Structure and Function

Learning Objectives

By the end of this section, you will be able to:

• Identify structures of the abdomen
• Recognize functions of the abdomen
• Recall effects of impaired function of the abdomen

A thorough assessment of the abdomen provides valuable information regarding the function of a patient’s gastrointestinal (GI) and genitourinary (GU) systems. Understanding how to properly assess the abdomen and recognizing both normal and abnormal assessment findings will allow the nurse to provide high-quality care to the patient. A thorough assessment of the abdominal system will help provide information about the reproductive system, the patient’s metabolism, and how the patient’s immune system is functioning.

Structures of the Abdomen

The abdomen is defined as the anterior region of a person’s trunk between the diaphragm and pelvis. The structure of the abdomen can be broken down into external and internal structures.

External Structures

The external structure of the abdomen has dual functions: to assist the body in movement and to protect internal organs (Table 27.1). The layers consist of skin, muscle, fat, and fascia (thin, fibrous connective tissue).

Structure	Function
Skin (outermost layer)	Protection, absorption of nutrients, retain moisture
Superficial fascia	Protection; houses nerves and blood vessels
Muscles	Protection, movement, and posture
Transversalis fascia	Protection; houses and anchors structures within the abdomen
Fat	Protection, thermoregulation, insulation
Peritoneum	Protection, insulation, anchors organs in place, secretes lubricating fluids to help reduce friction

Table 27.1 Summary of the External Structures of the Abdomen (Source: Wade & Streitz, 2023.)

Abdominal Quadrants

The abdomen is divided into four quadrants to provide a reference to the anatomic location of organs when signs and symptoms arise in the abdomen. This helps guide the provider in clinical decision-making. The two main reference lines intersect at the naval (umbilicus) to divide the abdomen into four quadrants (Figure 27.2).

Figure 27.2 To visualize the abdomen and its organs, the abdomen is divided into four quadrants for the physical exam. Within those four quadrants, the corresponding abdominal organs are indicated to help guide the physical exam. (credit: modification of “Abdominopelvic Quadrants” by “BruceBlaus”/Wikimedia Commons, CC BY 3.0)

When assessing the abdomen, consider the organs located in the quadrant you are examining:

• right upper quadrant (RUQ): houses the liver, gallbladder, right kidney (posterior abdomen), and a portion of the small intestine and large intestine
• left upper quadrant (LUQ): houses the stomach, the spleen, pancreas, left kidney (posterior abdomen), another portion of the small intestine and large intestine, and a portion of the liver
• right lower quadrant (RLQ): houses the appendix, part of the large intestine (cecum and ascending colon), the right ovary and fallopian tube in females, upper part of the right side of the bladder (the uterus is located below the lower quadrants in the abdominopelvic region)
• left lower quadrant (LLQ): houses the lower part of the large intestine (descending colon), sigmoid colon, and the left ovary and fallopian tube in females, and the upper part of the left side of the bladder

Abdominal Wall Muscles

The abdominal wall muscles work together to provide core stability, posture, movement, and trunk stabilization and support (Figure 27.3). The four main abdominal muscles are the rectus abdominus, external obliques, internal obliques, and transverse abdominus (Table 27.2).

Figure 27.3 Each muscle of the abdomen has a specific function for movement. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Muscle	Location and Function
External obliques	Located on each side of the rectus abdominus; run from the side to the front of the bodyEnable trunk rotation from side to side
Internal obliques	Located under the external obliques; run from the hip bones up to the front of the bodyEnable trunk rotation along with the external obliques
Rectus abdominis	Runs down the middle of the abdomen; often called a “six pack”Provides stability and posture; holds internal organs in place
Transverse abdominis	Located under the internal obliques; runs around the spineProvides trunk stability and maintains internal abdominal pressure

Table 27.2 Muscles of the Abdomen (Source: Cleveland Clinic, 2021.)

Internal Structures

Internal structures of the abdomen include vital organs and blood vessels that aid in digestion, metabolism, and overall body function (Figure 27.4). The two main blood vessels in the abdomen are the abdominal aorta and inferior vena cava. The abdominal aorta is the main artery that carries blood away from the heart into the circulatory system. The inferior vena cava (IVC) is the body’s largest vein; it carries blood to the heart from the bottom half of the body back to the heart.

Figure 27.4 Internal organs of the abdomen are either solid or hollow, and the structure of each will lead to its overall function. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Each organ in the abdomen has a specialized function, and can be classified as solid or hollow viscera. Solid organs are referred to as solid viscera. The solid organs of the abdomen include the spleen, kidneys, liver, gallbladder, and pancreas (Table 27.3).

