Chapter 1: Introduction to Anatomy

​ 1.1 Objectives

​ After completing this chapter, you should be able to:

1.2​ Introduction

 ​A healthy body is the product of sound decision making, physical activity, and genetics. But what does it mean to be “healthy”? At this point in your life, you have most likely been bombarded with health information from television, magazines, Google searches, and friends. It’s overwhelming, but increasingly important, to be able to evaluate the validity of the massive amount of medical information you receive on a daily basis. Especially if, eventually, you would like to be the person giving sound advice to others. Understanding your body begins with anatomy (physical structure) and physiology (how things work). Together, these two subjects can provide a foundation for making informed decisions about health, wellness, and the diagnosis and treatment of disease.

Consider that 10% of the United States population is living with a disease known as diabetes, which was characterized more than 2000 years ago as an untreatable and fatal disease of the kidney (since physicians observed that diabetics urinated excessively). Scroll through the timeline below to follow the history of the diagnosis and treatment of this once deadly disease.

​As a result of centuries of work that has only been superficially summarized here, a disease that was once a guaranteed death sentence is now manageable and soon to be cured – all because of anatomy and physiology.

​So, why are you getting a history lesson in your science textbook? Because it’s not uncommon for students learning about anatomy to lose perspective and get lost in the details of the many systems they study. The body is a marvel of complexity. You will inevitably wonder why you are learning this information – but you might find some comfort in knowing that the physicians and scientists who preceded you on this journey of discovery very likely asked themselves the same questions at some point in their career (and occasionally forgot to do their homework, too). Just as others have connected the dots between the renal, circulatory, digestive, and endocrine systems to solve the mystery of diabetes, you might one day apply the things you’re learning about here to do something great. At a minimum, you will learn quite a bit about how the organs and structures of the body relate to health and disease. And truthfully, why wouldn't you want to learn more about one of the greatest and most important gifts you have been given, your body?

1.3​ Structure Dictates Function

​Anatomy and physiology are essential to our ability to decipher the inner workings of the human body. Anatomy and physiology are complementary subjects because structure dictates function. At the cellular level, you will see how a change in the structure of proteins will affect their functions. More than 30 million people live with sickle cell disorder, which gives them more than a 300-fold increase in their risk of stroke in comparison to a healthy individual. Sickle cell disease is the result of a single amino acid substitution! How can an individual know that cells in your gut and kidney are specialized for absorption simply by looking at them under a microscope? How can we predict the action of a muscle just by looking at its attachment to bones? 

Anatomists carefully examine and describe the relationships between different structures and organs within the body. Physiologists study the processes that occur within organs or organ systems. If you are interested in careers such as medicine or nursing, you will most likely integrate anatomy and physiology with other basic sciences (such as biochemistry and pharmacology) in order to diagnose and treat your patients.

1.3.1 Different Approaches to Anatomy

Like many of the names of organs and structures you will learn about in this book, the word "Anatomy" comes form ancient Greek and means "to cut apart." We can cut apart the field of anatomy into two broad methods used to understand the human body: microscopic anatomy and gross (macroscopic) anatomy.

Microscopic anatomy focuses on understanding structures that cannot be seen with the naked eye. Different types of microscopes are used to understand how cells and tissues are shaped and see their internal features. The invention (or discovery) of the microscope in the 17^thcentury enabled scientists to explore structure and function at a microscopic level. Using a microscope, we can explore subjects like histology, which provide a detailed understanding of the fine structures within our bodies (see Figure 1.2).

