Homeostasis and Endocrine Signaling 32 Homeostasis and Endocrine Signaling

Multicellularity allows for cellular specialization with particular cells devoted to specific activities Specialization requires organization and results in an internal environment that differs from the external environment

Organisms must maintain homeostasis Organisms must maintain homeostasis. Why are homeostasis and regulation essential life functions for living things?

Homeostasis Organisms use homeostasis to maintain a “steady state” or internal balance regardless of external environment In humans, body temperature, blood pH, and glucose concentration are each maintained at a constant level Regulation of room temperature by a thermostat is analogous to homeostasis

Sensor/ control center: Thermostat turns heater off. Response: Figure 32.4 Sensor/ control center: Thermostat turns heater off. Response: Heating stops. Room temperature decreases. Stimulus: Room temperature increases. Set point: Room temperature at 20C Figure 32.4 A nonliving example of temperature regulation: control of room temperature Stimulus: Room temperature decreases. Room temperature increases. Response: Heating starts. Sensor/ control center: Thermostat turns heater on. 5

Which body systems are responsible for maintaining homeostasis in animals? How are their modes of action different?

Coordination and Control Functions of the Endocrine and Nervous Systems In the endocrine system, signaling molecules released into the bloodstream by endocrine cells reach all locations in the body In the nervous system, neurons transmit signals along dedicated routes, connecting specific locations in the body 7

Endocrine cell Cell body of neuron Figure 32.9 (a) Signaling by hormones (b) Signaling by neurons Stimulus Stimulus Endocrine cell Cell body of neuron Nerve impulse Axon Hormone Signal travels everywhere. Signal travels to a specific location. Blood vessel Nerve impulse Figure 32.9 Signaling in the endocrine and nervous systems Axons Response Response 8

Regulating and Conforming Faced with environmental fluctuations, animals manage their internal environment by either regulating or conforming An animal that is a regulator uses internal mechanisms to control internal change despite external fluctuation An animal that is a conformer allows its internal condition to change in accordance with external changes

Interpret the following figure.

(temperature conformer) Figure 32.3 40 River otter (temperature regulator) 30 Body temperature (C) 20 Largemouth bass (temperature conformer) 10 Figure 32.3 Regulating and conforming 10 20 30 40 Ambient (environmental) temperature (C) 11

An animal may regulate some internal conditions and not others For example, a fish may conform to surrounding temperature in the water, but it regulates solute concentrations in its blood and interstitial fluid (the fluid surrounding body cells)

How does an organism know if there is a disruption of homeostasis?

Animals achieve homeostasis by maintaining a variable at or near a particular value, or set point Fluctuations above or below the set point serve as a stimulus; these are detected by a sensor and trigger a response The response returns the variable to the set point

Homeostasis in animals relies largely on negative feedback, a control mechanism that reduces the stimulus Homeostasis moderates, but does not eliminate, changes in the internal environment Set points and normal ranges for homeostasis are usually stable, but certain regulated changes in the internal environment are essential

We need to know a few examples of how homeostasis is moderated for the AP exam. Let’s take a closer look…

Thermoregulation: A Closer Look Thermoregulation is the process by which animals maintain an internal temperature within a tolerable range

Describe the difference between endothermic and ectothermic organisms.

Endothermy and Ectothermy Endothermic animals generate heat by metabolism; birds and mammals are endotherms = “warm-blooded” Ectothermic animals gain heat from external sources; ectotherms include most invertebrates, fishes, amphibians, and nonavian reptiles = “cold-blooded”

Endotherms can maintain a stable body temperature in the face of large fluctuations in environmental temperature Ectotherms may regulate temperature by behavioral means Ectotherms generally need to consume less food than endotherms, because their heat source is largely environmental

How would you classify the following?

(a) A walrus, an endotherm Figure 32.5a Figure 32.5a Endothermy and ectothermy (part 1: endotherm) (a) A walrus, an endotherm 22

(b) A lizard, an ectotherm Figure 32.5b Figure 32.5b Endothermy and ectothermy (part 2: ectotherm) (b) A lizard, an ectotherm 23

Balancing Heat Loss and Gain Organisms exchange heat by four physical processes Radiation Evaporation Convection Conduction Heat is always transferred from an object of higher temperature to one of lower temperature

Generate definitions of each type of heat exchange from the figure.

