Disturbances in the Venous Drainage from the Intestine Can Greatly Affect the Mechanisms of Capillary Absorption in the Villi
With the exception of blood from the terminal colon and rectum, all venous blood from the GI tract is collected into the hepatic portal vein and passes through the liver before entering the vena cava and returning to the heart (Figure 30-23).
Because of this system, the nutrient-rich blood leaving the intestine may be modified by the liver. The liver can thus regulate the nutrient concentration of blood reaching general body tissues, keeping it relatively constant. This particular vascular arrangement of the GI system results in the passage of blood through two capillary beds, one in the gut wall and one in the liver, before its return to the heart. In most tissues, arterial hydrostatic pressure forces blood through the capillary beds. In the liver, however, this is not the case, because most of the arterial hydrostatic pressure has been dissipated during flow of blood through the intestinal capillaries. The following two circumstances tend to overcome this problem and allow hepatic blood flow to occur:1. The capillaries (referred to as sinusoids) of the liver are comparatively large and thus offer little resistance to flow; therefore they can function in a low-pressure system.
2. The venous outflow of the liver goes directly into the thoracic vena cava.
The bellows-like action of the thorax transmits a negative pressure to the thoracic vena cava, which tends to aspirate blood from the hepatic veins and abdominal vena cava. Under normal circumstances» these conditions allow blood to flow readily from the intestine through the liver. However» small changes in circulatory function can have a large impact on GI blood flow. If the pumping capacity of the heart becomes reduced, it cannot remove returning venous blood quickly. Accumulation of blood and an increase in pressure in the thoracic vena cava result. This increase in pressure interferes with the flow of blood out of the liver, which in turn reduces blood flow out of the intestine. This sequence of events makes the GI system particularly susceptible to right-sided heart failure, in which the heart’s pumping action is compromised.
Diffuse liver disease, in addition to right-sided heart failure, can also interfere with GI blood flow. In this condition the resistance to blood flow through the liver is increased because of pressure on the sinusoids. Small rises in hepatic flow resistance can have large effects on intestinal blood flow because the pressure gradient across the hepatic portal vein is normally small. When the flow of blood out of the intestine is impaired, hydrostatic pressure in the capillaries of the villi is increased; the higher pressure tends to offset the osmotic and hydrostatic forces promoting water absorption, and thus water absorption is impaired.