Gross or macroscopic anatomy focuses on naming and describing relationships between the structures of the body that can be seen with the naked eye. If we were to examine a dissected specimen, we would be see the teeth, esophagus, stomach, small intestine, liver, and pancreas without relying on any kind of magnifying device. If we consider where these different structures are located within the body, we would see that the teeth are located within the oral cavity (mouth), the esophagus passes posterior to the heart through the thorax to deliver food to the stomach in the abdomen, and the liver and pancreas have a series of ducts that deliver bile and pancreatic juice to the small intestine. These organs are supplied with blood by a network of arteries and veins that have a complex branching pattern. Traveling with those vessels is an equally complex web of nervous tissue that carries motor and sensory information to and from the brain. That may sound a little overwhelming but we can break gross anatomy down into the following subfields depending on what aspect of "normal" anatomy we are trying to understand:

• Comparative anatomy - looks at similarities and differences between different species.
  • Example: cats, pigs, and humans all have 4 chambered hearts.
• Systemic anatomy - examines the gross anatomy of each organ system.
  • Example: the structures and organs of the respiratory system include the nose, paranasal sinuses, larynx, trachea, lungs, and thoracic diaphragm.
• Regional anatomy - examines the bones, muscles, organs, vessels, nerves, and other associated structures found in relation to each other in one area of the body.
  • Example: head and neck, thorax, abdomen, pelvis, perineum, back and limbs.
• Surface anatomy - identifies landmarks and structures visible through the skin and relates them to the deeper structures within the body. 
  • Example: the jugular notch and sternal angle allow you to locate the intercostal spaces to ausculatate the heart valves.
• Developmental anatomy - studies how the human body forms and grows from conception to maturity. Embryology falls within developmental anatomy but focuses on a very specific part of development - the formation and growth of the embryo.

We can also study anatomy from a more medical or diagnostic perspective:

• Pathologic anatomy - focuses on anatomical changes that occur due to disease.
  • Example: cancer tumors may change the shape of an organ.
• Radiological anatomy - uses different imaging techniques to visualize normal and pathological anatomy. Imaging techniques include x-ray, ultrasound, computed tomgraphy (CT), and magnetic resonance imaging (MRI).
  • Example:  x-rays can reveal fractures in bones or cavities in teeth, CT scans can indicate cancer tumors, MRI can reveal torn ligaments.
• Surgical anatomy - focuses on how anatomy is altered or corrected using surgical techniques.
  • Example: gastric bypass surgery changes how food moves through the stomach, coronary bypass surgeries use segments of different vessels to restore blood flow to the heart.

Question 1.04

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1.3.2​ Levels of Organization

Here is a video introduction to the levels of organization within the human body

​A key to understanding how the body functions is to fully understand how it is organized from the molecular level, to the cellular level, to the level of tissues, then organs, organ systems, and then the entire body. Examining the assembly of the smallest units (atoms) into coordinated parts (biomolecules), into more complex components (cells, tissues, and organs) (see interactive diagram below) is often a helpful strategy for learning about a new body system. This is because each organism can be reduced to its functional organ systems, which are assembled from individual organs. At the foundation of the organ is a complex arrangement of tissues, which are composed of many cells. These cells are given their unique functions by the biomolecules and atoms found within them. Together, these molecules work to maintain homeostasis in the human body.

Question 1.05

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The assembly of smaller components to form a complex organism results in an independent human being.​ Interact with the diagram below to see how organisms exist within life's grand biosphere. 

1.3.3 Characteristics of Life

You are alive, your computer isn’t (not yet, anyways!). Have you ever considered what it is about living organisms that makes them different from non-living things? Here are some general characteristics:

