22.1 The Body’s Organization

• Millions of individual cells of different types come together to make an organism.

• Cells of the same type occur within a tissue, while an organ contains different types of tissues.

• Tissues are the structural and functional units of the body, and organ systems contain multiple organs that work together to perform a specific function.

• The urinary system is an example of an organ system composed of several individual organs, each playing a particular role in the overall process.

• The structure of cells, tissues, and organs directly aid function.

• Biologists have categorized animal tissues into four major types: epithelial, connective, muscular, and nervous.

• Each type of tissue is subdivided into even more types, except for nervous tissue.

• This chapter looks at the structure and function of each tissue type, as well as the organs and organ systems where they are used.

Epithelial Tissue Protects

• Epithelial tissue forms external coverings and internal linings of many organs and covers the entire surface of the body.

• For a substance to enter or exit the body, it must cross an epithelial tissue.

• Epithelial cells adhere to one another, but an epithelium is generally only one cell layer thick.

• Cuboidal epithelium lines the kidney tubules and the lumen of a portion of the kidney.

• Columnar epithelium lines portions of the lumen of the digestive tract.

• Epithelial cells can be classified by the number of layers these cells make in tissues.

• A layer that is only one cell thick is referred to as simple.

• Multiple layers of cells are called stratified.

• Pseudostratified epithelium is a special classification in which the tissue appears to have multiple layers of cells but actually has only one.

• Squamous epithelium is composed of thin, flattened cells.

• One or more types of epithelial cells are the primary components of glands, which produce and secrete products (mainly hormones).

• Epithelial tissue cells can go through mitosis frequently and quickly.

• Epithelial tissue is more likely than other tissue types to become cancerous.

Connective Tissue Connects and Supports

• Connective tissue types (Fig. 22.5) bind structures and provide support and protection.

• Connective tissue cells are separated by a matrix.

• The matrix varies from solid to semifluid to fluid.

• The matrix usually contains collagen fibers.

• Collagen is the most common protein in the human body.

• Collagen is mainly used for structural support.

Loose Fibrous and Related Connective Tissues

• Loose fibrous connective tissue occurs beneath an epithelium and connects it to other tissues within an organ.

• It forms a protective covering for many internal organs, such as muscles, blood vessels, and nerves.

• Fibroblasts are the cells that produce a matrix containing fibers, including collagen fibers and elastic fibers.

• Loose fibrous connective tissue in the walls of lungs and arteries gives these organs resilience.

• Adipose tissue is a type of loose connective tissue in which the fibroblasts enlarge and store fat, and there is limited matrix.

• Adipose tissue is located beneath the skin and around organs, such as the heart and kidneys, where it cushions and protects the organs and serves as long-term energy storage.

• Dense fibrous connective tissue contains more collagen fibers, which are packed closely together.

• Dense fibrous connective tissue is found in tendons, which connect skeletal muscles to bones, and in ligaments, which connect bones to other bones at joints.

• Cartilage is made up of cells that lie in small cavities called lacunae, separated by a semisolid yet flexible matrix.

• Hyaline cartilage is the most common type of cartilage and contains fine collagen fibers. Its matrix appears white and translucent when unstained.

• Hyaline cartilage is found in the nose, ends of long bones and ribs, respiratory passages, and the fetal skeleton.

• Cartilage is later replaced by bone, except in cartilaginous fishes like sharks.

• Bone is the most rigid connective tissue and consists of an extremely hard matrix of inorganic salts, primarily calcium salts, deposited around collagen fibers.

• Inorganic salts give bone rigidity, while collagen fibers provide elasticity and strength.

• Compact bone is the most common type of bone in humans and consists of cylindrical structural units called osteons.

• Bone cells are located in lacunae between the rings of matrix, and blood vessels in the central canal carry nutrients for bone renewal.

Blood

• Blood is composed of cells and plasma.

• Matrix in blood is not made by the cells of connective tissue.

• Blood is a connective tissue by definition.

• Blood has many functions for overall homeostasis of the body.

• Blood transports nutrients and oxygen to cells and removes their wastes.

• Blood helps distribute heat and plays a role in fluid, ion, and pH balance.

• Various components of blood help protect us from disease.

• Blood’s ability to clot prevents fluid loss.

• Blood contains three formed elements: red blood cells, white blood cells, and platelets.

• Red blood cells are small, biconcave, disk-shaped cells without nuclei.

• Hemoglobin makes the cells red and binds oxygen to transport it to cells.

• White blood cells are usually larger, have a nucleus, and fight infection in two primary ways.

• Platelets are fragments of giant cells present only in bone marrow.

• Platelets form a plug that seals a damaged blood vessel and release molecules that help the clotting process.

Muscular Tissue Moves the Body

• Muscular tissue and nervous tissue work together to enable animals to move.

• Muscular tissue contains contractile protein filaments, called actin and myosin filaments, that interact to produce movement.

• The three types of vertebrate muscles are skeletal, cardiac, and smooth.

