urinary system ii Flashcards
Tubular Reabsorption
-Most of tubular contents reabsorbed to blood
Selective transepithelial process
- almost all organic nutrients reabsorbed
- water and ion reabsorption hormonally regulated and adjusted
Includes active and passice tubular reabsoption
two routes: trancellular or paracellular
Transcellular tubular reabsobtion
- transport across apical membrane
- diffusion through cytosol
- transport across basolateral membrane (often involves the lateral intercellular spaces because membrane transporters transport ions into these spaces)
- movement through the interstitial fluid into the capillary
Paracellular route of tubular reabsobtion
movement through leaky tight junctions, particularly in the PCT
Movement through the interstitial fluid and into the capillary
Tubular reabsoption of sodium
Na+ = most abundant cation in filtrate
Transport across basolateral membrane
-Primary active transport out of tubule cell by Na+-K+ ATPase pump peritubular capillaries
Transport across apical membrane
-Na+ passes through apical membrane by secondary active transport or facilitated diffusion mechanisms
Reabsoption of nutrients, water, and ions
Na+ reabsorption by primary active transport provides energy and means for reabsorbing most other substances by secondary active transport
Creates electrical gradient passive reabsorption of anions
Organic nutrients reabsorbed by secondary active transport; cotransported with Na+
-Glucose, amino acids, some ions, vitamins
Passive tubular reabsoption of water
Movement of Na+ and other solutes creates osmotic gradient for water
Water reabsorbed by osmosis, aided by water-filled pores called aquaporins
Aquaporins always present in PCT obligatory water reabsorption. If solutes are reabsorbed, water is obliged to follow
Aquaporins inserted in collecting ducts only if ADH present facultative water reabsorption. Water is reabsorbed only if we need it.
Passive Tubular Reabsorption of Solutes
Solute concentration in filtrate increases as water reabsorbed concentration gradients for solutes
Fat-soluble substances, some ions and urea, follow water into peritubular capillaries down concentration gradients
– Lipid-soluble drugs, environmental pollutants difficult to excrete
Transport maximum
Transcellular transport systems specific and limited
Transport maximum (Tm) for ~ every reabsorbed substance; reflects number of carriers in renal tubules available
When carriers saturated, excess excreted in urine
E.g., hyperglycemia high blood glucose levels exceed Tm glucose in urine
Reabsorptive capabilities of PCT
- site of most reabsorption
- all nutrients (glucos, AA….)
- 65% of Na and water
- many ions
- almost all uric acid; 1/2 of the urea (later secreted back into filtrate)
reabsorptive capabilities of nephron loop
Descending limb: water can leave; solutes can’t
Ascending limb: water can’t leave; solutes can
- thin segment = passive sodium movement
- thick segment = Na–K–2Cl symporter and Na–H antiporter; some passes by paracellular route
reabsoptive capabilities of DCT and collecting duct
Hormonally regulated ADH = water Aldosterone = Na and water ANP = Na PTH = Ca
ADH
Released by posterior pituitary gland
Causes principal cells of collecting ducts to insert aquaporins in apical membranes water reabsorption
-As ADH levels increase increased water reabsorption
Aldosterone
Targets collecting ducts (principal cells) and distal DCT
Promotes synthesis of apical Na+ and K+ channels, and basolateral Na+-K+ ATPases for Na+ reabsorption; water follows
little Na+ leaves body; aldosterone absence leads to loss of 2% filtered Na+ daily - incompatible with life
Functions – increase blood pressure; decrease K+ levels
ANP
Reduces blood Na+ decreased blood volume and blood pressure
Released by cardiac atrial cells if blood volume or pressure elevated
Inhibits reabsorption of Na+
Inhibits release of aldosterone and ADH
PTH
acts on DCT to increase Ca reabsoption
Tubular secretion
Reabsorption in reverse; almost all in PCT
- Selected substances
- K+, H+, NH4+, creatinine, organic acids and bases move from peritubular capillaries through tubule cells into filtrate
- Substances synthesized in tubule cells also secreted – e.g., HCO3-
Disposes of substances (drugs) bound to plasma proteins
Eliminates undesirable substances passively reabsorbed (urea/uric acid)
Rids body of excess K (via aldosterone)
Controls blood pH by altering amounts of H or HCO3 in urine
regulation of urine concentration and volume
Osmolality of body fluids
- Expressed in milliosmols (mOsm)
- Kidneys maintain osmolality of plasma at ~300mOsm by regulating urine concentration and volume
Formation of dilute or concentrated urine
Osmotic gradient used to raise urine concentration > 300 mOsm to conserve water
Overhydration –> large volume dilute urine
- ADH production decreases; urine ~ 100 mOsm
- If aldosterone present, additional ions removed –> ~ 50 mOsm
Dehydration –> small volume concentrated urine
- Maximal ADH released; urine ~ 1200 mOsm
- Severe dehydration – 99% water reabsorbed
Urea Recycling and the Medullary osmotic gradient
Urea helps form medullary gradient:
- Enters filtrate in ascending thin limb of nephron loop by facilitated diffusion
- Cortical collecting duct reabsorbs water; leaves urea
- In deep medullary region now highly concentrated urea –> interstitial fluid of medulla –> back to ascending thin limb –> high osmolality in medulla
urinary role in pH balance
In tubular cell: that whole carbonic anhydrase reaction happens and H secretion is coupled to Na reabsorption via an antiporter
More H in blood leads to higher H secretion, so you need to buffer the H in the urine with NH3 from tubular cells, HCO3, or HPO4 which is the most important urine buffer
Diuretics
Chemicals that enhance urinary output
ADH inhibitors, e.g., alcohol
Na+ reabsorption inhibitors (and resultant H2O reabsorption), e.g., caffeine, drugs for hypertension or edema
Loop diuretics inhibit medullary gradient formation
Osmotic diuretics - substance not reabsorbed so water remains in urine, e.g., high glucose of diabetic patient, mannitol
Physical characteristics of urine transparency color ph specific gravity
Clear: if cloudy may indicate UTI
Pale to deep yellow from urochrome –> pigment from hemoglobin breakdown; more concentrated urine –> deeper color
Abnormal color from food, bile pigments, blood, drugs
pH is slightly acidic (4.5-8)
- acidic diet (protein and whole wheat decreases pH)
- alkaline diet (vegetarian), vomiting, or UTI increases pH
Specific gravity = 1.001 to 1.035 and is dependent on solute concentration
chem comp of urine
95% water; 5% solute
Nitrogenous wastes:
- urea (from AA breakdown) = biggest solute component
- uric acid (from nucleic acid metabolism)
- creatine (metabolite of creatine phosphate
other solutes:
Na–K–PO4–SO4–Ca–Mg–HCO3
Too much solute or weird solutes (WBCs, proteins, bile pigments) may indicate pathology
Ureters
Bring urine from kidney to bladder, starting at L2
Retroperitoneal
Enter at inferior, posterior bladder wall
(as bladder pressure increases, ends of ureters close)
3 layers
- mucosa = transitional epithelium
- muscularis = smooth muscle sheets which contract in response to stretch and propel urine into bladder
- adventitia = outer fibrous CT
