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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 deoxygenated, 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 deoxygenated 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.
