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Describe the physiology of CSF
CSF is the fluid that bathes the brain and spinal cord and is contained in the ventricles and subarachnoid space. The total volume of CSF is approximately 150mls but there is very regular turn over so the daily production is 500-600mls/.
Composition: (compared to plasma)
List the functions of CSF
Outline the physiological factors that influence cerebral blood flow
The normal brain is approximately 1.4kg and receives about 750ml/minute blood flow or 15% of total cardiac output despite comprising only 2% of body weight. This translates to about 50ml/100g/minute of blood flow. Cerebral blood flow is supplied by the carotid (about 70%) and the vertebral (remaining 30%) arteries.
The brain also has a high oxygen requirement, consuming 3 – 3.5ml of oxygen/100g/minute or about 50ml/minute which is about 20% of total basal oxygen consumption. It has an oxygen extraction ratio of 33%.
Cerebral blood flow is dependent on a number of factors which can be described by the Hagen Poiseuille law. This states that blood flow is proportional to the change in pressure (in this case the cerebral perfusion pressure) x pi x radius to the power of 4 divided by 8 x viscosity of the fluid x length of the vessel. Put another way this is the change in pressure divided by resistance.
Factors affecting blood flow can therefore be thought of in terms of factors affecting cerebral perfusion pressure and factors affecting cerebral vascular resistance.
Cerebral perfusion pressure: this is the difference between the mean arterial pressure and the intra-cranial pressure or CVP (whichever is higher). CPP is normally around 80mmHg. Factors affecting cerebral perfusion pressure:
Factors affecting cerebral vascular resistance include:
What are the factors that determine ICP?
Intracranial pressure is determined by the total volume of substance within the skull. This consists of 3 substances:
The normal ICP is 5 – 14mmHg and has rhythmic variations associated with respiration and blood pressure.
The Monroe Kellie Doctrine states that the skull is a rigid box and that the sum of volumes of the constituents within the skull, that is the brain, CSF and intracerebral blood, is constant. Consequently, for ICP to remain normal, an increase in the volume of any one of the constituents should result in a reciprocal decrease in either one or both of the remaining two constituents. Therefore, the volume of brain tissue, CSF and cerebral blood are the main determinants of ICP.
Increases in intracranial constituent volume can initially be compensated by:
After these compensatory mechanisms are exhausted, an increase in volume will cause an almost linear increase in ICP.
What are the consequences of increased ICP?
An increase in ICP can have the following effects:
Outline the structure and function of the blood-brain barrier
The blood-brain barrier is a selectively permeable membrane which regulates the passage of substances to and from the brain. The primary function of the blood-brain barrier is to maintain the homeostasis of the CNS environment. It regulates the uptake of electrolytes and nutrients into the brain, it buffers the brain from changes in blood composition, and it helps prevents toxins and pathogens from entering the brain. The blood-brain barrier consists of endothelial cells, pericytes, and astrocyte endfeet.
Certain areas of the brain are not protected by the blood-brain barrier including the posterior pituitary gland, pineal gland, the median eminence of the hypothalamus, and the area postrema.