The abdomen is a part of the body sometimes casually referred to as the belly or torso.
The organs within this area are said to be contained within a space known as the abdominal cavity, which is bounded by the musculo-aponeurotic walls anterolaterally.
Superiorly, the abdominal cavity extends into the thoracic cage to the 4th intercostal space and is separated from the thoracic cavity by the diaphragm.
Some organs found within the upper region of the abdominal cavity, such as the spleen, liver, stomach and parts of the kidneys, are actually protected by your rib cage.
Inferiorly, the lower portion of the abdominal cavity doesn't have a physical boundary because it’s continuous with the pelvic cavity; so sometimes, they’re lumped together under the term “abdominopelvic cavity”.
However, to better understand the anatomy, the inferior boundary, which separates the abdominal cavity from the pelvic cavity is an imaginary plane called the pelvic inlet, which divides the pelvis into a greater, or false pelvis, above, and a lesser, or true pelvis, below.
Similar to how the ribcage protects some of the superior organs of the abdominal cavity, the greater pelvis protects some of the lower organs of the abdomen, including portions of the ileum, cecum, appendix, and sigmoid.
You may have already noticed how many organs there are inside the abdominal cavity. So to make it easier to describe their location, the abdomen is often divided into anatomical quadrants, of which there are four, or regions, of which there are nine.
Let's start with the quadrants. To get those four quadrants, imagine a line running down from the xiphoid process, or tip of the sternum, all the way down to the pubic symphysis. This line is the median plane and divides the abdomen into a left and a right half.
The second imaginary line goes straight through the belly button or umbilicus, from left to right. This line creates the transverse or transumbilical plane and divides the abdomen into an upper and lower half. So the resulting quadrants are the right upper quadrant, left upper quadrant, right lower quadrant, and the left lower quadrant.
Now that we understand how the quadrants are divided, let's see what abdominal organs are contained within each quadrant.
Large organs located in the right upper quadrant are the right lobe of the liver, the gallbladder, the head of the pancreas, the first to third part of the duodenum, and the right colic flexure, while large organs located in the left upper quadrant are the left lobe of the liver, stomach, spleen, body of the pancreas, as well as the left colic flexure.
In the right lower quadrant, there’s the ascending colon, cecum and appendix, and the lower portion of the right kidney, while the left lower quadrant contains the descending and sigmoid colon and lower portion of the left kidney.
Finally, remember that the lower quadrants are also home to the ureters, which carry urine from the kidney to the bladder, as well as the internal reproductive structures, which are different between sexes. So in genetically female individuals, the lower quadrants also contain the ovaries and parts of the uterus, while in genetically male individuals, the spermatic cords can be found.
In comparison to the four quadrant system, the anatomy of the abdominal cavity can also be described using the regional system which divides the abdomen into 9 distinct regions. If you've ever played tic-tac-toe, you already know how to do that: you’ll need two vertical, or sagittal planes, intersecting with two transverse or horizontal planes. In other words, imagine vertical or sagittal lines descending from the middle of the left and right clavicle, these are known as the midclavicular lines.
Next, imagine there are also two horizontal or transverse lines - the first line goes from left to right just below the tenth rib's costal cartilage, and is therefore known as the subcostal plane. The second horizontal line, which forms the transtubercular plane, starts at the iliac tubercles on each side of the pelvis and passes through the L5 vertebrae.
Just as a reminder, the iliac tubercles are located at the widest portion of the outer border of the pelvis or iliac crest, and are approximately 5 centimeters posterior to the anterior superior iliac spine.