An organism is a plant, animal, or cell that can carry out all of the basic physiological functions of a living thing. Organisms grow, adapt, respond to stimuli, and reproduce. Organisms turn molecules into energy and then use that energy to maintain their structural organization and internal environment through a process called metabolism. Some single-celled life-forms carry out all of these functions on their own, but complex organisms, such as humans, operate differently. Humans are made up of trillions of individual cells that work together to carry out different functions. Each cell type may have its own specialized task. For example, cells in the stomach are specialized to secrete gastric acid, and cells in the heart are specialized to contract in unison.
The human body has a specific hierarchical structure of organization. It consists of six different levels. The first level, the chemical level, includes atoms, which are the basic units of matter. These atoms chemically combine to make molecules such as sugars, water, and proteins. These substances, and more, are found in all living things. The second level of organization is the cellular level. Cells are the basic unit of structure and function in the human body. Specialized cells that are grouped together to perform a common function form tissue. Tissues comprise the third level of organization. They also contain an extracellular matrix, which is a molecular framework secreted by the cells to support them structurally and biochemically. Tissues may be composed of one or more kinds of cells. There are four different types of tissues found in the human body:
- Connective tissue: tissues such as bone and cartilage that help shape the body's overall structure
- Epithelial tissue: tissues found in the inner and outer linings of various body surfaces such as organs and the skin
- Muscle tissue: tissues that contract and relax to facilitate body movement
- Nervous tissue: tissues made of nerve cells that make up the brain, spinal cord, and nerves found throughout the body
Hierarchical Organization of the Body
Integumentary, Skeletal, and Muscular Systems
The integumentary system is made up of the skin (the largest organ in the body) along with other associated structures such as hair and nails. This system protects the body's internal organs from infection, dehydration, and changes in temperature.
The skeletal system is made up of bones, cartilage, ligaments, and tendons, which provide a structure and framework for the body. Other functions of the skeletal system include providing a storage site for minerals, such as calcium and phosphorus, in bone tissue, and aiding in the formation of blood cells. Along with the muscular system, the skeletal system allows the body to move. It also protects the body's internal organs. For example, the rib cage protects the heart and lungs, and the skull protects the brain.
The muscular system is comprised of three types of muscle tissue that each function in a unique way within the body. The three different types of muscle tissue include:
- Skeletal muscle: With about 700 skeletal muscles in the human body, these primarily function to aid in body movement. These muscle fibers are attached to the skeleton. This tissue appears to have stripes (described as striated) and is under voluntary control, meaning the person can consciously choose to move the muscle.
- Cardiac muscle: This muscle tissue is under involuntary control, meaning a person is unable to consciously move it when so desired. As the name implies, this tissue is found in the heart. It is also striated like skeletal muscle tissue.
- Smooth muscle: This muscular tissue is found in the walls of organs, such as the small intestines. Smooth muscle does not appear striated; the cells are spindle shaped. Similar to cardiac muscle, this tissue is also under involuntary control.
In general, muscle is important because we rely on muscles to create movement through the actions of muscular contraction and pulling on the bones. Muscles function to move blood and food through the body as a result of muscular contractions in the heart and digestive systems.
Nervous and Endocrine Systems
The nervous system transmits information using both electrical and chemical signals throughout the body. This system is made up of the peripheral nervous system (PNS) and the central nervous system (CNS). The PNS contains sensory nerves, also called afferent nerves, and motor nerves, also called efferent nerves. An afferent nerve sends information about the body and its environment to the brain. An efferent nerve carries information from the brain to the rest of the body (in particular, to the muscles to initiate movement). The CNS consists of the brain and spinal cord and is responsible for storing and processing information and coordinating action.
The endocrine system is also responsible for signaling within the body. For example, the pancreas is an endocrine gland that is responsible for producing a hormone , or chemical messenger, called insulin. This hormone regulates how much glucose is taken up by a cell, which in turn affects blood sugar levels in the body. Unlike the nervous system, which transmits information along nerves using electrochemical signaling (a form of electrical and chemical communication), glandular organs in the endocrine system secrete hormones, also known as chemical messengers, to send information.
Respiratory, Digestive, and Reproductive Systems
The respiratory system consists of the respiratory tract and lungs. Within the lungs, many small air sacs called alveoli provide a large surface area. Oxygen and carbon dioxide gas are exchanged in the alveolar region between capillary blood and the air in the air sacs. Oxygen is obtained from the lungs. Also, carbon dioxide diffuses out of the bloodstream and is exhaled out the body.
The digestive system's main task is to play a role in how chemical building blocks are acquired and made available to the body. There are two major processes that facilitate this task: mechanical and chemical digestion of food, as well as the absorption of food. Through mechanical or chemical digestion, food is broken down into progressively smaller pieces. Eventually, the small pieces are absorbed into the bloodstream, where cells throughout the body can use them for various biological processes. Some of the organs in the digestive system include the mouth, stomach, spleen, liver, pancreas, and small and large intestines.
Reproduction is one of the key physiological functions of an organism. In the human body, the male reproductive system includes the male testes, which produce sperm, and the female reproductive system includes ovaries, which produce ova (eggs). External sex organs such as the penis in males and the vulva in females are used for copulation, or sexual intercourse. In females the uterus receives and nourishes the fertilized egg that later develops into an embryo and then a fetus. The vagina, which is an internal organ, serves as the birth canal through which the fetus travels during birth.
Cardiovascular, Lymphatic, and Urinary Systems
The heart and blood vessels comprise the cardiovascular system. This system moves oxygen-rich blood from the lungs to all the tissues in the body. It then carries the oxygen-depleted blood and carbon dioxide to the lungs, where carbon dioxide is removed from and oxygen is added to the blood. In addition to transporting oxygen and carbon dioxide, blood also transports other substances, such as hormones, white blood cells, nutrients, and waste products. Blood also transports heat throughout the body.
The lymphatic system also moves fluid through the body, via a network of lymphatic vessels and lymph nodes that carry lymph, a clear fluid, back toward the heart and the cardiovascular system. The lymphatic system also generates and contains lymphocytes, white blood cells that provide a key element of the body's immune response.
The urinary system is responsible for excretion, which is a process of ridding the body of metabolic waste products found in the bloodstream. The urinary system also regulates blood volume, pressure, and pH. Within the urinary system, kidneys filter waste products from the blood. This waste (urine) is stored in the bladder until it is released from the body (urination). The urinary system receives input from the nervous, endocrine, and cardiovascular systems.