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Field notes · chapter 41

The Animal Body and Principles of Regulation

pp. 899–922 · Raven Part VII

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41.1

Organization of Animal Bodies

pp. 900–901
41.2

Epithelial Tissue

pp. 901–904
  • Epithelium lines and covers organs and body cavities, forms the outer body surface (skin), and builds glands. Paired with connective tissue it forms membranes. Watch Tissues, Part 2 - Epithelial Tissue: Crash Course Anatomy & Physiology #3 on YouTube, from CrashCourse
  • Epithelia can come from any of the three germ layers: jellyfish gastrodermis from endoderm, vertebrate epidermis from ectoderm, blood-vessel lining from mesoderm.
  • Structure: a sheet of cells with a free apical surface facing the outside or a lumen, and an opposite basal surface anchored to the basement membrane.
  • The basement membrane is a thin mat of fine protein fibers and glycoproteins, secreted by the epithelial cells themselves, that glues epithelium to the connective tissue beneath.
  • Epithelium has almost no extracellular material. Cells are held tightly by tight junctions and desmosomes (adhesive junctions tied to intermediate filaments), and communicate through gap junctions.
  • Trade-off: blood vessels cannot invade epithelium. Vessels sit in the connective tissue just past the basement membrane, and O₂, CO₂, and nutrients must diffuse across. Since diffusion only reaches a few cell layers, epithelial thickness is capped.
  • Constant contact with the outside world and with internal lumens means a high mitotic rate — epithelium is continuously replaced and regenerates quickly during wound healing.
  • Classification uses two criteria: number of cell layers and cell shape. Watch Tissues, Part 2 - Epithelial Tissue: Crash Course Anatomy & Physiology #3 on YouTube, from CrashCourse
  • Simple epithelium = one layer; every cell touches the basement membrane and has an apical end. Stratified epithelium = two or more layers; only the bottom layer touches the basement membrane. Watch Tissues, Part 2 - Epithelial Tissue: Crash Course Anatomy & Physiology #3 on YouTube, from CrashCourse
  • Shapes: squamous = flattened with no clean geometry; cuboidal = height about equals width; columnar = taller than wide.
  • All simple epithelia are built for rapid diffusion of gases, nutrients, and water. Simple squamous lines alveolar sacs and blood capillaries; simple cuboidal lines kidney tubules and some glands and covers the ovaries; simple columnar lines the stomach and intestines. Watch Tissues, Part 2 - Epithelial Tissue: Crash Course Anatomy & Physiology #3 on YouTube, from CrashCourse
  • Stratified squamous forms the skin epidermis and mouth lining. Its outer layers are loaded with keratin, making a tough protective barrier.
  • Pseudostratified epithelium (parts of the respiratory tract) only looks layered — every cell reaches the basement membrane but only some reach the free surface. It is ciliated and secretes mucus, sweeping debris along. Watch Tissues, Part 2 - Epithelial Tissue: Crash Course Anatomy & Physiology #3 on YouTube, from CrashCourse
  • Glands are epithelia that manufacture metabolic products and release them into ducts or body fluids including blood.
  • A goblet cell is a unicellular gland dumping mucus into the digestive and respiratory tracts. Most glands are multicellular organs, sorted by secretion route.
  • Exocrine glands push product through ducts straight to the target — salivary glands, sweat glands. Endocrine glands release into the blood — thyroid and pituitary are small endocrine organs; liver, pancreas, and adrenal glands are larger.
  • Vertebrate glands develop from invaginated epithelia — epithelium folding inward during development.
41.3

