and the spinal cavity. Technically speaking there is no anatomical division as the cranial cavity flows directly into the spinal cavity. However, they are separated into two distinct cavities to assist in study and analysis.
Figure 1-4 Muscle, nerve, and blood vessel connections.
The cranial cavity is a fluid-filled spaced inside the skull occupied by the brain. The spinal cavity encompasses the spinal cord, which travels down the posterior aspect of the body and is continuous with the cranial cavity (the spinal cord is attached directly to the brainstem), extending down toward the base of the spine.
The ventral cavity also includes two sub-cavities, the thoracic cavity and the abdominopelvic cavity. These two cavities are separated anatomically by the diaphragm, with the thoracic cavity residing above the abdominopelvic cavity.
The thoracic cavity holds the heart, lungs, thymus, lower one-third of the esophagus, and an irregularly shaped, central compartment called the mediastinum.
The abdominopelvic cavity is further subdivided in some texts into the abdominal cavity and the pelvic cavity, with the caveat that there is no real anatomical partition separating the two cavities.
The abdominal portion contains both the main and accessory organs of digestion (stomach, small and large intestines, liver, pancreas, and gallbladder), as well as the kidneys and ureters. The pelvic portion is surrounded and guarded by the pelvic girdle, and encloses mainly the reproductive organs and the bladder.
Serous membranes
At this point, discussion of serous membranes, also called serosa, of the ventral cavity is appropriate. A membrane is a thin, flexible lining of tissues that secretes a lubricating fluid. Serous membranes form the lining that insulates each body cavity and all internal organs.8 The membrane itself is composed of two types of tissue – a superficial epithelial layer, called the mesothelium, and a deeper layer composed of connective tissue.
The epithelial layer is a single layer of simple squamous epithelial cells that have no blood supply. This layer is responsible for secreting the lubricating serous fluid. The deeper, second layer is composed of connective tissue, appropriately named the connective tissue layer, which provides vascular and nerve supply to the epithelial layer. The connective tissue layer also functions to bind the membrane to organs and cavities.
When a serous membrane covers and lines an organ, it is generally called a visceral membrane. When it covers and lines a body cavity, it is generally called a parietal membrane.
Note: Serous fluid is secreted and lubricates the space that exists between these two membranes.
Specific names are given to serous membranes in the ventral cavity when the membrane covers and lines a particular organ or cavity. For example, the serous membrane that coats the thoracic cavity is called the parietal pleura. Parietal distinguishes that it is referring to a cavity, and pleura distinguishes that it is referring to the thoracic cavity. Moreover, the serous membrane that lines the surface of each lung, which of course is an organ, is called the visceral pleura.
The term visceral distinguishes that it is referring to a particular organ – the lung – the term pleura again distinguishing that it remains located in the thoracic cavity. The area flanked by these two cavities, called the pleural space, is lubricated with serous fluid to protect the linings from the friction caused by the expansion and contraction of the lungs during breathing. This is similar to engine oil lubricating the cylinder within which a piston pumps up and down during internal combustion of a gasoline engine.
Another specific serous membrane is the lining or covering of the heart, called the visceral pericardium. The serous membrane that coats the pericardial cavity (the area that encases the heart) is called the parietal pericardium. (Remember that the term visceral defines the membrane that is covering an internal organ, and the term parietal means that the membrane is lining a cavity.)
In an effort to reduce the friction caused by the heart pumping inside its cavity, there exists a space joining these two membranes where another serous fluid is secreted.
The abdominal cavity is lined with a serous membrane called the peritoneum. The membrane overlaying each abdominal visceral organ is called a visceral peritoneum, whereas the parietal peritoneum is the membrane that insulates the entire abdominal cavity. The region conjoining these two membranes is called the peritoneal cavity, which too, is lined with serous fluid for lubrication purposes.
Figure 1-5 Circulation overview.
■Organ Systems Overview
The human body is made up of several organ systems that function together as one complex unit. Traditionally, the body is divided and studied in eleven different organ systems. Below is an overview of those organ systems, each with the respective structures involved and a brief description of the functions they perform.
Circulatory system
The body’s circulatory system, sometimes called the cardiovascular system (in an effort to distinguish it from some texts who include the lymphatic system as part of the circulatory system), is formed by the heart, blood, and blood vessels.
For our purposes, the anatomy and physiology of the body’s lymphatic system will be discussed separately in a later chapter.
The cardiovascular system’s main function is to pump blood, delivering oxygen and essential nutrients to all living cells via the channels of blood vessels throughout the body.
The heart’s cardiac output, or volume of blood that is pumped per minute, is an excellent way of measuring how well the heart is functioning.
The three main types of blood vessels, which are discussed in greater detail later, are the arteries, veins, and capillaries. The main component of the human body’s circulatory system, the cardiovascular system, is a closed organization, meaning that the blood never leaves the network of blood vessels. Instead, nutrients, gases, and hormones diffuse across the membrane of the cells in the capillaries and flow into the interstitial fluid (essentially lubricates cellular structures, filled with components such as amino and fatty acids, sugars, and regulatory substances), where they are passed along to target tissues and ultimately the target cells.
Figure 1-6 Digestive organs.
Digestive system
The digestive system, or gastrointestinal system as it’s sometimes called, is formed by the major organs of digestion – the stomach, small and large intestines, and rectum plus the accessory organs of digestion – the teeth, salivary glands, liver, gallbladder, and pancreas. Each of these organs will be described in greater detail in Chapter 10: The Digestive System. Also included are the mouth (oral cavity), esophagus, and anus. Together, these structures function to digest food and excrete waste.
Digestion is the mechanical and chemical breakdown, or catabolism, of food into smaller macronutrients so that they are more readily absorbed.
Mechanical digestion is initially performed by the teeth through chewing. Chemical digestion is initially started by the secretion of saliva, which contains various enzymes (such as amylase)
