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.

Membrane Potential

• Membrane potentials will occur across cell membranes if
  • 1) there is a concentration gradient of an ion
  • 2) there is an open channel in the membrane so the ion can move from one side to the other

The Sodium Pump Sets Up Gradients of Na and K Across Cell Membranes

• All cells have the Na pump in their membranes
  • Pumps 3 Nas out and 2 Ks in for each cycle
  • Requires energy from ATP
    • Uses about 30% of body's metabolic energy
  • This is a form of active transport- can pump ions "uphill", from a low to a high concentration
  • This produces concentration gradients of Na & K across the membrane
  • Typical concentration gradients:

•  
• The ion gradients represent stored electrical energy (batteries) that can be tapped to do useful work
• The Na pump is of ancient origin, probably originally designed to protect cell from osmotic swelling

Inhibited by the arrow poisons ouabain and digitalis

Regulation of glomerular filtration :

1. Extrinsic regulation : 

- Neural regulation : sympathetic and parasympathetic nervous system which causes vasoconstriction or vasodilation respectively .
- Humoral regulation : Vasoactive substances may affect the GFR , vasoconstrictive substances like endothelin ,Angiotensin II , Norepinephrine , prostaglandine F2 may constrict the afferent arteriole and thus decrease GFR , while the vasodilative agents like dopamine , NO , ANP , Prostaglandines E2 may dilate the afferent arteriole and thus increase the filtration rate .

2. Intrinsic regulation : 

- Myogenic theory ( as in the intrinsic regulation of cardiac output) .
- Tubuloglomerular feedback: occurs by cells of the juxtaglomerular apparatus that is composed of specific cells of the distal tubules when it passes between afferent and efferent arterioles ( macula densa cells ) , these cells sense changes in flow inside the tubules and inform specific cells in the afferent arteriole (granular cells ) , the later secrete vasoactive substances that affect the diameter of the afferent arteriole.

Production of Hormones

The kidneys produce and interact with several hormones that are involved in the control of systems outside of the urinary system.

Calcitriol. Calcitriol is the active form of vitamin D in the human body. It is produced by the kidneys from precursor molecules produced by UV radiation striking the skin. Calcitriol works together with parathyroid hormone (PTH) to raise the level of calcium ions in the bloodstream. When the level of calcium ions in the blood drops below a threshold level, the parathyroid glands release PTH, which in turn stimulates the kidneys to release calcitriol. Calcitriol promotes the small intestine to absorb calcium from food and deposit it into the bloodstream. It also stimulates the osteoclasts of the skeletal system to break down bone matrix to release calcium ions into the blood.
 
Erythropoietin. Erythropoietin, also known as EPO, is a hormone that is produced by the kidneys to stimulate the production of red blood cells. The kidneys monitor the condition of the blood that passes through their capillaries, including the oxygen-carrying capacity of the blood. When the blood becomes hypoxic, meaning that it is carrying deficient levels of oxygen, cells lining the capillaries begin producing EPO and release it into the bloodstream. EPO travels through the blood to the red bone marrow, where it stimulates hematopoietic cells to increase their rate of red blood cell production. Red blood cells contain hemoglobin, which greatly increases the blood’s oxygen-carrying capacity and effectively ends the hypoxic conditions.
 
Renin. Renin is not a hormone itself, but an enzyme that the kidneys produce to start the renin-angiotensin system (RAS). The RAS increases blood volume and blood pressure in response to low blood pressure, blood loss, or dehydration. Renin is released into the blood where it catalyzes angiotensinogen from the liver into angiotensin I. Angiotensin I is further catalyzed by another enzyme into Angiotensin II.

Angiotensin II stimulates several processes, including stimulating the adrenal cortex to produce the hormone aldosterone. Aldosterone then changes the function of the kidneys to increase the reabsorption of water and sodium ions into the blood, increasing blood volume and raising blood pressure. Negative feedback from increased blood pressure finally turns off the RAS to maintain healthy blood pressure levels.

Lipids:

• about 40% of the dry mass of a typical cell
• composed largely of carbon & hydrogen
• generally insoluble in water
• involved mainly with long-term energy storage; other functions are as structural components (as in the case of phospholipids that are the major building block in cell membranes) and as "messengers" (hormones) that play roles in communications within and between cells
• Subclasses include:
  • Triglycerides - consist of one glycerol molecule + 3 fatty acids (e.g., stearic acid in the diagram below). Fatty acids typically consist of chains of 16 or 18 carbons (plus lots of hydrogens).
  • phospholipids - Composed of 2 fatty acids, glycerol, phosphate and polar groups , phosphate group (-PO⁴) substitutes for one fatty acid & these lipids are an important component of cell membranes

steroids - have 4 rings- cholesterol, some hormones, found in membranes include testosterone, estrogen, & cholesterol

• There Are 12 Pairs of Cranial Nerves

• The 12 pairs of cranial nerves emerge mainly from the ventral surface of the brain
• Most attach to the medulla, pons or midbrain
• They leave the brain through various fissures and foramina of the skull

• Nerve	Name	Sensory	Motor	Autonomic Parasympathetic
  I	Olfactory	Smell
  II	Optic	Vision
  III	Oculomotor	Proprioception	4 Extrinsic eye muscles	Pupil constriction Accomodation Focusing
  IV	Trochlear	Proprioception	1 Extrinsic eye muscle (Sup.oblique)
  V	Trigeminal	Somatic senses (Face, tongue)	Chewing
  VI	Abducens	Proprioception	1 Extrinsic eye muscle (Lat. rectus)
  VII	Facial	Taste Proprioception	Muscles of facial expression	Salivary glands Tear glands
  VIII	Auditory (Vestibulocochlear)	Hearing, Balance
  IX	Glossopharyngeal	Taste Blood gases	Swallowing Gagging	Salivary glands
  X	Vagus	Blood pressure Blood gases  Taste	Speech Swallowing Gagging	Many visceral organs (heart, gut, lungs)
  XI	Spinal acessory	Proprioception	Neck muscles: Sternocleidomastoid Trapezius
  XII	Hypoglossal	Proprioception	Tongue muscles Speech

   

• Many of the functions that make us distinctly human are controlled by cranial nerves: special senses, facial expression, speech.

• Cranial Nerves Contain Sensory, Motor and Parasympathetic Fibers