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• Chapter 41 discusses animal bodies and homeostasis

Key Concepts

• Animal body organization
• Relationship between structure and function
• General homeostasis principles
• Homeostatic control of internal states

Organization of Animal Bodies

• All animal cells share similarities
• Animal cells exchange materials with their surroundings
• Animal cells obtain energy from organic nutrients
• Animal cells synthesize complex molecules
• Animal cells detect and respond to signals in their immediate environment
• Animal cells reproduce
• Animal life begins as a single, fertilized egg that divides multiple times
• Daughter cells specialize in a specific function

Internal Organization of Animals

• Cells with similar properties form tissues
• Tissues combine to form organs
• Organs make up organ systems
• Tissues are associations of cells with similar structure and function
• Muscle tissue facilitates movement of limbs and organs, and includes skeletal, cardiac, and smooth subtypes
• Nervous tissue is composed of specialized cells like neurons and glia, which process signals and communication
• Epithelial tissue covers body surfaces (skin) and internal surfaces, providing overall protection
• Connective tissue offers structural support and connections
• Skeletal muscle attaches to the skeleton, facilitating locomotion under voluntary control
• Smooth muscle surrounds tubes and cavities for involuntary control of propulsion
• Cardiac muscle is exclusive to the heart and works through involuntary control
• In asthma, bronchioles' smooth muscle tissue contracts

Types of Epithelial Tissues

• Epithelial tissues comprise densely packed cells
• Epithelial Tissues cover the body, enclose organs, and line body cavities
• Cell shapes include cuboidal, squamous, and columnar
• Tissue arrangements include simple (one layer), stratified (multiple layers), pseudostratified (one layer that appears stratified), transitional (multiple layers that expand/contract)
• Epithelial tissues are asymmetrical or polarized
• One side of epithelial rests on basal lamina, and the other faces the environment
• Epithelial tissue specializes in protection, secretion, and absorption of ions and organic molecules
• Epithelial tissue functions as selective barriers

Connective Tissues

• Connective tissues connect, surround, anchor, and support
• Loose connective tissue: Composed of fibroblasts, collagen, and elastic fibers, supporting organs/tissues with flexibility
• Dense connective tissue: Densely packed collagen fibers and fibroblasts, providing strong support; found in tendons, ligaments, and dermis
• Blood: Fluid of red/white blood cells and platelets in plasma, critical for oxygen/nutrient delivery and immune function
• Adipose tissue: Lipid-heavy cells insulating organs and storing energy
• Bone: Osteocytes in extracellular matrix of collagen with calcium salts, structurally supporting the body and aiding blood cell formation
• Cartilage: Composed of chondrocytes in a collagen/proteoglycan matrix, provides cushion and support

Organs

• Organs are made of two or more tissue types
• Organs form sheets, tubes, layers, bundles, or strips
• Organ systems are different organs working together for overall function
• Organ systems work together
• For example, nervous, circulatory, and endocrine systems influence kidney water retention
• Spatial arrangement of organs into organ systems contributes to the body plan
• Highly conserved genes control the body plan, which includes homologs in all animals

Major Organ Systems

• Circulatory: Transports solutes to all body parts and includes heart/vessels, blood/hemolymph
• Digestive: Breaks food into absorbable units, absorbs nutrients/ions/water, and eliminates waste using ingestion structures, storage structures, digestive/absorptive structures, and elimination structures
• Endocrine: Secretes hormones to regulate and coordinate growth, metabolism, blood/water balance, blood pressure, and reproduction
• Excretory: Removes soluble wastes and regulates body fluid volume/solute concentration, includes respiratory/urinary structures
• Immune/lymphatic: Defends against pathogens, involves white blood cells, lymph organs, and vessels
• Integumentary: Protects against dehydration/injury and pathogens, sometimes regulates temperature, includes body surfaces/skin
• Muscular-skeletal: Facilitates locomotion, generates force, propels material and supports body through muscles, bones/cartilage/exoskeleton, and connective tissues
• Nervous: Regulates/coordinates movement, sensation and organ functions for learning, involves brain, sensory structures, signal delivery system, and sense organs
• Reproductive: Produces gametes and provides nutritive environment for embryo/fetus via gonads and associated structures
• Respiratory: Exchanges O2 and CO2, regulates blood pH via gas-exchange sites

Hox Genes

• Hox genes are a family of ancient, highly conserved genes present in all animals
• Hox genes determine the timing and spatial patterns along the anteroposterior body axis during development
• Hox genes encode transcription factors that regulate other gene expression
• Mutant mice studies reveal that different Hox genes determine where organs form
• Hox genes are important for organ growth, development, and function
• Hox genes control cell proliferation, apoptosis, shape changes, cell migration, and cell-cell adhesion in organs

Relationship Between Structure and Function

• Comparing respiratory systems unveils structure-function relationships
• Similar structures suggest similar functions
• Insect respiratory systems have tubes connecting to the external environment
• Insect tracheoles (single-cell thick structures) serve as air conduits
• Thin cells provide the high surface area necessary for gas diffusion
• Mammalian respiratory systems use oxygenated blood via hemoglobin for tissue oxygenation
• Muscular contractions draw air into lungs, where gas exchange happens at alveoli (squamous epithelium)

