The superior mesenteric artery (SMA) is a major artery arising from the anterior surface of the abdominal aorta. It typically originates at the level of the first lumbar vertebra (L1), just below the celiac trunk. The SMA supplies blood to a significant portion of the gastrointestinal tract, including the small intestine and parts of the large intestine.
Location
The SMA originates from the aorta just below the celiac trunk and passes anterior to the renal veins. It runs downward and forward, crossing over the left renal vein, the duodenum, and the uncinate process of the pancreas. It travels in the mesentery toward the intestines, sending out several important branches that supply the jejunum, ileum, and colon. It is located in the upper abdomen, within the retroperitoneum, and extends into the mesentery of the small intestine.
Structure and Anatomy
Origin
The superior mesenteric artery (SMA) originates from the anterior surface of the abdominal aorta, typically at the level of the first lumbar vertebra (L1), just inferior to the celiac trunk. Its origin is about 1 to 2 cm below the celiac artery. The SMA emerges anteriorly from the aorta in a downward direction, supplying the majority of the small intestine and parts of the large intestine.
Course
After its origin, the SMA descends anterior to several important structures:
- Anterior to the left renal vein: The SMA passes over the left renal vein, an important landmark in abdominal anatomy.
- Posterior to the neck of the pancreas: It travels behind the pancreatic neck but anterior to the uncinate process of the pancreas.
- Anterior to the third part of the duodenum: It crosses the horizontal part of the duodenum (the third part), marking a key anatomical relationship in surgeries and imaging.
The artery then enters the mesentery of the small intestine, where it provides numerous branches to supply the intestines.
Branches
The SMA gives off several important branches that supply various portions of the gastrointestinal tract. These branches are categorized into primary and secondary branches:
- Inferior Pancreaticoduodenal Artery: This is the first major branch of the SMA. It supplies the head of the pancreas and the duodenum and forms an important anastomosis with the superior pancreaticoduodenal artery (from the celiac trunk).
- Jejunal and Ileal Arteries: Numerous branches arise from the left side of the SMA to supply the jejunum and ileum. These arteries form an intricate arcading system known as arterial arcades, which further give rise to small straight arteries called vasa recta. The jejunal arteries typically form fewer, larger arcades, while the ileal arteries form more complex, smaller arcades.
- Ileocolic Artery: This branch originates from the right side of the SMA and travels toward the cecum, ileum, and appendix. It gives off branches such as the anterior and posterior cecal arteries and the appendicular artery that specifically supply the appendix.
- Right Colic Artery: This artery supplies the ascending colon. It arises from the right side of the SMA and travels to the right toward the ascending colon, where it divides into ascending and descending branches that supply the upper and lower parts of the ascending colon.
- Middle Colic Artery: The middle colic artery arises from the anterior surface of the SMA and supplies the transverse colon. It splits into right and left branches, which provide blood to the right and left portions of the transverse colon. The left branch of the middle colic artery anastomoses with the left colic artery from the inferior mesenteric artery (IMA), forming part of the marginal artery.
Relations
- Anteriorly: As the SMA exits the aorta, it is initially covered by the pancreas and its uncinate process. Further down, it is covered by the mesentery of the small intestine.
- Posteriorly: The SMA lies on top of the left renal vein, the third part of the duodenum, and the uncinate process of the pancreas.
- Medially: The abdominal aorta and the superior mesenteric vein, which runs parallel to the artery, lie medially.
- Laterally: The SMA’s lateral relationships include various loops of small intestine and the kidneys.
Length and Diameter
The SMA is generally around 20 to 25 cm in length from its origin to its terminal branches in the mesentery. Its diameter typically ranges between 6 to 8 mm at its origin, making it a significant vascular structure supplying the midgut.
Anastomoses
The SMA forms several important anastomoses with other arteries:
- With the celiac trunk: Via the inferior and superior pancreaticoduodenal arteries, there is an anastomotic connection between the SMA and the celiac trunk, providing collateral circulation to the duodenum and pancreas.
- With the inferior mesenteric artery (IMA): Via the middle colic artery (SMA) and the left colic artery (IMA), forming the marginal artery of Drummond. This collateral system is vital for maintaining blood flow to the large intestine.
Termination
The SMA continues into the mesentery as it supplies the distal small intestine (ileum) and the proximal large intestine (cecum, ascending colon, and transverse colon). It eventually terminates in the region of the ileocolic junction, where it supplies the terminal ileum and cecum.
Function
The superior mesenteric artery (SMA) plays a critical role in supplying oxygenated blood to a large portion of the gastrointestinal (GI) tract. Specifically, it provides blood to most of the small intestine and parts of the large intestine, supporting digestion and nutrient absorption. Below is a detailed breakdown of its functional contributions through its branches.