• ​Energy production and consumption. Living things must eat. This brings nutrients into the body that can be used to produce energy. The energetic currency of the human body, our primary energy source, is ATP (adenosine triphosphate). Humans are heterotrophs; we don't make our own energy and must eat to survive. ATP can be generated from the breakdown of carbohydrates, fats, or proteins. It is used to drive numerous cellular processes that require energy input. As you are reading this, ATP is being consumed in the retina of your eye in order to give you the ability to detect light and distinguish colors. As you scroll with your mouse, the skeletal muscles in your arms and fingers are consuming ATP in order to produce movement. In nearly every chapter of this text, you will encounter examples of how the cells that form a particular tissue “spend” their ATP allowance.​
• Growth/repair. Consider the many differences between an infant and yourself, or between you and your parents and grandparents as proof that the human body changes over time. Cells, tissues, and organs form, grow, and age over time. During our long lives, injury is inevitable and the ability of a tissue to repair itself is essential.
• ​Adaptation. ​Humans respond and adapt to changes in our environment. These changes can be reversible or permanent. For example, hikers ascending Mount Everest may experience difficulty breathing due to the reduced O₂ levels at this very high altitude. Over time, however, stem cells in the bone marrow produce more red blood cells (erythrocytes), which help maintain O₂ flow to the tissues. Upon returning to their native elevation, the level of red blood cells in the hikers will return to normal. A more permanent change occurs in response to exposure to the carcinogens found in cigarette smoke, which damage lung tissues and alter their structure. In this case, breathing will remain challenging even if the smoker is able to eventually quit as happens in emphysema.
• ​Reproduction. Wouldn’t it be great if the money in your checking account could reproduce? Unfortunately, only living creatures have the ability to reproduce. Although individual organisms do not need to reproduce to survive, reproduction is required for the survival of the species.

In summary, humans and other living organisms are capable of sustaining life because of their innate structural organization, their ability to use energy to do work, interact with their environment, and their capability to grow, repair, and reproduce.

1.3.4​ Cells as the Living Unit of Life

The smallest functional unit of life, possessing all of the qualities essential to sustain life, is the cell. Cells are the building blocks of tissues and contain all the necessary machinery to sustain homeostasis through metabolism and maintenance/repair (the next chapter is dedicated to Cell Structure and Function). All individual cells comprise at least three shared components: a membrane surrounding genetic material that floats in cytosolic fluid (Figure 1.3). In human cells, these three components are joined by several additional membrane-bound organelles whose individual functions establish a higher level of cellular complexity (further discussed in the next chapter). Human cells rely on the complex steps of gene expression to become specialized. The multiple steps of gene expression is carried in the nucleus, cytosol, endoplasmic reticulum, and Golgi apparatus, and it results in production of the specific proteins required for a particular cell type to function. This specialization, better known as cell differentiation, is essential for the formation of many different types of cells. How this specialization occurs is of interest to biomedical researchers today. How does a muscle cell know to become a muscle cell, and a nerve cell to become a nerve cell? Deconstructing how the cues and signals that a cell receives are translated into the instructions for differentiation allows us to understand more complicated processes like development, tissue repair, and immunity.

1.3.5 Tissues

In the human body, there are approximately 200 different cell types that can be divided into 4 major categories of tissue according to their shared functions:

• Epithelial tissue
• Muscle tissue
• Nerve tissue
• Connective tissue

Within each category, the cells are further grouped by variations in function. For example, epithelial cells come in various shapes and arrange in single or multiple layers. These arrangements enable epithelial cells in various organs to carry out diverse roles in protection, secretion, and absorption.

​Tissues assemble from combinations of these four common cell types working together to serve a shared function. Tissues are the intermediate level of structural and functional organization between the cell and organ.  The coordination and assembly of common cells through cell junctions, cell adhesion molecules, and cell attachment to the extracellular matrix strengthens the integrity of the tissues. Proteins such as collagen and elastin localized within the extracellular matrix and junctional complexes act as tethering ropes to align and bind cells.

​
1.3.5.A  Epithelium

​Epithelial tissues can be found lining the walls of open tubes and they provide a secretory (outward) and/or absorptive (inward) surface. They are also found covering exposed surfaces as a source of protection.  The structure of an epithelial cell serves to create two important surfaces that are organized by the arrangement of junctional proteins to create segregated regions of the plasma membrane. The two surfaces are known as the apical and the basolateral surfaces. The apical surface faces the lumen of the organ it is lining and the basolateral surface is tethered to the extracellular matrix. This polarization of the two cellular surfaces is translated to the tissue level and creates an environment for movement of nutrients between the apical and basolateral surfaces. The basolateral surface is part of the underlying connective tissue layer, which contains a connection to the vasculature. The movement of ions, nutrients, and gases between these two spaces is critical for the homeostasis of body systems. Consider the example of absorption of nutrients from the partially digested stomach contents in the small intestinal lining. The apical surface of the intestinal lining contains carriers that enable nutrients and ions to be transported from the digestive system into the bloodstream.

