Human Body Structure and Organization
Levels of Human Body Organization
The human body is composed of many elements, atoms, molecules, cells, tissues, organs, and systems.
- Elements: Basic substances that make up matter.
- Atoms: The smallest units of elements that form living organisms.
- Molecules: Components from which cells build their structures.
- Cells: The basic unit of living beings.
- Tissue: A group of specialized cells associated to perform a specific function.
- Organs: Formed by different tissues working together for a specific function.
- Device or System: An association of organs that work together to perform a vital function.
Homeostasis
Homeostasis is a balance that maintains a constant internal environment within the organism. The nervous and endocrine systems are responsible for coordinating the regulatory mechanisms.
Cell Structure and Function
The cell is the basic unit of the structure and functioning of the human body. It comprises:
- Plasma Membrane: The outer boundary of the cell.
- Cytoplasm: The space between the plasma membrane and the nuclear membrane. It contains the cytoskeleton and various organelles, including ribosomes, endoplasmic reticulum, Golgi apparatus, lysosomes, and mitochondria.
- Nucleus: The most prominent structure of the cell.
The set of biochemical reactions within the cell is called cell metabolism.
The Nucleus and Genetic Information
The nucleus contains the information necessary for cell functioning and reproduction.
- Genetic Information: Contains the instructions for manufacturing proteins in the body.
- Nucleoplasm: Contains filamentous or chromatin material made of genetic material (DNA) and a rounded corpuscle called the nucleolus.
DNA molecules can be found in the form of chromatin in a resting cell or as condensed chromosomes during cell division. A gene is a portion of DNA that holds the genetic information needed to create a specific protein.
Tissue Types
Tissue consists of similar cells and intercellular substance. The main types are epithelial, connective, muscle, and nervous tissue.
Epithelial Tissue
Epithelial tissues are responsible for functions such as protection (skin), absorption (intestinal epithelium), filtration (kidney epithelium), secretion (glands), and excretion (epithelium of pulmonary alveoli).
- Lining Epithelial Tissue: Covers the entire body, including the epidermis of the skin, and lines internal cavities like the esophagus. It can be formed by one or several layers of cells (simple epithelium, stratified epithelium) and may have villi, cilia, or produce mucus.
- Glandular Tissue: Forms glands, which include:
- Endocrine Glands: Secrete products directly into the blood.
- Exocrine Glands: Secrete products outside the body or into the digestive tract.
- Mixed Glands: Secrete substances both internally and externally.
Connective Tissue
Connective tissues are the most abundant supporting structures in the body. They unite tissues and protect internal organs. They comprise cells called fibroblasts and an extracellular matrix that they secrete. This matrix is reinforced by natural protein fibers, providing elasticity, hardness, or resistance.
- Loose Connective Tissue: Spreads throughout the body, surrounds and protects organs, and forms a subcutaneous layer of the skin (including adipose tissue).
- Dense Connective Tissue: Very resistant due to collagen fibers; forms tendons and ligaments.
- Cartilaginous Tissue: Composed of a gelatinous matrix, collagen fibers, and elastic fibers. Forms the skeleton of the embryo and is later replaced by bone in adults. It is found in the ear, nose, and intervertebral discs.
- Bone Tissue: Formed by a hard matrix impregnated with mineral salts that provide rigidity.
- Blood Tissue: Comprises a liquid matrix (plasma) and mobile cells (white blood cells, platelets, and red blood cells).
Nervous Tissue
Nervous tissue consists of two cell types: neurons and glial cells.
Neurons
Neurons are the functional unit of the nervous system. They are connected and specialized to receive and transmit information (nerve impulses).
- Cell Body: Contains the nucleus and cytoplasm.
- Dendrites and Axon: Information always travels from the dendrites to the axon. The axons of many neurons bundle together to form nerves.
Glial Cells
Glial cells have functions similar to connective tissue within the nervous system, providing support and protection.
- Astrocytes: The most abundant glial cells, present in the central nervous system. They adhere to blood vessels, supplying nutrients to neurons.
- Microglia: Small cells with protrusions that have a protective function for neurons and defend against harmful substances or infectious agents.
- Schwann Cells: Found in the peripheral nervous system. They insulate some neurons by forming a myelin sheath around their axons.