Chapter 36 Notes

An Introduction to Animal Structure and Function

Lecture Notes:

There are several unifying themes in the study of animal anatomy and physiology.

· There is a correlation between form and function; functions are properties that emerge from the specific shape and order of body parts.

· A comparative approach allows us to see how species of diverse evolutionary history and varying complexity solve problems common to all.

· Animals, as all living organisms, have the capacity to respond and adjust to environmental change in two temporal scales:

→ Over the long term by adaptation due to natural selection.

→ Over the short term by physiological responses.

The purpose of this introductory chapter is to:

· Illustrate the hierarchy of structural order characterizing animals.

· Emphasize the importance of energetics in animal life (how animals obtain, process, and use chemical energy).

· See how animal body forms affect their interactions with the environment.

I. The functions of animal tissue and organs are correlated with their structures

There is a structural hierarchy of life:

· Atoms → molecules → supramolecular structures → cell.

· The cell is the lowest level of organization that can live as an organism.

The hierarchy of multicellular organisms is: cell → tissues → organs → organ systems.

A. Animal Tissue

Tissues = Groups of cells with common structure and function.

· Cells may be held together by a sticky coating or woven together in a fabric of extracellular fibers.

There are four main categories of tissues: epithelial tissue, connective tissue, muscle tissue, and nervous tissue.

1. Epithelial Tissue

Formed from sheets of tightly packed cells, epithelial tissue covers the outside of the body and lines organs and body cavities. Characteristics of epithelium include:

· Cells are closely joined and are riveted by tight junction in some. (See Campbell, Chapter 7).

· It functions as barriers against mechanical injury, invading microbes, and fluid loss.

· Its free surface is exposed to air or fluid. Cells at the base are attached to a basement membrane, which is a dense layer of extracellular material.

Epithelial tissue cells are categorized by the number of layers and shape of the free surface cells:

· Simple epithelium in one layer of cells.

· Stratified epithelium has multiple tiers of cells.

· Pseudostratified epithelium is one layer of cells that appear to be multiple since the cells vary in length.

· Cells shapes are: cuboidal (like dice), columnar (bricks on end), or squamos (like flat floor tile).

· A tissue may be described by a combination of terms such as stratified squamos epithelium.

Some epithelia are specialized for absorption or secretion of chemical solutions in addition to their protective role.

· Some epithelia are ciliated (e.g. the lining of the respiratory system).

· The mucus membranes lining the oral cavity and nasal passageways secrete mucus, which moistens and lubricates the surfaces.

· The structure fits function. For example, simple squamos epithelium is leaky and is specialized for exchange of materials by diffusion. It is found in blood vessel linings and air sacs.

2. Connective Tissue

Connective is characterized by a sparse cell population scattered through an extensive extracellular matrix.

· Functions to bind and support other tissues.

· Matrix is a web of fibers embedded in a homogenous ground substance.

· Major types of connective tissue include:

→ loose connective tissue

→ adipose tissue

→ fibrous connective tissue

→ cartilage

→ bone

→ blood.

Loose connective tissue holds organs in place and attaches epithelia to underlying tissue.

· Consists of a loose weave of 3 types of proteinaceous fibers:

→ Collagenous fibers are bundles of fibers containing 3 collagen molecules each. Great tensile strength; resist stretching.

→ Elastic fibers are long threads of the protein elastin. Elastic properties

lend tissue a resilience to quickly return to the original shape.

→ Reticular fibers are branched and form a tightly woven fabric joining

connective tissue to adjacent tissues.

· Consists of two types of cells:

→ Fibroblasts secrete the proteins of extracellular fibers.

→ Macrophages are phagocytic amoeboid cells that function in immune defense of the body.

Andipose tissue:

· Is loose connective tissue specialized to store fat in adipose cells distributed throughout its matrix.

· Insulates the body and stores fuel molecules.

· Each adipose cell has one large fat droplet which can vary in size as fats are stored and utilized.

