Day 1 5/12/15 Flashcards
Main Physiological Function of the Kidney
-the maintenance of the composition and volume of the extracellular fluid
Total Intracellular Volume
27 L
Total Extracellular Volume
15 L
Liquid input is ________ urine output.
- equal to
- ECF volume over the course of a typical day does not change
Things the Renal System Regulates
- ECF volume
- ECF osmolarity
- EF electrolyte composition
- ECF pH
- metabolic wastes
- foreign substances
Nephron Components
- 1 million nephrons per kidney
- consists of glomerular and peritubular capillaries and the tubule
Nephron Functions
- glomerular filtration
- tubular reabsorption
- excretion
Non-ECF Functions of the Kidney
- produce erythropoetin from precursor synthesized in liver
- regulate BP via angiotensin regulation
- secrete renin which inc. BP by cleaving angiotensinogen to angiotensin (see above)
Molecular Cut Off Size for Filtration
- 60,000 daltons (or smaller can be filtered)
- note that albumin is 67,000 daltons
(pi)gc
- 30 mm avg
- oncotic pressure
- on the blood side, the large dissolved proteins in the plasma (i.e. serum albumin, immunoglobulins) do not filter
- as water is extracted from the plasma by filtration, the protein concentration rises, resulting in a net negative osmotic pressure that opposes filtration.
- this is also called the colloid osmotic pressure or COP
- opposes glomerular filtration
Pt
- 10 mm avg
- results from the fact that filtrate must flow in narrow confines of the tubule
- results in backpressure at bowman’s capsule
- opposes glomerular filtration
Pgc
- 46mm avg
- filtration out of the capillary is driven by the value of the blood pressure where filtration occurs
- only significant driving force for filtration
Glomerular Filtration Rate (GFR) =
GFR = K (Pgc - Pt - pgc) K = 1 for men and 0.8 for women
Net Filtration Pressure
The sum of the forces (Pgc - Pt - pgc) is called net filtration pressure or NFP
-6 mm avg
Autoregulation
- works over BP range of 75 to 150
- ex. drop in MAP will cause afferent arterioles to dilate to restore Pgc, GFR and RBF to normal values
- responds to normal, rapid fluctuations in MAP within normal range
- only involves afferent arterioles
- mechanism of arteriolar changes: myogenic
- location of mechanism: intrinsic to kidney
Renal Solution to Chronic Hypovolemia
- mediated by external and renal baroreceptors
- responds to chronic, significant drops in MAP below normal due to hypovolemic events
- purpose: maintain GFR but reduce RBF during circulatory emergencies
- arterioles involved: afferent and efferent
- mechanism of arteriolar changes: baroreceptor mediated (direct neural + hormonal)
- location of mechanism: intrinsic + extrarenal
Glomerulotubular Balance
- refers to the ability of the obligatory reabsorption mechanisms in the proximal tubule to compensate for changes in filtered blood
- i.e. proximal tubule reabsorption readjusts to filtration changes so that a fixed proportion of the filtered load of water and NaCl is always reabsorbed. This proportion is 65%.
Tubuloglomerular Feedback
-directly regulates GFR of each nephron in response to changes in [NaCl] at a specialized group group of epithelial cells called the macula densa that can cause the arteriole to constrict or dilate
Reabsorption Range of Water and NaCl
- 86 - 99.95% of water is reabsorbed
- 98 - 99.99% of NaCl is reabsorbed
- most of the filtered water and salt is obligatorily reabsorbed, with only a small fraction under homeostatic control
Nearly all obligatory recapture of water and salt occurs in the:
-proximal segments (ie. proximal tubule and loop of Henle)
Homeostatically varied reabsorption primary takes place in:
-the fine tuning segments (ic. distal tubule and collecting duct)
Primary Energetic Event of Reabsorption
- when Na+ is actively extruded from interior of tubular epithelium by the basolateral Na/ ATPase pump
- reabsorption of Cl-, H2O and other solutes is coupled to the active reabsorption of Na+
Role of the Proximal Tubule
- reabsorbing most (65%) of the obligatorily reabsorbed load of water and NaCl
- recapture of important metabolites in filtrate
Role of the Loop of Henle (des. and asc. loop)
- descending loop: 15% of H2O reabsorption, H2O permeability is high but there is no permeability to NaCl
- ascending loop: 25% of NaCl reabsorption, water permeability in the ascending limb is quite low
Role of Distal Tubule & Collecting Duct
- 8-10% of NaCl reabsorption
- 6-20% of H2O reabsorption
- sensitive to aldosterone and ADH
Anti-Diuretic Hormone (ADH)
- also known as vasopressin
- increases water permeability of fine tuning segments
- in presence of ADH, small vesicles containing aquaporins are fused to apical membrane of epithelial cells
Aldosterone
-upregulates basic pathway for Na reabsorption
Role of Diuretics
-inc. tubular flow and excretion in most substances