1.3.5.B ​ Muscle

​Without the attachment of muscles to bone or the layering of smooth muscle in the digestive tract, essential processes like breathing and swallowing would not be possible. The assembly of contractile proteins within muscle cells makes them ideally suited for the generation of mechanical force. There are three types of muscle tissue in the human body:

• Skeletal muscle
• Smooth muscle
• Cardiac muscle

Skeletal muscle, which is voluntary , allows us to choose when and how to move. Smooth muscle, which is involuntary, is not directly controlled by conscious thoughts. The constriction and dilation of arteries is controlled by layers of smooth muscle in the artery walls. The wall of the heart is the only organ in the body composed of cardiac muscle. While in structural appearance, cardiac muscle looks similar to skeletal muscle, it is more specialized and under involuntary control.

1.3.5.C  ​Nervous Tissue

Nervous tissue comprises glia and neurons. Glial cells provide protection, support, and nourishment to the cells within the nervous system. Nerves provide the tracks for long distance communication within the body, and are made up of neurons, which transmit electrical signals from the nervous system to organs. In doing so, nerves provide control over many voluntary and involuntary processes in the body including muscle contraction and enzyme secretion.

1.3.5.D      Connective Tissue

 Connective tissues  are a collection of cells and proteins that provide support and  integrity to other tissues and structures in the body. Connective  tissues in the body differ in the arrangement of their cells, protein  fibers, and ground substance.  The ground substance and protein fibers  combine to form an  extracellular matrix  that contributes to the connective tissue's  tensile strength and defines the function for a given connective tissue.  Some connective tissues are less ordered, such as hyaline cartilage,  which is well suited for flexible structures like the earlobe. Other  connective tissues have rope-like structures with densely packed fibers  and cells, making this anatomical arrangement ideal for connecting bone  to bone, such as with a ligament. 

1.3.6​ Organs and Systems

The idea of “growing” a brain, heart, or kidney in a laboratory has been explored by many movies, and highlights a question that has given scientists many sleepless nights! If we know the cells and tissues that comprise an organ, then why can’t we create organs? The answer is complicated. It turns out that our bodies are more than just the sum of their parts. If you think of the human body as a jigsaw puzzle, then the easiest answer is that we have most of the pieces, but are still working out the details on how they fit together. Until we can make more connections between the individual pieces, the bigger picture is still a bit fuzzy (especially because, unlike a jigsaw puzzle, we are three dimensional!). Although this text won’t discuss how organs develop in detail, we will spend some time looking at the structure of the finished products. Each organ in the human body is assembled from tissues that are bundled together and folded into a three-dimensional shape (most are tube-like). Examining the tissues that form an organ will often reveal clues about its function within the body.

Similarly, organs work together in interdependent systems that serve a larger function for the body. Although we will discuss each in greater detail later in the book, the major systems are summarized in the table below:

​

There are eleven major organ systems in the human body, and they function independently to carry out interdependent processes. This means that that while each system carries out a particular function, eventually all systems intersect with each other in some way. As an example, let’s see if we can connect two systems that might appear to be very disjointed – the circulatory system and the musculoskeletal systems. The circulatory system contains the heart and blood vessels. The heart pumps to create pressure that pushes blood into arteries, capillaries, and eventually to our tissues. However, as blood travels further from the heart it is stripped of nutrients and O₂. In order to be replenished, it must be returned to the heart, but the pressure in the return vessels, the veins, is very low, making it difficult to move blood through. Although there are adaptations in veins that help to overcome this problem, this process is greatly helped by skeletal muscle. Because veins are situated between skeletal muscles, they get a pressure boost when skeletal muscles contract during normal movement and “squeeze” them.

