The bulk of the pancreas is an exocrine gland secreting pancreatic fluid into the duodenum after a meal. However, scattered through the pancreas are several hundred thousand clusters of cells called islets of Langerhans. The islets are endocrine tissue containing four types of cells. In order of abundance, they are the:

• beta cells, which secrete insulin and amylin;
• alpha cells, which secrete glucagon;
• delta cells, which secrete somatostatin, and
• gamma cells, which secrete a polypeptide of unknown function.

Beta Cells

Beta cells secrete insulin in response to a rising level of blood sugar

Insulin affects many organs. It

• stimulates skeletal muscle fibers to
  • take up glucose and convert it into glycogen;
  • take up amino acids from the blood and convert them into protein.
• acts on liver cells
  • stimulating them to take up glucose from the blood and convert it into glycogen while
  • inhibiting production of the enzymes involved in breaking glycogen back down (glycogenolysis) and
  • inhibiting gluconeogenesis; that is, the conversion of fats and proteins into glucose.
• acts on fat (adipose) cells to stimulate the uptake of glucose and the synthesis of fat.
• acts on cells in the hypothalamus to reduce appetite.

Diabetes Mellitus

Diabetes mellitus is an endocrine disorder characterized by many signs and symptoms. Primary among these are:

• a failure of the kidney to retain glucose .
• a resulting increase in the volume of urine because of the osmotic effect of this glucose (it reduces the return of water to the blood).

There are three categories of diabetes mellitus:

• Insulin-Dependent Diabetes Mellitus (IDDM) (Type 1) and
• Non Insulin-Dependent Diabetes Mellitus (NIDDM)(Type 2)
• Inherited Forms of Diabetes Mellitus

Insulin-Dependent Diabetes Mellitus (IDDM)

IDDM ( Type 1 diabetes)

• is characterized by little or no circulating insulin;
• most commonly appears in childhood.
• It results from destruction of the beta cells of the islets.
• The destruction results from a cell-mediated autoimmune attack against the beta cells.
• What triggers this attack is still a mystery, although a prior viral infection may be the culprit.

Non Insulin-Dependent Diabetes Mellitus (NIDDM)

Many people develop diabetes mellitus without an accompanying drop in insulin levels In many cases, the problem appears to be a failure to express a sufficient number of glucose transporters in the plasma membrane (and T-system) of their skeletal muscles. Normally when insulin binds to its receptor on the cell surface, it initiates a chain of events that leads to the insertion in the plasma membrane of increased numbers of a transmembrane glucose transporter. This transporter forms a channel that permits the facilitated diffusion of glucose into the cell. Skeletal muscle is the major "sink" for removing excess glucose from the blood (and converting it into glycogen). In NIDDM, the patient's ability to remove glucose from the blood and convert it into glycogen is reduced. This is called insulin resistance. NIDDM (also called Type 2 diabetes mellitus) usually occurs in adults and, particularly often, in overweight people.

Alpha Cells

The alpha cells of the islets secrete glucagon, a polypeptide of 29 amino acids. Glucagon acts principally on the liver where it stimulates the conversion of glycogen into glucose (glycogenolysis) which is deposited in the blood.

Glucagon secretion is

• stimulated by low levels of glucose in the blood;
• inhibited by high levels, and
• inhibited by amylin.

The physiological significance of this is that glucagon functions to maintain a steady level of blood sugar level between meals.

Delta Cells

The delta cells secrete somatostatin. Somatostatin has a variety of functions. Taken together, they work to reduce the rate at which food is absorbed from the contents of the intestine. Somatostatin is also secreted by the hypothalamus and by the intestine.

Gamma Cells

The gamma cells of the islets secrete pancreatic polypeptide. No function has yet been found for this peptide of 36 amino acids.

