In the common setup of the circulatory system, oxygenated blood from the heart flows through
arteries to capillaries – the smallest blood vessels where nutrient and gas exchange takes
place. A network of capillaries that nourish an area is called a capillary bed. Blood
from capillary beds, now DE-oxygenated, drains into veins to return to the heart.
A portal venous system is a DEVIATION from this configuration. It occurs when a capillary bed
drains into ANOTHER capillary bed BEFORE going back to the heart. It’s a VENOUS system because
the vessels that connect the 2 capillary beds are VEINS: they contain DE-oxygenated blood.
With this arrangement, a portal system allows DIRECT transportation of substances from one organ
to another WITHOUT spreading them all over the body. An example is the hypophyseal portal system,
which connects the hypothalamus and pituitary gland. Hormones produced by the hypothalamus
are SECRETED into the portal system to REACH the anterior pituitary, where they regulate
production of pituitary hormones. But the better known portal system is perhaps the
one that involves the liver. In fact, when not specified otherwise, the term “portal system”
usually refers to the hepatic portal system. In the hepatic portal system, venous drainage
from MOST of the gastrointestinal tract, plus the spleen and pancreas, pools into the portal
vein to reach the liver, BEFORE returning to the heart. This way, all substances ABSORBED
through the GI tract, including nutrients, toxins and pathogens, are FIRST processed in the liver
BEFORE they can reach the GENERAL circulation. The liver acts like a GATEKEEPER to the body,
it serves 2 major functions in THIS context. First, the liver processes the nutrients and
regulates the AMOUNT of nutrients that can ENTER the blood. For example, after a meal, when glucose
SPIKES from digestion of carbs, the liver converts EXCESS glucose into glycogen for storage. When the
body is fasting, glycogen is converted back to glucose to be released to the blood. In
other words, the liver CONTROLS the BALANCE of blood sugar, preventing excessive fluctuations.
The free amino acids resulting from protein digestion are also processed in the liver,
where they are synthesized into NEW proteins and pro-enzymes. EXCESS free amino acids,
which can be HARMFUL, are converted to other forms of energy storage, or BROKEN DOWN to urea to be
REMOVED in waste. This brings us to the SECOND function of the liver as a DETOXIFICATION organ.
The liver SCREENS the blood for potentially toxic substances and pathogens, and REMOVES them BEFORE
they can reach the rest of the body. It can, for example, remove alcohol and drugs from the blood.
An important pharmacological implication of liver functions is that most MEDICINES
administered orally are METABOLIZED in the liver, and MAY become DEactivated, before reaching the
general circulation and TARGET organs. This is known as the first pass effect. For this reason,
some medicines MUST be taken via OTHER routes to BYPASS liver metabolism. On the other hand,
some drugs are SPECIFICALLY designed as Pro-drugs and must be taken orally,
as they require CONVERSION in the liver to become functional.