• Skeletal muscle is attached by tendons to the bones of the skeleton, and when it contracts, bones move.

• Contraction of skeletal muscle occurs faster than in the other two muscle types and is under voluntary control.

• The cells of skeletal muscle, called fibers, are cylindrical and quite long, and have multiple nuclei.

• The fibers have alternating light and dark bands running across the cell, giving them a striated appearance.

• Cardiac muscle is found only in the walls of the heart, and its contraction pumps blood and accounts for the heartbeat.

• Like skeletal muscle, cardiac muscle has striations, but the contraction of the heart is autorhythmic and involuntary.

• Cardiac muscle cells have a single, centrally placed nucleus and are branched and seemingly fused with one another.

• Smooth muscle is named because the cells lack striations and form layers in which the thick middle portion of one cell is opposite the thin ends of adjacent cells.

• Smooth muscle is involuntary and is found in the walls of the viscera and blood vessels.

• Smooth muscle contracts more slowly than skeletal muscle but can remain contracted for a longer time.

Nervous Tissue Communicates

• Nervous tissue coordinates body functions and responses to internal and external environments.

• The nervous system depends on sensory input, integration of data, and motor output.

• Nerves conduct impulses from sensory receptors to the spinal cord and brain for integration.

• Sensation occurs only in the brain.

• Nerves conduct impulses away from the spinal cord and brain to muscles and glands for response.

• Neuron has three parts: dendrites, cell body, and axon.

• Dendrites conduct signals toward the cell body.

• The cell body contains cytoplasm and a nucleus.

• Axon conducts nerve impulses away from the cell body to other cells.

• The brain and spinal cord contain many neurons, while nerves contain bundles of axons.

• Neuroglia support and nourish neurons, outnumbering neurons 9 to 1.

• Schwann cells are a type of neuroglia that form a protective coating called myelin sheath on axons.

• Myelin sheaths insulate axons and allow nerve impulses to travel more quickly.

22.2 Organs and Organ Systems

• Organs are composed of multiple tissues.

• The structure and function of an organ depend on its tissues.

• An organ can perform a function that none of its tissues can do alone.

• The bladder's function is to store and expel urine.

• The bladder's function is dependent on its individual tissues.

• The bladder's epithelium lining helps store urine and prevents leakage.

• The bladder's muscles propel urine into the urethra for removal.

• Organ systems' functions depend on their organs.

• The urinary system produces, stores, and transports urine.

• The kidneys produce urine, the bladder stores it, and tubes transport it.

• The body's systems are divided into 5 categories.

• These systems contribute to homeostasis, the constancy of the internal environment.

Transport and Protection

• The cardiovascular system:

  • Consists of blood, heart, and blood vessels.

  • Transports nutrients and oxygen to interstitial fluid for cells.

  • Removes waste molecules from interstitial fluid.

  • Surrounds the body's cells with interstitial fluid.

• The lymphatic system:

  • Consists of lymphatic vessels, lymph, lymph nodes, and other lymphatic organs.

  • Absorbs fat from the digestive system.

  • Collects excess interstitial fluid.

  • Returns excess fluid to the blood in the cardiovascular system.

• Both systems:

  • Involved in protecting the body against disease.

  • Certain cells in the lymph and blood are part of the immune system.

  • Along with the thymus and spleen, specifically protect the body from disease.

Maintenance of the Body

• Three systems (respiratory, urinary, and digestive) add or remove substances from the blood.

• If blood composition remains constant, so does that of the interstitial fluid.

• The respiratory system consists of the lungs, trachea, and other structures that take air to and from the lungs.

• The respiratory system brings oxygen into the body and takes carbon dioxide out through the lungs.

• Respiratory system exchanges gases with the blood.

• The urinary system consists of kidneys, a urinary bladder, and structures that transport urine.

• The urinary system rids the blood of wastes and helps regulate the fluid level and the chemical content of the blood.

• The digestive system consists of organs along the digestive tract, teeth, salivary glands, liver, and pancreas.

• The digestive system receives food and digests it into nutrient molecules.

• Nutrient molecules then enter the blood.

Control

• The nervous system includes the brain, spinal cord, and associated nerves.

• Nerves conduct nerve impulses from receptors to the brain and spinal cord and from the brain and spinal cord to muscles and glands.

• Sensory receptors and sense organs are sometimes considered part of the nervous system.

• The endocrine system consists of hormonal glands, such as the thyroid and adrenal glands, which secrete hormones.

• Hormones serve as messengers between body parts.

• Both the nervous and endocrine systems coordinate and regulate the functions of the body's other systems.

• The nervous system causes quick responses, while the endocrine system's responses tend to last longer.

• The endocrine system helps maintain the proper functioning of the male and female reproductive organs.

Sensory Input and Motor Output

• The integumentary system:

  • Consists of the skin and its accessory structures.

  • Contains sensory receptors that respond to external stimuli.

  • Communicates with the brain and spinal cord via nerve fibers.

  • It may cause the brain to respond to a stimulus.

• The skeletal system:

  • Enables movement of body parts.