Fibrous connective tissue:

· Is dense due to the arrangement of a large number of collagenous fibers in parallel bundles, which imparts great tensile strength.

· Found in tendons (attach muscles to bones) and ligaments (attach bones together at joints).

Cartilage:

· Is composed of collagenous fibers embedded in chondroitin sulfate, a protein-carbohydrate ground substance.

· Cartilage cells, or chondrocytes:

→ Secrete both collagen and chondroitin sulfate, which make cartilage both strong and flexible.

→ Are confined to lacunae, scattered spaces within the ground substance.

· Comprises the skeleton of all vertebrate embryos.

→ Some vertebrates (e.g. sharks) retain the cartilaginous skeleton as adults.

→ Most vertebrates eventually replace most of the cartilage with bone. Cartilage is retained in areas such as the nose, ears, trachea, intervertebral discs and ends of some bones.

Bone is a mineralized connective tissue.

Ÿ Osteoblasts, bone forming cells, deposit a matrix of collagen and calcium phosphate which hardens into the mineral hydroxyapatite. The combination of collagen and hydroxyapatite makes the bone harder than cartilage, but not brittle.

Ÿ Bone consists of repeating Haversian systems (concentric layers or lamellae deposited around a central canal containing blood vessels and nerves).

Ÿ Osteocytes are located in spaces called lacunae surrounded by a hard matrix and are connected to each other by cell extensions caled canaliculi.

Ÿ In long bones, only the outer area is hard and compact; the inner area is filled with spongy bone tissue called marrow.

Blood is a connective tissue composed of:

Ÿ Liquid extracellular matrix of plasma, which contains water, salts, and proteins.

Ÿ Cellular component which contains:

→ Leukocytes, white blood cells that function in immune defense.

→ Erythrocytes, red blood cells that transport oxygen.

→ Platelets, cell fragments that function in blood clotting.

Ÿ Blood cells are made in red marrow near the ends of long bones.

3. Muscle Tissue

Muscle Tissue consists of long, excitable cells capable of contraction.

Ÿ In the muscle cell, cytoplasm are parallel bundles of microfilaments made of the contractile proteins, actin and myosin.

Ÿ Muscle is the most abundant tissue in most animals.

There are three types of vertebrate muscle tissue:

Ÿ Skeletal muscle is responsible for voluntary movements.

→ Attached to bones is tendons.

→ Microfilaments are aligned to form a banded striated appearance.

Ÿ Cardiac muscle form the contractile wall of the heart.

→ Cells are striated and branched.

→ Ends of cells are joined by intercalated disks, which relay the contractile impulse from cell to cell.

Ÿ Visceral muscle is smooth (unstriated) tissue in walls of internal organs.

→ Spindle-shaped cells contract slowly, but can retain contracted condition longer than skeletal muscle.

→ Responsible for involuntary movements (e.g. churning of the stomach).

4. Nervous Tissue

Nervous Tissue senses stimuli and transmits signals from one part of the animal to another.

Neuron = Nerve cell specialized to conduct an impulse of bioelectric signal.

Consists of:

Ÿ Cell body

Ÿ Dendrites, extensions that conduct impulses to the cell body.

Ÿ Axons, extensions that transmit impulses away from the cell body. (See Campbell, Chapter 44)

B. Organs and Organ Systems

Tissues are organized into organs in all but the simplest animals.

Ÿ May be layered, such as the dermis in humans

Ÿ Many organs are suspended by sheets of connective tissue called mesenteries.

Ÿ Organs may be organized into organ systems.

Organ systems = Several organs with separate functions that act in a coordinated manner (e.g. digestive, circulatory, and respiratory systems).

Ÿ Systems are interdependent: an organism is a living whole greater than the sum of its parts.

II. Bioenergetics is fundamental to all animal functions

Animals, as living organisms, exchange energy with the environment. Since they are heterotrophic, animals acquire energy from organic molecules synthesized by other organisms.