Blood Supply to the Small Intestine
The SMA is the primary source of blood to the small intestine, including the jejunum and ileum. This function is essential for:
- Nutrient absorption: The SMA ensures that the small intestine receives the oxygen and nutrients required for its cells to absorb nutrients from digested food.
- Peristalsis: Adequate blood supply from the SMA supports the muscle contractions (peristalsis) that move food through the small intestine.
The jejunal and ileal arteries, which branch from the SMA, form arterial arcades and straight vessels called vasa recta, supplying the entire length of the small intestine from the duodenojejunal junction to the ileocecal valve. This extensive blood supply ensures efficient nutrient absorption and transport of digested food.
Blood Supply to the Pancreas and Duodenum
The first branch of the SMA, the inferior pancreaticoduodenal artery, plays a significant role in supplying blood to the lower part of the pancreas and the duodenum (first part of the small intestine). The pancreas and duodenum are involved in producing digestive enzymes and hormones, such as insulin, which are essential for digestion and metabolic regulation.
This artery anastomoses with the superior pancreaticoduodenal artery (from the celiac trunk), creating a dual blood supply to the pancreas and duodenum. This ensures continuous perfusion to these organs, which is essential for the release of digestive enzymes and further breakdown of food as it enters the small intestine.
Blood Supply to the Cecum and Appendix
The ileocolic artery, a branch of the SMA, provides blood to the terminal ileum, cecum, and appendix. Its specific functions include:
- Supporting immune function: The appendix, a lymphoid organ, requires a steady blood supply to maintain its immune functions.
- Ileocecal junction function: This junction, where the small intestine meets the large intestine, relies on the ileocolic artery for proper muscle tone and function to control the passage of digested material into the large intestine.
The ileocolic artery also ensures that the terminal ileum, which is responsible for absorbing vitamin B12 and bile acids, receives an adequate blood supply.
Blood Supply to the Ascending Colon
The right colic artery, another branch of the SMA, provides blood to the ascending colon. The function of the SMA in this region includes:
- Facilitating water absorption: The ascending colon is involved in absorbing water and electrolytes from digested food. Adequate blood flow is crucial for these absorptive processes to occur efficiently.
- Maintaining peristalsis: Blood flow supports the muscular contractions that move the contents of the colon toward the transverse colon.
Blood Supply to the Transverse Colon
The middle colic artery arises from the anterior surface of the SMA and supplies the transverse colon. Its functional roles include:
- Maintaining colon motility: The blood flow from the middle colic artery ensures that the transverse colon can contract and move waste products along the large intestine.
- Supporting absorption: Like the ascending colon, the transverse colon absorbs water and some nutrients. The middle colic artery provides the necessary blood supply for this function.
Collateral Circulation
The SMA has several anastomotic connections, allowing for collateral circulation. This redundancy ensures that even if one artery is compromised, another can supply blood to the same area, preventing ischemia. Key examples include:
- Pancreaticoduodenal anastomosis: The inferior pancreaticoduodenal artery of the SMA anastomoses with the superior pancreaticoduodenal artery from the celiac trunk, ensuring that the pancreas and duodenum continue to receive blood.
- Marginal artery of Drummond: The middle colic artery of the SMA forms an anastomosis with the left colic artery of the inferior mesenteric artery (IMA), providing collateral blood flow to the colon.
Oxygen and Nutrient Delivery
One of the overarching functions of the SMA is to deliver oxygen and essential nutrients to the intestinal tissues. This is vital for:
- Cellular metabolism: Cells of the intestinal wall require a constant supply of oxygen and nutrients to maintain their metabolic activities, including energy production for digestion, absorption, and peristalsis.
- Tissue repair and maintenance: The high turnover rate of intestinal cells demands an uninterrupted blood supply, facilitated by the SMA, to support rapid cell regeneration and repair.
Clinical Significance
The superior mesenteric artery (SMA) is vital in maintaining blood flow to a large portion of the gastrointestinal tract, making it clinically significant in several conditions. One of the primary concerns is mesenteric ischemia, which occurs when the blood flow through the SMA is reduced due to conditions like atherosclerosis, embolism, or thrombosis. This can lead to severe abdominal pain, bowel ischemia, and in extreme cases, bowel infarction, which can be life-threatening.
In surgical procedures, especially involving the intestines, pancreas, or duodenum, the SMA’s location and its branches must be carefully navigated to avoid vascular injury. Additionally, SMA syndrome, a rare condition where the artery compresses the duodenum, can lead to symptoms such as nausea, vomiting, and weight loss.
The artery’s extensive network of anastomoses, such as the connection with the inferior mesenteric artery through the marginal artery, provides crucial collateral circulation, which helps in maintaining blood supply even when part of the SMA is compromised.