Organ	Function
Gallbladder	Aids in fluid balanceFilters and removes damaged or old red blood cells from the bloodstreamProduces white blood cells and antibodies to combat infectionStores bile from the liver and releases to the duodenum to aid in digestionStores some blood to aid the circulatory system in the event of hemorrhagic crisis
Kidneys	Aids in fluid balanceControls the production of red blood cellsProduce active vitamin D to maintain healthy bonesReleases hormones that regulate blood pressureRemoves waste products and drugs from the body
Liver	Filters the blood and breaks down toxinsMetabolizes carbohydrates, fats, and proteinsProduces clotting factors to prevent bleeding and albumin to aid in fluid balanceSecretes bile to aid in digestionStores, produces, and releases vitamins and minerals into the blood
Pancreas	Makes digestive enzymes used to digest food in the small intestineSecretes insulin and glucagon to aid in blood sugar control
Spleen	Aids in fluid balanceFilters and removes damaged or old red blood cells from the bloodstreamMakes digestive enzymes used to digest food in the small intestineProduces white blood cells and antibodies to combat infectionSecretes insulin and glucagon to aid in blood sugar controlStores some blood to aid the circulatory system in the event of hemorrhagic crisis

Table 27.3 Solid Organs of the Abdomen (Source: U.S. Department of Health and Human Services, 2017.)

Organs that are hollow tubes or pouches within the abdominal cavity are the hollow viscera. When injured, the hollow viscera will spill its contents into the abdominal cavity. The hollow organs of the abdomen include the stomach, small intestine, large intestine, and bladder (Table 27.4).

Organ	Function
Stomach	Contracts and relaxes to mix and break down food, then push the food to the small intestineProduces enzymes to digest foodTemporarily stores ingested food
Small intestine	Absorbs nutrients and extracts water from foodBreaks down foodMoves food along the GI tract to the large intestine
Large intestine	Absorbs electrolytes and water from food, turning it into stoolHouses bacteria that further feed on the waste and complete the chemical digestion process
Bladder	Collects urine from the kidneys and stores urine until it is ready to be expelled from the body

Table 27.4 Hollow Organs of the Abdomen (Source: U.S. Department of Health and Human Services, 2017.)

Vascular Structures

In addition to the aorta and the inferior vena cava (IVC), the abdomen contains several other important vascular structures. The three main branches of the aorta supply different organs of the abdomen. These arteries include the:

• celiac trunk: supplies blood to the stomach, spleen, liver, gallbladder and pancreas
• inferior mesenteric artery: supplies blood to the intestines
• superior mesenteric artery: supplies blood to the intestine and pancreas

The branches of the inferior vena cava are referred to as tributaries. Different tributaries drain blood of organs and tissues from those structures back into the IVC. The IVC then carries that blood back to the heart. Important veins of the abdominal cavity that flow into the IVC include:

• gonadal veins: drain reproductive organs of both sexes
• hepatic veins: carry blood away from the liver
• mesenteric veins: carry blood away from the intestines and to the portal veins
• portal vein: carries blood away from the stomach, intestines, and spleen to the liver
• renal veins: carry blood away from the kidneys

Link to Learning

A breakdown of the venous system of the abdominal cavity is provided in this video. It explains and illustrates how the veins are connected to the inferior vena cava and which organs are supported by each venous tributary.

Functions of the Abdomen

The abdomen functions as a cavity to house vital organs of the digestive, endocrine, urinary, reproductive, and circulatory systems. It also aids in functions such as maintaining intra-abdominal pressure (IAP), forced expiration, bladder emptying, vomiting, excretion, pregnancy, and childbirth.

Protect Abdominal Viscera

The organs within the abdomen are protected by multiple layers of tissue and muscle. When external trauma occurs to the abdomen, the layers of fascia, muscle, and fat collectively work to shield and absorb trauma to the abdomen to protect it.

Increase Intra-Abdominal Pressure

The steady pressure within the abdominal cavity that occurs from the interaction of the abdominal wall and viscera is called intra-abdominal pressure (IAP). Because of the enclosed nature of the abdominal cavity, any deviations in the internal volume caused by things such as organ inflammation, obstruction, ascites (fluid), pregnancy, or a tumor can cause the pressure to increase. The abdominal wall can stretch and evenly distribute the pressure to preserve organ function. In some instances, when the abdominal wall cannot properly compensate for the increase in volume, intra-abdominal pressure can increase and cause organ dysfunction.