Energy input - the ingestion of food

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Digestion - enzymatic hydrolysis of food

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Absorption - body cells absorb small energy-containing molecules

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Catabolism - Cellular respiration and fermentation harvests chemical energy from food molecules.

Some energy stored in ATP Some energy lost as heat to surroundings

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Energy used - chemical energy of ATP powers cellular work. After the needs of staying alive are met, leftover chemical energy and carbon skeletons from food molecules can be used in biosynthesis.

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Energy lost - cellular work generates heat, which is lost to the surroundings.

A. Metabolic Rate

Bioenergetics, the study of the dynamic balance between energy intake and loss is an organism, gives clues to how an animal adapts to its environment. By measuring the rate of energy use, physiologists can determine:

Ÿ How much food energy an animal needs just to stay alive.

Ÿ The energy costs for specific activities such as walking or running.

Metabolic rate = total amount of energy an animal uses per unit of time; usually measured in kilocalories (kcal or CAL = 1000 calories). Can be determined by:

Ÿ Measuring the amount of oxygen used for an animal’s cellular respiration.

Ÿ Measuring an animal’s heat loss per unit of time.

® Heat loss, a by product of cellular work, is measured with a calorimeter - a closed, insulated chamber with a device that records heat production.

® Calorimeters are effectively used with small animals that have high metabolic rates, but are less precise with small animals that have low metabolic rats and with large animals.

Every animal has a range of metabolic rates:

Ÿ Minimal rates support basic life functions, such as breathing.

Ÿ Maximal rates occur during peak activity, such as all-out running.

Ÿ Between these extremes, metabolic rates can be influenced by many factors, such as:

® age, sex, and size

® body temperature

® environmental temperature

® food quality and quanity

® activity level

® amount of available oxygen

® hormonal balance

® time of day

Endotherms = Animals that generate their own body heat metabolically.

Ÿ Examples include birds and mammals

Ÿ Require more kilcalories to sustain minimal life functions than ectotherms.

Ÿ Many are also homeothermic, that is their body temperature must be maintained within narrow limits.

Basal Metabolic Rate (BMR) = An endothermic animal’s metabolic rate measured under resting, fasting, and stress-free conditions.

Ÿ Average human BMR is 1600-1800 kcal/day for adult males; 1300-1500 kcal/day for adult females.

Ectotherms = Animals that acquire most of their body heat from the environment.

Ÿ Include most fish, amphibians, reptiles, and invertebrates.

Ÿ Are energetically different from endotherms; body temperature and metabolic rate changes with environmental temperature.

Ÿ Because it is influenced by temperature, an ectoderm’s minimal metabolic rate (SMR) must be determined at a specific temperature.

Standard Metabolic Rate = An ectotherm’s metabolic rate measured under controlled temperature and under resting, fasting, and stress-free conditions.

B. Body Size and Metabolic Rate

There is an inverse relationship between metabolic rate and size.

Ÿ Smaller animals consume more calories per gram than larger animals

Ÿ Correlated with a higher metabolic rate and a need for faster oxygen delivery to the tissues, small animals also have higher:

® breathing rates

® blood volume

® heart rate

Ÿ This inverse relationship between metabolic rate and body size holds true for both endotherms and ectotherms, and is not simply a function of surface area to volume ratio.

III. An animal’s size and shape affect its interactions with the external environment

An animal’s body plan results from a developmental pattern programmed by its genome - a product of millions of years of evolution due to natural selection.

A. Body Size, Proportions, and Posture

Body proportions and size-weight relationships change in animal bodies as they become larger.

Ÿ Body design must accommodate the greater demand for support that comes with increasing size. (The strain on body support depends on an animal’s weight, which increases as the cube of its height or other linear dimension.)

Ÿ In mammals and birds, the most important design feature in supporting body weight is posture - leg position relative to the main body - rather than leg bone size. For example:

® The legs of an elephant are in a more upright position that those of small mammals.