Question 1.15

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​The interdependency of body systems also has a significant impact on the diagnosis and treatment of disease. For example, observing someone with asthma might lead you to the conclusion that asthma is a breathing disorder that affects the lungs in the respiratory system. However, this complicated disorder also involves the nervous, immune, and muscular systems as well. In actuality, for individuals that suffer from asthma, physical and environmental factors such as exercise, pollen, chemical fumes, dust, and individualized allergens can trigger constriction of the smooth muscle surrounding small airways and increase mucus production. This causes the airways to narrow and leads to difficulties with breathing, coughing, wheezing, and shortness of breath that characterize the disease. In order to manage these symptoms during an attack, asthma sufferers often carry an inhaler filled with a medication known as albuterol, which binds to receptors used by the nervous system to relax smooth muscle and dilate the airways. In order to prevent the attacks from happening in the long term, they might take medication to desensitize or suppress their immune response to these benign environmental factors.

Often, exploring disease and clinical situations for disorders like asthma enables you to dive deeper into the fascinating world of anatomy and physiology by reinforcing the connections between organ systems, which is why you can expect to hear about some in every chapter of this book!

1.4​ Mapping the Human Body

As you know, our bodies have many parts – many of which you will learn about in this book. Locating and describing them is often going to be the first step. But this might not always be as straightforward as it appears. Here’s an example: where is your appendix? Telling someone that your appendix is in the abdomen gives them about as much information as if you told them that you live in North America. It’s helpful, but not nearly as precise as it could be – do you live in Boston? Mexico City? Toronto? Do you live in the north end? The suburbs? We have the same problem when describing anatomical structures. Even though we have bilateral symmetry, there are many asymmetrical features on the inside. The appendix, for example, is located on the right side of the body, beneath the skin, belly fat, and abdominal muscles, at the intersection of the small intestine and large intestine. If it becomes inflamed, as in appendicitis, is the inflammation on the outside or the inside? Is it on the region attached to the intestine or the free edge? Hopefully, this example demonstrates the need for precise terminology!​

​This is particularly true when making comparisons between two or more individuals. We study generalized anatomy, but all of us have anatomical variations that make us unique – some have additional structures, e.g., a bone, blood vessel, or tooth, and others have variation in the shape of the structure, e.g., the orientation of your ear canal. In these circumstances, it is essential that there be a standard of reference for comparison. This standard of reference is anatomical position. Anatomical position (Figure 1.5) refers to a pose where the body is standing upright, feet together, with eyes facing forward, arms by the side, and palms facing forward. Imagining (and studying) the body in this position gives us a reference point that we can use to describe the locations of structures regardless of what position the body is actually in. Although you may see patients that are curled up on their sides in pain or laying face down on an exam table, you always have to imagine them in anatomical position and use anatomical position when writing your notes.

The terms "left" and "right"are also used a bit differently when describing the human body in anatomy. Left and right are always used in relation to the patient's perspective. The patient's left hand will be to your right if you are facing them. The same is true of how the body and organs are illustrated in anatomical drawings. In an anterior drawing, the left side will appear on the right side of the page. With practice, you will get used to flipping left and right in your head, but sometimes it is useful to imagine yourself in the patient's or anatomical drawings place in order to orient yourself.

The figure below also introduces some terminology used to describe the various body regions. If it sounds like Greek to you, in many cases it is! Putting some effort into memorizing these terms that we will be using throughout the text can save you some time later on.

1.4.1​ Body Cavities

​What do your stomach, intestine, and liver have in common with respect to their location? Your heart and lungs? They are located within the same body cavities. The body is divided into two general spaces: anterior (ventral) body cavity and posterior (dorsal) body cavity. The anterior body cavity is further subdivided in the thoracic cavity and the abdominopelvic cavity by a flat dome-shaped muscle called the diaphragm. Many of the body cavities can be further subdivied into smaller spaces that contain specific organs.