Connective Tissue

pp. 904–906
41.4

Muscle Tissue

pp. 907–908
  • The defining muscle trait: it converts chemical energy into mechanical energy to produce force. Watch Types of Tissue Part 3: Muscle Tissue on YouTube, from Professor Dave Explains
  • Muscle is a metazoan hallmark. It enables self-initiated movement and behavior, which is what separates animals from plants and fungi.
  • Myosin, a contractile protein, apparently first appeared in sponges, where it seems to help regulate water flow — before true muscle tissue existed.
  • Two broad appearances: smooth (uniform look, evolved first) and striated (striped, from a highly ordered arrangement of protein fibers).
  • Striation evolved for complex movement needing more nervous control, faster action, and much greater force than smooth muscle can supply.
  • Smooth muscle: sheets of long spindle-shaped cells with one nucleus each. It sits in the organs of the internal environment (the viscera), hence the alternate name visceral muscle. Found in blood vessel walls, the stomach, the intestines, and the iris of the eye. Watch Types of Tissue Part 3: Muscle Tissue on YouTube, from Professor Dave Explains
  • In some smooth tissues cells fire only on nerve stimulation and the whole sheet contracts as a unit — for example the iris, which contracts in bright light. In others, such as the digestive tract wall, cells generate their own electrical impulses spontaneously, and nerves only modulate the slow steady contraction rather than trigger it.
  • Skeletal muscle is voluntary, striated, and drives locomotion, lifting, and talking. It is homologous to striated muscle elsewhere in the animal kingdom and usually attaches to bone by tendons. Watch Types of Tissue Part 3: Muscle Tissue on YouTube, from Professor Dave Explains Watch Muscles, Part 1 - Muscle Cells: Crash Course Anatomy & Physiology #21 on YouTube, from CrashCourse
  • Skeletal muscle cells are long tubular muscle fibers. They form in development when several cells fuse end to end, which is why a mature fiber is multinucleate with nuclei pushed to the cell edge. Fibers contain contractile myofibrils.
  • Cardiac muscle is the most recently evolved type, is striated, and is exclusive to vertebrate hearts. Watch Types of Tissue Part 3: Muscle Tissue on YouTube, from Professor Dave Explains
  • Cardiac cells are arranged unlike skeletal fibers: they are smaller, heavily interconnected cells with one nucleus each, not long multinucleate fibers.
  • Junctions between adjacent cardiac cells appear as dark lines called intercalated disks. These are actually gap-junction regions, letting small molecules and ions pass cell to cell so the heart behaves as one functional unit. Watch Muscles, Part 1 - Muscle Cells: Crash Course Anatomy & Physiology #21 on YouTube, from CrashCourse
  • Some specialized cardiac cells fire impulses spontaneously; the nervous system normally just sets the rate. Fig 41.7 (Scientific Thinking): an excised frog heart placed in a nutrient and oxygen bath keeps contracting with no nerve connection, proving the heartbeat is intrinsic to the heart.
41.5

Nerve Tissue

pp. 908–909
41.6

Overview of Vertebrate Organ Systems

pp. 909–911
  • The 11 principal organ systems are grouped into five functional categories: communication and integration; support and movement; regulation and maintenance; defense; reproduction and development. Watch Human Body Systems Overview (Updated 2024) on YouTube, from Amoeba Sisters
  • Communication and integration: the nervous system (brain, spinal cord, nerves, sensory organs) detects internal feedback and external stimuli, integrates the information, and drives the response. Sensory systems are a nervous-system subset covering vision, hearing, smell, and so on. The endocrine system runs in parallel, sending chemical signals that regulate and fine-tune chemistry in every other system.
  • Support and movement: the musculoskeletal system. Muscles produce movement but need something rigid to pull against, and the skeleton is that framework. Vertebrates use internal skeletons; insects use external skeletons; earthworms use hydrostatic skeletons.
  • Regulation and maintenance: the digestive system (eating, nutrient absorption, solid waste elimination), the circulatory system (heart and vessels distributing blood), the respiratory system (takes in O₂, expels CO₂), and the urinary system (osmoregulation, tight control of body-fluid concentration). Watch Human Body Systems Overview (Updated 2024) on YouTube, from Amoeba Sisters
  • Defense: the integumentary system — unbroken skin — is the first barrier. Pathogens that get past it meet the immune (lymphatic) system, including antibody production and specialized attack cells.
  • Reproduction and development: the reproductive system maintains biological continuity. It holds the organs where gametes form plus the glands and tubes that nourish gametes and let opposite-sex gametes meet. Many vertebrate female systems also include structures for nurturing embryo and fetus.
  • Once gametes fuse into a zygote, an extensive program of cell division and development turns that single diploid cell into a multicellular adult.
  • Organ systems overlap. The pancreas serves both digestive and endocrine roles, muscles and skeleton form one musculoskeletal unit, and the hypothalamus links nervous and endocrine control.
  • Organs listed in Fig 41.9 worth knowing: nervous = brain, spinal cord, nerves; endocrine = hypothalamus, pituitary, thyroid, thymus, adrenal, pancreas, gonads; skeletal = skull, sternum, pelvis, femur; digestive = salivary glands, esophagus, liver, stomach, intestines; circulatory = heart, arteries, veins; respiratory = trachea, lungs; urinary = kidney, ureter, bladder, urethra; integumentary = hair, skin, nails; lymphatic/immune = lymph nodes, thymus, spleen, bone marrow, lymphatic vessels.
41.7

