Week 11 Flashcards
What are the main sources of water loss?
REGULATED PROCESSES
- Urination, sweating feces
UNREGULATED PROCESSES
- respiration
Extracellular fluid composition
- Sodium
- Chloride
- Bicarbonate
Intracellular Fluid Composition
- Potassium
- Magnesium
- Organic Ions
- Proteins
How does the composition of of interstitial fluid differ from plasma
Greater protein concentration in plasma due to capillary filtration
- Plasma also contains several non-ionic substances
What are the two core concepts underlying the regulation of body fluid volumes and composition?
FLOW DOWN GRADIENTS: Osmosis, diffusion of ions and other solutes due to chemical and/or electrical gradients
MASS BALANCE: The contents of water/solute in any body compartment is determined by inputs and outputs
- Accumulation = input + generation - output - consumption
Mass balance of Sodium with intake
Excretion delayed compared to intake
- Sodium retention occurs (+ accumulation)
Isotonic
Same Ion concentration inside and out - no change in cell size
Hypotonic
More ion in cell compared to environment
- Cell swells
Hypertonic
More ions outside the cell compared to inside
- cell shrinks
What is the effect of adding an isotonic NaCl solution to normal state conditions on osmolarity and volume
Increases overall volume
What is the effect of adding a hypotonic NaCl to normal state conditions on osmolarity and volume
- Decrease overall osmolarity
- increase volume of intracellular
- smaller increase in extracellular volume
What is the effect of adding a hypertonic NaCl to normal state conditions on osmolarity and volume?
- Increase overall osmolarity
- Increased extracellular volume
- decrease in intracellular volume
What are the main purposes of the kidneys
- Regulation of body fluid volume
- Regulation of electrolyte composition
- Excretion of metabolic wastes
- foreign substances
Kidney Nephron
- The functional unit of the kidney
- Approximately 6 nephrons per cortical collecting duct, 8-10 cortical ducts per medullary collecting but, which merge into progressively larger ducts then discharge into renal pelvis
Renal Capillary beds
Two capillary beds in series
- Glomerular capillaries
- Peritubular capillaries
What is the blood flow rate to the kidneys at rest
20% of CO
Cortical Nephrons
- Glomeruli in the outer cortex
- Loops of Henle only penetrate outer medulla
Juxtamedullary Nephrons
- Glomeruli lie deeper in the cortex
- Loops of Henle descend into inner medulla, alongside vasa recta (specilized peritubular capillaries)
- Specilized for producing concentrated urine
What are the 3 urine formation processes?
- Glomerular Filtration
- Reabsorption from tubules
- Secretion into tubules
What is the rate of glomerular filtration and how does it compare to urine volume
Glomerular filtration of fluid: 180L/day
Only 1.5 L/day of urine
What substances are just filtered
Many waste products
What substances are filtered and partially reabsorbed
Water and many electrolytes
What substances are filtered and completed reabsporped
Many nutritional substances (macro components)
What substances are filtered and secreted
Some organic acids and bases
The Bladder
- Urine travels from renal pelvis to bladder via ureters
- Smooth muscle chamber, formed by the detrusor muscles, whose contraction empties the bladder
- Internal (involuntary, smooth) and external (voluntary, skeletal) muscle sphincters prevent emptying
- bladder and internal sphincter are innervated by ANS
- External sphincter in innervated by motor neurons
Micturition Reflex
- Stretch receptor on bladder senses filling
- Sensory PNS sends signal to sacral spinal cord
- Sensory neuron synapses with motor neuron of PNS and ascending fibers that synapse in the pons and then cortex for conscious control
- Motor neuron of PNS causes a contraction of detrusor muscles and relaxes internal sphincter, Central command innervates somatic motor fiber of pudendal nerve to control external sphincter
What is glomerular filtration rate
20% of renal plasma flow rate
- Filtration fraction is much greater than most other capillaries
What pressures determine fluid flow across capillaries?
- Hydrostatic pressure
- Colloid osmotic pressure
Pc> Pif = filtration
piC>piIF = absorption
Net filtration pressure
(Pc-Pif) - (piC - piIF)
Net filtration rate
NFP x Kf (Volume/min)
where Kf is the permeability coefficient of the capillaries
What accounts for the high filtration rate of glomerular capillaries?
High permeability coefficient relative to other capillary beds (400x higher)
How does the NFP in the glomerular capillaries compare to other capillaries?
- NFP: same
- Glomerular Hydrostatic Pressure: higher
- Bowman’s capsule pressure: Higher
- Glomerular colloid osmotic pressure: about same
Layers of glomerular capillaries
- Endothelial cell
- Basement Membrane
- Epithelial cells (podocytes)
What accounts for the high permeability in the glomerular capillaries
Fenestrations: Wide endothelial perforations
Slit Pores: wide opening between podocytes
Why do proteins tend not to get filtered through glomerular capillaries
Although wide enough for plasma proteins, the negative charged molecules on surface of endothelial cells, basement membrane and podocytes deter negatively charged proteins
How will obstruction of urinary outflow affect glomerular filtration rate
Increase capsule pressure causing decrease in net filtration pressure
What is the primary way the GFR is regulated
Glomerular hydrostatic pressure
What 3 variables determine glomerular hydrostatic pressure
- Arterial Pressure
- Afferent arteriole resistance
- Efferent arteriole resistance
How is renal blood flow calculated
= (Pa - Pv)/ total renal vascular resistance
What happens if resistance increases in the afferent arterioles of the glomerulus
- Flow decreases and glomerular pressure decreases causing increase in change in pressure
What happens if resistance increases in the efferent arterioles of the glomerulus
- Increase glomerular pressure
- decrease in flow
- increase in change in pressure
- decrease in peritubular capillary pressure
Control of filtration by afferent arterioles
- Constriction of afferent arterioles increase renal vascular resistance
- leads to reduce in renal blood flow and glomerular pressure
- decreasing glomerular filtration rate
Control of filtration by efferent arterioles
- Constriction of efferent arterioles increase renal vascular resistance, thus reducing renal blood flow
- Increasing resistance to outflow from glomerular capillaries initially increases glomerular pressure and GFR
- With larger reductions in renal blood flow, colloid osmotic pressure in glomerular capillaries can start to rise and limit GFR
Autoregulation of filtration rate
- Feedback mechanism intrinsic to the kidneys normally keep renal blood flow and GFR relatively constant
- Tubulo-glomerular feedback mechanism acts to maintain relatively constant delivery of NaCl to the distal tubule (GFR is proportional to NaCl delivery)
- Mechanism involves the juxtaglomerular complex between the distal tubule and glomerular arterioles
Tubulo-Glomerular feedback
- Decrease in GFR due to decrease in arterial pressure
- Macula Densa, which are specialized epithelial cells that release prostaglandins in response to low NaCl in the distal tubule
- Prostaglandins cause afferent arterioles to vasodilate, and stimulate renin release from juxtaglomerular cells into blood
- Renin catalyzes production of angiotensin I, which is converted to angiotensin II
- Angiotensin II stimulates constriction of efferent arterioles
Effect of SNS on renal blood flow
- Causes constriction
- decreased flow
- opposed by autoregulation
