What is a hydrostatic head

 
If an acceleration force acts on the body - an everyday situation, since the surface on which we stand prevents free fall (otherwise the body would be "weightless") and thus exerts an upward acceleration effect - then occurs in fluid-filled spaces a hydrostatic pressure gradient: The pressure increases from "above" (in the direction of the acceleration effect) to "below" (usually towards the center of the earth).

Now what happens when a body changes its position, e.g. is tilted? Where are the places where the pressure stays the same? Does this point differ, for example, between the venous and arterial systems? How does their position relate to the corresponding pressure receptors?

The hydrostatic indifference point HIP (or the hydrostatic indifference line, see below) is the location in a fluid-filled space in which the
hydrostatic pressure with change of position (of the body) remains unchanged. The hydrostatic pressure drops in places above (related to the direction of the acting acceleration) and increases in places below.

The position of the HIP (depending on the specific change in position) depends on several factors in the body:

Which fluid space is meant? (Arterial system, venous system, pleural space, liquor space, etc.) - see below
 
The change from which to which body position is meant? (e.g. lying to standing, lying on your side, head-down position, ...)
 
What is the state of the fluid space in question? E.g. depending on the filling volume (infusion, blood loss, dehydration), stress (cardiac output, vasoconstriction), etc.
 

The pressure changes relate to the purely hydrostatic effect, i.e. to the point in time immediately after the change in position - before adaptive adaptation processes (change in vascular tone, heart activity, neural and humoral reactions) take place.

The physiological effects of a change in position can be considerable (e.g. the cardiac output decreases after a tilt lying → upright physiologically by up to ~ 35%). The position of the corresponding mechanoreceptors relative to that of the HIP determines the type and extent of reflex reactions (e.g. baroreceptors, see below).

    For the human venous system, the HIP for a change between lying and upright body position (when standing up or standing up) is a hand's breadth under the diaphragmatic dome, i.e. at the level of the upper abdominal viscera (liver, spleen, pancreas). This means, among other things,

   that the microcirculation is largely spared from pressure fluctuations when the body changes position. It also means

   that when straightening up the venous pressure at heart level (central venous pressure, right cardiac preload) decreases, which on the one hand lowers the diastolic filling (reduced stroke volume!), on the other hand


> Figure: Pressure / strain receptors are above "their" respective hydrostatic indifference point
According to: Hinghofer-Szalkay H. Gravity, the hydrostatic indifference concept and the cardiovascular system. Eur J Appl Physiol 2011; 111: 163-74

Therefore, the size of the stimulus at the receptors changes when the body position changes accordingly

       In the arterial system

the hydrostatic indifference point (between lying down and upright) is at the level of the atria. That in turn means

     that the overlying carotid receptors are less stimulated when standing up / standing up than when lying down, which in turn increases the heart rate when standing up (Schellong test: orthostatic response);

   that with the classic arterial blood pressure measurement (congestive cuff on the upper arm, which is automatically at the level of the heart), hardly any falsifications of the arterial pressure can occur (unlike measurements in the periphery, e.g. on the wrist, when there is a difference in height to the position of the atria).

   
Hydrostatic pressure gradients and a point of hydrostatic indifference exist in all fluid-filled body cavities, e.g. in liquor-filled spaces.
 

From: Hinghofer-Szalkay H. Gravity, the hydrostatic indifference concept and the cardiovascular system. Eur J Appl Physiol 2011; 111: 163-74



Errors in the textbooks - Unfortunately, a factually inaccurate answer is expected here: The concept of a hydrostatic plane of difference has been haunted in German-language physiology books for years. It is claimed that it will remain in the event of a change of position the hydrostatic pressure constant within this plane (like a transverse section through the abdomen about 10 cm below the diaphragm).

This concept is wrong: Such a "level of indifference"
can not give it (> Review). For example, if a person changes from standing to the supine position, the hydrostatic pressure decreases anteriorly within the plane shown in the textbooks, and increases dorsally - anything else would contradict physics.

There are two possibilities. The hydrostatic pressure remains constant

with simple tilting of the body along a line (
or with a combined change in body position in one point (hydrostatic indifference point, HIP).
 
By the way: In the weightless state, the hydrostatic indifference concept loses its meaning - without (gravity) acceleration there is no hydrostatic pressure gradient.


Body position and blood volume

> Figure: Blood density and body position
From: Hinghofer-Szalkay & Greenleaf: J Appl Physiol 1987; 63: 1003-7

Time course of the mass density of the blood (corresponds to the hematocrit) of a test person on two different test days (6.8) sitting (left), standing (middle) and lying (right). Standing up leads to haemoconcentration within minutes (capillary fluid loss, hence increase in hematocrit), lying down leads to haemodilution (~ 10% change in blood volume)

The mass density was determined continuously from venous blood by measuring the oscillation frequency of a tuning fork-shaped hollow body made of glass through which blood flows ("mechanical oscillator technique")




The straightening of the body also has consequences for the balance of the filtration forces in the microcirculation. As the> figure shows, when the body position is changed from sitting (or lying) to standing, the hematocrit increases rapidly (blood thickening) because there is more fluid in the body regions below the hydrostatic indifference (lower abdomen, legs, forearms) due to the increased capillary pressure that prevails here is pressed into the surrounding tissue (Starling equilibrium!) than migrates more into the capillaries above (head, shoulder region, thorax).

The filtration equilibrium over the entire body therefore changes with the position of the body. If the test subject is tilted into a lying position after standing, the situation is reversed: More fluid now migrates back into the capillaries from the "lower" body regions than is also exiting in the "upper" regions - the plasma volume increases, the hematocrit decreases.

The change in plasma volume can be up to ~ 20%, in blood volume ~ 10%. This is of practical relevance: the position of the body also determines the degree of dilution (the concentration) of many components dissolved in the blood that do not move through the capillary walls at the same speed as the filtrate (e.g. hormones, plasma proteins).

Since the protein concentration of the blood components mainly determines their mass density (specific weight), the plasma density (at ~ 70 g / l and body temperature: ~ 1020 g / l) and the blood density (this depends on the hematocrit: erythrocytes have a Protein concentration - MCHC - of ~ 340 g / l, which results in a blood density of ~ 1050 g / l with a hematocrit of ~ 45% - see the values ​​in the figure).



© Helmut Hinghofer-Szalkay