Radiation Evaporation Convection Conduction Figure 32.6 Figure 32.6 Heat exchange between an organism and its environment Convection Conduction 26

How might the circulatory system collaborate in this thermoregulation process?

Circulatory Adaptations for Thermoregulation In response to changes in environmental temperature, animals can alter blood (and heat) flow between their body core and their skin Vasodilation, the widening of the diameter of superficial blood vessels, promotes heat loss How? Vasoconstriction, the narrowing of the diameter of superficial blood vessels, reduces heat loss 28

In aquatic environments, we see adaptations for how thermoregulation takes place.

The arrangement of blood vessels in many marine mammals and birds allows for countercurrent exchange Countercurrent heat exchangers transfer heat between fluids flowing in opposite directions and reduce heat loss 30

Canada goose 1 3 Artery Vein 35C 33 30 27 20 18 Key Warm blood Figure 32.7 Canada goose 1 3 Artery Vein 35C 33 30 27 Figure 32.7 A countercurrent heat exchanger 20 18 Key Warm blood 10 9 Cool blood Blood flow 2 Heat transfer 31

Other mechanisms for thermoregulation?

Acclimatization in Thermoregulation Birds and mammals can vary their insulation to acclimatize to seasonal temperature changes Acclimatization in ectotherms often includes adjustments at the cellular level Some ectotherms that experience subzero temperatures can produce “antifreeze” compounds to prevent ice formation in their cells

In mammals (FYI: we are mammals, in case you didn’t know)… where is the physiological thermostat for thermoregulation?

Physiological Thermostats and Fever Thermoregulation in mammals is controlled by a region of the brain called the hypothalamus The hypothalamus triggers heat loss or heat-generating mechanisms Fever is the result of a change to the set point for a biological thermostat Animation: Negative Feedback Animation: Positive Feedback

Sensor/control center: Thermostat in hypothalamus Response: Sweat Figure 32.8a Sensor/control center: Thermostat in hypothalamus Response: Sweat Response: Blood vessels in skin dilate. Stimulus: Increased body temperature Body temperature decreases. Figure 32.8a The thermostatic function of the hypothalamus in human thermoregulation (part 1: increase) Homeostasis: Internal body temperature of approximately 36–38C 36

Sensor/control center: Thermostat in hypothalamus Response: Shivering Figure 32.8b Homeostasis: Internal body temperature of approximately 36–38C Body temperature increases. Stimulus: Decreased body temperature Response: Blood vessels in skin constrict. Figure 32.8b The thermostatic function of the hypothalamus in human thermoregulation (part 2: decrease) Sensor/control center: Thermostat in hypothalamus Response: Shivering 37

Hormones, released in the endocrine system, are released to moderate a series of feedback mechanisms in varying organ systems.

Hormones may have effects in a single location or throughout the body Only cells with receptors for a certain hormone can respond to it The endocrine system is well adapted for coordinating gradual changes that affect the entire body 39

We need to know a few examples of endocrine pathways within the mammalian system. Let’s take a closer look…

Simple Endocrine Pathways Digestive juices in the stomach are extremely acidic and must be neutralized before the remaining steps of digestion take place Coordination of pH control in the duodenum relies on an endocrine pathway 41

Pathway Example  Stimulus Low pH in duodenum S cells of duodenum Figure 32.10 Pathway Example  Stimulus Low pH in duodenum S cells of duodenum secrete the hormone secretin ( ). Endocrine cell Negative feedback Hormone Blood vessel Figure 32.10 A simple endocrine pathway Target cells Pancreas Response Bicarbonate release 42

The release of acidic stomach contents into the duodenum stimulates endocrine cells there to secrete the hormone secretin This causes target cells in the pancreas to raise the pH in the duodenum The pancreas can act as an exocrine gland, secreting substances through a duct, Which substances??? or as an endocrine gland, secreting hormones directly into interstitial fluid Which hormones??? 43

Neuroendocrine Pathways Hormone pathways that respond to stimuli from the external environment rely on a sensor in the nervous system In vertebrates, the hypothalamus integrates endocrine and nervous systems Signals from the hypothalamus travel to a gland located at its base, called the pituitary gland 44

