Tissue, Organ, and Organ System Levels of Organization
Levels of Biological Organization
Cells: Composed of molecules.
Tissues: Groups of similar specialized cells.
Organs: Contain several different types of tissues.
Organ Systems: Multiple organs working together.
Tissues: Definition and Major Types
Definition: A tissue is composed of specialized cells of the same type that perform a common function in the body.
Four Major Types of Human Body Tissues:
Connective Tissue: Binds and supports body parts.
Muscular Tissue: Moves the body and its parts.
Nervous Tissue: Receives sensory information and conducts nerve impulses.
Epithelial Tissue: Covers body surfaces and lines body cavities.
Connective Tissues
General Definition and Functions:
Binds structures together.
Provides support and protection.
Fills spaces.
Stores fat.
Forms blood cells.
Examples of Connective Tissues: Adipose tissue, cartilage, bone, and blood.
Common Components (diverse in structure and function, but all types share these three):
Specialized Cells.
Ground Substance.
Protein Fibers.
Ground Substance
A noncellular material that separates the cells.
Varies in consistency:
Solid (e.g., bone).
Semifluuid (e.g., cartilage).
Fluid (e.g., blood).
Components of Connective Tissues (Detailed):
Adipose cell: Stores fat.
Ground substance: Fills space between cells and fibers.
Elastic fiber: Branched and stretchable.
Stem cell: Divides to produce other types of cells.
Collagen fiber: Unbranched, strong, but flexible.
Fibroblast: Divides to produce other types of cells.
Reticular fiber: Branched, thin, and forms networks.
White blood cell: Engulfs pathogens or produces antibodies.
Protein Fibers
Three Possible Types of Fibers:
White Collagen Fibers:
Contain collagen, a protein that gives them flexibility and strength.
Reticular Fibers:
Very thin collagen fibers.
Highly branched proteins that form delicate supporting networks.
Yellow Elastic Fibers:
Contain elastin, a protein.
Not as strong as collagen, but more elastic.
Can return to their original shape and may stretch to over a 100 times their relaxed size without damage.
Inherited Connective Tissue Disorders
Arise when individuals inherit genes leading to malformed fibers.
Example: Marfan Syndrome:
Caused by mutations in the fibrillin gene.
Fibrillin is a protein component of elastic fibers.
These mutations cause a decrease in the elasticity of connective tissues normally rich in elastic fibers.
Clinical Significance: Affects structures like the aorta (a major blood vessel).
The aorta cannot expand in response to increased blood pressure, potentially leading to rupture and death.
Fibrous Connective Tissue
Exists in two forms: Loose fibrous tissue and Dense fibrous tissue.
Both contain fibroblasts.
Fibroblasts: Cells in connective tissues that produce fibers and other substances.
Located some distance from one another and separated by a jelly-like ground substance.
Contain white collagen fibers and yellow elastic fibers.
Matrix: Includes both the ground substance and the fibers.
Loose Fibrous Connective Tissue
Composition: Mainly fibroblasts widely separated by a matrix containing collagen and elastic fibers.
Types: Includes areolar and reticular connective tissue.
Functions and Locations:
Supports epithelium and many internal organs.
Its presence in lungs, arteries, and the urinary bladder allows these organs to expand.
Forms a protective covering, enclosing many internal organs (e.g., muscles, blood vessels, nerves).
Adipose Tissue
Definition: A special type of loose connective tissue in which the cells enlarge and store fat.
Characteristics:
Has little extracellular matrix.
Cells, called adipocytes, are crowded and each is filled with liquid fat.
Functions:
The body uses stored fat for energy.
Provides insulation.
Offers organ protection.
Releases a hormone that regulates appetite control centers in the brain.
Primary Locations: Beneath the skin, around the kidneys, and on the surface of the heart.
Dense Fibrous Connective Tissue
Composition: Contains many collagen fibers packed closely together.
Functions: Has more specific functions than loose connective tissue.
Examples and Locations:
Tendons: Connect muscles to bones.
Ligaments: Connect bones to other bones at joints.
Supportive Connective Tissue
Main Types: Cartilage and Bone.
General Functions: Provide structure, shape, protection, and leverage for movement.
Cartilage
Characteristics:
More flexible than bone because it lacks mineralization of the matrix.
Cells lie within small chambers called lacunae.
Cells are separated by a solid yet flexible proteinaceous matrix.
