- Capillary pressure in the glomerulus controls renal output
- Incr. Blood Volume à incr. CO à incr. AR (CP) à incr. GFR à Decr. H2O absorption à incr. Urinary Output à decr. Blood Volume
- 3 Major Topics
1. Filtration – driven by capillary pressure
2. Absorption – controlled by diffusion and osmosis
3. Secretion –
Absorption – is always from the tubules back to the blood à tissues
Secretion – is always waste to be excreted (drugs, Potassium, and H+ ions)
Capillaries
- the greatest exchange surface
- Vasomotion – fluid moves intermittent
- Allows for the time needed to diffuse
- @ 4 billion capillaries
- Very thin endothelial cells with a basement membrane
- Liver – loose tissue for a lot of perfusion
- Brain - loose tissue for minimal perfusion
- Intracellular cleft are the main mechanism for diffusion
- H20, small uncharged molecules
- Reflection – negative charges on the basement membrane doesn’t allow other negative molecules through (protein)
- Glucose can’t get past capillary without insulin à creates osmotic pressure pulling fluid back into capillary à increasing blood pressure = hypertension
- Permeability
- Water 18 1.00
- NaCl 58.5 0.96
- Urea 60 0.80
- Glucose 180 0.60
- Sucrose 842 0.40
- Inulin 5000 0.20
- Some molecules are absorbed via pinocytosis à allows for molecules which shouldn’t normally pass the membrane to get across
- Molecular Weight determines diffussion (under 200 MW usually pass easily
Starling’s Forces
1. CP = Capillary Pressure (hydrostatic)
2. pp = Plasma Colloid Osmotic Pressure (protein molecules)
- The number of molecules determine osmotic forces – the size does not matter
3. IFP = Interstitial Fluid Pressure (-3 mm Hg)
- Controlled by the lymphatic system
- If IFP rises then Edema occurs
4. pif – Interstitial Fluid Colloid Osmotic Pressure
- Driving forces for hydrostatic pressure is mostly controlled by CP & pp.
- Low fluid levels – Hypertonic/Dehydrated
- Thirst center recognizes dehydration
Capillary Pressure
- pushes fluid out of capillaries (30 mm Hg)
Plasma colloid osmotic pressure (28 mm Hg)
- pulls fluid into capillaries
- Na+ ions are pulled into the vessels by the (-) charges on the protein
Interstitial (Free) Fluid Pressure
- Pushes fluid into capillaries (-3 mm Hg)
Interstital Colloid Osmotic
- pulls fluid Out of capillaries (8 mm Hg)
Net inward flux of 7 mm Hg
Systemic Net OUTWARD flux 0.3
If lymphatics fail systemically à 24 hrs until death
Elephantiasis à worm (philaria) enters lymph system and obstructs lymph flow
- severe risk of gangrene due to poor circulation to legs
Edema (4 causes)
- Hypertension
- Reduced Interstitial Colloid pressure
- Injury (mechanical)
- Blocked Lymphatics
LYMPHATICS
- Interstitial fluid pressure drives the fluid into the lymphatic system
- Flap-like valves of lymphatics are one-way valves allow fluid and keeps fluid from flowing backward.
- Work very similar to venous flow
Total Body Water 60% of body weight
- 1/3rd of total body water is extracellular fluid
- ¾ Interstitial fluid
- ¼ plasma fluid
- 2/3rd of total body water is intracellular
Plasma is the only controllable fluid. Therefore, 1/8th of our total body water controls the rest of our fluids.
- Intake of salt increases the osmolarity in plasma which makes the body think it need more H2O to dilute
*** Plasma osmotic pressure is about 300 milli-osmoles (in normal human)
Water toxicity – cellular tissues swell as they intake water
Hematocrit
- anemic – hypotonic
- polycythemia – hypertonic
Adipose tissue takes up space that water would normally occupy
- obese individuals are usually dehydrated
Positive ions in intra/extra – cellular fluids
- Ca++
- Mg++
- K+
- Na+
Negative ions in intra/extra – cellular fluids
- Cl-
- Phosphate/organic ions
- Bicarbonate
- Protein
Protein and phosphate are the two major intracellular buffers