Homeostasis

pp. 912–914
41.8

Regulating Body Temperature

pp. 914–919
  • Temperature is one of the most important environmental variables an organism faces, so it is the best test case for the homeostatic principles of 41.7. Some organisms let body temperature match the environment (conformers); others actively regulate. Watch Thermoregulation on YouTube, from Bozeman Science
  • Reaction rates rise with temperature and fall as it drops. Enzyme-catalyzed reactions follow the same kinetics, and the enzyme protein itself is temperature-sensitive.
  • Q10 quantifies this: the ratio of reaction rates at two temperatures 10 °C apart. Q10 = R(T+10) / R(T).
  • A typical enzyme Q10 is about 2 — a 10 °C rise roughly doubles the rate. This cannot continue indefinitely: at high temperature enzyme structure is disrupted and activity is lost.
  • Q10 also applies to whole-organism metabolism, and measured values are usually about 2 to 3, implying temperature acts mainly through the enzymes of metabolism.
  • Rare cases — some intertidal invertebrates — have Q10 near 1, meaning metabolic rate is essentially unaffected by temperature. The likely adaptation is having several enzymes in one pathway with widely different temperature optima, so one covers for the others.
  • By the second law of thermodynamics, no energy transaction is 100% efficient, so metabolic reactions continuously generate heat as a byproduct. That heat must either be shed or used to warm the body.
  • Metabolic rate and body temperature are mutually linked: low body temperature blocks high metabolic rates, while high metabolic rates can overheat the body and force cooling.
  • Simplest model: body heat = heat produced + heat transferred. The transfer term can be positive or negative — the same term covers warming and cooling.
  • Four heat-transfer mechanisms: radiation (electromagnetic, needs no contact, hot to cold), conduction (direct contact, kinetic energy passing between touching molecules), convection (heat carried by a flowing gas or liquid), and evaporation (loss only). Watch Thermoregulation on YouTube, from Bozeman Science
  • Convection can be driven externally by wind, or internally by density differences — warmed air or water is less dense and rises.
  • Every substance has a heat of vaporization, the energy needed to go from liquid to gas. Water has a high one, and many animals exploit that for cooling.
  • Three other factors set overall heat-transfer rate. Surface area: more surface relative to mass means faster exchange, so small organisms swap heat with the environment quickly; posture and limb position adjust this. Temperature difference: the bigger the gap between body and ambient, the greater the transfer. Heat conductance: high conductance drags body temperature toward ambient, so regulating animals benefit from low-conductance insulation — feathers, fur, blubber — while behavioral regulators benefit from high conductance.
  • Old classification by constancy: homeotherms regulate around a set point, poikilotherms let body temperature track the environment. Colloquially these were called warm-blooded and cold-blooded. Watch Thermoregulation on YouTube, from Bozeman Science
  • That scheme fails, because a poikilotherm in a thermally stable habitat — many deep-sea fishes — can have a steadier body temperature than some homeotherms.
  • Newer classification by heat source: endotherms use metabolism to generate heat and stay above ambient; ectotherms have relatively low metabolic rates and do not use metabolism for heat. Endotherms usually have low thermal conductivity thanks to insulation; ectotherms usually have high thermal conductivity and no insulation. Watch Thermoregulation on YouTube, from Bozeman Science
  • These are idealized endpoints of a spectrum. Many animals fall in between and count as heterotherms, and classifying a given species is partly a judgment call.
  • Ectotherms do regulate — mostly by behavior. Butterflies must hit a threshold body temperature before flight, so on cool mornings they orient to absorb maximum sunlight. Moths and many insects use a shivering reflex to warm thoracic flight muscles (Fig 41.14: thorax goes from ~25 °C to ~40 °C in about 2 minutes, then plateaus when heat production balances heat loss). Watch Thermoregulation on YouTube, from Bozeman Science
  • Non-mammal, non-bird vertebrates are ectothermic but can hold high, fairly constant temperatures behaviorally — homeothermic ectotherms. Reptiles do this by moving between sun and shade, which is why lizards bask.
  • The marine iguana adds circulatory control: faster heart rate plus vasodilation maximizes warming on land, while slower heart rate plus vasoconstriction minimizes cooling while diving to feed.
  • Some large fishes — tuna, swordfish, certain sharks — keep parts of the body well above water temperature by countercurrent heat exchange: cool venous blood returning from the surface is warmed by heat radiating from adjacent arteries carrying warm blood out from the core. Fig 41.15 shows the same arrangement in a killer whale flipper (core 36 °C, water 5 °C).