Surface Area and Diffusion

• Diffusion requires extensive surface area in cells, tissues, and organs
• Increasing surface area comes at the expense of increasing volume
• Volume grows comparatively more than surface area
• Volume grows to the power of 3, where surface area grows to the power of 2
• This is overcome by changes in shape
• High surface-area-to-volume better facilitates material exchange
• Animals develop specialized structures to maximize surface area

Vertebrates and Physiological Variables

• Most physiological functions are constantly changing and are called variables
• Homeostasis stabilizes internal environments despite external changes
• Homeostasis maintains relatively stable internal conditions
• Normally blood sugar (glucose) remains at steady, predictable levels
• Levels of ingested glucose in the blood can increase quickly
• If you skip a meal, your blood glucose may slightly drop
• Homeostatic mechanisms restore normal blood glucose levels

Homeostatic Control Systems

• Cell, tissue, and organ activities are regulated/coordinated, so any extracellular fluid change prompts correction
• Control Systems feature a set point (normal value for variable)
• Control Systems have a sensor (monitors variable)
• Control Systems have an integrator (sensor input compared to set point)
• Control Systems have an effector (compensates deviations between actual and set point value)
• Example: body temperature regulation

Negative Feedback

• A variable triggers responses that move the variable in the opposite direction back to equilibrium
• Negative Feedback Prevents overcompensation by going too far in the opposite direction
• It occurs at organ, cellular, or molecular levels

Blood Pressure

• Blood pressure decreases due to blood loss sustained in a fight
• Sensors are blood vessels and heart sense the pressure change
• Integrator is the brain
• The body heals with neural pathways and hormones with vasoconstriction
• Blood pressure sensors in the heart and arteries sense a pressure drop.
• Signals go to the brain for hormonal secretions.

Positive Feedback

• It does not achieve homeostasis
• It increases the change/output
• It results in a very quick action (e.g., blood clotting and child birth)

Feedforward Regulation

• The body prepares for change to some variable before it occurs
• The body prepares anticipatory changes when seeing/smelling food or during exercise
• It speeds up homeostatic responses and minimizes deviations
• It commonly comes from or is modified by learning

Local and Long-Distance Chemical Signals

• Communication between cells links homeostatic processes
• Some responses are highly localized such as from an injury
• Paracrine signaling uses molecules released into interstitial fluid to impact neighboring cells
• Neurotransmitters are released to traverse an adjacent nerve cell
• Hormones are chemical messengers produced by the endocrine system to act on distant cells by secreting into the blood

Homeostatic Control of Internal Fluids

• Animal bodies consist mostly of water contained in two compartments
• Intracellular fluid is inside cells
• Extracellular fluid is outside cells
• Plasma is the fluid portion of the blood
• Interstitial fluid is between cells
• Vertebrates maintain separate intracellular/extracellular fluids within a closed circulatory system
• In invertebrates, the fluids mix as hemolymph
• Intracellular and extracellular fluid can vary widely in terms of solute composition

Movement of Solutes and Water

• Passive transport occurs when solutes move down a concentration gradient and does not require ATP hydrolysis
• Simple diffusion: Substances move across a membrane without assistance
• Only nonpolar/gaseous substances (O2 and CO2) use simple diffusion
• Facilitated diffusion: Uses protein transporters
• Active transport: Transport of solutes against concentration gradients
• The movement of water generally occurs readily between compartments
• While water crosses biological membranes slowly through simple diffusion, aquaporins improve permeability
• Water movement depends on pressure differences and osmosis
• Swollen/shrunken cells are fragile and die if membrane ruptures due to exposure to very dilute/concentrated extracellular fluids

Water-Ion Balances

• Maintaining water levels is important for animals
• Water participates in essential chemical reactions within the body
• Dehydration can be life-threatening, decreasing blood volume
• Ion balance is also necessary for animals
• Even a few percentage points shift in the concentration of certain ions can disrupt basic cellular activities
• Osmolarity describes the solute concentration of an aqueous solution (expressed in milliosmoles/liter)

Obligatory Exchanges and Water

• Many processes have the potential to disturb ion and water homeostasis
• Minimizing or reversing the disturbances needs extra energy expenditure
• Obligatory exchanges are so named because the animal is obligated to make them
• Regulated exchanges compensate for obligatory exchanges and are physiologically controlled

Exchanges Due to Respiration / Feeding / Water Evaporation

• Requirements for respiration and for water and ion balance are different for air- and water-breathing animals
• Air-breathing animals take in O2 and release CO2
• Breathing is associated with water loss
• Water-breathing animals move water over their respiratory organs (gills)
• Fresh water and salt water create reverse challenges
• Foods contain salts and water; eating also involves obligatory exchanges
• Some marine animals (not fish) drink seawater
• Salty environments lead to specialized epithelial cells called salt glands
• Endotherms (generate their own body heat) use body water to cool in active or hot conditions
• When sweat evaporates, water draws body heat from body
• Sweat is salty

