Ventilation simply means inhaling and exhaling of air from the atmospheric air into lungs and then exhaling it from the lung into the atmospheric air.
Air pressure gradient has to exist between two atmospheres to enable a gas to move from one atmosphere to an other.
 

During inspiration: the intrathoracic pressure has to be less than that of atmospheric pressure. This could be achieved by decreasing the intrathoracic pressure as follows:
 

Depending on Boyle`s law , the pressure of gas is inversely proportional to the volume of its container. So increasing the intrathoracic volume will decrease the intrathoracic pressure which will allow the atmospheric air to be inhaled (inspiration) . As decreasing the intrathoracic volume will increase the intrathoracic pressure and causes exhaling of air ( expiration)

So. Inspiration  could be actively achieved by the contraction of inspiratory muscles : diaphragm and intercostal muscles. While relaxation of the mentioned muscles will passively cause expiration.
 

Contraction of diaphragm will pull the diaphragm down the abdominal cavity ( will move inferiorly)  , and then increase the intrathoracic volume ( vertically)  . Contraction of external intercostal muscle will pull the ribs upward and forward which will additionally increase the intrathoracic volume ( transversely  , the net result will be increasing the intrathoracic volume and decreasing the intrathoracic pressure.
 

Relaxation of diaphragm will move it superiorly during expiration, the relaxation of external intercostal muscles will pull the ribs downward and backward , and the elastic lungs and chest wall will recoil. The net result is decreasing the intrathoracic volume and increasing intrathoracic pressure.
 

All of this occurs during quiet breathing. During forceful inspiration an accessory inspiratory muscle will be involved ( scaleni , sternocleidomastoid , and others) to increase negativity in the intrathoracic pressure more and more.
 

During forceful expiration the accessory expiratory muscles ( internal intercostal muscles and abdominal muscles ) will be involved to decrease the intrathoracic volume  more and more and then to increase  intrathoracic pressure more and more.

The pressure within the alveoli is called intralveolar  pressure . Between the two phases of respiration it is equal to the atmospheric pressure. It is decreased during inspiration ( about 1 cm H₂O ) and increased during expiration ( about +1 cm H₂O ) . This difference allow entering of 0.5 L of air into the lungs.

Intrapleural pressure is the pressure of thin fluid between the two pleural layers . It is a slight negative pressure. At the beginning of inspiration it is about -5 cm H2O and reachs -7.5 cm H₂O at the end or inspiration.

At the beginning of expiration the intrapleural pressure is -7.5 cm H₂O and reaches -5 cmH₂O at the end of expiration.
The difference between intralveolar pressure and intrapleural pressure is called transpulmonary pressure.

Factors , affecting ventilation :
 

Resistance : Gradual decreasing of the diameter of respiratory airway increase the resistance to air flow.
 

Compliance : means the ease , which the lungs expand.It depends on both the elastic forces of the lungs and the elastic forces , caused by the the surface tension of the fluid, lining the alveoli.
 

Surface tension: Molecules of water have tendency to attract each other on the surface of water adjacent to air. In alveoli the surface tension caused by the fluid in the inner surface of the alveoli  may cause collapse of alveoli . The surface tension is decreased by the surfactant .

Surfactant is a mixture of phospholipids , proteins and ion m produced by type II pneumocytes.

Immature newborns may suffer from respiratory distress syndrome , due to lack of surfactant which is produced during the last trimester of pregnancy.
 

The elastic fibers of the thoracic wall also participate in lung compliance.

Maintenance of Homeostasis

The kidneys maintain the homeostasis of several important internal conditions by controlling the excretion of substances out of the body. 

Ions. The kidney can control the excretion of potassium, sodium, calcium, magnesium, phosphate, and chloride ions into urine. In cases where these ions reach a higher than normal concentration, the kidneys can increase their excretion out of the body to return them to a normal level. Conversely, the kidneys can conserve these ions when they are present in lower than normal levels by allowing the ions to be reabsorbed into the blood during filtration. (See more about ions.)
 
pH. The kidneys monitor and regulate the levels of hydrogen ions (H+) and bicarbonate ions in the blood to control blood pH. H+ ions are produced as a natural byproduct of the metabolism of dietary proteins and accumulate in the blood over time. The kidneys excrete excess H+ ions into urine for elimination from the body. The kidneys also conserve bicarbonate ions, which act as important pH buffers in the blood.
 
Osmolarity. The cells of the body need to grow in an isotonic environment in order to maintain their fluid and electrolyte balance. The kidneys maintain the body’s osmotic balance by controlling the amount of water that is filtered out of the blood and excreted into urine. When a person consumes a large amount of water, the kidneys reduce their reabsorption of water to allow the excess water to be excreted in urine. This results in the production of dilute, watery urine. In the case of the body being dehydrated, the kidneys reabsorb as much water as possible back into the blood to produce highly concentrated urine full of excreted ions and wastes. The changes in excretion of water are controlled by antidiuretic hormone (ADH). ADH is produced in the hypothalamus and released by the posterior pituitary gland to help the body retain water.
 
