Hydrostatic pressure of interstitial fluid
In most natural cavities of the body, where free fluid is in dynamic equilibrium with the surrounding fluid, the measured pressures are negative.
The main function of microcirculation is to transport nutrients to tissues and remove cellular waste products. Small arterioles control blood flow to individual tissues and their local condition, by controlling the diameter of the arterioles. Thus, in most cases, each tissue's regulation of flow is related to its own needs.
The walls of the capillaries are very thin and are composed of a layer of highly permeable endothelial cells. Thus, water, cellular nutrients and cellular secretion products can be exchanged quickly and easily between tissues and circulating blood.
The human body's peripheral circulatory system has about 10 billion capillaries with an estimated total surface area of 500 to 700 square meters (about one-eighth of the surface area of a football field). Thus, any functioning cell has a capillary that feeds it no more than 20-30 micrometres away.
There are many methods for measuring interstitial fluid hydrostatic pressure, each of which yields slightly different values, depending on the method used and the tissue pressure being measured.
In loose subcutaneous tissue, the interstitial fluid pressure measured by various methods is usually a few mmHg - less than atmospheric pressure; Therefore, this value is called the interstitial negative pressure. In other tissues surrounded by cysts, such as the kidney, interstitial fluid pressure is usually positive (i.e. greater than atmospheric pressure). The most widely used methods that have been used are: (1) pressure measurement with a micropipette inserted into tissues, (2) measurement of free interstitial fluid pressure in tissue perforated cysts (3). ) measure the free interstitial fluid pressure with a cotton wick.
Different methods provide different values for the interstitial hydrostatic pressure, even in similar tissues.
Measure interstitial fluid pressure using a micropipette
The same micropipette used to measure capillary pressure can also be used in some tissues to measure interstitial fluid pressure. The tip of the micropipette is about 1 micrometre in diameter but is also 20 times larger than the size of the interstitial proteoglycan filaments in the interstitial fluid. Therefore, the pressure being measured can be the pressure in a pocket of free liquid.
Pressures measured using the micropipette method range from -2 to 2 mmHg in loose tissues, such as skin, but in many cases the mean is slightly below atmospheric pressure.
Measurement of free interstitial fluid pressure in tissue perforated cysts
The free interstitial fluid pressure measured when using a 2 cm diameter capsule in the liquid subcutaneous tissue is normally about -6 mmHg, but with smaller capsules, the values do not differ by more than -2 mmHg from with micropipette measurements.
Interstitial fluid pressure in tissues with a rigid framework
Some tissues of the body are surrounded by rigid frameworks, such as the skull around the brain, the fibrous capsule around the kidneys, the fibrous membranes surrounding the muscles, and the sclera around the eyes. In most tissues, regardless of the method used for measurement, the interstitial fluid pressure is positive. However, that pressure is always less than the pressure outside the rigid frame. For example, the pressure of the cerebrospinal fluid that surrounds the brain of an animal averages about 10 mmHg, while the average interstitial fluid pressure is about 4-6 mmHg. In the kidney, the mean peritubular pressure is 13 mmHg, while the observed interstitial fluid pressure is about 6 mmHg.
Thus, keeping in mind that the pressure outside the skin is atmospheric pressure, which is considered non-pressure, it is possible to formulate a general rule that normally the interstitial fluid pressure is usually a few millimetres of mercury negative for the pulse pressure. around each cell.
In most natural cavities of the body, where free fluid is in dynamic equilibrium with the surrounding fluid, the measured pressures are negative. Some of the chambers and the measured pressures are as follows:
Pleural space: -8 mmHg
Intra-synovial space: -4 to -6 mmHg
Epidural space: -4 to -6 mmHg
Interstitial fluid pressure in the subcutaneous fluid is always lower than atmospheric pressure
Although the different methods mentioned above give slightly different values for the interstitial fluid pressure, most physiologists believe that under normal conditions the interstitial fluid pressure in the subcutaneous laxity is lower than the pressure. The average atmosphere is about -3 mmHg.
The underlying cause of negative interstitial fluid pressure is the pumping of the lymphatic system
The lymphatic system is discussed later in this chapter, but first, we need to understand the fundamental role it plays in generating interstitial fluid pressure. The lymphatic system is a "street sweeper" system that removes excess fluid, excess protein molecules, debris, and other substances from tissue. Normally, when fluid enters the ends of the lymphatic capillaries, the lymphatic vessel walls automatically contract within a few seconds and pump the fluid into the blood circulation. This overall process produces a slight negative pressure that has been measured in the interstitial fluid.