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Interstitial and Intravascular Spaces: Volume Control |
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As hard as it is to explain the origin of the cell with all the complex structures and inner workings it needs to live and reproduce, a multicellular organism (MCO), like your body, represents a causal hurdle several orders of magnitude harder. Take water for example.
Think Again!
The last article showed that for the body’s cells to work properly, the intracellular fluid (ICF) must have the right volume and chemical content. Also, for the body to live, the extracellular fluid (ECF), the fluid outside the cells (but within the body), must have the right volume and chemical content too. These are very different from each other but must be maintained for life.
This is a hard problem.
The cell membrane is the interface between the ICF and ECF. It allows water and Na+ and K+ ions to pass through, but not protein. If the effects of diffusion and osmosis were to progress to their natural ends, they would permanently alter the volume and chemical content of the ICF and ECF. And you’d be dead!
But you’re not dead!
That’s because (as noted in the last article) at rest about one-quarter of the body’s energy is used by your cells to power about a million sodium-potassium pumps in the cell membrane to prevent this from happening. This innovation is the main reason why the ICF:ECF ratio stays at 2/3:1/3.
But that’s not all!
The ECF can be divided into what is between your cells, the interstitial fluid (ISF), and what is within the circulation, the intravascular fluid (IVF). The normal ISF: IVF ratio is about 80:20.
This must stay within a narrow range for survival.
In particular, too much fluid in the ISF relative to the IVF can cause low blood volume, low blood pressure and low blood flow to the tissues, all of which can lead to death.
This means that the mere presence of liquid water, although necessary for life is not, in and of itself, sufficient for life. To live, the body must have the right amounts of water, in the right places (e.g. ICF: ECF, ISF: IVF), all the time.
Following the Rules of MCO (Human) Life
A unicellular organism is like an island of life because it can get what it needs and get rid of what it doesn’t need through its surroundings. But a MCO is like a huge dark continent which needs a way to transport the same chemicals to and from the cells deep in its interior.
It is the cardiovascular system (CVS) that performs this function for the body. The heart pumps blood through the arterial system into the capillaries in the tissues. Capillaries are microscopic blood vessels lined by a single layer of cells, often with microscopic pores in the walls (Fig.1).
As the blood from the arterial side moves through the
capillaries to the venous side, some of the fluid (with chemicals in solution)
is squeezed out of the blood (IVF) into the ISF. The cell can then get what it
needs from, and get rid of what it doesn’t need to, the ISF through the cell
membrane. This is how the chemical exchange takes place within the tissues.
(Fig.2)
One
can see that without the capillaries the CVS would be useless. It would be like
a highway without exit ramps. It is important to note that the ISF acts as a
bridge between the IVF and the ICF and the capillary wall is the interface
between the ISF/IVF.
The
Hard Problem
Filtration
is the process by which fluid pushed through a permeable membrane under
pressure leaves behind particles too big to pass through the pores. When this
involves separating larger solutes (proteins) from smaller solutes (Na+ + K+ ions) the process is called ultrafiltration (see Fig. 3)
Using
ultrafiltration, the CVS feeds the tissues via the arterial system by pushing
fluid through the capillary walls. As blood leaves the heart into the aorta its
pressure is about 100 mmHg. By the time this arterial blood reaches the
arterioles which lead into the capillaries in the tissues, the pressure is 35
mmHg. After blood passes through to the venous end the pressure is 18 mmHg.
Why
is this important?
Blood
flowing through the capillary, with a hydrostatic pressure of 35 mmHg at the
beginning and 18 mmHg at the end, tends to constantly push water out of the IVF
into the ISF.
This
dynamic results in another hard problem.
If
the hydrostatic pressure causing ultrafiltration of water from the IVF to the
ISF is not resisted, and is allowed to continue to its natural end, all the
water from the IVF would go into the ISF and there would be no blood volume, no
blood pressure, no blood flow, and no human life!
What
type of innovation do you think is needed to solve this hard problem?
What
force of nature makes water move in a given direction? (see the last article)
Could
it potentially offset some of the force of hydrostatic pressure?
The
Innovative Solution
Osmosis
is the force of nature that makes water move in a given direction. It can counter
the hydrostatic pressure that is constantly filtering water out of the blood (IVF)
into the ISF.
How?
Osmosis
is a passive process of transport across a semi-permeable membrane in which
water can pass through, but the solute can’t. Since the solute can’t move
across the membrane, water, instead, moves from the area of lower solute concentration
to the area of higher solute concentration.
The
difference in the solute concentration between the two solutions determines the
osmotic pressure. A bigger difference results in a higher osmotic pressure
being applied to the membrane and more water moving across it from a solution
of lower to higher solute concentration.
So
how does this apply to blood and how does it solve the hard problem?
Blood
consists of a complex yellow pale fluid, called plasma with different types of
blood cells suspended within it. Plasma consists mostly of water with many
different chemicals in solution, in particular plasma proteins.