1. Automatic control (sensory) of respiration is in - brainstem (midbrain) 

2. Behavioral/voluntary control is in - the cortex

3. Alveolar ventilation -the amount of atmospheric air that actually reaches the alveolar per breath and that can participate in the exchange of gasses between alveoli and blood

4. Only way to increase gas exchange in alveolar capillaries - perfusion-limited gas exchange 

5. Pulmonary ventiliation not effected by - concentration of bicarbonate ions

6. Central chemoreceptors - medulla -  CO2, O2 and H+ concentrations

7. Peripheral chemoreceptors - carotid and aortic bodies- PO2, PCO2 and pH 

8. Major stimulus for respiratory centers - arterial PCO2 

9. Rhythmic breathing depends on 
1. continuous (tonic) inspiratory drive from DRG (dorsal respiratory group)
2. intermittent (phasic) expiratory input from cerebrum, thalamus, cranial nerves and ascending spinal cord sensory tracts

10. Primary site for gas exchange - type I epithelial cells for alveoli

A rise in blood pressure stretches the atria of the heart. This triggers the release of atrial natriuretic peptide (ANP). ANP is a peptide of 28 amino acids. ANP lowers blood pressure by:

• relaxing arterioles
• inhibiting the secretion of renin and aldosterone
• inhibiting the reabsorption of sodium ions in the collecting ducts of the kidneys.

The effects on the kidney reduce the reabsorption of water by them thus increasing the flow of urine and the amount of sodium excreted in it (These actions give ANP its name: natrium = sodium; uresis = urinate). The net effect of these actions is to reduce blood pressure by reducing the volume of blood volume in the system.

The Heartbeat

During rest, the heart beats about 70 times a minute in the adult male, while pumping about 5 liters of blood.

The stimulus that maintains this rhythm is self-contained. Embedded in the wall of the right atrium is a mass of specialized heart tissue called the sino-atrial (S-A) node. The S-A node is also called the pacemaker because it establishes the basic frequency at which the heart beats.

The interior of the fibers of heart muscle, like all cells, is negatively charged with respect to the exterior. In the cells of the pacemaker, this charge breaks down spontaneously about 70 times each minute. This, in turn, initiates a similar discharge of the nearby muscle fibers of the atrium. A tiny wave of current sweeps over the atria, causing them to contract.

When this current reaches the region of insulating connective tissue between the atria and the ventricles, it is picked up by the A-V node (atrio-ventricular node). This leads to a system of branching fibers that carries the current to all parts of the ventricles.

The contraction of the heart in response to this electrical activity creates systole.

A period of recovery follows called diastole.

• The heart muscle and S-A node become recharged.
• The heart muscle relaxes.
• The atria refill. 

The Electrocardiogram

The electrical activity of the heart can be detected by electrodes placed at the surface of the body. Analysis of an electrocardiogram (ECG or EKG) aids in determining, for example, the extent of damage following a heart attack. This is because death of a portion of the heart muscle blocks electrical transmission through that area and alters the appearance of the ECG

Control of the Heart

Although the A-V node sets the basic rhythm of the heart, the rate and strength of its beating can be modified by two auxiliary control centers located in the medulla oblongata of the brain.

• One sends nerve impulses down accelerator nerves.
• The other sends nerve impulses down a pair of vagus nerves

Accelerator Nerves

The accelerator nerves are part of the sympathetic branch of the autonomic nervous system, and  like all post-ganglionic sympathetic neurons  release noradrenaline at their endings on the heart.

They increase the rate and strength of the heartbeat and thus increase the flow of blood. Their activation usually arises from some stress such as fear or violent exertion. The heartbeat may increase to 180 beats per minute. The strength of contraction increases as well so the amount of blood pumped may increase to as much as 25-30 liters/minute.

Vigorous exercise accelerates heartbeat in two ways;

• As cellular respiration increases, so does the carbon dioxide level in the blood. This stimulates receptors in the carotid arteries and aorta, and these transmit impulses to the medulla for relay  by the accelerator nerves  to the heart.
• As muscular activity increases, the muscle pump drives more blood back to the right atrium. The atrium becomes distended with blood, thus stimulating stretch receptors in its wall. These, too, send impulses to the medulla for relay to the heart.

