Like many parts of the body, the digestive system and circulatory system are related in a number of integral ways. When most people think of the circulatory system, they picture the heart and lungs acting to pump blood throughout the body. Only few understand the enteric requirement of the circulatory system. The enteric system, or the gut, which contains many digestive organs, requires about 30% of all cardiac output. This large amount of blood, needed for ongoing digestive functions, is the basis for the interrelation of the two systems.
Understanding the complex dynamics among bodily systems requires a basic understanding of each system independently. Generally speaking, the digestive system breaks down food that is consumed. This system acts to divide the food into necessary components, like vitamins and nutrients, and allows for absorption of these nutrients into the bloodstream. The circulatory system acts to move these necessary nutrients around the body as well as transport unwanted materials away.
Although the relationship between the digestive system and circulatory system is extensive, there are two primary ways in which these systems rely on each other. The first is the need of the circulatory system for digestion to continue to function. The second is the delivery of nutrients from the digestive system to the body's bloodstream for circulation.
Like all organs and systems of the body, blood is constantly needed for performance. The blood itself is not crucial, but the nutrients within this vital fluid are. These nutrients supply the tissues and cells with chemicals they need to continue living. The bottom line is that, without nutrients, there is no life, and without circulation, there are no nutrients.
Although the digestive system needs blood from the circulatory system to work properly, the circulatory system also needs the digestive system. As food breaks down, it moves from the esophagus to the stomach and into the intestines. Eventually, the broken-down foodstuffs reach the vascular small intestines, where absorption occurs. It is in this region where many vital substances move from the food in the intestines into the circulatory system.
Digestion is almost constantly occurring, so in part due to the absorptive properties of the intestines, there remains a high demand for blood flow to the enteric region. This is the physiological reason behind the high percentage of flow to this area. There may be more links between the two systems but, in simple terms, this is how they interact.
How Does Food Move from the Digestive to Circulatory System?
To understand the link between the digestive and the circulatory system, it is essential to know about the structure of the intestinal wall (small intestine). The walls of the small intestine are adapted to perform the duty ideally. Micro-projections called villi play a crucial part in the delivery of nutrients from the small intestine to the circulatory system.
As per studies, the mechanism of internal absorption (of nutrients such as amino acids and vitamins) is highly regulated and efficient.
The villi have a dense supply of capillaries inside. The capillary bed provides a high surface area of vessels for the nutrients to get absorbed into the circulation. Two of the three major nutrients, i.e., amino acids and sugars, are transported via this method (capillaries); however, fat absorption takes longer.
Lacteals for Fat Absorption
A lacteal is a specialized vessel inside the villi that connects the digestive system to the lymphatic system. The lacteal is, in fact, a lymphatic vessel that absorbs the large fat globules broken down in the intestine. This is because the fat molecules are so large that they can not pass directly into the capillary bed.
Research suggests that lymphatic vessels are crucial for the absorption of dietary fats.
The lymphatic vessels then empty the contents, i.e., lymph and fat molecules, into the subclavian vein, which connects directly to the heart. The fats make their way from the lymphatic vessels into the lymphatic ducts (thoracic ducts), which connect them directly to the heart.
The lymph also absorbs excess tissue fluid leaking out of the vessels, thus, preventing fluid buildup (edema).
Digestive System and the Liver
In addition to the direct connection, the digestive and circulatory systems are also connected via an organ, i.e., the liver.
The liver is the largest organ of the body and performs several roles inside the human body. The digestive system transfers its contents to the liver. This massive organ then analyzes the contents and decides what processes are necessary. Once inside, nutrients can undergo alterations, detoxification, processing, and storage.
The liver efficiently removes toxic metabolites (breakdown products) delivered to it by the digestive system. The liver does not let toxic metabolites (such as alcohol and by-products of medicines) become part of the circulation and excretes them.
Another important duty allotted to the liver is regulating blood glucose levels. Even consuming high amounts of sugars and carbohydrates will not affect your blood sugar levels if you are healthy. This is because the liver stores excess sugars in the form of fats (adipose tissues) in the body. The hormones insulin and glucagon govern this regulation.
Insulin increases the utilization and storage of sugar (reducing sugar in the blood), while glucagon increases blood sugar levels.
Hepatic Portal Vein
The hepatic portal vein establishes the nexus between the digestive system and the liver. It is not a true vein (it does not carry blood back to the heart). It brings blood full of nutrients from the gastrointestinal tract to the liver.
Persistently high blood pressure in the portal vein is termed portal hypertension, leading to liver cirrhosis.
How Circulatory System Impacts the Digestive System
The connection between both systems is two-way. All the organs in the digestive system require blood (nutrients) to work optimally, therefore, they have their blood supply. Abruptions in the communication between these systems can lead to the development of diseases.
The condition is called intestinal ischemia when blood flow to the intestines becomes compromised. Potential causes of this halting or sluggishness of the blood include blood clots in the major arteries or narrowing of the blood vessels by cholesterol deposition.
Different types of intestinal ischemia are based on the region of the intestine involved. Ischemic colitis arises due to the slowing blood flow to the large intestine (colon). Anemia, bowel twisting (volvulus), and heart medications may trigger it.
Acute mesenteric ischemia has a high mortality rate and is thus a medical emergency. It affects the small intestine and is attributed to emboli, atherosclerosis, shock, etc. Intestinal angina (chronic mesenteric ischemia) is mainly due to the narrowing and blocking of the arteries.
Thus, it is clear that digestive and circulatory systems rely on each other for the normal and harmonious working of the body.