40. Cerebral circulation. The concept of “blood-brain barrier”

Cerebral circulation

The cerebral circulation supplies the brain with blood. It originates from four large arteries, two carotid and two vertebral arteries. These four arteries form a structure called the circle of Willis at the base of the brain. From the circle of Willis arteries which supply the brain originate.

The brain constitutes about 2% of the body weight, but it receives 15% of cardiac output in rest, around 700 mL/min.

Regulation of cerebral circulation:

Like the coronary circulation the cerebral circulation is autoregulated between MAP of 60 mmHg to 160 mmHg. This means that the cerebral blood flow remains constant as long as the MAP is between those values.

The Monro-Kellie principle states that the sum of the volume of the brain tissue, the volume of the blood in the brain and the volume of the cerebrospinal fluid is always constant. This is because they reside inside the skull, and the space inside the skull cannot be enlarged. So, if the amount of blood flow to the brain increases, the brain tissue and CSF will be compressed.

This principle explains the importance of autoregulation. The MAP increases many times during the day. If the cerebral blood flow would increase every time the MAP increased the brain would be compressed, which is not good. Autoregulation is also important to prevent hypoxic damage to the brain if the MAP were to drop suddenly.

Several molecules regulate the vasodilation and vasoconstriction of cerebral arteries:

• Vasodilators
  • Increased CO2 (hypercapnia)
  • H+
  • K+
  • Adenosine
  • Nitric oxide (NO)
• Vasoconstrictors
  • Decreased CO2 (hypocapnia)

The cerebral circulation also redirects blood flow to the more active parts of the brain. For example, when speaking the “speaking centre” of the brain will receive more blood flow, at the expense of other brain areas.

Blood brain barrie

The blood brain barrier is a barrier between the blood and the brain. Its function is to prevent toxic molecules in the blood from entering the brain. Almost all parts of the brain are protected by the blood brain barrier, except for a few parts. These uncovered parts are called the circumventricular organs and include the area postrema, the pineal gland and the neural lobe of the pituitary gland, and some others. The circumventricular organs need to receive unfiltered blood for various reasons.

Permeability:

• Permeable to
  • Water
  • CO2
  • O2
  • Lipid soluble molecules
    • Alcohol
  • Electrolytes
    • Na+
    • Cl–
    • K+
  • Non-lipid-soluble small molecules
    • Glucose
    • Ketone bodies
• Impermeable to
  • Proteins
  • Non-lipid-soluble large molecules

Structure:

Unlike normal capillaries the capillaries of the cerebral circulation have tightly connected epithelial cells. There are no pores between the epithelial cells which allow molecules to pass between the cells. The capillaries are also surrounded by so-called astrocytic feet, which are projections of a type of cell called the astrocyte.

Gases and lipid-soluble molecules can diffuse through the epithelial cells and into the brain tissue. Electrolytes and molecules like glucose cross the blood brain barrier with the help of transport proteins, proteins on the surface of the endothelial cells.