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Communication and

HomeostasisPart 3

The Pancreas

The Pancreas is a small organ lying below the stomach. It has both

exocrine and endocrine functions.

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 neutralisethe contents of the digestive system which have just left the acidic stomach)

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.

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.

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.

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

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

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

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

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

vesicles (containing insulin) move to the cell surface membrane to be secreted by

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.

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

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.

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.

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

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

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)

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 blood.

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.

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.

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.