portaalijärjestelmä

A portaalijärjestelmä, often translated as portal system, refers to a network of blood vessels that carries blood from one capillary bed to another before returning it to the heart. This unique arrangement bypasses the general circulation. The most well-known example is the hepatic portal system, which transports nutrient-rich blood from the digestive organs (stomach, intestines, spleen, and pancreas) directly to the liver.

In the hepatic portal system, capillaries in the digestive organs merge to form larger veins, which then converge to form the hepatic portal vein. This vein enters the liver and branches into smaller vessels, eventually leading to sinusoids, which are a specialized type of capillary. Within the sinusoids, the blood from the digestive organs interacts with liver cells, allowing the liver to process absorbed nutrients, detoxify harmful substances, and store glycogen. After passing through the sinusoids, the blood collects in the hepatic veins, which then drain into the inferior vena cava, returning the blood to the general circulation and subsequently to the heart.

Other portal systems exist in the body, though they are less extensive. For instance, the hypophyseal portal system connects the hypothalamus to the anterior pituitary gland, allowing hormones from the hypothalamus to be transported directly to the pituitary to regulate its function. Understanding portal systems is crucial in medicine, particularly in conditions affecting the liver or endocrine glands.