Major Endocrine Glands and Their Hormones Hypothalamus Figure 32.11a Major Endocrine Glands and Their Hormones Hypothalamus Pituitary gland Anterior pituitary Pineal gland Melatonin Posterior pituitary Oxytocin Vasopressin (antidiuretic hormone, ADH) Thyroid gland Thyroid hormone (T3 and T4) Calcitonin Adrenal glands (atop kidneys) Parathyroid glands Parathyroid hormone (PTH) Adrenal medulla Epinephrine and norepinephrine Ovaries (in females) Estrogens Progestins Figure 32.11a Exploring the human endocrine system (part 1: glands and hormones) Adrenal cortex Glucocorticoids Mineralocorticoids Testes (in males) Androgens Pancreas Insulin Glucagon 45

Hypothalamus Anterior pituitary Neurosecretory cells of the Figure 32.11b Hypothalamus Neurosecretory cells of the hypothalamus Portal vessels Hypothalamic hormones HORMONE Posterior pituitary Anterior pituitary Endocrine cells TARGET Pituitary hormones Figure 32.11b Exploring the human endocrine system (part 2: hypothalamus and pituitary) FSH TSH ACTH Prolactin MSH GH Mammary glands Testes or ovaries Thyroid gland Adrenal cortex Melanocytes Liver, bones, other tissues 46

Let’s consider another example…

Stimulus TSH circulation throughout body Sensory neuron Thyroid gland Figure 32.11c Stimulus TSH circulation throughout body Sensory neuron − Hypothalamus Thyroid gland Neurosecretory cell TRH Negative feedback Thyroid hormone Thyroid hormone circulation throughout body Figure 32.11c Exploring the human endocrine system (part 3: hormone cascade) − Anterior pituitary TSH Response 48

Low level of iodine uptake High level of iodine uptake Thyroid scan Figure 32.11d Low level of iodine uptake High level of iodine uptake Figure 32.11d Exploring the human endocrine system (part 4: thyroid scan) Thyroid scan 49

It also secretes antidiuretic hormone (ADH) Hormonal signals from the hypothalamus trigger synthesis and release of hormones from the anterior pituitary The posterior pituitary is an extension of the hypothalamus and secretes oxytocin, which regulates release of milk during nursing in mammals It also secretes antidiuretic hormone (ADH) 50

Another example…Oxytocin

Pathway Example Stimulus Suckling Sensory neuron Hypothalamus/ Figure 32.12 Pathway Example  Stimulus Suckling Sensory neuron Hypothalamus/ posterior pituitary Neuro- secretory cell Posterior pituitary secretes the neurohormone oxytocin ( ). Positive feedback Neuro- hormone Figure 32.12 A neuroendocrine pathway Blood vessel Smooth muscle in breasts Target cells Response Milk release 52

Feedback Regulation in Endocrine Pathways A feedback loop links the response back to the original stimulus in an endocrine pathway While negative feedback dampens a stimulus, positive feedback reinforces a stimulus to increase the response 53

There are different types of hormones : water soluble or lipid soluble Their receptors are located in different places at their target cells. They also have very different modes of action.

Pathways of Water-Soluble and Lipid-Soluble Hormones The hormones discussed thus far are proteins that bind to cell-surface receptors and that trigger events leading to a cellular response The intracellular response is called signal transduction A signal transduction pathway typically has multiple steps 55

Lipid-soluble hormones have receptors inside cells When bound by the hormone, the hormone-receptor complex moves into the nucleus There, the receptor alters transcription of particular genes 56

Multiple Effects of Hormones Many hormones elicit more than one type of response For example, epinephrine is secreted by the adrenal glands and can raise blood glucose levels, increase blood flow to muscles, and decrease blood flow to the digestive system Target cells vary in their response to a hormone because they differ in their receptor types or in the molecules that produce the response 57

Same receptors but different intracellular proteins (not shown) Figure 32.13 Same receptors but different intracellular proteins (not shown) Different receptors Different cellular responses Different cellular responses Epinephrine Epinephrine Epinephrine  receptor  receptor  receptor Glycogen deposits Figure 32.13 One hormone, different effects Vessel dilates. Vessel constricts. Glycogen breaks down and glucose is released from cell. (a) Liver cell (b) Skeletal muscle blood vessel (c) Intestinal blood vessel 58

Evolution of Hormone Function Over the course of evolution the function of a given hormone may diverge between species For example, thyroid hormone plays a role in metabolism across many lineages, but in frogs it has taken on a unique function: stimulating the resorption of the tadpole tail during metamorphosis Prolactin also has a broad range of activities in vertebrates 59

Tadpole Adult frog Figure 32.14 Figure 32.14 Specialized role of a hormone in frog metamorphosis Adult frog 60