All vertebrates share the same basic body plan, with tissues and organs functioning in a similar manner. Here, we will focus on the human body, studying form (anatomy) and function (physiology). The two go hand-in-hand and are an extension of one of biology's central themes, evolution.

Animals are made of complex systems of cells, which must be able to perform all of life's processes and work in a coordinated fashion to maintain a stable internal environment. Early in a human's development, groups of cells specialize into three fundamental embryonic or germ layers: endoderm, mesoderm, and ectoderm. These embryonic layers differentiate into a number of specialized cells and tissues. Tissues are groups of cells similar in structure and function and may be held together by some sort of matrix. There are four primary groups of tissues: epithelial, connective, muscular, and nervous.

Different tissues functioning together for a common purpose are called organs (eg, stomach, kidney, lung, heart).

Organ systems are composed of individual organs working together to accomplish a coordinated activity. For example, the stomach, small intestine and large intestine all play a role in digestion.

Tissues are groups of cells similar in structure and function. The four primary categories of tissues are epithelial, connective, muscular, and nervous.

• Epithelial tissues cover the outer and inner surfaces of the body, serving as "gatekeepers" for the passage of materials. Epithelial cells have glandular roles, secreting substances such as mucous and hormones. Some simple epithelial tissues consist of only one layer, while stratified epithelial tissues are comprised of multiple layers. The shape of the cell on the outer surface (columnar, cuboidal, or squamous) facilitates identification of epithelial tissues.
• Connective tissues help to bind, support, and join other tissues. The cells of connective tissues are sparsely arranged in a profuse extracellular matrix which can be fluid (as in blood), flexible (as in cartilage) or hard and crystalline (as in bone). 
• Muscle tissues, composed of cells called fibers, are unique in the abundance and arrangement of actin and myosin filaments, which enable muscle to contract. In humans, there are three types of muscle tissues: smooth, cardiac, and skeletal.
• Nerve tissues are comprised primarily of neurons (nerve cells), which detect stimuli and send responses in the form of electrochemical charges called impulses. Nerve tissues include a variety of supporting cells called neuralgia and Schwann cells. These cells support, insulate, and eliminate foreign materials in and around neurons.

Groups of organs working together to perform major activities of the body are called organ systems.

• The skeletal system, made of bones, cartilage and joints, is the framework of the body. It protects internal organs, stores minerals and provides a place for muscles to attach.
• The human muscular system is composed of smooth, cardiac and skeletal muscle tissue. Skeletal muscle, attached to the skeleton with dense strips of connective tissue called tendons, is responsible for the movement of body parts. Smooth muscle, sometimes called visceral muscle, is found in internal organs (eg, lines the walls of many blood vessels, makes up the iris of the eye and forms the wall of the gut). Cardiac muscle forms the bulk of the heart which controls blood circulation.
• The circulatory system-blood, blood vessels and the heart-is the body's transportation system, moving oxygen, carbon dioxide, nutrients, wastes, hormones, vitamins, minerals and water throughout the body. It also aids in regulation of temperature.
• The digestive system converts foods to simple substances that can be absorbed and used by the cells of the body. It is composed of the mouth, pharynx, esophagus, stomach, small intestine and large intestine and is aided by several accessory organs (liver, gall bladder, and pancreas).
• Made up of the skin, lungs, sweat glands and the kidneys, the excretory system removes metabolic wastes from the body. The kidneys are responsible for eliminating the bulk of wastes from the human body.
• The reproductive system generates reproductive cells (gametes) and provides a mechanism for them to be fertilized and maintained until the developing embryo can survive outside the body. The primary reproductive organs are the ovaries (female) and the testes (male).
• The nervous system regulates and coordinates the body's responses to changes in the internal and external environment. Major structures of the nervous system are the brain, spinal cord and nerves.
• The endocrine system consists of the hypothalamus, pituitary, thyroid, parathyroid and adrenal glands, as well as the pancreas, ovaries and testes. This system helps to maintain homeostasis, regulate temperature, and control growth, development, metabolism and reproduction by secreting and releasing hormones.
• The first line of defense in protecting the body is the integumentary system, which is composed of the skin, hair, nails, sweat and oil glands. It protects against injury, infection and fluid loss and also aids in temperature regulation.

The skeleton forms a sturdy internal framework of 206 bones and associated tissues - cartilage, tendons, and ligaments. Bones provide the base to which muscles attach and also the leverage required to accomplish external movement. The skeleton protects vital organs such as the brain, spinal cord, heart and lungs. As a living, dynamic tissue, bone stores vitamins and minerals (especially calcium and phosphorus) and houses red bone marrow, which produces blood cells.

Anatomists divide the skeleton into two parts, axial and appendicular. The axial skeleton (upright, or core of the body) includes the skull, ribs, sternum and vertebral column. Comprised of the shoulders, arms, hips and legs, the appendicular skeleton forms the appendages that attach to the axial skeleton.

