Abstract:

Cerebral circulation

The principal arterial inflow to brain in humans is via four arteries. Two internal carotid and two vertebral arteries. vertebral arteries unit to form the basilar artery The basilar and the carotids artery form the circle of Willis below the hypothalamus.

The cerebral vessels have a number of anatomic features:

1- In choroid plexus there are gaps between the endothelial cells of capillary walls, but the choroid epithelial cells that separate them from CSF are connected to one another by tight junction.  

2- Capillaries in the brain substance are non-fenestrated resemble capillaries of muscles, tight substance between endothelial cells. Substance transport through capillaries is by vesicular transport.

3-Brain capillaries are surrounded by end feet of astrocytes.

Motor autonomic nervous system supply mainly large arteries while sensory nerves are found on most distal arteries. Brain is sensitive to hypoxia, and occlusion of its blood supply produces unconsciousness in short period as 10 second.

Pulmonary circulation

   Systolic pulmonary arterial pressure is 25 mmHg, diastolic pulmonary arterial pressure 8 mmHg. The pulmonary vascular resistance is about 1/8 that of systemic circulation, and this why pulmonary arterial pressure is much lower than aortic pressure

Factors that help to avoid the development of pulmonary edema:

1-    The normal negativity of interstitial fluid of lung.

2-    The lymphatic pumping of fluid out the interstitial space.

3-    The increased osmosis of fluid into the pulmonary capillaries. 

Placental and fetal circulation

Placental circulation:

1-  The placenta is fetal lung where O2 and CO2 are exchanged, but the cellular covering the villi are thicker and less permeable than alveolar membrane in the lungs, exchange is much less efficient.