  • Ectotherm advantage: low metabolic rate means low food demand — a lizard needs only about 10% of the energy intake of a similar-sized mouse. The trade-off is no sustained high-energy activity.
  • Endotherm response to heat and cold starts with blood flow to the surface. Vasodilation sends more blood to the skin, raising heat loss; vasoconstriction cuts surface flow and limits it. Watch Homeostasis - negative and positive feedback (thermoregulation and lactation) on YouTube, from Armando Hasudungan
  • When ambient temperature rises, many endotherms use evaporative cooling: sweating (some mammals including humans) pushes water from sweat glands onto the surface, while panting (some mammals and birds) uses respiratory surfaces. Either way the animal must tolerate the water loss. Large endotherms also use behavior — elephants flap their ears to boost convection. Watch Homeostasis - negative and positive feedback (thermoregulation and lactation) on YouTube, from Armando Hasudungan
  • Endothermy buys sustained high-energy activity at the cost of a steady, high food intake.
  • Body size matters enormously. The mouse-to-elephant curve (Fig 41.16) shows metabolic rate is not proportional to size: mass-specific metabolic rate is highest in the shrew and approaches zero in the elephant. Smaller animals burn far more energy per unit mass.
  • Small endotherms also have a high surface-to-volume ratio, so in the cold they lose heat faster than they can produce it — hence heavy insulation, thicker further north and in winter. Large endotherms in hot climates have the reverse problem: a lot of total heat and relatively little surface to shed it, so they carry little insulation.
  • Thermogenesis kicks in below a critical low temperature, when ordinary responses cannot keep the animal warm. Normal energy metabolism is redirected specifically to make heat. Two forms.
  • Shivering thermogenesis: antagonistic muscles work against each other so there is little net movement, and ATP hydrolysis yields heat. Seen in some insects and in endothermic vertebrates.
  • Nonshivering thermogenesis: fat metabolism is rerouted to make heat instead of ATP. It occurs in brown adipose tissue (BAT), distinct from white adipose tissue (WAT). Brown fat cells are rich in UCP1, a membrane channel that lets protons leak, uncoupling electron transport from oxidative phosphorylation — heat, no ATP. Humans keep small BAT deposits in the neck and between the shoulders, heavily vascularized so heat moves away efficiently. Beige fat is a third type, convertible from WAT into BAT.
  • Hypothalamic control (Fig 41.17) above 37 °C: hypothalamic neurons detect the rise and activate the heat-losing center. Nerves dilate peripheral vessels, other nerves drive sweat production, and metabolism-stimulating hormone production is suppressed. Watch Homeostasis - negative and positive feedback (thermoregulation and lactation) on YouTube, from Armando Hasudungan
  • Below 37 °C: the heat-promoting center produces the antagonistic set of responses. Nerves constrict vessels and block sweating; the adrenal medulla releases epinephrine and the anterior pituitary releases TSH, both raising metabolism. TSH makes the thyroid produce thyroxine, a metabolic stimulant. Epinephrine plus autonomic stimulation of fat tissue can trigger thermogenesis. As temperature climbs back, negative feedback dials the response down. Watch Homeostasis - negative and positive feedback (thermoregulation and lactation) on YouTube, from Armando Hasudungan
  • Central thermoreceptors in the brain and abdominal interior sense core temperature and synapse on hypothalamic neurons, which drive effectors through sympathetic nerves.
  • Fever: pyrogens raise temperature by resetting the hypothalamic set point higher. Many gram-negative bacteria carry cell-wall endotoxins that act as pyrogens, and substances released by circulating white blood cells are pyrogens too.
  • Fever appears adaptive because higher temperature suppresses bacterial growth. Evidence from ectotherms: desert iguanas injected with pyrogen-producing bacteria spent more time in the sun, producing a behaviorally induced fever. This has prompted rethinking whether routine fever should be suppressed — though very high fevers remain dangerous, causing seizures or delirium.
  • Torpor: endotherms lower both metabolic rate and body temperature into a dormant state, cutting food requirements. Hummingbirds and some other birds drop body temperature by as much as 25 °C at night. Torpor suits smaller endotherms; large mammals have too much mass to cool quickly.
  • Hibernation is extreme deep torpor lasting weeks to months, with temperature dropping up to 20 °C below the normal set point. Hibernators tend to be mid-sized: very small endotherms burn more energy than they can store even with reduced metabolism, and very large mammals apparently do not hibernate at all.
  • Bears do not truly hibernate. They drop temperature only a few degrees and enter a long winter sleep; their large thermal mass and slow heat loss make hibernation-level savings unnecessary.