41 physiology, homeostasis, and temperature regulation

41Physiology, Homeostasis, and

Temperature Regulation

41 Homeostasis:Maintaining the Internal Environment

• Homeostasis is the maintenance of constant conditions in the internal environment of an organism.

• Single-celled organisms and simple multicellular animals meet all of their needs by direct exchange of substances with the external environment.

41 Homeostasis:Maintaining the Internal Environment

• Complex, multicellular organisms have specialized cells that help maintain an internal environment.

• The internal environment consists of extracellular fluid that bathes every cell. Cells exchange materials with this environment.

• Homeostasis is an essential feature of complex animals.

Figure 41.1 Maintaining Internal Stability while on the Go

41 Tissues, Organs, and Organ Systems

• Cells grouped together with the same characteristics or specializations are called tissues.

• The four basic types of tissue are epithelial, connective, muscle, and nervous.

• An organ is composed of tissues, usually of several different types.

Figure 41.2 Four Types of Tissue

Lining, transport, secretion, & absorption

Support, strength, elasticity

Movement

Information synthesis, communitcation, control


• Epithelial tissues are sheets of densely packed and tightly connected cells that cover inner and outer body surfaces.

• Specialized functions:

Secretion of hormones, milk, mucus, digestive enzymes, sweat

Contain cilia to move substances.

Chemoreceptors for taste, smell, etc.

Protective, absorptive, or transport functions.


• Epithelial tissues have distinct inner and outer surfaces.

• The outer surfaces are the apical ends of the epithelial cells. They face the air (skin, lungs) or a fluid-filled organ cavity (the lumen of the gut).

• Apical ends may have cilia or be highly folded to increase surface area.

• The inner surfaces are the basil ends; they rest on an extracellular matrix called a basal lamina.

• Some epithelial tissue, such as skin, gets much wear and tear, and thus has a high rate of cell division and replacement.


• Connective tissue consists of cells embedded in an extracellular matrix that they secrete.

• Protein fibers is an important component.

• The most common is collagen, a very strong fiber.

very dense in tough tendons and ligaments.

forms a netlike framework for organs, to give shape and strength.


• Other protein fibers include elastin which can be stretched to several times its resting length and then recoil.

• Tissues that are regularly stretched, such as lung walls and artery walls, have abundant elastin.

Lung ElastinAorta Elastin


• Cartilage and bone connective tissue provide rigid structural support.

• Cartilage is a network of collagen fibers embedded in a flexible matrix of proteins and carbohydrates. It is found in the external ears, nose, and trachea, and lines joints of vertebrates.

• The extracellular matrix of bone is hardened by the deposition of calcium phosphate.

Cartilage

Bone


• Adipose tissue is a connective tissue that forms and stores droplets of lipids.

Serves as a: fuel reserve cushion to protect internal organs help insulate against heat loss.

• Blood is also a connective tissue made up of blood plasma.

Adipose tissue: fat

cells

Blood cells


• Muscle tissues are made of elongated cells capable of contracting and causing movement by a sliding of protein filaments past each other.

• They are the most abundant tissues in the body and use most of the energy the body produces.


• Nervous tissue is composed of neurons.

• Neurons are extremely diverse in size and form. They function by generating electrochemical signals in the form of nerve impulses.

• These impulses are conducted via long extensions to other parts of the body where they communicate with other neurons, muscle cells, or secretory cells to control activities of organ systems.


• A discrete structure that carries out a specific function in the body is an organ.

• Most organs include all four tissue types.

• Most organs are part of an organ system, a group of organs that function together.

• Review the major organ systems in your book (page 784).

41 Physiological Regulation and Homeostasis

• Homeostasis depends on the ability to regulate the functions of organs and organ systems.

• Maintenance of homeostasis is dependent on information received, specifically feedback information that signals any discrepancy between the set point (the particular desired condition or level) and the conditions present.

• The difference between the set point and the feedback information is the error signal.

41 Physiological Regulation and Homeostasis

• Cells, tissues, and organs are controlled effectors that respond to commands from regulatory systems.

• Regulatory systems obtain, process, and integrate information, then issue commands to controlled systems, which effect change.

• Regulatory systems receive feedback information.

• Feedforward information signals the system to change the setpoint.

41 Temperature and Life

• Living cells tolerate only a narrow range of temperature. Most cell function is limited to the range between 0°C and 45°C.

• Even within this range, temperature change may create problems for animals.

• Heat always moves from a warmer to a cooler object, so any environmental temperature change will cause change in the temperature of an organism—unless the organism can regulate its temperature.

41 Temperature and Life

• Most physiological processes are temperature-sensitive, going faster at higher temperatures.

• Reaction rates double or triple as temperature increases by 10°C.

• Temperature change can disrupt physiological functioning, throwing off the balance and integration that cell processes require.

• To maintain homeostasis, organisms must either compensate for or prevent temperature change.

41 Maintaining Optimal Body Temperature

• Animals may be classified by how they respond to environmental temperatures:

Homeotherms maintain a constant body temperature.

In poikilotherms, body temperature changes when environmental temperature changes.

A third category, heterotherm, fits animals that regulate body temperature at a constant level some of the time, such as hibernating mammals.


• Animals may also be classified according to the sources of heat that determine their body temperature:

Ectotherms (most animals aside from mammals and birds) depend on external heat sources to maintain body temperature.