The thoracic cavity is divided into three compartments, two lateral spaces and one central space. The two lateral spaces contain the lungs and are called the pleural cavities. The central space is called the mediastinum. The mediastinum extends from the base of the neck to the diaphragm between the left and right lungs. The pericardial cavity ,which contains the heart, is located within the mediastinum. The mediastinum also contains the esophagus, trachea, thymus gland, major blood vessels, nerves, and lymphatic structures. 

The abdominopelvic cavity is arbitrarily divided into the abdominal cavity and the pelvic cavity using a bony landmark called the pelvic brim. However, there is no physical separation between the abdomen and the pelvis so the small intestine can drape down into the pelvis and the uterus and bladder can move up into the abdomen when enlarged. Almost all of the subdivisions of the ventral body cavity (with the exception of part of the pelvis) contain serous membranes. 

Serous membranes prevent friction between mobile organs (such as the heart and the stomach) and the walls of the container that holds them. Serous membranes are sometimes called serous sacs because they are one continuous layer that will line both the walls of the body and cover (or invest) organs. 

Imagine blowing up a balloon and then setting it into a large salad bowl. The bowl represents the walls of the body. The balloon is the serous sac. Now imagine pushing your fist into the balloon until you can almost feel the bottom of the bowl. Your fist represents an organ. The part of the balloon that is touching the bowl walls is the parietal layer of the serous membrane. The parietal layer of the serous membrane will line the body wall. The part of the balloon that is touching your fist is the visceral layer of serous membrane. The visceral layer of the serous membrane will cover an organ or organs. The air inside of the balloon represents the serous cavity. A serous cavity is a potential space between the parietal and visceral layers of the serous membranes. In real life, serous membranes take on much more complicated shapes than our simple balloon in a bowl but this is a good analogy to make sense of serous membranes at first. Serous membrane is given specific names for the organs it is associated with: 

• Pericardium - serous membrane of the heart
• Pleura - serous membrane of the lungs
• Peritoneum - serous membrane of many of the digestive organs

 Serous membranes prevent friction by covering organs and lining the spaces that contain organs.  a)  Serous membranes are continuous sheets which organs push into similar to how the fist is pushing into the balloon pictured above.  The part of the balloon that the hand (representing the organ) touches is the visceral layer while the outer part of the balloon represents the parietal layer of the serous membrane.  The air inside the balloon represents the cavity between the parietal and visceral layers of serous membrane. b​​) The pericardium is the serous membrane lining the pericardial cavity and covering the heart. c) The left and right pleural cavities are each lined by their own serous membrane called pleura.  d)  The serous membrane of the abdomen and pelvis is called peritoneum.  Peritoneum will line the abdomen and part of the pelvis and will cover some of the organs of the abdomen and pelvis.

The posterior body cavity is subdivided into the cranial cavity and the spinal cavity (or vertebral canal). The cranial cavity is located inside the skull and contains the brain. The spinal cavity is located within the vertebral column and contains the spinal cord. Unlike the subdivisions of the anterior body cavity, the dorsal body cavity does not have serous membranes. Instead, the brain and spinal cord are surrounded by cerebrospinal fluid and layers of meninges. 

Welcome to your first interactive diagram! By hovering your cursor over the diagram below, you should see multiple blue hotspots appear. You can interact with each spot by hovering over it. Sometimes you will uncover additional text to read, sometimes an additional image, and in some cases you will see a video to watch. In this example, the hotspots will help you identify and remember the cavities and sub-cavities of the human body. 

