The pancreas has both exocrine and endocrine functions. Its exocrine function involves secreting digestive enzymes into the small intestine. Its endocrine function involves the islets of Langerhans secreting the hormones insulin and glucagon directly into the bloodstream. Insulin lowers blood glucose levels while glucagon raises them, and they work together via negative feedback to maintain homeostasis of blood glucose concentrations. Diabetes occurs when blood glucose levels cannot be properly regulated.

1. S
Communication and
Homeostasis
Part 3

2. The Pancreas
The Pancreas is a small organ lying below the stomach. It has both
exocrine and endocrine functions.

3. The Exocrine Function
The majority of cells in the pancreas manufacture and release digestive
enzymes. The cells are found in small groups surrounding tiny tubules. They
secrete the digestive enzymes into these tubules. The tubules join up to
make the pancreatic duct. The pancreatic duct carries the fluid containing
the enzymes into the first part of the small intestine.
Fluid Contents…
S Amylase (carbohydrase)
S Trypsinogen (inactive protease)
S Lipase
S Sodium Hydrogencarbonate (this makes the fluid alkaline to neutralise
the contents of the digestive system which have just left the acidic
stomach)

4. The Endocrine Function
In the Pancreas, there are islets of Langerhans which contain two
different types of cells.
S Alpha cells: these manufacture and secrete Glucagon. Glucagon
causes blood glucose levels to rise.
S Beta cells: these manufacture and secrete Insulin. Insulin causes
blood glucose levels to drop.
The islets are well supplied with blood capillaries, and these hormones
(glucagon and insulin) are secreted directly into the blood.

6. The Control of Blood Glucose
The cells in the islets of Langerhans monitor the concentration of
glucose in the blood. The normal blood concentration of glucose is
90mg 100cm-3 / 4-6 mmol dm-3.
If the concentration rises or falls, the cells in the islets of Langerhans
respond as necessary. Nevertheless, blood glucose levels will never
remain absolutely constant, but using negative feedback the body is
able to keep it fairly well controlled.

7. If the Concentration Rises
A high blood glucose concentration is detected by the beta cells,
which release insulin into the blood. The target cells are the
hepatocytes (liver cells) and muscle cells which possess the specific
membrane-bound receptors for insulin. When the blood passes these
cells, the insulin binds to the receptors. This activates the adenyl
cyclase inside each cell which converts ATP to cAMP. In turn, cAMP
then activates a series of enzyme-controlled reactions.

8. The Effect of Insulin On the
Cell
1. More glucose channels are places into the cell surface
membrane
2. More glucose enters the cell
3. Glucose is converted to glycogen for storage
4. More glucose is concerted to fats
5. More glucose is used in respiration

9. If the Concentration Falls
A low blood glucose concentration is detected by the alpha cells which
secrete glucagon into the blood. Its target cells are also the
hepatocytes which possess the specific receptor for glucagon.
The effects of glucagon on the cell includes…
S The conversion of glycogen to glucose
S The use of more fatty acids in respiration
S The production of glucose by the conversion from amino acids and
fats

10. Terms
S Glycogenesis  the conversion of glucose to glycogen
S Glycogenolysis  the conversion of glycogen to
glucose
S Gluconeogenesis  the production of glucose by
conversion from amino acids and fats

11. How is the ultrastructure of the
alpha and beta cells
specialised?
S Lots of ribosomes and RER for protein synthesis
S A lot of Golgi apparatus for packaging hormones into vesicles
S They will have many secretory vesicles as these vesicles
transport the hormone to the cell surface membrane for secretion
by exocytosis
S Many mitochondria to supply ATP (from aerobic respiration) as an
energy source for the active processes

12. Insulin Secretion
1. The potassium ion channels in the cell membrane of the beta cells are normally
open so potassium ions diffuse out of the cell making the inside more negative.
The calcium ion channels are normally closed.
2. When glucose concentrations outside the cell are high, glucose molecules diffuse
into the cell
3. The glucose is quickly used in metabolism to produce ATP
4. The extra ATP causes the potassium ion channels to close
5. The potassium ions can no longer diffuse out, so the inside becomes less
negative.
6. This change opens the calcium channels
7. Calcium ions enter the cell and cause the secretion of insulin by making the

13. Terms
S Diabetes Mellitus  a disease in which blood glucose
concentrations cannot be controlled effectively.
S Hyperglycaemia  the state in which the blood glucose
concentration is too high.
S Hypoglycaemia  the state in which the blood glucose
concentration is too low.

