1. Endocrine & Cell Communication Part IV:
Maintaining Balance (Homeostasis)

2. AP Biology Curriculum Framework
EK 3.D.2 Cells communicate with each other through direct contact with other cells or from a distance via chemical signaling. c. Signals released by one cell type can travel long distances to target cells of another cell type. 1. Endocrine signals are produced by endocrine cells that release signaling molecules, which are specific and can travel long distances through the blood to reach all parts of the body. illustrative example-insulin
When you ingest carbohydrates your blood glucose level rises, which stimulates your pancreas to secrete insulin which in turn promotes cellular uptake of glucose into the liver and muscle cells where it is stored as glycogen. When your blood sugar level decreases between meals, the pancreas secretes glucagon which promotes the hydrolysis of glycogen to release glucose and fatty acids to raise your blood sugar levels.

3. Simple Hormone Pathways
Hormones are released from an endocrine cell, travel through the bloodstream, and interact with specific receptors within a target cell to cause a physiological response

4. Simple Hormone Pathways
For example, the release of acidic contents of the stomach into the duodenum stimulates endocrine cells there to secrete secretin.
This causes target cells in the pancreas, a gland behind the stomach, to raise the pH in the duodenum.
The increased pH results in a decrease of secretin secretion.
The pancreas releases sodium bicarbonate to raise the pH which neutralizes acid chyme from the stomach thereby raising the pH (making the environment more alkaline).

5. Simple Hormone Pathways
Example
Pathway
Low pH in duodenum
Stimulus
Endocrine cell
S cells of duodenum secrete the hormone secretin ( ).
Negative feedback
In this simple endocrine pathway a low duodenum pH stimulates endocrine cells in the small intestine (S cells) to secrete the hormone secretin. Secretin travels through the blood stream to its target cells (pancreatic cells) causing them to release bicarbonate solution resulting in an increase in the pH. The increase serves as a negative feedback mechanism resulting in lower levels of secretin released.
Hormone
Blood vessel
Target cells
Pancreas
Response
Bicarbonate release

6. Negative Feedback
Secretin secretion regulation is an example of negative feedback in action.

7. Feedback Regulation
A negative feedback loop inhibits a response by reducing the initial stimulus, thus preventing excessive pathway activity.
Positive feedback reinforces a stimulus to produce an even greater response.
For example, in mammals oxytocin causes the release of milk, causing greater suckling by offspring, which stimulates the release of more oxytocin.

8. An example of positive feedback
Oxytocin stimulates the uterus to contract. This causes the placenta to make more prostaglandins which signal more vigorous uterine contractions which cause more oxytocin to be produced thereby amplifying the contraction process.

9. Insulin and Glucagon: Control of Blood Glucose
Hormones work in pairs to maintain homeostasis.
Insulin (decreases blood glucose) and glucagon (increases blood glucose) are antagonistic hormones that help maintain glucose homeostasis.
The pancreas has clusters of endocrine cells called pancreatic islets with alpha cells that produce glucagon and beta cells that produce insulin.

10. Body cells take up more glucose.
Figure 45.13
Insulin
Body cells take up more glucose.
Beta cells of pancreas release insulin into the blood.
Liver takes up glucose and stores it as glycogen.
STIMULUS: Blood glucose level rises (for instance, after eating a carbohydrate-rich meal).
Blood glucose level declines.
Homeostasis: Blood glucose level (70–110 mg/100mL)
Describe the actions that occur when blood glucose levels decline and when they rise. Glucagon and insulin are paired hormones that work together to maintain blood glucose levels between 70 and 110 mg/100mL
STIMULUS: Blood glucose level falls (for instance, after skipping a meal).
Blood glucose level rises.
Liver breaks down glycogen and releases glucose into the blood.
Alpha cells of pancreas release glucagon into the blood.
Glucagon

11. AP Curriculum Framework
EK 3.D.4 Changes in signal transduction pathways can alter cellular response.
A. Conditions where signal transduction is blocked or defective can be deleterious, preventative or prophylactic.
Illustrative example - diabetes

12. Out of Balance: Diabetes Mellitus
Diabetes mellitus is perhaps the best-known endocrine disorder.
It is caused by a deficiency of insulin or a decreased response to insulin in target tissues.
It is marked by elevated blood glucose levels.
Ask students to explain how a lack of insulin leads to elevated levels of glucose in the blood. Then ask them to suggest reasons this increased level of glucose is harmful to the person with diabetes.

13. Out of Balance: Diabetes Mellitus
Type 1 diabetes mellitus (insulin-dependent) is an autoimmune disorder in which the immune system destroys pancreatic beta cells.
Type 2 diabetes mellitus (non-insulin-dependent) involves insulin deficiency or reduced response of target cells due to change in insulin receptors.
Type 1 has in the past been referred to as Juvenile Diabetes. Just as a point of interest, the incidence varies from 8 to 17 per 100,000 in Northern Europe and the U.S. with a high of about 35 per 100,000 in Scandinavia to a low of 1 per 100,000 in Japan and China.

14. Action of Insulin
When insulin receptors respond properly to the presence of insulin, the result is the transport of glucose from outside the cell to inside the cell via transport protein. People with Type I diabetes do not produce sufficient insulin to maintain a proper level of glucose transport. The disorder is typically treated by providing the patient with insulin.

15. Increases Ca2 uptake in intestines Active vitamin D
Stimulates Ca2 uptake in kidneys
PTH
Parathyroid gland (behind thyroid)
Blood calcium levels need to be approximately 10 mg/100 mL. Two hormones, PTH and calcitonin work in tandem to regulate the blood glucose in mammals.
Stimulates Ca2 release from bones
STIMULUS: Falling blood Ca2 level
Blood Ca2 level rises.
Homeostasis: Blood Ca2 level (about 10 mg/100 mL)

16. Homeostasis in blood calcium levels
PTH increases the level of blood Ca2+
It releases Ca2+ from bone and stimulates reabsorption of Ca2+ in the kidneys.
It also has an indirect effect, stimulating the kidneys to activate vitamin D, which promotes intestinal uptake of Ca2+ from food.
Calcitonin decreases the level of blood Ca2+
It stimulates Ca2+ deposition in bones and secretion by kidneys.
Describe how calcitonin and PTH work together to maintain blood calcium levels. High calcium levels can cause mental confusion, nausea, fatigue. Low blood calcium causes muscle cramps, spasms, twitching and tingling in the fingers and around the mouth.

17. Practice Blood calcium levels rise Blood calcium level falls
Parathyroid releases PTH
Thyroid releases calcitonin
If calcium rises above set point
If calcium falls below set point
Ask students to match the events on the right with the numbers in the picture. The next slide shows the answers.

18. Solution 3. Blood calcium level falls 5. Parathyroid releases PTH
1. Blood calcium levels rise
3. Blood calcium level falls
5. Parathyroid releases PTH
2. Thyroid releases calcitonin
6. If calcium rises above set point
4. If calcium falls below set point
The two hormones, calcitonin and parathyroid hormone work together to keep blood calcium levels within a homeostatic range.(10 mg/100 mL)

19. Created by:
Debra Richards
Coordinator of Secondary Science Programs
Bryan ISD
Bryan, TX