Approximately four times as strong as concrete, bone is one of the strongest materials produced by nature. It is a connective tissue composed of cells called osteocytes, which are embedded in a hard, calcified matrix. Bones are made of a dense outer layer of compact material that surrounds a core of loosely structured spongy bone. The compact layer of bone is covered by a fibrous membrane called the periosteum. Cavities within each bone contain red bone marrow (blood-forming tissue) or yellow bone marrow (fat storage).

Movement of the skeleton occurs at the joints where two or more bones meet. There are three categories of joints. Slightly movable joints allow some movement but function mainly as a cushion (eg, joints between the vertebra). Freely movable, or synovial, joints allow a range of movement determined by the structure of the joint.  Examples of movable joints are the ball and socket (shoulder), hinge (elbow), pivot (between radius, ulna and humerus), and saddle joint (thumb). A few joints found in the skull are non-movable (sutures). Ligaments are inelastic connective tissues which hold bones together in a joint.

The skeleton’s most important function is to provide support for the body. The skull, ribs, sternum and vertebral column make up the “upright,” rigid axial skeleton, while the arms, legs, shoulders and hips compose the more movable appendicular portion of the skeleton. The articulation of bones forms joints and provides the skeleton with flexibility, enabling it to be moved as muscles contract, extend and relax.

Nearly two-thirds of bone is made of various salts, primarily compounds containing phosphorus and calcium, which provide rigidity and hardness. Collagen makes up the remainder of the matrix (nonliving portion of bone tissue). New bone is produced by osteoblasts, which secrete collagen. After the calcium phosphate is deposited in the matrix, some of the osteoblasts become trapped in their own secretion and then are called osteocytes (osteo = bone, cyte = cell). Bone is a dynamic tissue and constantly is reorganized by cells called osteoclasts. Severe bone loss is called osteoporosis.
 
Compact bone consists of concentric rings of calcium and other minerals (lamellae) surrounding a haversian canal (containing blood vessels and nerves that support the osteocytes). The circular structure helps to form columns that allow the bone to withstand stress, forming a basic structure of compact bone called a Haversian system.

The primary role of the muscular system is to produce movement. As muscle tissue contracts, energy is used and heat is generated. Muscles also maintain body positions and postures, such as supporting your head or sitting.

Muscle is a unique tissue in its ability to contract (shorten). The functional unit of skeletal muscle tissue is a sarcomere, composed of actin and myosin protein filaments. When a muscle contracts, the sarcomere is shortened by actin filaments "sliding" over myosin filaments. Since a muscle fiber moves by shortening (it pulls and cannot push), muscles must work in antagonistic pairs. For example, a flexor contracts (shortens) and decreases the angle of joint while an extensor is stretched, increasing the angle of the joint. One muscle group contracts while an opposing muscle group extends. Muscle tissue is controlled and coordinated by stimuli from the nervous system.

The three types of muscle tissue are skeletal, smooth and cardiac. Skeletal muscle is attached to the skeleton with tendons and is controlled consciously. Skeletal muscle cells are long, fiber-like and multinucleated. The number of muscle fibers is fixed at birth, but protein can be added to increase the size the fiber with use and exercise. Lack of use causes muscle fibers to atrophy. Movement of smooth muscle tissue, found in internal organs, is usually involuntary. The cells of smooth muscle tissue are spindle-shaped and contain a single nucleus. Cardiac muscle, also involuntary muscle, is found only in the heart. Cardiac muscle tissue contains "gap" junctions that allow the diffusion of ions and the spread of electrical impulses from one cell to another.

There are more than 600 voluntary muscles in the body, the strongest of which are the skeletal muscles. Skeletal muscles account for approximately 23% of a woman's weight and approximately 40% of a man's.  Muscles work together to accomplish even simple movements.

Skeletal muscle tissue, usually found attached to the skeleton, consists of long, straight multinucleated cells with bands, or "striations." Skeletal muscle is under voluntary, conscious control.

Smooth muscle tissue is made of spindle-shaped cells containing a single nucleus. Smooth muscle surrounds hollow internal structures, including the stomach, intestines, arteries and veins. Because of the diagonal arrangement of parallel filaments of actin and myosin proteins, smooth muscle has a greater capacity than other muscle types to be stretched while retaining the ability to contract. Both smooth and cardiac muscle are considered involuntary since they generally are not under our conscious control.

Cardiac muscle tissue is found only in the heart and has several attributes in common with both skeletal and smooth muscle. It has striations or banding like skeletal muscle but is under involuntary control like smooth muscle.

A whole muscle is composed of bundles made of muscle fibers or cells. Actin and myosin are proteins which are found in muscle cells. During a contraction, the knoblike head of a myosin filament attaches to a binding site on actin to form a "crossbridge." Powered by ATP, the crossbridge changes shape, pulling the actin filament toward the center of the sarcomere.