The Hydrostatic Pressure: General Principles, Application in Medicine and Differences With Oncotic Pressure

In health, we have exciting advances regarding the study and application of hydrostatic pressure and the functionalities that it reverses in subjects associated with health.

However, before entering into the subject of its relationship with the field of health and medicine, we briefly define the general features of what is and what hydrostatic pressure consists of. Let’s see…

What is hydrostatic pressure?

Hydrostatic pressure refers to the stress that any fluid in a confined space exerts. If the liquid is in a container, there will be some pressure on the wall of that container.

Hydrostatic pressure is the pressure generated by the liquid’s weight on a measurement point when the liquid is at rest.

The height of a liquid column of uniform density is directly proportional to the hydrostatic pressure.

The hydrostatic properties of a liquid are not constant, and the main factors that influence it are the density of the liquid and the local gravity.

It is necessary to know both quantities to determine the hydrostatic pressure of a particular liquid.


Hydrostatic pressure is the force that fluid molecules exert on each other due to the Earth’s gravitational attraction.

This force occurs if the fluid is in motion or at a complete stop and forces the fluids forward or outward when encountering an area of ​​least resistance in their field.

This energy forces the water from a hole in a paper cup, the gas from a leak in a pipe, and the blood from the vessels to the surrounding tissues.

Increasing the elevation increases the amount of hydrostatic pressure.

The fluid that flows downwards also increases the pressure, which causes the water that travels through the waterfalls to flow faster than the water that flows through the stream until it falls.

Temperature is another factor that affects pressure because when temperatures rise, the molecules move faster, increasing the pressure.

Industries commonly use hydrostatic pressure testing methods to ensure that liquids remain in confined environments.

The tests ensure that the pipes and other types of containers do not leak and verify that the materials can withstand a more significant pressure of possible environmental changes.

It is not uncommon for companies to exert internal forces 150 times more than usual while controlling pressure changes with instrumentation.

If we imagine a container in the form of a column, we can see that the pressure that pushes against its wall is more significant in the background, which will be in the upper part. This is related in part to the force of gravity.

The capillaries are the equivalent of a container in the form of a column, rotated on its side. The pressure that blood exerts on the veins is known as blood pressure.

The force of the hydrostatic pressure means that the blood moves along the capillary, and the fluid moves through its pores and into the interstitial space.

This movement means that the pressure exerted by the blood will become lower, and the blood moves along the capillary from the arterial to the venous end.

Fluid or hydrostatic statics are fluid mechanics branches that study incompressible fluids at rest.

It covers the study of the conditions under which the fluids are at rest in stable equilibrium against fluid dynamics and the analysis of fluids in motion.

Hydrostatics is classified as part of the static fluid, which is the study of all fluids, incompressible or not, at rest.

Hydrostatics is fundamental for hydraulics, the engineering of equipment for storing, transporting, and using fluids.

It is also relevant to geophysics and astrophysics (for example, in understanding plate tectonics and the anomalies of the Earth’s gravitational field), meteorology, medicine (in the context of blood pressure), and many other fields.

Hydrostatics offers physical explanations for many phenomena of everyday life, such as why atmospheric pressure changes with altitude, why wood and oil float on water, and why the water’s surface is always flat and horizontal, whatever the shape of your container.

Pressure in resting liquids

Due to the fundamental nature of the fluids, a fluid can not remain at rest under the presence of shear stress. However, fluids can exert normal pressure on any contact surface.

If it is considered that a point of the fluid is an infinitesimally small cube, then it follows from the equilibrium principles that the pressure on each side of this fluid unit must be equal.

If it were not so, the fluid would move in the direction of the resultant force.

Thus, the pressure on a fluid at rest is isotropic; it acts with equal magnitude in all directions.

This feature allows fluids to transmit force through the length of pipes or tubes; That is, a force applied to a liquid in a pipeline is transmitted, through the fluid, to the other end of the line.

This principle was formulated first, in a slightly extended form, by Blaise Pascal and is now called Pascal’s law.

In a fluid at rest, all frictional and inertial forces disappear, and the state of tension of the system is called hydrostatic.

When this condition of V = 0 is applied to the Navier-Stokes equation, the pressure gradient becomes a function of the body’s forces only.

For a barotropic fluid in a field of conservative force as a gravitational force field, the pressure exerted by a fluid in equilibrium becomes a function of the force exerted by gravity.

Hydrostatic pressure in the field of medicine

Blood vessels have a unique way of maintaining adequate pressure throughout the body. Hydrostatic capillary arterial pressure usually measures 35 millimeters of mercury, or 35 mm Hg. Venous capillary pressure typically measures 15 mm Hg.

The force behind the heart’s contractions and the gravity that pulls the blood away from the core causes increased pressure.

The porous nature of the venous capillaries also decreases the pressure of the flowing blood.

The liquid components of the blood flow naturally through the pores into the interstitial tissues due to this pressure, leaving behind lipids, proteins, and particles too large to escape.

This usually decreases venous pressure. On the contrary, the pressure increases within the tissues exert forces on the capillaries, called hydrostatic osmotic pressure.

