Physiology Flashcards
Structure of the urinary tract
Kidney
Paired organ
Hilum- entry point for vessels, nerves & lymphatics
Inner - medulla- 10-19 pyramids
Cortex- outer cortical, inner juxtamedullary
Minor -> major calyces to ureter
Filter ~150L of blood per day, 1/4 of CO
Ao -> renal artery-> segmental -> interlobar -> arcuate artery -> afferent artery -> efferent artery -> peritubular arteries -> venules
800-2000ml/day urine
Renal corpuscule = glomerulus + Bowman’s capsule
- Podocytes with filtration slits- block large proteins & RBCs
- Allow small molecules- salts, glucose to pass into Bowman’s capsule (becomes filtrate)
Nephron
- PCT
- Loop of Henle (DLH, ALH)
- DCT
- Collecting ducts
Juxtaglomerular complex (DCT & afferent arteriole)
Regulates GFR & BP
- Macula densa: sense low Na/Cl
- Extraglomerular mesangial cells
- Juxtaglomerular cells: in afferent arteriole, secrete renin- increases Na+ reabsorption/water, increases blood volume, vasoconstricts- increases blood pressure
Ureters
- Connect kidney to bladder, one way valve at bladder
Bladder
- Muscular bag- rugae, transitional epithelium (stretchy), detrusor- contracts during urination
- Trigone (uretovesical junctions, internal urethral orifice/outlet at base)- stretch indicates time to pee (>300-400ml)
- Infront of rectum, uterus
- Micturition reflex via S2/S3, pontine storage centre inhibits
Urethra
Internal urethral orifice -> external opening
- Males: prostatic, intermediate spongy, also carries
- Internal sphincter involuntary, external sphincter voluntary
Water balance requirements, homeostasis, osmolarity
Water- blood volume, lubrication, digestion, forms synovial fluid, eliminating toxins, regulating body temp
60% body weight is water
- higher if skinny/muscular, lower if fat
Intracellular 40% BW- 2/3
- Cations Mg2+, K+, Anions- protein, phophates
- Na/K+ ATPase keeps K+ in, Na+ out
Extracellular 20% BW- 1/3
- Cations Na+, Anions- Cl-, HCO3-
Osmolarity= overall solute in blood
~300mosm/L
Input: fluid intake 80%, food 20%
Elimination: insensible- breathing (600-900), sweating (500-700), urinating (800-2000ml/day), defecation (200ml/day)
Kids 4-13yrs 1.7L/day
https://www.osmosis.org/learn/Movement_of_water_between_body_compartments?from=/md/foundational-sciences/physiology/renal-system/fluid-compartments-and-homeostasis
Renal clearance,and GFR
Renal clearance: rate of substance removal from plasma and excretion in urine
C = concentration in urine x urine flow rate/concentration in plasma
Sum of reabsorption and secretion
- Can be equivalent to GFR if minimal secretion or reabsorption
- Compare to inulin as completely filtered
- Comparing substance x/inulin = clearance ratio
- <1= not freely filtered, net reabsorption (ie glucose)
- >1 = freely filtered, net secretion (ie albumin)
- Radioactive tracers- Cr-EDTA/DTPA gold standard for measuring GFR/clearance in clinical setting
Can also use Schwartz equation to estimate GFR in CKD patients (eGFR 15-75)
- *eGFR = 36.5 x height (cm) x creatinine (umol/L)**
- Creatinine largely filtered, not reabsorbed, small amount of secretion 10-15% so overestimates GFR
Reabsorption of water
- Mainly in collecting duct
- ADH dependent
- Released in response to high serum Na+, hypovolemia
- AVP binds to vasopressin receptor V2R, triggers G protein mediated cAMP pathway, releases aquaporins into CD membrane
- Free water reabsorption to vessels
Sodium homeostasis
Most Na+ in ECF (~135mEq/L)
Relative to water in body
- Determines blood volume
- Affects nerve & muscle function
Reabsorption
67% reabsorbed at PCT
- water also follows = isoosmotic reabsorption
1. Early PCT
- Na-gluc cotransporter
- Na-aa cotransporter
- Na-PO4 cotransporter (regulated by PTH)
- Na-H exchanger (regulated by angiotensin 2- if hypotensive, renin stimulates ang2 release → increases Na reabsorption/H20)
2. Late PCT
- Paracellular absorption- follows Cl- across gradient
25% in thick ascending loop of Henle
- NKCC2 (ADH- posterior pituitary, decreased BV or increased solutes/osmolality)
- Impermeable to water
~5% Early DCT
- Impermeable to water
- Na-Cl- co-transporter
~3% Late DCT (fine tuning)
- eNAC in principal cells
- In response to aldosterone, decreased BP, increases eNAC production
Other
- SNS: activates in decreased BP, vasoconstricts afferent arteriole/Na+ reabsorption in PCT
- *Secretion**
- Atriopeptin (ANP): activates in increased BP, dilates afferent arteriole, inhibits eNAC in DT/CD
- Urodilatin, BNP (also decrease Na+ reabsorption)
Volume regulation
Activated when hypovolemia/decreased intravascular volume:
- RAAS, Renin/sympathetic system: increased Na+/water reabsorption, vasoconstriction, increased ECF/blood volume
- Key players: aldosterone, angiotensin, noradrenaline
Activated by volume expansion (atrial expansion):
- Naturetic peptides: increase GFR/filtration, decrease Na+ reabsorption in DCT/CD (eNAC)
Potassium homeostasis
Outline renin-angiotensin system, ADH, AVP, aldosterone
Blood pressure regulation
Glucose reabsorption/filtration
Acid-base balance
Kidneys contribute to acid base balance via
- Reabsorption of HCO3-
- Excretion of H+
Bicarbonate reabsorption
- Occurs in PCT 90%
- Acid secreted from tubule lumen (via Na+/H+ exchanger) on apical surface
- 10% in ALH/DCT (H+ ATPase active transport in intercalated cells)
- HCO3- binds to H+ and forms carbonic acid/H2CO3 → (carbonic anhydrase IV) → H2O + CO2
- Water/carbon dioxide diffuses into cell → (carbonic anhydrase ii) → converts back to H+/HCO3
- HCO3- absorbed into blood stream via NA+ HCO3- cotransporter or HCO3-, Cl- exchanger
H+ excretion/buffers
- Ammonia (NH3+) → ammonium (NH4+) in PCT
- Ammonium excreted via NH4+-Cl
- Phosphate- HPO4- → H2PO4
Bloo