Forced Expiration

Forced breathing, also known as hyperpnea, is a mode of breathing that can occur during exercise or actions that require the active manipulation of breathing, such as singing. During forced breathing, inspiration and expiration occur due to muscle contractions. In addition to the contraction of the diaphragm and intercostal muscles, other accessory muscles must also contract. During forced inspiration, muscles of the neck, including the scalenes, contract and lift the thoracic wall, increasing lung volume. During forced expiration, accessory muscles of the abdomen, including the obliques, contract, forcing abdominal organs upward against the diaphragm. This helps push the diaphragm further into the thorax, pushing more air out. In addition, accessory muscles (primarily the internal intercostals) help compress the rib cage, which also reduces the volume of the thoracic cavity. During sneezing and coughing, the abdominal muscles contract and the diaphragm relaxes, moving upward into the thoracic cavity, forcing air outside of the lungs.

Link to Learning

The abdomen and its muscles assist the diaphragm during coughing. What occurs during the cough reflex is illustrated in this video.

Bladder Emptying

When the bladder becomes full, the lower abdomen becomes taut and feels full. During urination, the smooth muscles at the base of the bladder relax to allow urine to flow out. To fully empty the bladder, the bladder contracts to fully expel the urine. Abdominal wall contractions assist with bladder compression and emptying.

Vomiting

Vomiting, also known as emesis, is the forceful expulsion of the food contents out of the stomach through the mouth. Vomiting is a protective mechanism to rid the body of harmful substances such as pathogens, irritants, or spoiled food. The abdominal wall muscles assist with vomiting by contracting and increasing intra-abdominal pressure (IAP), encouraging the stomach to contract and force the food up the esophagus and out of the mouth.

Excretion

Part of the digestive system’s job is removing waste from the body, which may be stool or excess gas. When a person passes excess intestinal gas through the rectum, this is called flatulence. Excess gas may be a byproduct from bacterial breakdown of food; foods that produce gas as a natural byproduct, such as complex carbohydrates; or carbonated beverages.

Flatulence is a normal bodily function that results in malodorous gas. The final step of the digestive system’s job is removing waste in the act of defecation. Defecation is the process of eliminating waste from the body through the rectum and anus. Indigestible food, also known as feces, accumulates in the last part of the large intestine (rectum). When the rectum fills, the smooth muscle stretches, signaling to the brain it is time to empty the rectum. When it is time to defecate, the smooth muscle of the anal sphincter relaxes and the rectum contracts, expelling the feces from the body. Abdominal muscles aid in peristalsis (the wavelike movements of the intestines to move contents forward) by contracting to increase intra-abdominal pressure during defecation. For a comprehensive abdominal assessment, the nurse should ask the patient questions related to defecation; for example: What was the consistency of the stool? Was there blood in the stool? Was this the first bowel movement in the past 24 hours?

Life-Stage Context

Constipation in Older Adults

Chronic constipation increases with age, especially after age 60 years. Risk factors for constipation in older adults include weakness of the abdominal and pelvic floor muscles, immobility, malnutrition, chronic medical conditions, aging effects on colonic motility, and chronic medication use. Chronic constipation can cause complications such as hemorrhoids, rectal bleeding, fecal impaction, anal pain, anal fissures, and rectal ulcers. Dietary and lifestyle modifications are the primary interventions in the treatment and prevention of chronic constipation (Mari et al., 2020).

Pregnancy and Childbirth

During pregnancy, the abdominal wall muscles and ligaments stretch to accommodate the growing baby and uterus, keep internal organs in place, and maintain a healthy intra-abdominal pressure. The rectus abdominus muscles are held together by a ligament band called the linea alba, which softens and stretches, separating the two rectus muscles away from each other (National Health Service, 2024). This is called recti divarication (Figure 27.5). This divarication usually resolves in the first few months after delivery.

Figure 27.5 The rectus abdominis muscles spread apart during pregnancy to maintain a healthy intra-abdominal pressure during pregnancy. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Effects of Impaired Function of the Abdomen

Dysfunction of one or more intra-abdominal organs will manifest in different ways. The patient may feel pain, experience bloating (feeling of fullness), have a change in bowel habits, or have abdominal distention. Many different things can cause the abdominal organs to not function correctly. Some common causes of impaired function include:

• Medications: Some medications have known side effects that cause stomach discomfort. Opioids cause constipation. Antibiotics may cause diarrhea. Overuse of antacids and nonsteroidal anti-inflammatory drugs change the amount of acid the stomach produces, which leads to stomach discomfort, reflux, bloating, or diarrhea.
• Poor diet and lack of exercise: A diet low in fiber and lack of exercise can cause constipation, which leads to abdominal distention and bloating. Conversely, a high-fiber diet, or sometimes greasy foods, can cause the patient to experience diarrhea (frequent, loose, watery stool).
• Reflux: Gastroesophageal reflux (GERD) may occur when there is too much acid in the stomach, or the muscle from the esophagus to the stomach is weak. When this occurs, the patient may experience “heartburn,” which is acid that moves out of the stomach back into the esophagus. Patients may experience pain, bloating, or frequent belching.
• Stress: Poorly managed stress in a patient’s life may manifest as stomach pain and discomfort.

