Hormones and the Endocrine System

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secrete secretin ( )

Endocrine

cell

Blood

vessel

Pancreas

Target

cells

Response

Bicarbonate release

Negative feedback

–

Slide 31

Insulin and Glucagon: Control of Blood Glucose

A negative feedback loop inhibits a response by reducing the initial stimulus.

Negative feedback reverses a trend to regulate many hormonal pathways involved in homeostasis.

Insulin and glucagon are antagonistic hormones that help maintain glucose homeostasis.

The pancreas has endocrine cells called islets of Langerhans with alpha cells that produce glucagon and beta cells that produce insulin.

Slide 32

Insulin Lowers Blood Glucose Levels

Homeostasis:

Blood glucose level

(about 90 mg/100 mL)

Insulin

Beta cells of

pancreas

release insulin

into the blood.

STIMULUS:

Blood glucose level

rises.

Liver takes

up glucose

and stores it

as glycogen.

Blood glucose

level declines.

Body cells

take up more

glucose.

Slide 33

Glucagon Raises Blood Glucose Levels

Homeostasis:

Blood glucose level

(about 90 mg/100 mL)

Glucagon

STIMULUS:

Blood glucose level

falls.

Alpha cells of pancreas

release glucagon.

Liver breaks

down glycogen

and releases

glucose.

Blood glucose

level rises.

Slide 34

Maintenance of glucose homeostasis by insulin and glucagon

Homeostasis:

Blood glucose level

(about 90 mg/100 mL)

Glucagon

STIMULUS:

Blood glucose level

falls.

Alpha cells of pancreas

release glucagon.

Liver breaks

down glycogen

and releases

glucose.

Blood glucose

level rises.

STIMULUS:

Blood glucose level

rises.

Beta cells of

pancreas

release insulin

into the blood.

Liver takes

up glucose

and stores it

as glycogen.

Blood glucose

level declines.

Body cells

take up more

glucose.

Insulin

Slide 35

Target Tissues for Insulin and Glucagon

Insulin reduces blood glucose levels by

Promoting the cellular uptake of glucose

Slowing glycogen breakdown in the liver

Promoting fat storage.

Glucagon increases blood glucose levels by

Stimulating conversion of glycogen to glucose in the liver

Stimulating breakdown of fat and protein into glucose.

Slide 36

Diabetes Mellitus

Diabetes mellitus is an endocrine disorder caused by a deficiency of insulin or a decreased response to insulin in target tissues.