Human Biology: Tissues, Organ Systems, and Homeostasis
Biological Levels of Organization and Types of Tissues
Definition of Tissue: A tissue is a collection of cells of the same type that perform a common function.
Biological Levels of Organization: Tissues represent a level of organization above cells and below organs. Cells form tissues, which in turn form organs.
Major Tissue Types: There are four primary classes of tissues in the human body: * Connective Tissue: This tissue type functions to bind and support various body parts. * Muscular Tissue: This tissue is specialized for movement, both of the entire body and its individual parts. * Nervous Tissue: This tissue is responsible for conducting nerve impulses. * Epithelial Tissue: This tissue covers body surfaces and lines various body cavities.
Connective Tissue: Components and Composition
Primary Function: Connective tissue is primarily involved in connecting and supporting structures within the body.
General Components: Connective tissue consists of three main components: * Specialized Cells. * Ground Substance: This is the noncellular material found between the cells. Its consistency varies significantly among different types of connective tissue, ranging from solid (as in bone) to fluid (as in blood). * Protein Fibers.
Specific Cell and Fiber Types (Figure 4.1): * Adipose Cell: Specialized for storing fat. * Stem Cell: Divides to produce other types of cells. * Fibroblast: Divides to produce other types of cells and is responsible for producing the protein fibers. * White Blood Cell: Functions to engulf pathogens or produce antibodies. * Ground Substance: Fills the spaces between cells and fibers. * Collagen Fiber: Unbranched, strong, but flexible. * Elastic Fiber: Branched and stretchable, containing the protein elastin which allows for stretching and recoiling. * Reticular Fiber: Thin, highly branched collagen fibers that form a supportive network.
Major Categories of Connective Tissue: There are three main types: Fibrous, Supportive, and Fluid.
Fibrous Connective Tissue
Forms of Fibrous Tissue: * Loose Fibrous Connective Tissue: Fibers create a loose, open framework. This includes areolar connective tissue, reticular connective tissue, and adipose tissue. * Loose Connective Tissue Function: Supports the epithelium and many internal organs. * Adipose Tissue: Specialized loose tissue for storing fat. It contains very little extracellular matrix. The primary cells are adipocytes, which are filled with liquid fat. Its functions include energy storage, insulation, and cushioning of organs. It is found primarily under the skin and around some organs. * Dense Fibrous Connective Tissue: Fibers (primarily collagen) are densely packed together. This tissue has more specific functions compared to loose tissue. * Tendons: Connect muscles to bones. * Ligaments: Connect bones to other bones at joints.
Matrix Structure: Both forms contain fibroblasts separated by a matrix composed of ground substance and fibers.
Supportive Connective Tissue: Cartilage and Bone
Functions: Supportive connective tissue provides structure, shape, protection, and leverage for movement.
Cartilage: * Cell types: Chondrocytes and chondroblasts, which reside in small chambers called lacunae (singular: lacuna). * Matrix: Solid yet flexible. Unlike many other tissues, it lacks a direct blood supply, resulting in slow healing processes. * Types of Cartilage: * Hyaline Cartilage: Contains fine collagen fibers. Found at the tip of the nose, the ends of long bones, and in the fetal skeleton. * Elastic Cartilage: Contains a high volume of elastic fibers. Found in the outer ear. * Fibrocartilage: Contains strong collagen fibers. Found in the disks located between vertebrae.
Bone: * Characteristics: The most rigid of all connective tissues. The matrix is composed of collagen and calcium salts. * Structural Composition: The calcium salts surround protein fibers to provide both elasticity and strength, comparable to steel rods in reinforced concrete. * Cell Types: Osteoblasts and osteoclasts are responsible for forming the bone matrix. * Types of Bone Tissue: * Compact Bone: Makes up the shafts of long bones. It consists of cylindrical structural units called osteons. A central canal within the osteon contains blood vessels and nerves. Bone cells are located in lacunae. * Spongy Bone: Found inside the ends of long bones. It is lighter than compact bone but remains strong.
Fluid Connective Tissue: Blood and Lymph
Blood: * Composition: Consists of a fluid matrix called plasma and cellular components known as formed elements. * Formed Elements: * Red Blood Cells (Erythrocytes): Cells that carry oxygen. * White Blood Cells (Leukocytes): Cells that fight infection. * Platelets (Thrombocytes): Cellular fragments involved in blood clotting.
Lymph: * Origin: Derived from the interstitial fluid surrounding the tissues. * Composition: Contains white blood cells. * Function: Lymphatic vessels absorb excess interstitial fluid and return the lymph to the cardiovascular system.
Muscular Tissue: Movement and Contraction
General Function: Specialized for contraction to move the body and its parts. Muscle cells are referred to as muscle fibers.
