Before you begin to study the different structures and functions of the human body, it is helpful to consider its basic architecture; that is, how its smallest parts are assembled into larger structures. It is convenient to consider the structures of the body in terms of fundamental levels of organization that increase in complexity: subatomic particles, atoms, molecules, organelles, cells, tissues, organs, organ systems, organisms and biosphere (Figure 1).
Figure 1. Levels of Structural Organization of the Human Body. The organization of the body often is discussed in terms of six distinct levels of increasing complexity, from the smallest chemical building blocks to a unique human organism.
The Chemical level
To study the chemical level of organization, scientists consider the simplest building blocks of matter: subatomic particles, atoms and molecules. All matter in the universe is composed of one or more unique pure substances called elements, familiar examples of which are hydrogen, oxygen, carbon, nitrogen, calcium, and iron. The smallest unit of any of these pure substances (elements) is an atom. Atoms are made up of subatomic particles such as the proton, electron and neutron. Two or more atoms combine to form a molecule, such as the water molecules, proteins, and sugars found in living things. Molecules are the chemical building blocks of all body structures.
Cells
A cell is the smallest independently functioning unit of a living organism. Even bacteria, which are extremely small, independently-living organisms, have a cellular structure. Each bacterium is a single cell. All living structures of human anatomy contain cells, and almost all functions of human physiology are performed in cells or are initiated by cells.
A human cell typically consists of flexible membranes that enclose cytoplasm, a water-based cellular fluid together with a variety of tiny functioning units called organelles. In humans, as in all organisms, cells perform all functions of life.
Tissues
In the body’s organizational hierarchy, tissues occupy a place between cells and organs. That is, a tissue is a group of cells with a similar shape and function. In turn, organs (which make up the body) are comprised of various tissues.
The component cells of a tissue are a specific cell type. A tissue’s cells may be identical, but are not necessarily so. Broadly speaking there are four types of tissues.
- Muscle tissue (in turn divided into skeletal, smooth and cardiac) is contractile. It allows locomotion of the body. It also allows necessary contractions of various organs such as the heart and of respiratory and digestive systems.
- Nerve tissue comprises the body’s wiring system. It conducts signals between the nervous system and various organs.
- Connective tissue holds the body together. It is found in most organs, anchoring them to the skeleton and other organs. Types of connective tissue include fibrous tissue, fatty tissue, loose tissue and cartilage. Connective tissue also includes bone, blood and lymph.
- Epithelial tissue is the body’s protection against the outside environment. Skin tissue helps to maintain homeostasis. It helps monitor and control temperature, and resists abrasion, foreign bodies and damaging chemicals. Internally, epithelial tissue lines most internal cavities, secreting or absorbing nutrients.
Tissues form during development. Stem cells in the embryo differentiate into various cell types. The necessary genes in the cells turn on or off, resulting in the production of proteins that characterize a cell’s structure and function. Early in embryonic growth, the cells migrate to the appropriate location in the body. Once there, they proliferate so that the tissue can perform its needed function.
Different tissues arise from the source cells in each of the three primary germ cell layers. For example, the epithelium is derived from the ectoderm and endoderm. Connective tissue arises largely from the mesoderm. Gastrointestinal and respiratory tissues arise mostly from the endoderm. Programmed cell death, or apoptosis, may take place to eliminate transitory tissues in the embryo, such as the pronephros, a simple excretory organ that is later replaced by the kidney.
Organs
The organ level of organization in the body may be the most familiar to us from our everyday experiences. Many of the common ailments we hear about—an upset stomach, a broken bone, lung disease, skin cancer—are named for the organs they affect.