® Large mammals run with legs nearly extended, which reduces strain; whereas, small mammals run with legs bent and crouch when standing.

Ÿ Bioenergetics also plays an important role in load-bearing, since crouched posture is partly a function of muscle contraction, powered by chemical energy.

B. Body Plans and Exchange with the Environment

Animal cells must have enough surface area in contact with an aqueous medium to allow adequate environmental exchange of dissolved oxygen, nutrients, and wastes. This requirement imposes constraints on animal size and shape.

Ÿ Unicellular organisms, such as protozoans, must have sufficient surface area of plasma membrane to service the entire volume of cytoplasm and are thus limited in size. Recall that:

® The upper limits of cells size are imposed by the surface area to volume ratio.

® As cell size increases, volume increases proportionately more than surface area.

Ÿ Some multicellular animals have a body plan that places all cells in direct contact with their own aqueous environments. Two such body plans include:

® Two-layered sac - a body wall only two cell layers thick. For example, the body cavity of Hydra opens to the exterior, so both outer and inner layers of cells are bathed in water.

® Flat-shaped body with maximum surface area exposed to the aqueous environment. For example, tapeworms are thin and flat, so most cells are bathed in the intestinal fluid of the worm’s vertebrate host.

Ÿ Most complex animals have a smaller surface area to volume ratio and thus lack adequate exchange area on the outer surface.

® Instead, highly folded, moist internal surfaces exchange materials with the environment.

® The circulatory system shuttles materials between these specialized exchange surfaces.

Though logistical problems exist with environmental exchange, there are some distinct advantages to a compact body form.

Ÿ Environmental exchange surfaces are internal and protected from desiccation, so the animal can live on land.

Ÿ Cells are bathed with internal body fluid, so the animal can control the quality of the cells’ immediate environment.

IV. Homeostatic mechanisms regulate an animal’s internal environment

Interstitial fluid = The internal environment of vertebrates, composed of fluid between the cells:

Ÿ Fills spaces between cells.

Ÿ Exchanges nutrients and wastes with blood carried in capillaries.

Homeostasis = Dynamic state of equilibrium in which internal conditions remain relatively stable; steady state.

Ÿ French physiologist Claude Bernard first described the “constant internal milieu” in animals; he recognized many animals can maintain constant conditions in their internal environment - even when the external environment changes.

Ÿ Maintained by control systems which include three components:

® receptor - detects internal change

® control center - processes information from the receptor and directs the effector to respond.

® effector - provides response.

Ÿ As a control system operates, the effector’s response feeds back and influences the magnitude of the stimulus by either depressing it (negative feedback) or enhancing it (positive feedback).

Negative Feedback = Homeostatic mechanism that stops or reduces the intensity of the o original stimulus and consequently causes a change in a variable that is opposite in direction to the initial change.

Ÿ Most common homeostatic mechanism in animals.

Ÿ There is a lag time between sensation and response, so the variable drifts slightly above the set point.

Ÿ A non-biological example is the thermostatic control of room temperature. (See Campbell, Figure 36.11.) Human examples include hormonal control of blood glucose levels and the regulation of body temperature by the hypothalamus.

® If the human hypothalamus detects a high blood temperature, it sends nerve impulses to sweat glands, which increase sweat output and cause evaporative cooling.

® When the body temperature returns to normal, no additional signals are sent.

Set Point = A variable’s range of values that must be maintained to preserve homeostasis.

Positive Feedback = Homeostatic mechanism that enhances the initial change in a variable.

Ÿ Rarer than negative feedback and usually controls episodic events.

Ÿ Examples include blood clotting and the heightening of labor contractions during which childbirth.

® During childbirth, the baby’s head against the uterine opening stimulates contractions which cause greater pressure of the head against the uterine opening.

® The greater pressure, in turn, enhances uterine contractions further.