While the osmotic pressure pushes fluids into the capillary pores, the electrical charges of the solids inside the vessel cause the molecules to bind as they flow into the blood.

This reaction is called the Gibbs-Donnan effect.

The osmotic pressure and the Gibbs-Donnan effect together extract fluids from the interstitial tissues into the plasma, known as colloid osmotic pressure.

When the body perceives an abnormally low venous pressure, the arteries usually narrow.

When damage occurs in the vessel, the plasma contains an insufficient number of solids or lowers blood pressure, then edema or swelling occurs.

Capillary hydrostatic pressure:

This pressure drives fluid out of the capillary (i.e., filtration) and is higher at the arteriolar end of the capillary and lower at the venular end.

Depending on the organ, the pressure can fall along the capillary at 15-30 mmHg (axial or longitudinal pressure gradient).

The axial gradient favors filtration at the arteriolar end and reabsorption at the venular end of the capillary.

Tissue pressure (interstitial):

This hydrostatic pressure is determined by the volume of interstitial fluid and the compliance of the tissue interstitium, which is defined as the change in volume divided by the change in pressure.

The more fluid seeps into the gap, the greater the interstitial space volume and the hydrostatic pressure within that space. In some organs, interstitial compliance is low, which means that small increases in interstitial volume lead to significant increases in pressure.

Examples include the brain and the kidney coated by rigid bone (brain) or a capsule (kidney).

Conversely, soft tissues such as skin, muscles, and lungs have high compliance and, therefore, the interstitial space can undergo a significant expansion with a relatively small increase in pressure.

As the interstitial volume increases, the interstitial pressure increases, limiting the amount of leakage in the interstitium because this pressure opposes hydrostatic capillary pressure.

In other words, as the hydrostatic pressure gradient decreases due to the increase in interstitial pressure, the fluid filtration will be attenuated. However, significant increases in interstitial tissue pressure can lead to tissue damage and cell death.

Usually, the interstitial pressure is close to zero. In some tissues, it is slightly subatmospheric, while it is somewhat optimistic in others.

Capillary oncotic pressure:

Because the capillary barrier is easily permeable to ions, the osmotic pressure within the capillary is mainly determined by plasma proteins that are relatively impermeable.

Therefore, instead of talking about “osmotic” pressure, this pressure is called “oncotic” or “colloidal osmotic” pressure because colloids generate it.

Albumin generates approximately 70% of the oncotic pressure. This pressure is typically 25-30 mmHg.

Oncotic pressure increases throughout the capillary, particularly in capillaries with a high net filtration (for example, in renal glomerular capillaries) because the filtering fluid leaves proteins that increase protein concentration.

Usually, when oncotic pressure is measured, it is measured through a semipermeable membrane that is permeable to fluids and electrolytes but not too large for protein molecules.

However, in most capillaries, the wall (mainly endothelium) has a finite permeability to proteins.

The actual permeability to the protein depends on the type of capillarity and the nature of the protein (size, shape, charge).

Due to this finite permeability, the actual oncotic pressure generated through the capillary membrane is more minor than calculated from the protein concentration.

The effects of finite protein permeability on physiological oncotic pressure can be determined by knowing the capillary wall’s reflection coefficient (σ).

If the capillary is impervious to the protein, it equals 1.

When the value for σ is meager, the oncotic pressures of the plasma and tissue may have a negligible influence on the net driving force.

Wound (interstitial):

The oncotic pressure of the interstitial fluid depends on the interstitial protein concentration and the reflection coefficient of the capillary wall.

The more porous the capillary barrier to proteins, the greater the interstitial oncotic pressure.

This pressure is also determined by fluid filtration in the gap. For example, increased capillary filtration decreases interstitial protein concentration and reduces oncotic stress.

A reduction in interstitial oncotic pressure increases the net oncotic pressure through the capillary endothelium, which opposes filtration and promotes reabsorption, thus limiting capillary leakage.

In a “typical” tissue, the oncotic pressure of the tissue is about five mmHg (that is, much lower than the oncotic pressure of the capillary plasma).

What is the difference between oncotic and hydrostatic pressure?

Hydrostatic pressure increases filtration by pushing the fluid and solute out of the capillaries. At the same time, oncotic capillary pressure (also known as colloid osmotic pressure) draws fluid into the veins and prevents hydrostatic pressure.

The hydrostatic pressure is based on the pressure exerted by the blood pressure against the walls of the capillaries. In contrast, oncotic pressure exists due to proteins, such as albumin, globulins, and fibrinogen, which do not leave the capillary and extract water.

The same forces also act on the interstitial fluid.

The arteries transport oxygenated blood and nutrients to the body’s metabolic tissues. This oxygenated blood travels through the capillary network within the tissues.

The exchange of fluids in the blood capillaries is called microcirculation. Hydrostatic and oncotic pressure are the two driving forces that intervene in the movement of fluids during microcirculation.

The main difference between hydrostatic and oncotic pressure is that hydrostatic pressure is the force that pushes the fluid out of the blood capillaries. In contrast, oncotic pressure is the force that drives the liquid into the blood capillaries.

The Starling principle describes the general interaction between hydrostatic pressure and oncotic pressure.