Physiology Lecture (18) PDF

Physiology Dynamics of filtration LECTURE (18) DR. El-Sawy 1  Physiology Dynamics of filtra...

Physiology Dynamics of filtration LECTURE (18) DR. El-Sawy 1  Physiology Dynamics of filtration Definitions:  Space between cells. Interstitium  It constitutes about 1/ of body tissue. 6 Interstitial  Gel like fluid that entrapped in interstitium. fluid  Body fluids continuously move out of blood into interstitial fluid Filtration and in opposite directions from interstitium to blood. DR. El-Sawy 2  Physiology Dynamics of filtration Definition:  Passage of bulk flow of fluid across capillary membrane under control of hydrostatic and osmotic pressure difference. Starling 's principle The rate & direction of fluid movement is proportional to algabric sum of hydrostatic and osmotic forces" DR. El-Sawy 3  Physiology Dynamics of filtration Formation and drainage of interstitial fluid  20 liters of fluid are filtered every day at arterial ends of capillaries. 18 liters of them are reabsorbed back at venous ends. 2 liters are drained by lymphatic system. Factors determine movement of fluid:  Four primary factors determine whether fluid move out of the blood into the interstitial fluid or in the opposite direction.  These factors are : 1. Capillary hydrostatic pressure 2. Interstitial fluid hydrostatic pressure 3. Plasma osmotic pressure 4. Interstitial fluid colloid pressure DR. El-Sawy 4  Physiology Dynamics of filtration capillary hydrostatic interstitial fluid plasma colloid osmotic = interstitial fluid colloid pressure (Pc) hydrostatic pressure (Pi) oncotic pressure (πc) osmotic pressure (πi) The force which tends to move fluid → The force which tends to cause osmosis of fluid →  Outward through  Inward through  Inward through  Outward through Definition capillary membrane. capillary membrane capillary membrane i.e. capillary membrane. when it is positive. it is the main absorbing  Outward when it is force. negative.  30 mmHg in arterial end  -3 mmHg  negative  28 mmHg  Average protein conc. of  10 mmHg in venous end. interstitial fluid pressure. 9 mmHg by plasma interstitial fluid is about proteins 3 gm/100 ml. Value  The functional mean  it is positive in tissues e’ 9 by accompanying capillary pressure: tight fibrous coverings as Na ions due to  Resulting in average 17.3 mmHg (average +6 mmHg in kidney. (Donnan effect). colloid osmotic pressure effective pressure). +4 mmHg in brain. of about 8 mmHg. DR. El-Sawy 5  Physiology Dynamics of filtration A. Forces moving fluid outward At arterial end At venous end  Capillary pressure 30 10  Negative interstitial fluid pressure 3 3  Interstitial fluid colloid osmotic pressure 8 8 Total outward force 41 21 B. Forces moving the fluid inward At arterial end At venous end  Plasma colloid osmotic pressure 28 28 Summation of the forces: At arterial end At venous end  Outward force 41 21  Inward force 28 28  Net force 13 7 13 mmHg filtration pressure occurs at arterial ends of capillaries. 7 mmHg is the reabsorbing pressure at venous ends of capillaries. It is less than the filtration pressure, but venous capillaries are more numerous and more permeable so it reabsorbs about 9/10 of the fluid, the remainder flows into lymph vessels. DR. El-Sawy 6  Physiology Dynamics of filtration Principle:  Under normal conditions: A state of near equilibrium exists at capillary membrane Slight disequilibrium → account for the small amount of fluid that is eventually returned by way of lymphatics. The pressure in arterial and venous capillaries are averaged to calculate the mean functional capillary pressure to be 17.3 mmHg. Mean forces tending to move fluid outward mmHg  Mean effective capillary pressure 17.3  Negative interstitial fluid pressure 3  Interstitial colloid osmotic pressure 8 Total outward force 28.3 Mean force tending to move fluid inward mmHg  Plasma colloid osmotic pressure 28.0 Summation of mean forces: mmHg  Outward force 28.3  Inward force 28  Net outward force 0.3 DR. El-Sawy 7  Physiology Dynamics of filtration 0.3 mmHg represents the slight imbalance of forces that causes slightly more filtration of fluid into the interstitial spaces than reabsorption. This slight excess of filtration is called the net filtration, and it is what causes fluid to return to the circulation through the lymphatics. DR. El-Sawy 8