1.4.2​ Terms of Relative Position

​Further precision is often needed when clinicians are describing where a problem is located so that an adequate diagnosis and treatment can be defined. For example, when an orthopedist is describing where on a bone there might be a fracture, it is more specific to say the fracture is located on the distal (far) third of the radius bone. Table 1.2 lists the most common terms that describe one location on the human body with respect to another and Figure 1.7 exemplifies several of these directional terms on the human body in anatomical position.

Directional  terms can seem a little confusing sometimes.  Some terms have very  similar meanings or may be used interchangeably.  For example,  anterior/ventral and posterior/dorsal are often used interchangeably to name the same  structures.  When discussing structures of the nervous system, you will see structures labeled as the anterior root or the ventral root.  Both names refer to the exact same structure.   It is important to recognize which  anatomical terms have synonyms as these are very common in anatomy.

Another  frustrating aspect of directional terms is that some terms are  considered to be more appropriate to describe certain regions of the body  than others.  A great example of this is the use of proximal/distal  versus superior/inferior.  Most anatomists and health care professionals  will use proximal and distal to describe the relationship of structures  located on the same limbs (or extremities).   The terms superior/inferior are usually used to describe the  relationship of structures located on the head,  neck, and trunk.  For  example, anatomists would say the the knee is proximal to the ankle and  the chest is superior to the abdomen.  

Now you might be asking  yourself, "why can't I use superior and inferior to describe the  relationship of the knee and the ankle? Clearly, the knee is superior to  the ankle." The answer lies in comparative anatomy.  Most of our directional terms were developed to describe relationships between different organisms with different body plans (such as comparing a starfish to a human being).  While superior/inferior works to describe relationships for bipedal organisms, those terms can't be used to describe the same relationships in quadrupeds or animals with other body plans.  Human anatomy has maintained the convention of using proximal and distal to describe the relationships of structures located on the same limb.   

1.4.3​ Planes of Section and Compartmentalization

​Remember how we said earlier that humans have bilateral symmetry? If we wanted to prove that, we would have to identify the axis, or line, that we could use to cut the body into equal halves. This is known as a midsagittal anatomical section. Sectioning the body along various planes allows us to examine the internal arrangement of our anatomical parts. In other words, do you want your onions sliced or diced? Some common sections are shown in the figure below:

​The transverse section specifies a cut that divides the body into upper/superior and lower/inferior sections like a belt. Transverse sections run perpendicular to the long axis of the body (or structure). We can draw transverse planes anywhere from the top of the head all the way down to the feet. Some transverse planes are given specific names including:

• Transverse thoracic plane at the T4/T5 disc space
• Subcostal plane just inferior to rib 10
• Transumbilical plane through the umbilicus
• Supracristal plane- across the top of the hip bones

The sagittal section is a lengthwise cut that divides the body into right and left sections at the midline or lateral to the midline. Sagittal planes run parallel to the long axis of the body. The plane that divides the body into equal right and left halves can be called the midsagittal or median plane. Sagittal planes that are not at midline can also be referred to as parasagittal planes. A few parasagittal planes have specific names, including:

• Midclavicular line (plane) - through the midpoint of the clavicle
• Midinguinal line (plane) - through the midpoint of the inguinal ligament

 Finally, a coronal section also run parallel to the long axis of the body but divides the body into anterior and posterior portions. Because the human body is not symmetrical from anterior to posterior, coronal (or frontal) planes do not create equal halves.  

In addition to the three standard planes, there are also oblique planes. Oblique planes are planes that pass through the body at an angle.  

Although it is possible to prepare anatomical specimens cut in the planes listed above, these planes probably play a more important role in radiological anatomy and surface anatomy. MRIs and CT scans are be taken in transverse, frontal, sagittal, or oblique planes depending on the reason for conducting the scan. Planes are also used as part of surface anatomy to divide the body into quadrants or regions. The abdomen is divided into four quadrants using the median plane and the transumbilical plane (Figure 1.9). The abdomen and pelvis can also be divided into nine regions using the left and right midclavicular planes, the subcostal plane, and the supracristal plane (Figure 1.9).

