
drugs, vitamin supplements, and diet can alter color
cloudy urine may indicate a urinary tract infection
What should the odor of urine be?
slightly aromatic when fresh, but not fruity of like ammonia
develops ammonia odor upon standing as bacteria metabolizes the urea
may be altered by some drugs and vegetables
What is the pH of Urine? What is the specific gravity of urine?
Slightl acdic ~pH 6 with a range of 4.5-8.0
diet, prolonged vomiting, or urinary tract infections may alter the pH
Specific gravity should be 1.001 to 1.035, dependent on the solute concentration (distilled water is 1)
What is the chemical composition of Urine?
95% water and 5% solutes
Nitrogenous wastes: urea, uric acid, and creatinine
- Other normal solutes:
- -Na+, K+, PO43-, and SO42-
- -Ca2+, Mg2+, and HCO3-
abnormally high concentrations of any constituent may indicate pathology
Where are the ureters and what do they do?
Convey urine from kidneys to bladder
enter the base of the bladder through the posterior wall
As bladder pressure increases, distal end of the ureters close, preventing backflow of urine
What are the 3 layers wall of the ureters?
1. Lining of transitional epithelium
- 2. Smooth muscle muscularis
- -contracts in response to stretch
- -peristalsis waves bring urine to bladder
- -internal longitudinal layer and external circular layer
- 3. outter adventitia of fibrous connective tissue
What are Renal Calculi?
Kidney stones that form in the renal pelvis
- -crystallizes calcium, magnesium, or uric acid salts
- -larger stones block the ureter, cause pressure and pain in kidneys
- -may be due to chronic bacterial infection, urine retention, rise in blood calcium, or rise in pH of urine
best treatment is to break it up by shock wave lithotripsy and give pain medication
What is the urinary bladder and where is it located?
Muscular sac for temporary storage of urine
- Retroperitoneal, on pelvic floor posterior to the pubic symphysis
- -males: prostate gland surrounds the neck inferiorly
- -females: anterior to the vagina and uterus
collapses when empty-rugae appear
- expands and rises superiorly during filling and becomes pear shaped
- -no significant rise in internal pressure
What is the trigone?
Smooth triangular area outlines by teh openings for the ureters and the urethra
infections tend to persist in this region
What are the layers of the bladder wall?
1. Inner Transitional epithelial mucosa
2. Thick detrusor muscle (3 layers of smooth muscle)
3. Fibrous Adventitia (peritoneum on the superior surface only)
What is the urethra? What is it lined with?
Muscular tube leading from bladder to outside of body
- Lining epithelium
- -Mostly Pseudostratified columnar epithelium EXCEPT:
- *transitional epithelium near bladder
- *stratified squamous epithelium near urethral orifice
What are the sphincters in the urethra?
- Internal Urethral Sphincter
- -involuntary (smooth muscle) at the bladder-urethral junction
- -contracts to open (relaxation keeps bladder closed)
- External urethral sphincter
- -voluntary (Skeletal muscle) surrounding the urethra as it passes through the pelvic floor
What is the difference between the male and female urethras?
- Female (3-4cm)
- -tightly bound to anterior vaginal wall
- -external urethral orifice is anterior to the vaginal opening, posterior to the clitoris
- -carries both semen and urine
- Three named regions
- 1. Prostatic Urethra (2.5 cm)- within prostate gland
- 2. Membranous urethra (2 cm)-passes through the urogenital diaphram
- 3. Spongey Urethra (15 cm)-passes through the penis and opens to the external urethral orifice
What is Micturition? What causes it?
Urination or voiding of the bladder
- Three simultaneous events:
- 1. Conrtaction of the detrusor muscle by the ANS
- 2. Opening of the internal urethral sphincter by ANS
- 3. Opening of the external urethral sphincter by somatic nervou system
What is reflexive urination?
Urination in infants
- -distention of the bladder activates the stretch receptors
- -excitation of parasymphathetic neurons in reflex center in sacral region of spinal cord
- -contraction of the detrusor muscle
- -contraction (opening) of the internal sphincter
- -inhibition of somatic pathways to external sphincter, allowing its relaxation (opening)
How do the pontine control centers help micturition in toddlers?
Pontine control centers mature between ages 2 and 3
- 1. Pontine stage center inhibits micutrition
- -hibits parasympathetic pathways
- -excites sympathetic and somatic efferent pathways
- 2. Pontine micturition center promotes micturition
- -excites the parasympathetic pathways
- -inhibits symptathetic and somatic pathways
What are the three sets of embryonic kidneys that form in succession?
- 1. Pronephros degenerates but pronephric duct persists
- 2. Mesonephros claims this duct and it becomes the mesonephric duct. The mesonephrose degenerates but leaves aspects of the male reproductive system.
- 3. Metanephros develeops by the 5th week, develops into adults kidneys and descends
What are the developmental aspects of the urinary system?
- Metanephros develops as ureteruc buds that induce mesoderm of urogenital ridge to form nephrons
- -distal ends of ureteric buds form renal pelvis, calyces, and collecting ducts
- -proximal end becomes ureters
kidneys excrete urine into amniotic fluid by 3rd month
cloaca subdivides into rectum, anal canal, and urogenital sinus
What other developmental aspects?
frequent micturition of infants is due to small bladders and less-concentrated urine
incontinence is normal in infants: control of the voluntary urethral sphincter develops witht he nervous system
E.coli bacteria account for 80% of all urinary tract infections
Stretococcal infections may cause long term renal damage
sexually transmitted diseases can also inflame the urinary tract
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