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Atoms are the smallest chemical units of an element. They are the building blocks of all matter. Each atom contains three components: negatively charged electrons, positively charged protons, and neutrally charged neutrons. A molecule is a chemical combination of two or more atoms. A cell is the basic unit of life, not of a chemical element. An organelle is a specialized structure within a cell.

Chemical compounds are made up of molecules. A molecule is a chemical combination of two or more atoms. Water, for example, is composed of two hydrogen atoms and one oxygen atom. A molecule is not a physical or biologic combination of atoms. Bonds in a molecule may be covalent or ionic.

Cells are the smallest and most fundamental unit of life. Every structure in the human body is made of cells. Cells combine to form tissues, tissues form organs, and organs make up organ systems. Organelles are specialized structures within a cell (such as mitochondria or the nucleus), not the basic building blocks of the body. Tissues are groups of similar cells working together. Organs are made up of multiple tissue types performing complex functions.

A cell consists of several organelles. Several cells performing a similar function make up a tissue. A collection of tissues performing a specialized function make up an organ. Several organs work together to make up a system. The other sequences have the organizational levels out of order. Remember: organelles live inside cells, cells group into tissues, tissues build organs, organs form systems.

Epithelial tissues perform specialized functions including excretion, absorption, secretion, diffusion, and protection. Connective tissues provide a supporting network for organs and muscles. Nerve tissues control and coordinate body activities. Muscle tissue mediates movement. None of these perform excretion.

Several organs working together to carry out a specialized function make up a system. For example, the respiratory system includes the nose, pharynx, larynx, trachea, bronchi, bronchioles, alveoli, and lungs. A cell consists of organelles. Several cells working together form a tissue. Several tissues working together form an organ. The system is the highest organizational level.

A transverse plane (also called a horizontal plane) divides the body into upper (superior) and lower (inferior) portions. A frontal (coronal) plane divides the body into anterior and posterior portions. A sagittal plane divides the body into left and right portions. A median (midsagittal) plane runs through the midline, dividing the body into equal left and right halves.

The dorsal cavity contains two subdivisions: the cranial cavity and the spinal (vertebral) cavity. The thoracic, abdominal, and pelvic cavities are all part of the ventral cavity. Body cavities are hollow spaces containing organs, divided into dorsal and ventral compartments.

Posterior is an anatomical term referring to the back of the body or a body part. It is the opposite of anterior. Medial describes a position closer to the midline (opposite of lateral). Lateral refers to the sides of the body. Anterior describes the front of the body.

Inferior means below. The abdomen lies below the chest. Superior means above, and the abdomen is not above the chest. Ventral means front. Dorsal means back or behind. Neither describes the relationship between abdomen and chest.

Lymph moves through the body in lymphatic vessels and empties into large veins. Blood travels through arteries, veins, and capillaries and returns to the heart. Lymph specifically drains into the venous system, not arteries or capillaries.

Formed elements in blood are the cellular components: erythrocytes (red blood cells), leucocytes (white blood cells), and thrombocytes (platelets). Plasma is the fluid portion of blood, not a formed element. Proteins and hormones are transported by blood but are not formed elements. Oxygen and carbon dioxide are respiratory gases carried by plasma and erythrocytes.

Antigens sit on the surface of red blood cells and determine blood group. A person with group A carries A antigens on the RBC surface and anti-B antibodies in plasma. You develop antibodies to the antigens you do not have. Antibodies are in plasma, not on red blood cells. Antigens are on the RBC surface, not in plasma. A person with blood group A would not have anti-A antibodies, as that would trigger an immune reaction against their own cells.

Successful blood transfusion requires both blood group and rhesus factor compatibility. Rhesus (Rh) factors are antigens on the surface of some individuals' red blood cells. Those with the factor are Rh-positive; those without are Rh-negative. Matching blood group alone is not sufficient. Rhesus factor alone is not sufficient. Incompatible transfusions cause clotting, hemolysis, and other transfusion reactions.

Primary lymphoid organs are bone marrow and thymus. Lymphoid organs are where lymphocytes mature. Lymph nodes, spleen, and tonsils are all secondary lymphoid organs. Secondary lymphoid organs also include lymphoid tissues in mucous membranes.

T lymphocytes are produced in the bone marrow but mature in the thymus. B lymphocytes are produced in the bone marrow and mature primarily in the bone marrow. The spleen and lymph nodes are secondary lymphoid organs where mature lymphocytes encounter antigens.

Barrier defenses are physical barriers that prevent pathogens from entering body tissues. They include skin and mucous membranes. Neutrophils, macrophages, and mast cells are part of the innate immune response, not barrier defenses. They act after pathogens have breached the physical barriers.

Phagocytes are part of the body's first line of immunological defense. They protect against pathogens by engulfing them. Phagocytes include dendritic cells, neutrophils, eosinophils, macrophages, and monocytes. Natural killer cells, T lymphocytes, and plasma cells are not phagocytes. They participate in immunity through other mechanisms.

Natural killer cells resemble lymphocytes. Their granules contain chemicals that can destroy virally infected cells. Neutrophils are phagocytes that engulf pathogens like bacteria. B lymphocytes and plasma cells do not directly destroy cells. Instead, they produce antibodies.

Plasma cells are formed from B lymphocytes and they secrete antibodies. T lymphocytes release chemical messengers called cytokines. Macrophages and dendritic cells are phagocytes and do not secrete antibodies.

The epidermis has four layers in most places. In thickened areas like the palms and soles, there is an additional layer: the stratum lucidum. The cells are clear, hence the name lucidum (from "lucid," meaning clear). Stratum corneum, granulosum, and basale are found throughout the skin, not only in thickened areas.

Apocrine glands function as scent glands. They are one of two types of sudoriferous (sweat) glands, along with eccrine glands. Sebaceous glands secrete sebum, an oily waxy substance that lubricates the skin. Ceruminous glands are modified apocrine glands that secrete ear wax. Eccrine glands are sweat glands that help cool the body.

The hypodermis (also called subcutaneous tissue) is the innermost layer. It stores fat, insulates the body, and stores energy. The stratum basale is the innermost layer of the epidermis, not of the entire skin. The dermis is the middle layer. The stratum corneum is the outermost epidermal layer.

The dermis is the middle layer of the skin containing blood vessels and glands. The epidermis is the outermost layer and has no blood vessels. It is nourished by the dermis. The hypodermis (subcutaneous layer) is rich in fat.

Cellulitis is inflammation of the subcutaneous connective tissue. The suffix -itis means inflammation. Folliculitis is inflammation of the hair follicle. Dermatitis is inflammation of the skin ("derma" refers to skin). Hidradenitis results from inflammation of the sweat glands ("adenitis" refers to gland inflammation).