Distention of the wall of the right atrium also triggers the release of atrial natriuretic peptide (ANP) which initiates a set of responses leading to a lowering of blood pressure

The Vagus Nerves

The vagus nerves are part of the parasympathetic branch of the autonomic nervous system. They, too, run from the medulla oblongata to the heart. Their activity slows the heartbeat.

Pressure receptors in the aorta and carotid arteries send impulses to the medulla which relays these  by way of the vagus nerves  to the heart. Heartbeat and blood pressure diminish.

Red Blood Cells (erythrocytes)

• Women average about 4.8 million of these cells per cubic millimeter (mm³; which is the same as a microliter [µl]) of blood.
• Men average about 5.4 x 10⁶ per µl.
• These values can vary over quite a range depending on such factors as health and altitude.
• RBC precursors mature in the bone marrow closely attached to a macrophage.
• They manufacture hemoglobin until it accounts for some 90% of the dry weight of the cell.

• The nucleus is squeezed out of the cell and is ingested by the macrophage.

RBC have characteristic biconcave shape

Thus RBCs are terminally differentiated; that is, they can never divide. They live about 120 days and then are ingested by phagocytic cells in the liver and spleen. Most of the iron in their hemoglobin is reclaimed for reuse. The remainder of the heme portion of the molecule is degraded into bile pigments and excreted by the liver. Some 3 million RBCs die and are scavenged by the liver each second.

Red blood cells are responsible for the transport of oxygen and carbon dioxide.

An anti-diruetic is a substance that decreases urine volume, and ADH is the primary example of it within the body. ADH is a hormone secreted from the posterior pituitary gland in response to increased plasma osmolarity (i.e., increased ion concentration in the blood), which is generally due to an increased concentration of ions relative to the volume of plasma, or decreased plasma volume.

The increased plasma osmolarity is sensed by osmoreceptors in the hypothalamus, which will stimulate the posterior pituitary gland to release ADH. ADH will then act on the nephrons of the kidneys to cause a decrease in plasma osmolarity and an increase in urine osmolarity.

ADH increases the permeability to water of the distal convoluted tubule and collecting duct, which are normally impermeable to water. This effect causes increased water reabsorption and retention and decreases the volume of urine produced relative to its ion content.

After ADH acts on the nephron to decrease plasma osmolarity (and leads to increased blood volume) and increase urine osmolarity, the osmoreceptors in the hypothalamus will inactivate, and ADH secretion will end. Due to this response, ADH secretion is considered to be a form of negative feedback.

SPECIAL VISCERAL AFFERENT (SVA) PATHWAYS

Taste

Special visceral afferent (SVA) fibers of cranial nerves VII, IX, and X conduct signals into the solitary tract of the brainstem, ultimately terminating in the nucleus of the solitary tract on the ipsilateral side.

Second-order neurons cross over and ascend through the brainstem in the medial lemniscus to the VPM of the thalamus.

Thalamic projections to area 43 (the primary taste area) of the postcentral gyrus complete the relay.

SVA VII fibers conduct from the chemoreceptors of taste buds on the anterior twothirds of the tongue, while SVA IX fibers conduct taste information from buds on the posterior one-third of the tongue.

SVA X fibers conduct taste signals from those taste cells located throughout the fauces.

Smell

The smell-sensitive cells (olfactory cells) of the olfactory epithelium project their central processes through the cribiform plate of the ethmoid bone, where they synapse with mitral cells. The central processes of the mitral cells pass from the olfactory bulb through the olfactory tract, which divides into a medial and lateral portion The lateral olfactory tract terminates in the prepyriform cortex and parts of the amygdala of the temporal lobe.

These areas represent the primary olfactory cortex. Fibers then project from here to area 28, the secondary olfactory area, for sensory evaluation. The medial olfactory tract projects to the anterior perforated sub­stance, the septum pellucidum, the subcallosal area, and even the contralateral olfactory tract.

Both the medial and lateral olfactory tracts contribute to the visceral reflex pathways, causing the viscerosomatic and viscerovisceral responses.