Matrix is formed by cells called chondroblasts (immature) and chondrocytes (mature).
Healing: Heals slowly due to a lack of direct blood supply.
Three Types of Cartilage (distinguished by the type of fiber in the matrix):
Hyaline Cartilage:
Composition: Cells lie in lacunae within a white, translucent matrix containing very fine collagen fibers (only fine collagen fibers).
Appearance: Matrix has a glassy, translucent appearance.
Locations: Found in the nose, at the ends of long bones, in the ribs, and forms rings in the walls of respiratory passages.
Development: The fetal skeleton is also made up of this type of cartilage, which is later replaced by bone.
Elastic Cartilage:
Composition: Contains a greater number of elastic fibers.
Characteristic: More flexible.
Location: Found in the framework of the outer ear.
Fibrocartilage:
Composition: Matrix containing strong collagen fibers.
Function: Found in structures that withstand tension and pressure.
Locations: Examples include the discs between the vertebrae in the backbone and the cushion in the knee joint.
Bone
Characteristics: The most rigid connective tissue.
Matrix Composition:
Extremely hard matrix of inorganic salts (notably calcium salts).
These salts give bone its rigidity.
Deposited around protein fibers, especially collagen fibers.
Protein fibers provide elasticity and strength (similar to steel rods in reinforced concrete).
Cells: Osteoblasts (bone-forming cells) and osteoclasts (bone-resorbing cells) are responsible for forming the matrix.
Types of Bone:
Compact Bone:
Structure: Makes up the shaft of a long bone.
Consists of cylindrical structural units called osteons.
The central canal of each osteon is surrounded by concentric rings of hard matrix.
Bone cells (osteocytes) are located in lacunae between the rings of the matrix.
The central canal contains nerve fibers (carry nerve impulses) and blood vessels (carry nutrients), allowing bone to renew itself.
Spongy Bone:
Structure: Porous bone found at the ends of long bones.
Appears as an open bony latticework with numerous bony bars and plates separated by irregular spaces.
Strength: Although lighter than compact bone, it is designed for strength, with solid portions following lines of stress (like braces in buildings).
Location: The ends of long bones, also surrounds the bone marrow cavity (covered by compact bone, forming a sandwich structure).
Function: Site where red bone marrow is sometimes located.
Fluid Connective Tissue
The human body has two types: Blood and Lymph.
Blood
Definition: A fluid connective tissue located in blood vessels.
Components: Consists of formed elements and plasma.
Functions:
Transports nutrients and oxygen to the interstitial fluid that surrounds body cells.
Removes carbon dioxide and other waste products.
Helps distribute heat.
Plays a role in fluid, ion, and pH balance (regulated by various body systems).
Formed Elements (each has a specific function):
Red Blood Cells (Erythrocytes):
Characteristics: Small, biconcave disc-shaped cells that lose their nuclei as they mature.
Hemoglobin: Presence of the red pigment hemoglobin makes cells and blood red.
Hemoglobin is composed of 4 units.
Each unit has the protein globin and a complex iron-containing structure called heme.
The iron forms a loose association with oxygen.
Function: Transport oxygen from lungs to the tissues.
White Blood Cells (Leukocytes):
Characteristics: Have a nucleus (distinguishes them from red blood cells).
Appearance: Translucent without staining.
Function: Protect the body from invasion by foreign substances and organisms (infection).
Many are phagocytic cells (engulf infectious agents like bacteria).
Others produce antibodies (molecules that combine with foreign substances to inactivate them).
Some directly attack specific invading agents or infected body cells.
Platelets:
Characteristics: Fragments of giant cells present only in the bone marrow.
Function: When a blood vessel is damaged, platelets form a plug that seals the vessel.
Injured tissues release molecules that help the clotting process.
Lymph
Definition: A clear, sometimes faintly yellow, fluid connective tissue.
Derived from the interstitial fluid (fluid surrounding the tissues).
Contains white blood cells.
Lymphatic Vessels:
Absorb excess interstitial fluid and various dissolved solutes in the tissues.
Transport lymph to the particular vessels of the cardiovascular system.
Absorb fat molecules from the small intestine.
Lymph Nodes:
Composed of fibrous connective tissue.
Occur along the length of the lymphatic vessels.
Function: Lymph is cleansed as it passes through lymph nodes.
White blood cells congregate there, causing lymph nodes to enlarge during an infection.