Except
for the gammaglobulins (antibodies) made in
specialized immune cells, most of the plasma proteins are produced in the
liver. They consist mainly of albumin, globulins and the clotting factors. When
the clotting factors are removed from the plasma the fluid leftover is called
serum.
Albumin
is a soluble protein made of 585 amino acids (see Fig. 4). It represents 50% of
the liver’s total protein output. Albumin transports lipids, hormones,
vitamins, enzymes, ions, drugs and other molecules in the blood. But it also
provides 80% of the plasma’s osmotic activity.
Blood
(IVF), mainly due to the amount of albumin, has a lot more protein than the
ISF. Since most protein molecules are too large to pass through the capillary
wall, osmosis then naturally tends to make water move from the ISF to the IVF.
The
osmotic pressure applied by albumin in the blood offsets the hydrostatic
pressure pushing water from the IVF to the ISF as blood goes through the
capillaries (see Fig.5).
But
how much osmotic pressure is enough to maintain the right ISF: IVF ratio of
80:20?
When
the albumin level in the blood is normal, it applies an osmotic pressure
throughout the capillary of about 25 mmHg. As noted above, the hydrostatic
pressure pushing water out of the blood at the beginning of the capillary is about
35 mmHg and about 18 mmHg at the end.
As
blood enters the capillary, water tends to move from the IVF to the ISF due to
a net outward pressure of +10 mmHg (35-25) forcing it out of the circulation.
At the end of the capillary, water tends to flow in the reverse direction, from
the ISF to the IVF, due to a net pressure of -7 mmHg (18-25) pulling it back
into the circulation.
This
is a graduated process as blood traverses the capillary. Somewhere between
where blood enters and exits the capillary the hydrostatic and osmotic
pressures are equal and there is zero net flow of filtered water between the
IVF and ISF (see Fig.6).
Due
to the osmotic pressure of albumin pulling water back into the circulation,
only about 10% of the filtered water is lost. But most of this ends up, not in
the ISF, but in the lymphatics which serve the immune system. Another elegant
feature of albumin to be sure.
Real
Numbers Have Real Consequences
Physicians
and engineers do their work within the real world where real numbers have real
consequences—even death! Here’s how Steve Laufmann and I expressed this in our
book, Your Designed Body.
“Physicians
don’t get to make stuff up. They don’t have the luxury to merely observe how
life looks or theorize about its superficial qualities. They need to know how
the body really works, how the parts affect each other, and what it takes in
practical terms to keep it all working over a (hopefully) long lifetime. Though
their mistakes sometimes take longer to discover than those of physicians,
engineers also must live in the real world. Engineers design, build, deploy,
and operate complex systems that do real work in the real world. And it takes
yet more work to keep the systems from failing.”
As
opposed to physicians and engineers, the concept of “functional capacity” seems
to be totally absent from the mindset of evolutionary biologists. That’s
because their theoretical constructs always lack the objective criteria needed
to verify that a given biological structure works well enough for survival—in
other words its functional capacity and the control mechanisms needed to
maintain it are good enough.
Yet,
no matter how complex the genetics leading to a sophisticated biological
structure, if it can’t control and maintain the functional capacity to combat and/or
use the laws and forces of nature to its advantage, the organism in which it is
housed is as good as dead.
The
same applies to albumin.
This
complex and versatile plasma protein is made in the liver. To do this each
liver cell must use the information in its DNA to know how to hook up the right
combination of 585 amino acids.
But
how much albumin is enough?
How
is it controlled?
How
the liver knows how much albumin to make is as yet poorly understood. It is
thought to be related to the osmotic pressure that albumin applies across the
walls of the capillaries within the liver itself. Production of albumin also
seems to be affected by hormones like insulin, cortisol and thyroid hormone.
The
normal range of albumin in serum is 35 to 50 grams per liter (g/L). Low levels
of serum albumin (hypoalbuminemia) can occur due to conditions such as
malnutrition and diseases of the liver, kidneys and gastrointestinal tract.
Albumin
is the main plasma protein responsible for providing enough osmotic pressure to
pull enough water back into the blood (IVF) from the ISF after it’s been pushed
out by hydrostatic pressure. If the serum albumin level drops significantly
below the normal range, the osmotic pull of water back into the circulation is
significantly reduced as a consequence.
The
further below normal the serum albumin, the less osmotic pressure and the more
water tends to move out of the blood and stay inside the ISF forming what is
called edema in the tissues. And the more water that tends to stay as
edema in the tissues and doesn’t go back into the circulation, the lower the
blood volume and blood pressure, and ultimately blood flow to the tissues which
can compromise survival.
Although
the body can compensate for some of the drop in serum albumin if the level goes
much below 25 g/L, people usually experience severe weakness, fatigue and dizziness
on standing up. It is said that a serum albumin < 10 g/L is incompatible
with life.
When
it comes to human life, real numbers have real consequences!
Evolutionary
“Explanations”
Questions
Onward!
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