Blood Pressure. The kidneys monitor the body’s blood pressure to help maintain homeostasis. When blood pressure is elevated, the kidneys can help to reduce blood pressure by reducing the volume of blood in the body. The kidneys are able to reduce blood volume by reducing the reabsorption of water into the blood and producing watery, dilute urine. When blood pressure becomes too low, the kidneys can produce the enzyme renin to constrict blood vessels and produce concentrated urine, which allows more water to remain in the blood.

The Parathyroid Glands

The parathyroid glands are 4 tiny structures embedded in the rear surface of the thyroid gland. They secrete parathyroid hormone (PTH) a polypeptide of 84 amino acids. PTH increases the concentration of Ca²⁺ in the blood in three ways. PTH promotes

• release of Ca²⁺ from the huge reservoir in the bones. (99% of the calcium in the body is incorporated in our bones.)
• reabsorption of Ca²⁺ from the fluid in the tubules in the kidneys
• absorption of Ca²⁺ from the contents of the intestine (this action is mediated by calcitriol, the active form of vitamin D.)

PTH also regulates the level of phosphate in the blood. Secretion of PTH reduces the efficiency with which phosphate is reclaimed in the proximal tubules of the kidney causing a drop in the phosphate concentration of the blood.

Hyperparathyroidism

Elevate the level of PTH causing a rise in the level of blood Ca²⁺ .Calcium may be withdrawn from the bones that they become brittle and break.

 Patients with this disorder have high levels of Ca²⁺ in their blood and excrete small amounts of Ca²⁺ in their urine. This causes hyperparathyroidism.

Hypoparathyroidism

This disorder have low levels of Ca²⁺ in their blood and excrete large amounts of Ca²⁺ in their urine.

White Blood Cells (leukocytes)

White blood cells

• are much less numerous than red (the ratio between the two is around 1:700),
• have nuclei,
• participate in protecting the body from infection,
• consist of lymphocytes and monocytes with relatively clear cytoplasm, and three types of granulocytes, whose cytoplasm is filled with granules.

Lymphocytes: There are several kinds of lymphocytes, each with different functions to perform , 25% of wbc The most common types of lymphocytes are

• B lymphocytes ("B cells"). These are responsible for making antibodies.
• T lymphocytes ("T cells"). There are several subsets of these:
  • inflammatory T cells that recruit macrophages and neutrophils to the site of infection or other tissue damage
  • cytotoxic T lymphocytes (CTLs) that kill virus-infected and, perhaps, tumor cells
  • helper T cells that enhance the production of antibodies by B cells

Although bone marrow is the ultimate source of lymphocytes, the lymphocytes that will become T cells migrate from the bone marrow to the thymus where they mature. Both B cells and T cells also take up residence in lymph nodes, the spleen and other tissues where they

• encounter antigens;
• continue to divide by mitosis;
• mature into fully functional cells.

Monocytes : also originate in marrow, spend up to 20 days in the circulation, then travel to the tissues where they become macrophages. Macrophages are the most important phagocyte outside the circulation. Monocytes are about 9% of normal wbc count

Macrophages are large, phagocytic cells that engulf

• foreign material (antigens) that enter the body
• dead and dying cells of the body.

Neutrophils

The most abundant of the WBCs. about 65% of normal white count  These cells spend 8 to 10 days in the circulation making their way to sites of infection etc  Neutrophils squeeze through the capillary walls and into infected tissue where they kill the invaders (e.g., bacteria) and then engulf the remnants by phagocytosis. They have two types of granules: the most numerous are specific granules which contain bactericidal agents such as lysozyme; the azurophilic granules are lysosomes containing peroxidase and other enzymes

Eosinophils : The number of eosinophils in the blood is normally quite low (0–450/µl). However, their numbers increase sharply in certain diseases, especially infections by parasitic worms. Eosinophils are cytotoxic, releasing the contents of their granules on the invader.

Basophils : rare except during infections where these cells mediate inflammation by secreting histamine and heparan sulfate (related to the anticoagulant heparin). Histamine makes blood vessels permeable and heparin inhibits blood clotting. Basophils are functionally related to mast cells.  . The mediators released by basophils also play an important part in some allergic responses such as hay fever and an anaphylactic response to insect stings.

Thrombocytes (platelets):

Thrombocytes are cellular derivatives from megakaryocytes which contain factors responsible for the intrinsic clotting mechanism. They represent fragmented cells  which contain residual organelles including rough endoplasmic reticulum and Golgi apparati. They are only 2-microns in diameter, are seen in peripheral blood either singly or, often, in clusters, and have a lifespan of 10 days.

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.

Serum Lipids

• Total cholesterol is the sum of
  • HDL cholesterol
  • LDL cholesterol and
  • 20% of the triglyceride value
• Note that
  • high LDL values are bad, but
  • high HDL values are good.
• Using the various values, one can calculate a
  cardiac risk ratio = total cholesterol divided by HDL cholesterol
• A cardiac risk ratio greater than 7 is considered a warning.