The Endocrine System Pancreatic Islets

Pancreatic Islets The pancreas is a mixed gland and has both endocrine and exocrine functions The pancreatic islets produce hormones Insulin —allows glucose to cross plasma membranes into cells from beta cells Glucagon —allows glucose to enter the blood from alpha cells These hormones are antagonists that maintain blood sugar homeostasis

Pancreatic Islets Insulin —allows glucose to cross plasma membranes into cells from beta cells Glucagon —allows glucose to enter the blood from alpha cells Figure 9.14a–b

Homeostasis: Normal blood glucose levels (90 mg/100ml) Insulin-secreting cells of the pancreas activated; release insulin into the blood Uptake of glucose from blood is en- hanced in most body cells Blood glucose levels decline to set point; stimulus for insulin release diminishes Elevated blood sugar levels Liver takes up glucose and stores it as glycogen Stimulus: rising blood glucose levels (e.g., after eating four jelly doughnuts) Imbalance Homeostasis: Normal blood glucose levels (90 mg/100ml) Stimulus: declining blood glucose levels (e.g., after skipping a meal) Imbalance Low blood sugar levels Rising blood glucose levels return blood sugar to homeostatic set point; stimulus for glucagon release diminishes Glucagon-releasing cells of pancreas activated; release glucagon into blood; target is the liver Liver breaks down glycogen stores and releases glucose to the blood Figure 9.15

Homeostasis: Normal blood glucose levels (90 mg/100ml) Figure 9.15, step 1

Homeostasis: Normal blood glucose levels (90 mg/100ml) Elevated blood sugar levels Stimulus: rising blood glucose levels (e.g., after eating four jelly doughnuts) Imbalance Homeostasis: Normal blood glucose levels (90 mg/100ml) Imbalance Figure 9.15, step 3

Homeostasis: Normal blood glucose levels (90 mg/100ml) Insulin-secreting cells of the pancreas activated; release insulin into the blood Elevated blood sugar levels Stimulus: rising blood glucose levels (e.g., after eating four jelly doughnuts) Imbalance Homeostasis: Normal blood glucose levels (90 mg/100ml) Imbalance Figure 9.15, step 4

Homeostasis: Normal blood glucose levels (90 mg/100ml) Insulin-secreting cells of the pancreas activated; release insulin into the blood Uptake of glucose from blood is en- hanced in most body cells Elevated blood sugar levels Stimulus: rising blood glucose levels (e.g., after eating four jelly doughnuts) Imbalance Homeostasis: Normal blood glucose levels (90 mg/100ml) Imbalance Figure 9.15, step 5

Homeostasis: Normal blood glucose levels (90 mg/100ml) Insulin-secreting cells of the pancreas activated; release insulin into the blood Uptake of glucose from blood is en- hanced in most body cells Elevated blood sugar levels Liver takes up glucose and stores it as glycogen Stimulus: rising blood glucose levels (e.g., after eating four jelly doughnuts) Imbalance Homeostasis: Normal blood glucose levels (90 mg/100ml) Imbalance Figure 9.15, step 6

Homeostasis: Normal blood glucose levels (90 mg/100ml) Insulin-secreting cells of the pancreas activated; release insulin into the blood Uptake of glucose from blood is en- hanced in most body cells Blood glucose levels decline to set point; stimulus for insulin release diminishes Elevated blood sugar levels Liver takes up glucose and stores it as glycogen Stimulus: rising blood glucose levels (e.g., after eating four jelly doughnuts) Homeostasis: Normal blood glucose levels (90 mg/100ml) Figure 9.15, step 7

Homeostasis: Normal blood glucose levels (90 mg/100ml) Imbalance Stimulus: declining blood glucose levels (e.g., after skipping a meal) Homeostasis: Normal blood glucose levels (90 mg/100ml) Imbalance Low blood sugar levels Figure 9.15, step 9

Homeostasis: Normal blood glucose levels (90 mg/100ml) Imbalance Stimulus: declining blood glucose levels (e.g., after skipping a meal) Homeostasis: Normal blood glucose levels (90 mg/100ml) Imbalance Low blood sugar levels Glucagon-releasing cells of pancreas activated; release glucagon into blood; target is the liver Figure 9.15, step 10

Homeostasis: Normal blood glucose levels (90 mg/100ml) Imbalance Stimulus: declining blood glucose levels (e.g., after skipping a meal) Homeostasis: Normal blood glucose levels (90 mg/100ml) Imbalance Low blood sugar levels Glucagon-releasing cells of pancreas activated; release glucagon into blood; target is the liver Liver breaks down glycogen stores and releases glucose to the blood Figure 9.15, step 11

Homeostasis: Normal blood glucose levels (90 mg/100ml) Stimulus: declining blood glucose levels (e.g., after skipping a meal) Homeostasis: Normal blood glucose levels (90 mg/100ml) Low blood sugar levels Rising blood glucose levels return blood sugar to homeostatic set point; stimulus for glucagon release diminishes Glucagon-releasing cells of pancreas activated; release glucagon into blood; target is the liver Liver breaks down glycogen stores and releases glucose to the blood Figure 9.15, step 12

Homeostasis: Normal blood glucose levels (90 mg/100ml) Insulin-secreting cells of the pancreas activated; release insulin into the blood Uptake of glucose from blood is en- hanced in most body cells Blood glucose levels decline to set point; stimulus for insulin release diminishes Elevated blood sugar levels Liver takes up glucose and stores it as glycogen Stimulus: rising blood glucose levels (e.g., after eating four jelly doughnuts) Imbalance Homeostasis: Normal blood glucose levels (90 mg/100ml) Stimulus: declining blood glucose levels (e.g., after skipping a meal) Imbalance Low blood sugar levels Rising blood glucose levels return blood sugar to homeostatic set point; stimulus for glucagon release diminishes Glucagon-releasing cells of pancreas activated; release glucagon into blood; target is the liver Liver breaks down glycogen stores and releases glucose to the blood Figure 9.15, step 13