Three Types of Muscular Tissue: * Skeletal Muscle: * Attachment: Attached to the skeleton by tendons. * Control: Voluntarily controlled. * Structure: Fibers are very long (can run the length of the muscle), contain multiple nuclei, and appear striated (striped). * Smooth Muscle: * Location: Found in the walls of viscera (internal organs). * Control: Involuntarily controlled. * Structure: Spindle-shaped cells with a single nucleus; no striations are present. * Cardiac Muscle: * Location: Found exclusively in the walls of the heart. * Control: Involuntary. * Structure: Cells are striated, have a single nucleus, and are connected by specialized structures called intercalated disks.
Nervous Tissue: Communication and Support
Functions: Sensory input, integration of data, and motor output.
Component Types: Neurons and Neuroglia.
Neurons: * Structure: Compromised of three parts: dendrites, a cell body, and an axon. * Dendrites: Carry information toward the cell body. * Cell Body: Contains the nucleus and other organelles. * Axon: Conducts nerve impulses away from the cell body. Some axons are covered in a fatty substance called myelin. * Nerves: Bundles of axons traveling to and from the brain and spinal cord.
Neuroglia: * Ratio: Outnumber neurons by a ratio of to . * Volume: Take up more than half the volume of the brain. * Function: Main role is to support and nourish neurons.
Epithelial Tissue: Protection, Lining, and Glands
Characteristics: Made of tightly packed cells. It is anchored by a basement membrane on one side while being free on the other side.
Locations: Lines body cavities, covers body surfaces, and comprises glands.
Classification Criteria: Named for the number of cell layers (Simple vs. Stratified) and the shape of the cells.
Cell Shapes: * Squamous: Flattened cells. * Cuboidal: Cube-shaped cells. * Columnar: Column-shaped cells.
Specific Types of Epithelia: * Simple Squamous: Single layer of flattened cells. Found in lungs for gas exchange. * Simple Cuboidal: Single layer of cube-shaped cells. * Simple Columnar: Single layer of column-shaped cells. * Pseudostratified Columnar: Appears stratified due to the location of nuclei, but every cell touches the basement membrane. Often features cilia to move mucus. * Stratified Squamous: Several layers of cells. Forms the outer layer of the skin and lines the mouth, esophagus, nose, anal canal, and vagina. Function is protection. * Transitional Epithelium: Cells change shape from cuboidal to squamous in response to tension. Found lining the urinary bladder.
Glands: One or more cells that make and secrete a product. * Exocrine Glands: Secrete products into ducts. * Endocrine Glands: Ductless; they secrete products (hormones) directly into the bloodstream.
Organ Systems and Anatomical Terms
Definitions: * Organ: A group of tissues performing a common function. * Organ System: A group of organs with a similar function.
Anatomical Terminology (Upright Standing Position): * Ventral or Anterior: Toward the front. * Dorsal or Posterior: Toward the back. * Superior: Toward the head. * Inferior: Toward the feet. * Medial: Closer to the midline of the body. * Lateral: Away from the midline of the body. * Proximal: (For appendages) Closer to the trunk of the body. * Distal: (For appendages) Away from the trunk.
System Cooperation: Organ systems work together; some organs serve multiple systems (e.g., the pancreas functions in both the endocrine and digestive systems).
Body Cavities
Major Cavities: The body is divided into the Ventral cavity and the Dorsal cavity.
Ventral Cavity (Coelom): * Thoracic Cavity: Contains lungs, heart, and esophagus. Separated from the abdominal cavity by the diaphragm. * Abdominal Cavity: Contains the stomach, liver, spleen, pancreas, gallbladder, and most of the small and large intestines. * Pelvic Cavity: Contains the rectum, urinary bladder, internal reproductive organs, and the remainder of the intestines. * Scrotum: An external extension of the abdominal wall in males containing the testes.
Dorsal Cavity: * Cranial Cavity: Contains the brain. * Vertebral Canal: Contains the spinal cord.
Body Membranes
Function: Line cavities and internal spaces of organs and tubes opening to the outside.
Types of Membranes: * Mucous Membranes: Line tubes of the digestive, respiratory, urinary, and reproductive systems. They consist of epithelium over loose fibrous connective tissue and contain goblet cells that secrete mucus. * Serous Membranes: Line closed cavities and cover organs within them. * Pleurae: Line thoracic cavity and cover lungs. * Pericardium: Forms the pericardial sac and covers the heart. * Peritoneum: Lines the abdominal cavity and covers its organs. The double layer is called the mesentery. * Synovial Membranes: Composed only of loose connective tissue. Line freely moveable joints and secrete synovial fluid for lubrication. * Meninges: Composed only of connective tissue. Found in the dorsal cavity around the brain and spinal cord. Inflammation of these is called meningitis.
Integumentary System: Skin and Accessory Organs
Components: Includes the skin and accessory organs: hair, nails, and glands. It contains all four tissue types.