Kidney Specialist's Work

• Robert Cade (kidney specialist) studied University of Florida football players in the heat
• Cade found that athletes should replenish what is lost by drinking an artificial solution that resembles sweat
• This led to Gatorade
• Replacement of ions and solutes can improve athletic performance because athletic performance can be enhanced by maintaining body ion and H2O balance

Osmoregulators and Osmoconformers

• Osmoregulators maintain stable ion concentrations and osmolarities while osmo-conforming to water conditions that highly vary from blood content
• They drink from and excrete water and ions as they have to
• Osmoregulators include all terrestrial animals, all freshwater fishes, and many marine animals
• Osmoconformers' osmolarity conforms to the environment
• Osmolarity in extra/intracellular fluids in the same, which is usually seawater
• Most marine invertebrates and some vertebrates, like sharks, are osmoconformers
• Osmoconformers are generally limited to the marine environment

Regulation of Body Temperature and The Nervous System

• Animals survive in a narrow range of temperatures
• Chemical reactions relate to animal temperatures
• Rate increases with increase in body temperature
• Enzymes have optimal temperatures
• Proteins can become denatured at high temps
• Heat alters structures of plasma and intracellular membranes
• Low temperatures cause membranes to become less fluid, more rigid
• High temperatures can cause membranes to become leaky
• Sources of heat are both internal and environmental
• To maintain stable body temperature, animals either fluctuate or remain stable in variable temperatures

Animal Classification

• Endothermic Homeotherms, such as mammals maintain a high body temperature
• Ectotherms require heat for an external source to warm themselves
• Body adjustments maintain chemical reactions at optimal levels when temperature imposes challenges
• Endotherms (warm themselves) are instantly capable of intense activity, whereas reptiles need to wait to warm up
• Endotherms have a need for food and energy to be used to create ATP
• Endotherms are also at risk or overheating and losing fluid, they need fluid-rich environments

Heat Exchange

• Animals exchange heat with the environment in these 4 ways:
• Radiation: Emission of electromagnetic thermal waves by the surfaces of objects determined by temperature of radiating surface
• Evaporation: Vaporization of water from the body for cooling
• Convection: Transfer of heat by air or fluid movement
• Direct contact heat transfer from cooler substances

Heat Gain and Loss

• Core temperature is constant, but the skin surface varies
• Body surface provides insulation only in animals with blubber
• Insulation in the form of hair, fur, or feathers, contributes to heat balance
• The amount of blood that flows to the skin will increase or decrease to heat gain or loss
• Vessels dilate to dissipate heat and constrict to retain it as seen in birds and small mammals

Countercurrent Heat Exchange

• Heat moves through fluids that flow in opposite directions from arteries to veins
• Arteries moving to adjacent veins is efficient
• Returns heat to the body's core and keeps core much warmer than extremities
• Countercurrent Heat Exchanges are seen in dolphin flippers and bird legs

Adaptations / Heat Production

• Animals maintain heat through evaporation and by altering body parts or behavior
• Evaporation changes the rate of perspiration
• Sweating means a dilute loss of solution
• Humidity determines the actual rate of evaporation
• Panting is an alternative to sweat glands
• This involves changing surface area by ruffling feathers or raising wings
• They can be huddling in groups for cold weather
• They can seek shade, or have migration patterns
• Muscle activity changes with temperature regulation
• Decreases in blood flow to regions limit conduction of heat while permitting it, as does Primary Response
• Muscle contractions without limb movements during are shivering thermogenesis
• Energy shows as internal heat
• Non-shivering thermogenesis increases metabolic rate without actually increasing muscle activity and occurs in brown adipose tissue

Three main questions to consider moving forward

• Describe the components of a homeostatic control system.
• Explain how negative and positive feedback are related to homeostasis.
• Describe osmolarity and how it is regulated by the body.

The Nervous System

• The nervous system contains the Central Nervous System and Peripheral Nervous System
• Central Nervous System is the brain and nerve cord and is localized in vertebrates
• Peripheral nervous system are all neurons outside the Central Nervous System
• The Autonomic Nervous System controls involuntary functions such as heart rate and digestion
• A simple version of this system in invertebrates blurs what we see as the system in the vertebrates

Cells of The Nervous System

• Nervous systems have neurons and glia
• Neurons use electricity and chemicals to send and receive to all systems through organs and connections
• Animals have neurons except sponges

Neuron Construction

• Neurons have a cell body, dendrites, and axons
• The cell body has a nucleus and organelles
• The branching extensions are dendrites These can have signals and impulses
• Axon, a body of a cell, usually single can is insulated by myelin

Glia

• Glia surround neurons and perform functions such as structural and metabolic support
• Glia form the blood-brain barrier and regulate blood flow+
• Glia can also maintain ion concentration and remove cellular debris and synapse buildup
• Glial cells also have protective or regulatory effects for sending impulses (Myelin Sheath)

There are 3 types of neurons

• Sensory neurons from the outside are central to the transmission to the nervous system
• Motor neurons send signal away from CNS to encourage a response
• Interneurons help those two interconnect information between each other