14. Type 1 Diabetes
S “Insulin-dependent diabetes”
S “Juvenile-onset diabetes”
S It it thought to be the result of an autoimmune response in which
the body’s immune system attacks and destroys the beta cells. It
could also result from a viral attack.
S The body cannot manufacture sufficient insulin or store excess
glucose as glycogen.
S It is treated using insulin injections

15. Type 2 Diabetes
S “Non-insulin dependent”
S An individual can still produce insulin, but as people age, their
responsiveness to insulin declines. This could be because the
specific receptors on the surface of the liver and muscle cells
decline and the cells lose their ability to respond to the insulin in
the blood.
S The levels of insulin secreted may also decline.
S It is thought anyone who lives long enough will become diabetic –
but not until you are 120 years old!
S It is treated though careful monitoring and control of your diet,
especially with regards to carbohydrate intake.

16. The Source of Insulin
Insulin used to be extracted from the pancreas of pigs as it matches
human insulin most closely. However, due to a difference in the base
sequence of amino acids between humans and pigs, it wasn’t an ideal
source.
Recently, insulin can be produced by bacteria that have been
genetically engineered.

17. Advantages of Genetically
Engineered Insulin
1. It is an exact copy, so faster acting and more effective
2. Less chance of developing tolerance
3. Less chance of rejection
4. Lower risk of infection
5. Cheaper to manufacture
6. Less ethical issues

18. Control of Heart Rate
Some terms you need to know…
S Cell Metabolism  the result if all the chemical reactions taking
place in the cytoplasm
S Myogenic  the muscle tissue can initiate its own contrations
S Medulla Oblongata  the region at the base of the brain that
coordinates the unconscious functions of the body (e.g. breathing)
S Cardiovascular Centre  a specific region of the medulla
oblongata that receives sensory inputs about levels of physical
activity, blood CO2 concentration and blood pressure. It sends
nerve impulses to the SAN in the heart to alter the frequency of
excitation waves

19. How the Heart Adapts to Supply
More Oxygen and Glucose
S An increase in the heart rate (beats per minute)
S The heart can increase the strength of its contractions
S Increase the stroke volume (volume of blood pumped per beat)

20. Control of Heart Rate
S The heart muscle is myogenic
S The heart has a pacemaker called the SAN (found in the right
atrium) which initiates an action potential. This travels as a wave of
excitation over the atria walls, through the AVN and down the
Purkyne fibres to the ventricles. The ventricles then contract.
S The heart is supplied by nerves from the medulla oblongata which
connect to the SAN. They are called the accelerator nerve and
the vagus nerve. These do not initiate a contraction, but can affect
the frequency of the contractions. Action potentials down the
accelerator nerve increase heart rate; down the vagus nerve
reduce heart rate.
S The heart muscle responds to the presence of adrenaline in the

21. Control Mechanisms:
Interactions
The various factors that affect heart rate must interact in a
coordinated way to ensure that the heart beats at the most
appropriate rate. Under rest, the heart rate is controlled by the SAN.
This has a set frequency at which it initiates waves of excitation. The
frequency of these waves can be controlled by the cardiovascular
centre (CC) in the medulla oblongata.

22. Factors Affecting Heart Rate
S The movement of the limbs is detected by stretch receptors in the
muscles. These send impulses to the CC informing it that extra
oxygen may be needed and so the heart rate is increased.
S When we exercise, the muscles produce more carbon dioxide.
Some CO2 reacts with the water in the blood plasma and reduces
the pH. This change in pH is detected by chemoreceptors in the
carotid arteries, the aorta and the brain. These send impulses to
the CC to increase heart rate.

23. Factors Affecting Heart Rate
S When we stop exercising the concentration of CO2 in the blood
falls. This reduces the activity of the accelerator pathway, so the
heart rate declines.
S Adrenaline is secreted in response to stress, shock, anticipation
or excitement. The presence of adrenaline in the blood increases
heart rate to prepare the body for activity.
S Blood pressure is monitored by stretch receptors in the walls of
the carotid sinus (a small swelling in the carotid artery). It blood
pressure rises too high the stretch receptors send signals to the
cardiovascular centre, which responds by reducing heart rate.