Endotherms (all mammals and birds) regulate body temperature by generating metabolic heat and/or preventing heat loss.

Figure 41.7 Ectotherms nd Endotherms (Part 1)

If a lizard (an ectotherm) and a mouse (an endotherm) are placed in a closed chamber in which the temperature is gradually raised, the body temperature of the lizard will equilibriate with that of the chamber, whereas the body temperature of the mouse will remain constant.

Figure 41.7 Ectotherms nd Endotherms (Part 2)

The metabolic rates also respond differently.

• Ectotherm = metabolism decreases as air temperature decreases.• Endotherm = metabolic rate increases as temperature decreases, which increases production of

body heat.


• Ectotherms such as the lizard can use behavior to regulate body temperature in the natural environment.

• Behaviors include basking in the sun, seeking shade, burrowing, or orienting the body with respect to the sun.

• Endotherms also use behavioral thermoregulation. Most animals select the best thermal environment whenever possible.

Figure 41.8 An Ectotherm Uses Behavior to Regulate Its Body Temperature

Figure 41.9 Endotherms Use Behavior to Thermoregulate


• If the body temperature of an animal is to remain constant, the heat entering the animal must equal the heat leaving the animal.

Heatin = Heatout

• Heatin = metabolism + solar radiation (Rabs)

• Heatout = radiation (Rout) + convection + conduction

+ evaporation

Figure 41.10 Animals Exchange Heat with the Environment


• Heat exchange between the internal environment and the skin occurs largely through blood flow.

• When blood is close to the surface of the skin, heat energy carried by the blood is lost to the environment.

• When a person is exposed to cold, blood vessels of the skin constrict, decreasing blood flow and heat transport to the skin and reducing heat loss.

• Some ectotherms, such as the marine iguana, control blood flow to the skin as an adaptation for survival in cold water and hot sun.

Figure 41.11 Some Ectotherms Regulate Blood Flow to the Skin (Part 1)

Figure 41.11 Some Ectotherms Regulate Blood Flow to the Skin (Part 2)


• Some ectotherms raise their body temperature by producing heat.

• The flight muscles of insects must be warmed before flight can occur. This is achieved by flight muscle contractions.

• Honeybees regulate temperature in a hive by group clustering to produce metabolic heat so the brood temperature stays at about 34°C even as temperatures outside of the hive drop well below freezing.


• In most fish, blood passing through the gills comes in close contact with water, so the temperature of the blood tends to be about the same temperature as the water.

• Some large fish, such as bluefin tuna and great white shark, can raise body temperature 10–15°C above the water temperature.

• In the large swimming muscles, heat is exchanged through a countercurrent heat exchanger, a structural plan that allows cool blood returning from the gills to be warmed by warm blood from the muscles.

Figure 41.12 “Cold” and “Hot” Fish

41 Thermoregulation in Endotherms

• Endotherms respond to environmental temperature change by changing rates of heat production.

• Within a narrow range of temperatures, the thermoneutral zone, the metabolic rate of endotherms is low and independent of temperature.

• The metabolic rate of a resting animal within the thermoneutral zone is called the basal metabolic rate (BMR).

• The BMR of an endotherm is about six times that of an ectotherm of the same size and at the same body temperature.


• The thermoneutral zone is bounded by a lower critical and upper critical temperature.

• When environmental temperature falls below the lower critical temperature, mammals thermoregulate by generating heat through shivering and nonshivering heat production.

• In shivering, skeletal muscles use ATP to release only heat. Active body movement also generates heat.

Figure 41.14 Environmental Temperature and Mammalian Metabolic Rates


• Endotherms have many adaptations for reducing heat loss in cold environments:

Reduction of surface-to-volume ratios of the body by short appendages and round body shapes

Thermal insulation by thick layers of fur, feathers, and fat.

Decreasing blood flow to the skin by constricting blood vessels, especially in appendages


• In any climate, getting rid of excess heat may also be a problem, especially during exercise.

• Reduction or loss of fur or hair allows for easier loss of heat from the body to the environment.

• Seeking contact with water cools the skin because water absorbs heat to a greater capacity than does air.

• Sweating or panting to increase evaporation provides cooling.

41 The Vertebrate Thermostat

• The regulatory system for body temperature in vertebrates can be thought of as a thermostat.

• This regulator is at the bottom of the brain in a structure called the hypothalamus.

• The temperature of the hypothalamus itself is the major source of feedback information in many species.


• A fever is a rise in body temperature in response to pyrogens.

• Exogenous pyrogens come from foreign substances such as invading bacteria or viruses.

• Endogenous pyrogens are produced by cells of the immune system when they are challenged.

• Pyrogens cause a rise in the hypothalamic set point, and body temperature rises until it matches the new set point.

• Evidence suggests that moderate fevers help the body fight infections, but extreme fevers can be dangerous.


• Animals can save energy by turning down the thermostat to below normal (hypothermia).

• Many animals use regulated hypothermia as a means of surviving periods of cold and food scarcity.

• Regulated hypothermia lasting days or weeks with drops to very low temperatures is called hibernation. The reduction in metabolic rate results in enormous energy savings.

Figure 41.19 A Ground Squirrel Enters Repeated Bouts of Hibernation during Winter

41 physiology, homeostasis, and temperature regulation

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