Why does hydrostatic pressure decrease

Hydrostatic pressure is lower at the venule end of the capillary. Hydrostatic pressure is greater at the venule end of the capillary. Correct answer: Hydrostatic pressure is lower at the venule end of the capillary. Explanation : Hydrostatic pressure is the force of the fluid volume against a membrane, while osmotic pressure is related to the protein concentration on either side of a membrane pulling water toward the region of greater concentration.

Human blood has an osmolarity of roughly: At this concentration, the osmolarity inside the cell is equal to the osmolarity of the surrounding environment; therefore, it is considered to be in an isotonic solution. Possible Answers: The seawater pulled water from James' cells, which left him more dehydrated and caused him to urinate more.

The seawater was not processed by the kidneys because James' body lacked electrolytes. James' body was malnourished and unable to absorb the seawater, which left as urine. Correct answer: The seawater pulled water from James' cells, which left him more dehydrated and caused him to urinate more. Explanation : Ocean water has a higher osmolarity more units of solute per unit of solvent than human blood. Copyright Notice. View AP Biology Tutors.

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Top Subjects. Our Company. Varsity Tutors. During immersion, the buoyancy caused by the upward force of the liquid on the body counteracts gravity and thereby reduces the gradient for extravasation. Decreased extravasation means more fluid is retained in the circulation, and this accounts for the approximately — ml increase in circulating volume as measured during submersion Weston et al.

The increased urine output that follows will tend to normalize circulating volume over time. In a hyperbaric chamber, the pressure increase is equal all around the body. During immersion and submersion, this is not the case. Non-immersed parts of the body experience the atmospheric pressure of the air above the water, while immersed parts experience higher pressures, depending on the level of immersion. If the immersion is in water, the pressure gradient will be cm H 2 O 9.

It is exactly this pressure gradient that causes buoyancy as explained above. What now is the effect of this vertical pressure gradient, reiterating that at every given level of immersion, the pressure inside the tissues at that level is equal? One might be inclined to think of this increase in pressure in the blood vessels and tissues as acting as a resistance to blood flow.

During submersion in a vertical position, on its way from the heart to the legs the blood encounters increasingly greater pressures, resulting in incremental decreases of flow. This would then lead to preferential perfusion of the least immersed parts of the body.

However, this is not the case. The reason for this is that the circulation is a siphon, i. In a siphon, flow is determined by the difference between the inlet and the outlet pressure and the resistance of the system; intermediary pressure has no effect Munis and Lozada, Consider a garden hose through which water flows at a specific rate.

If a portion of the hose is now lowered or elevated while inlet and outlet remain at the same level, the flow rate will not change. If the pressure inside the hose would be measured, it would be greater in the lower portion of the hose, but this increase in intermediate pressure does not affect flow rate. It should be borne in mind that lowering part of the hose is not the same as squeezing the hose. Squeezing the hose is equivalent to external compression on the dependent body parts.

As explained above, this is not what happens during immersion. Also, it is important to realize that the gradient of hydrostatic pressure due to immersion is different from the gradient of intravascular pressure in a standing non-immersed person. As mentioned above, in a standing non-immersed person, intravascular pressure increases on the way down from the heart, due to the effect of gravity on the blood. This has no effect on flow again, because the circulation is a siphon but since in this case, a pressure difference between the blood in the vessel and extravascular tissues does exist, it promotes extravasation of fluid.

In most situations of submersion and immersion, there will be a pressure difference between the air in the lung and the rest of the body. For instance, during immersion with the head above the water head-out-water-immersion and snorkeling, air pressure in the lung is equal to the atmospheric pressure of the air breathed, and the pressure in the tissue surrounding the lung depends on the level of immersion Figure 1.

With head-out-water-immersion, the lung will be some 20 cm below the water, resulting in a pressure differential of 20 cm H 2 O 2. The pressure gradient between the air in the lungs and the surrounding tissues promotes extravasation of fluid from the pulmonary vasculature.

This is one of the supposed mechanisms in immersion pulmonary edema Koehle et al. Figure 1. Pressures in and around the lung during various types of immersion or submersion. Negative pressure differential means the pressure in the lung is lower than the pressure in the surrounding tissue. C Swimming under water breath holding , no pressure differential.

Of note, a pressure difference between the lungs and the surrounding tissues is absent when a person is fully submerged and not breathing, such as when swimming below the surface. During diving a pressure difference between lung and surrounding tissues may exist, depending on the pressure at which the breathing gases are delivered to the diver.

Usually, some amount of resistance has to be overcome in order to draw breathing gas into the lung and expel it out of the lungs. If this resistance is too high, excessive negative airway pressure will exist during inspiration, again promoting pulmonary extravasation of fluid.

Through this and other mechanisms, immersion pulmonary edema can also occur during diving Coulange et al. The special case of breath-hold diving must be briefly mentioned here. At a certain point, lung volume will reach the residual volume so the lung cannot collapse any further. Further descent causes extravasation of fluid from the pulmonary vasculature, which is a cause of pulmonary edema in extreme breath-hold diving Lindholm and Lundgren, However, in most cases of immersion or submersion, this is not a realistic assumption.

When immersion or submersion occurs in cold water, stimulation of autonomic nerve fibers will result in peripheral vasoconstriction in order to prevent heat loss. This will — together with the effect of buoyancy as explained above — increase centralization of circulating volume.

If, on the other hand, peripheral vasodilation occurs, such as during bathing in hot water, this may counteract the centralization of circulation volume as seen in thermoneutral or cold water. For the sake of completeness, we will briefly mention two circulatory effects that may occur during immersion or submersion, although they are not due to hydrostatic pressure and therefore not the aim of this paper.

The first is the mammalian diving reflex, which consists of parasympathetic stimulation leading to bradycardia, apnea, and vasoconstriction upon facial contact with liquid Bosco et al. In many tissues, the post-to-precapillary resistance ratio is about 0. When this ratio is 0. If this ratio increases, as occurs with arteriolar vasodilation, then arterial pressure has a greater influence on capillary pressure, which rises.

Conversely, arteriolar constriction decreases this ratio and decreases capillary pressure. This hydrostatic pressure is determined by the interstitial fluid volume and the compliance of the tissue interstitium, which is defined as the change in volume divided by the change in pressure. The more fluid that filters into the interstitium, the greater the volume of the interstitial space V i and the hydrostatic pressure within that space P i.

In some organs, the interstitial compliance is low, which means that small increases in interstitial volume lead to large increases in pressure. Examples of this include the brain and kidney, which are encased by rigid bone brain or by a capsule kidney.

In contrast, soft tissues such as skin, muscle and lung have a high compliance and therefore the interstitial space can undergo a large expansion with a relatively small increase in pressure. As interstitial volume increases, interstitial pressure increases, which can limit the amount of filtration into the interstitium because this pressure opposes the capillary hydrostatic pressure.

In other words, as the hydrostatic pressure gradient P C - P i decreases owing to the rise in interstitial pressure, fluid filtration will be attenuated. However, large increases in tissue interstitial pressure can lead to tissue damage and cellular death. Normally, P i is near zero. In some tissues it is slightly subatmospheric, whereas in others it is slightly positive.