An organ is made up two or more tissues that work together to perform a specific function for the body as a whole. For example, the contractile cells of skeletal muscle are bundled together to make muscle fiber tissue. In turn, connective tissue wraps around bundles of muscle fibers, like a tortilla around the filling of a burrito. Several of these bundles are are in turn wrapped by another tissue, perimysium. Finally, bundles of these are surrounded by a sheath of yet another tissue, epimysium, which covers the outside of the whole muscle. Yet more tissue is necessary for the muscle to function in the body. Connective tissue comprising ligaments attaches the muscle to the skeleton, and nerve tissue conducts impulses from the nervous system to signal the muscle to contract. Groups of organs that perform related functions are organized into organ systems, which perform more general functions. Table 1 describes the structures and functions of some common organs.
| Table 1. Structure and Function of Organs | |||
|---|---|---|---|
| Organ | Primary function(s) | Tissues it contains | Organ system(s) it is a part of |
| brain | control of body systems and behavior; cognition | nervous, connective, epithelial | nervous system; endocrine system |
| skin | protection; support and containment; temperature and fluid regulation | epithelial, nervous, connective, muscular | integumentary system |
| stomach | chemical and mechanical digestion of food | epithelial, connective, muscular, nervous | digestive system |
| sternum (breastbone) | support; protection; blood cell production | epithelial, connective, nervous | skeletal system; immune system; cardiovascular system |
| kidney | waste removal; fluid regulation | epithelial, connective, nervous | urinary system |
Organ Systems, The Whole Body, and Populations
Organ systems are made up of organs that work together to perform a specific function for the body as a whole. Table 2 describes the organ systems and their primary organs and physiological functions.
| Table 2. Organ Systems | ||
|---|---|---|
| Organ system | Key Organ(s) | Primary function(s) |
| integumentary | skin | support; protection; regulation of fluid levels |
| skeletal | bones, cartilage | support; protection; movement; blood cell production |
| muscular | muscles, tendons | support; movement |
| urinary | kidneys, bladder, urethra | waste removal; regulation of fluid levels |
| digestive | tongue, esophagus, stomach, small intestine, large intestine, gallbladder, rectum | digestion of food; waste removal |
| respiratory | trachea, lungs | gas exchange; regulation of temperature |
| cardiovascular | heart, blood vessels | transport of materials through the body; regulation of temperature |
| nervous | brain, spinal cord | control of behavior and body systems; cognition |
| endocrine | glands | control of the body systems and development |
| immune | thymus, tonsils, spleen | defense agains infection |
| lymphatic | lymph nodes, lymphatic vessels | immunity; regulating fluid balance |
| reproductive | penis, testes, prostate (male); uterus, ovaries, vagina (female) | reproduction |
The Whole Body
The organ systems of the body all work together to maintain proper physiological functions. Many times in the arena of anatomy and physiology, including in this course, we closely examine the molecules, cells, tissues and organs of the body to learn their forms and functions. However, it is important to consider that every molecule works as part of the entire system. Endocrine disorders such as diabetes affect glucose levels in the body. Altered blood glucose levels can affect many organ systems. For example, the immune system may not heal as well, the urinary system may experience kidney damage, and the cardiovascular system can experience vascular damage, even to the point of causing blindness. In the body, everything is interconnected.
Populations
Beyond the body, populations and environment can impact physiology and health. Some diseases and disorders are common to certain populations, most likely because of genetic connections. Also, environmental conditions can impact health. Particulates in the air can impact respiratory function. We are also affected by foods, exercise, sun exposure and other environmental conditions.
A tissue is a group of many similar cells (though sometimes composed of a few related types) that work together to perform a specific function. An organ is an anatomically distinct structure of the body composed of two or more tissue types. Each organ performs one or more specific physiological functions. An organ system is a group of organs that work together to perform major functions or meet physiological needs of the body.
This book covers eleven distinct organ systems in the human body (Figure 2 and Figure 3). Assigning organs to organ systems can be imprecise since organs that “belong” to one system can also have functions integral to another system. In fact, most organs contribute to more than one system.
Figure 2. Organ Systems of the Human Body. Organs that work together are grouped into organ systems.
Figure 3. Organ Systems of the Human Body (continued). Organs that work together are grouped into organ systems. The organism level is the highest level of organization. An organism is a living being that has a cellular structure and that can independently perform all physiologic functions necessary for life. In multicellular organisms, including humans, all cells, tissues, organs, and organ systems of the body work together to maintain the life and health of the organism.