Life-Stage Context

Normal Age-Related Changes in the Abdomen

As people age, the abdomen goes through various changes, such as in muscle tone, metabolism, digestive, and ligament or tissue changes (Table 27.5). It is important to encourage older adults to continue to maintain an active lifestyle as well as follow a diet full of fresh fruits, vegetables, and fiber.

Change	Effect on the Body
Digestion changes	Slower gut motility, decreased production of digestive enzymes, slower metabolism, changes in bowel habits
Hormone changes	Especially in older women when there is less estrogen in the body, which causes fat distribution to the body (Ruiz, 2019)
Increased fat accumulation	Older adults accumulate more fat, and this can increase the waist circumference, causing abdominal obesity. This can result from visceral fat accumulation as well.
Muscle tone	Decline in muscle tone, causing the muscles to become weaker and provide less support to the abdominal cavity
Weakening of the abdominal ligaments	The ligaments that support the abdominal organs weaken and may be displaced downward; this can cause hernias or prolapse of pelvic organs.

Table 27.5 Normal Age-Related Changes of the Abdomen

Pain

Abdominal pain is experienced by everyone at some point. Frequent ways people describe abdominal pain include:

• colicky or wavelike: starts and ends suddenly
• cramp-like: most commonly caused by GI disturbances
• generalized: pain felt over more than half of the abdomen
• localized: pain is felt in only one area of the abdomen

Abdominal pain can be caused independently and simultaneously by many different things because multiple organs and structures are contained within the abdomen. Examples include food poisoning, constipation, stress, food allergies, cancer, menstrual cramps, endometriosis, ulcers, appendicitis, pancreatitis, kidney stones, and muscle strain.

Bloating

Bloating is the feeling of fullness and tightness in the abdomen. It is often accompanied by discomfort or a stomach distention. Common causes of bloating include excess gas in the digestive system, overeating, and constipation. Bloating may be temporary or chronic.

Nausea

The sensation of discomfort and unease in the stomach that is often accompanied by the urge to vomit is nausea. It is triggered by multiple stimuli from the nervous system that is meant to be protective. Nausea may or may not be accompanied by vomiting. Common causes of nausea include motion sickness, pregnancy, food poisoning, infection, medications, or other medical conditions.

Loss of Appetite

Loss of appetite is referred to as anorexia. It may be caused by various factors, such as illness, stress, emotional factors, medications, or digestive issues. It is often a symptom of an underlying health issue. Prolonged loss of appetite may lead to unintended weight loss and malnutrition.

Diarrhea or Constipation

Diarrhea is a common digestive problem characterized by frequent, loose, watery stool. Diarrhea may be acute and temporary or due to a chronic condition. It can be caused by infection (bacterial, parasitic, or viral), food poisoning, diet, medications, or GI conditions such as irritable bowel syndrome, Crohn’s disease, and celiac disease. The intestines fail to absorb water and nutrients, which can lead to dehydration due to water loss. Often, a stool sample is needed to determine exact cause or rule out bacterial infections.

The term constipation refers to a condition characterized by uncomfortable, infrequent bowel movements. The stool is often dry, hard, and painful to pass. It may be caused by a lack of dietary fiber, poor water intake, sedentary lifestyle, medications, or other chronic medical conditions.

Patient Conversations

Collecting a Stool Sample from an Embarrassed Patient

Scenario: Nurse walks into the emergency room bay to assess a patient being admitted for chronic diarrhea. The provider has also ordered a stool sample to be collected. The patient seems anxious and guarded, answering questions with one-word answers.

Nurse: Hi, my name is Collette and I’m going to be your nurse today. Would you mind verifying your name and date of birth for me?

Patient: Okay… Lisa Washington, 5/22/92.

Nurse: Nice to meet you, Lisa. I understand you’re here for diarrhea and abdominal pain. I’m going to do an assessment, then I’d like to get a stool sample from you. I have a kit here and I’ll show you how to collect it the next time you go to the bathroom.

Patient: Is that necessary? That sounds really gross, and I really don’t want you to have to touch that.

Nurse: Oh no! I wouldn’t have to touch anything. In this collection kit, you have a plastic collection device to place on the toilet to collect your stool. You then scoop some up with this little spatula, put it in this cup, then screw the lid on. I won’t have to touch anything but the container.

Patient: I guess that’s okay, then. But can you make sure no one sees you leave the room with it in your hand? I don’t want anyone to know that it’s mine.

Nurse: Not a problem, Lisa. I’ll put the container in a collection bag that you can’t see through. No one will know what’s in the bag. So, are you ready for your assessment, now?

Patient: Sure. Thanks so much, Collette.