1. Explaining the Pathological Changes
In the normal conditions, meridians and collaterals can circulate qi and blood, and respond and conduct, but when the evil-qi disturbs, they become the pathways through which the evil-qi transmits. It is reflected in pathological changes. Chapter 56 in Plain Questions says: "while the evils attack the skin, the interspace of muscle will be effected, thus, the evils go further to the collaterals, and then empty into meridians, and finally arrive at the zang-fu viscera. "This states that the meridians are the pathways whereby exogenous-evil attacks from the skin and muscle interspace to the five-zang and six-fu viscera. Because the viscera communicate through meridians and collaterals, meridians and collaterals can be the routes of unhealthy pathological changes among the viscera. For example, the Liver Meridian of the Foot-Jueyin curves around the stomach and fills into the lung, the liver disease can affect the stomach and lung; the Kidney Meridian of Foot-Shaoyin enters into the lung and connects with the heart, so the excessive water due to kidney-deficiency can attack the heart and lung.

Because interior-exteriorly related meridians pertain to or connect with the same viscera, interiorly-exteriorly zang-viscera and fu-viscera are inter-influenced in pathology. For example, the heart-fire may extend down to the small intestine, the excessive heat of the large intestine and the obstruction of fu-viscera can lead to a dysfunction of lung-qi marked by asthma, cough, and stiffness in the chest.

The meridians and collaterals are the routes of interaction of pathological changes arising not only from the exterior to the interior, or between the viscera, but also between the viscera and the tissues of the body surface. Through the conduction of meridians and collaterals, internal visceral diseases can be reflected on the body's surface, manifested at special positions or corresponding orifices. For example, stagnation of liver-qi with distention and pain in both hypochondriac regions and lower abdomen is due to the Liver Meridian of the Foo-Jueyin. The FootJueyin extends to the lower abdomen and spreads through the hypochondriac region. True heart pain (angina pectoris) manifests not only from the pain on the anterior part of the heart, but also from the radiating pain extending to the ulnar border in the interior side of the upper extremities. This is because the Heart Meridian of Hand-Shaoyin travels along this region. Other examples are swelling and pain in the gum due to stomach fire, and red eyes due to a flaring up of liver-fire. All of those are the reflections of the meridian conduction.