Functions: Protects from trauma, pathogens, and water loss; regulates body temperature; contains sensory receptors (touch and temperature); and synthesizes Vitamin D (important for calcium and phosphorus regulation).
Regions of the Skin: * Epidermis: Thin, outermost layer made of stratified squamous epithelium. Deepest layer contains stem cells for replacement. * Skin Replacement Terminology: Autograft (from self), Allograft (from another person), or lab-grown skin. * Epidermal Cells: Keratinocytes (dead, waterproofed with keratin in upper layers), Langerhans cells (white blood cells), and Melanocytes (produce melanin for color and UV protection). * Dermis: Thick, inner layer made of dense fibrous connective tissue. Contains collagen and elastic fibers, blood vessels, sensory receptors, and glands. * Subcutaneous Layer (Hypodermis): Technically not part of the skin. Composed of loose connective tissue and adipose tissue. Functions in energy storage, insulation, and protection.
Accessory Organs: * Nails: Protective covering for digits. Grow from the nail root over the nail bed. Features includes the cuticle (covers root) and lunula (white half-moon base). * Hair: Follows growth from hair follicles (epidermal structures). The hair shaft protrudes from the skin. Color is from melanin. Arrector pili muscles attached to follicles cause "goosebumps" upon contraction. * Glands: * Oil Glands (Sebaceous Glands): Produce sebum to soften hair/skin and retard bacterial growth. Acne is the inflammation of these glands. * Sweat Glands (Sudoriferous Glands): Located in the dermis; ducts open to the skin surface to regulate temperature.
Homeostasis
Definition: The maintenance of a relatively constant internal environment despite fluctuating external conditions. Variables include blood glucose, pH, and body temperature. Illness results if conditions vary too much.
Internal Environment: Consists of blood (delivers $O_2$/nutrients, removes $CO_2$/wastes) and interstitial fluid (surrounds cells for substance exchange).
Control Systems: The nervous and endocrine systems coordinate other systems. The nervous system provides faster responses; the endocrine system (via hormones) has longer-lasting effects.
Negative Feedback Mechanisms: The primary mechanism for homeostasis. Components include a sensor and a control center. The system's output turns down or off the stimulus. * Example: Temperature regulation. If temperature is above normal, blood vessels dilate and sweat glands secrete. If below normal, blood vessels constrict and shivering may occur. The set point for humans is typically .
Positive Feedback Mechanisms: Brings about change in the same direction as the original stimulus. * Example: Childbirth. Fetal head pushes on the cervix, signals go to the brain, oxytocin is released, causing stronger contractions and more oxytocin release until delivery stops the cycle. * Risks: Positive feedback can be harmful in situations like a high fever.
Questions & Discussion (Check Your Progress)
Question: Explain the relationship among cells, tissues, and organs in the biological levels of organization.
Response: Cells are the basic unit, a collection of similar cells forms a tissue, and a group of tissues performing a common function forms an organ.
Question: Distinguish between muscular tissue and nervous tissue.
Response: Muscular tissue is specialized for contraction and movement, while nervous tissue is specialized for conducting nerve impulses and communication.
Question: Distinguish between connective tissue and epithelial tissue.
Response: Connective tissue binds and supports body parts; epithelial tissue covers body surfaces and lines cavities.
Question: Describe the three general categories of connective tissue and provide examples of each.
Response: 1. Fibrous (e.g., tendons, ligaments), 2. Supportive (e.g., bone, cartilage), 3. Fluid (e.g., blood, lymph).
Question: Explain the difference in the composition of the matrix in each of the three classes of connective tissue.
Response: Fibrous matrix has fibroblasts and fibers; supportive matrix is solid (bone) or flexible/solid (cartilage); fluid matrix is liquid (plasma or interstitial fluid).
Question: Describe how each of the two fluid connective tissues is important to homeostasis.
Response: Blood transports nutrients and wastes; lymph maintains fluid balance by returning excess interstitial fluid to the blood.
Question: Explain the difference in the structure and function of skeletal, smooth, and cardiac muscle.
Response: Skeletal is striated/voluntary/multi-nucleated (moves skeleton); smooth is non-striated/involuntary/spindle-shaped (walls of organs); cardiac is striated/involuntary/intercalated disks (pumps heart).
Question: List the functions of epithelial tissue.
Response: Protection, covering surface, lining cavities, and forming glands.
Question: Highlight the major body cavities and their locations.
Response: Ventral (front) contains thoracic, abdominal, and pelvic; Dorsal (back) contains cranial and vertebral.
Question: List the four types of body membranes and describe their structure and function.
Response: Mucous (lining tubes, secretes mucus), Serous (lines closed cavities, secretes watery fluid), Synovial (lines joints, secretes lubricating fluid), Meninges (around brain/spinal cord, protection).