1.4.3.A  Quadrants

• Right upper quadrant
• Left upper quadrant
• Right lower quadrant
• Left lower quadrant 
  

1.4.3.B ​​Regions

•  Epigastric region
• Left and right hypochondriac regions
• Umbilical region
• Left and right lumbar regions
• Hypogastric (pubic) region
• Left and right iliac (inguinal) regions 
  

The four quadrants are useful for describing the normal location of organs within the abdomen and pelvis. Some organs, such as the spleen, kidneys, gallbladder, and appendix are contained by only one quadrant. Other, larger organs such as the liver, stomach, and intestines occupy two or more quadrants. During a physical exam, medical professionals may palpate the abdomen to feel the position and size of organs within each quadrant. Patients may also be able to localize pain to a particular quadrant which can help a medical professional narrow down the possibilities of which organ may be causing the pain. 

The nine abdominopelvic regions are smaller than the four quadrants and most do not contain whole organs. The nine abdominoplevic regions can be used to describe where the phenomenon of referred pain from organs may be perceived by patients. Pain concentrated in one of the three central regions (epigastric, umbilical, and hypogastric) is often related to the gastrointestinal tract organs (stomach, intestines) while pain perceived in the lateral regions (hypochondriac, lumbar, and iliac) is often related to accessory organs like the liver and gallbladder, or organs from the urinary system (kidneys and ureters). 

Question 1.28

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Hover here to see the answer to Thought Question 1.01.

A 25-year-old female patient comes to urgent care complaining of abdominal pain, fever, and nausea. She tells the medical professional that earlier in the day she had felt a dull pain near her belly button but now the pain has moved lower and to the right.  She can point to a specific spot on her abdomen to indicate where the pain is localized.

Question 1.31

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​Hover here to see the answer to Thought Question 1.02.

Question 1.33

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Question 1.34

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1.5 In-Chapter Feedback​

1.6 Answers to Discussion Questions

​​Answer to Question 1.04

The ability to effectively 'see' smaller and smaller structures within the body has allowed us to understand physiological function as a consequence of anatomical structures.

Click here to return to Question 1.04

​​​Answer to Question 1.05

Expression of different proteins (from genes) allows cells to specialize for one task versus another. Since an organ is made from many diverse cell types, it's possible for it to have more than one function.

Click here to return to Question 1.05

​Answer to Question 1.15

Blood. The kidney filters the blood and produces urine, which is the waste from many metabolic processes. The filtered blood returns to circulation and the urine waste is expelled from the body.

Click here to return to Question 1.15

​​Answer to Question 1.28

Using the head as an example, the anterior section would contain structures such as the nose, lips, eyes, and frontal lobe of the cerebral cortex. The posterior section would contain the cerebellum, brain stem, occipital bones, etc.

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​​Answer to Question 1.31

The patient described first feeling pain in the umbilical region of the nine abdominopelvic regions.  Pain in one of the three central regions (epigastric, umbilical, and hypogastric) is often related to the gastrointestinal tract organs.  As her illness progressed, she indicates that the pain moved to the right lower quadrant.  She became able to localize the pain (point to a specific spot where it hurt).  The right lower quadrant contains the appendix, cecum, ureter, uterine tube in females, right ovary, and parts of the small intestine.  Her symptoms are consistent with appendicitis although ectopic pregnancy would also need to be ruled out.

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Answer to Question 1.33

Liver, gallbladder, right kidney, stomach, intestines.

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Answer to Question 1.34

Stomach, duodenum, pancreas.

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1.7 Image Citations

[1] Image courtesy of Wellcome Images under CC BY 4.0.

[2] Image courtesy of ​Darryl Leja, NHGRI under CC BY 2.0.

[3] Image courtesy of Harold E. Dougherty in the Public Domain.

[4] Image courtesy of Arielinson under CC BY-SA 4.0.

[5] Image courtesy of Erik1980 under CC BY-SA 3.0.