Unit Six. Animal Life
22. The Animal Body and How It Moves
22.2. Organization of the Vertebrate Body
All vertebrates have the same general architecture: a long internal tube that extends from mouth to anus that is suspended within an internal body cavity called the coelom. The coelom of many terrestrial vertebrates is divided into two parts: the thoracic cavity, which contains the heart and lungs, and the abdominal cavity, which contains the stomach, intestines, and liver. The vertebrate body is supported by an internal scaffold, or skeleton, made up of jointed bones. A bony skull surrounds and protects the brain, while a column of bones, the vertebrae, surrounds the spinal cord.
Like all animals, the vertebrate body is composed of cells—over 10 to 100 trillion of them in your body. It’s difficult to picture how large this number actually is. A line of 10 trillion cars would stretch from the earth to the sun and back 50 million times! Not all of these cells in your body are the same, of course. If they were, we would not be bodies but amorphous blobs. Vertebrate bodies contain over 100 different kinds of cells.
Groups of cells of the same type are organized within the body into tissues, which are the structural and functional units of the vertebrate body. A tissue is a group of cells of the same type that performs a particular function in the body.
Tissues form as the vertebrate body develops. Early in development, the growing mass of cells that will become a mature animal differentiates into three fundamental layers of cells: endoderm, mesoderm, and ectoderm. These three kinds of embryonic cell layers, in turn, differentiate into the more than 100 different kinds of cells in the adult body.
It is possible to assemble many different kinds of tissue from 100 cell types, but biologists have traditionally grouped adult tissues into four general classes: epithelial, connective, muscle, and nerve tissue. The bird pictured in figure 22.1 contains all four classes of tissues, and as you can see in the circled enlargements, each class of tissue contains different types of cells. Of these, connective tissues (indicated by the light green arrows) are particularly diverse.
Figure 22.1. Vertebrate tissue types.
The four basic classes of tissue are epithelial, nerve, connective, and muscle.
Organs are body structures composed of several different tissues grouped together into a larger structural and functional unit, just as a factory is a group of people with different jobs who work together to make something. The heart is an organ. It contains cardiac muscle tissue wrapped in connective tissue and joined to many nerves. All of these tissues work together to pump blood through the body: The cardiac muscles contract, which squeezes the heart to push the blood; the connective tissues act as a bag to hold the heart in the proper shape and ensure that the different chambers of the heart squeeze in the proper order; and the nerves control the rate at which the heart beats. No single tissue can do the job of the heart, any more than one piston can do the job of an automobile engine.
You are probably familiar with many of the major organs of a vertebrate body. Lungs are organs that terrestrial vertebrates use to extract oxygen from the air. Fish use gills to accomplish the same task from water. The stomach is an organ that digests food, and the liver an organ that controls the level of sugar and other chemicals in the blood. Organs are the machines of the vertebrate body, each built from several different tissues and each doing a particular job. How many others can you name?
An organ system is a group of organs that work together to carry out an important function. For example, the vertebrate digestive system is an organ system composed of individual organs that break up food (beaks or teeth), pass the food to the stomach (esophagus), break down the food (stomach and intestine), absorb the food (intestine), and expel the solid residue (rectum). If all of these organs do their job right, the body obtains energy and necessary building materials from food. The digestive system is a particularly complex organ system with many different organs consisting of many different types of cells, all working together to carry out a complex function. The circulatory system illustrated in figure 22.2 involves fewer different types of organs, but the level of organization is the same—organ systems are made up of organs, that are made up of tissues, that are made up of cells.
Figure 22.2. Levels of organization within the vertebrate body.
Similar cell types operate together and form tissues. Tissues functioning together form organs. Several organs working together to carry out a function for the body are called an organ system. The circulatory system is an example of an organ system.
The vertebrate body contains 11 principal organ systems:
1. Skeletal. Perhaps the most distinguishing feature of the vertebrate body is its bony internal skeleton. The skeletal system protects the body and provides support for locomotion and movement. Its principal components are bones, skull, cartilage, and ligaments. Like arthropods, vertebrates have jointed appendages— the arms, hands, legs, and feet.
2. Circulatory. The circulatory system transports oxygen, nutrients, and chemical signals to the cells of the body and removes carbon dioxide, chemical wastes, and water. Its principal components are the heart, blood vessels, and blood.
3. Endocrine. The endocrine system coordinates and integrates the activities of the body through the release of hormones. Its principal components are the pituitary, adrenal, thyroid, and other ductless glands.
4. Nervous. The activities of the body are coordinated by the nervous system. Its principal components are the nerves, sense organs, brain, and spinal cord.
5. Respiratory. The respiratory system captures oxygen and exchanges gases and is composed of the lungs, trachea, and other air passageways.
6. Immune and lymphatic. The immune system removes foreign bodies from the bloodstream using special cells, such as lymphocytes, macrophages, and antibodies. The lymphatic system provides vessels that transport extracellular fluid and fats to the circulatory system but also provides sites (lymph nodes and thymus, tonsils, and spleen) for the storage of immune cells.
7. Digestive. The digestive system captures soluble nutrients from ingested food. Its principal components are the mouth, esophagus, stomach, intestines, liver, and pancreas.
8. Urinary. The urinary system removes metabolic wastes from the bloodstream. Its principal components are the kidneys, bladder, and associated ducts.
9. Muscular. The muscular system produces movement, both within the body and of its limbs. Its principal components are skeletal muscle, cardiac muscle, and smooth muscle.
10. Reproductive. The reproductive system carries out reproduction. Its principal components are the testes in males, ovaries in females, and associated reproductive structures.
11. Integumentary. The integumentary system covers and protects the body. Its principal components are the skin, hair, nails, and sweat glands.
Key Learning Outcome 22.2. Groups of cells of the same type are organized in the vertebrate body into tissues. Organs are body structures composed of several different tissues. An organ system is a group of organs that work together to carry out an important function.