Chapter 4 Cell injury, inflammation and repair/Fig 4.14.jpg

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Exchange of fluid by extra-filtration across the wall of small blood vessels. HP and OP represent the difference between the hydrostatic and colloid osmotic pressures (mmHg) of plasma and extravascular space. The solid arrows indicate the net movement of fluid in and out of vessels along their length. The interrupted arrows indicate the direction of forces exerted by HP, OP and tissue pressure (TP). Upper figure, normal tissue: fluid movement across vessel wall approximates to equilibrium. Lower figure, acute inflammation: much more fluid leaves vessels than is returned to them. The values of HP and OP are approximations. In inflammation, HP may be less than indicated because of rise of TP and OP will also be reduced due to escape of plasma protein (via endothelial gaps) into the extravascular space which increases OP in the extravascular fluid (shown as 10 mmHg). The level of TP varies depending upon the nature of the tissue involved. In loose tissue TP will show no increase, whereas in tissues which are tightly tethered or have fibrous capsules TP can rise considerably (hence the question mark in this figure).