  • Serves as a place of attachment for skeletal muscles.

  • Protects body parts, such as the skull for the brain and rib cage for the heart and lungs.

• The muscular system:

  • Accounts for the movement of body parts.

  • Works with the skeletal system to enable movement.

• All three systems:

  • Protect and support the body.

  • Work together to enable movement and respond to external stimuli.

  • Are essential for overall body function.

Reproduction

• The reproductive system involves different organs in males and females.

• The male reproductive system consists of:

  • Testes

  • Other glands.

  • Various ducts, such as the ductus deferens.

  • Conducts semen to and through the penis.

  • Testes produce sex cells called sperm.

• The female reproductive system consists of:

  • Ovaries.

  • Uterine tubes.

  • Uterus.

  • Vagina.

  • External genitals.

  • Ovaries produce sex cells called eggs.

• Fertilization of an egg by a sperm leads to the development of offspring.

22.3 Homeostasis

• External environment: everything outside the body.

• Internal environment: cells, tissues, fluids, and organs.

• Cells live in interstitial fluid, which is constantly renewed with nutrients and gases via exchanges with the blood.

• Blood and interstitial fluid constitute part of the body’s internal environment.

• Volume and composition of interstitial fluid remain relatively constant as long as blood composition remains near normal levels.

• Maintenance of the relatively constant condition of the internal environment within acceptable ranges is called homeostasis.

• Homeostasis keeps internal conditions within a narrow range despite external changes.

• Examples: body temperature maintained near 37°C, blood pH stays about 7.4 even after eating acidic foods, and blood sugar remain at about 0.1% even after eating a candy bar.

Organ Systems and Homeostasis

• All systems of the body contribute to maintaining homeostasis.

• The cardiovascular system conducts blood to and away from capillaries.

• Red blood cells transport oxygen and participate in the transport of carbon dioxide.

• White blood cells fight infection, and platelets participate in the clotting process.

• Lymphatic capillaries collect excess interstitial fluid and return it via lymphatic vessels to the cardiovascular system.

• Lymph nodes help purify lymph and keep it free of pathogens.

• The digestive system takes in and digests food, providing nutrient molecules that enter the blood to replace those that are constantly being used by the body’s cells.

• The respiratory system removes carbon dioxide from and adds oxygen to the blood.

• Kidneys remove metabolic wastes and regulate blood volume, salt balance, and pH.

• The liver regulates the glucose concentration of the blood, removes excess glucose from the blood for storage as glycogen, and makes urea, a nitrogenous end product of protein metabolism.

• The nervous and endocrine systems regulate the other systems of the body.

• Sensory receptors send nerve impulses to control centers in the brain, which then direct effectors (muscles or glands) to become active.

• Endocrine glands secrete hormones that bring about slower, more lasting changes that keep the internal environment relatively stable.

Negative Feedback

• Negative feedback is the primary homeostatic mechanism that maintains internal environment stability.

• The negative feedback mechanism has two components: the sensor and the control center.

• The sensor detects a change in the internal environment (stimulus).

• The control center initiates an effect that brings conditions back to normal.

• The negative feedback mechanism is present when the output of the system dampens the original stimulus.

• Example: Pancreas secretes insulin when the blood glucose level is too high, causing cells to take up glucose and return the blood sugar level to normal.

• When feedback mechanisms cannot compensate, illness results.

• Example: In diabetes mellitus, the pancreas is unable to produce insulin, causing dangerously high blood sugar levels and serious illness.

• The study of homeostatic mechanisms is medically important.

Mechanical Example

• A home heating system is used to illustrate a negative feedback mechanism.

• The thermostat is set at 20°C (68°F), which is the set point.

• The thermostat has a thermometer that detects when the room temperature is above or below the set point.

• The thermostat also has a control center that turns the furnace off when the room is warm and turns it on when the room is cool.

• When the furnace is off, the room cools down a bit, and when the furnace is on, the room warms up a bit.

• This results in fluctuation above and below the set point due to the negative feedback system.

Human Example: Regulation of Body Temperature

• The thermostat for body temperature is located in the hypothalamus.

• When the core body temperature falls below normal, the control center directs the blood vessels of the skin to constrict, conserving heat.

• If the core body temperature falls even lower, the control center sends nerve impulses to the skeletal muscles, and shivering occurs.

• Shivering generates heat, and gradually body temperature rises toward 37°C (98.6°F).

• When the temperature rises to normal, the control center is inactivated.

• When body temperature is higher than normal, the control center directs the blood vessels of the skin to dilate.

• More blood is then able to flow near the surface of the body, where heat can be lost to the environment.

• The nervous system activates the sweat glands, and the evaporation of sweat helps lower body temperature.

• Gradually, body temperature decreases to 37°C (98.6°F).

• A negative feedback mechanism prevents continued change in the same direction.

• Body temperature does not get warmer and warmer because warmth stimulates changes that decrease body temperature.

• Body temperature does not get colder and colder because a body temperature below normal causes changes that bring body temperature up.