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Blood-Brain barrier- Cerebrospinal fluid- circulation in birds

Blood-Brain Barrier (BBB)

Structure and Function

The Blood Brain Barrier refers to the mechanisms in place around the microvasculature of the brain to ensure optimal neural functioning. Endothelial cells are the structural basis of the blood brain barrier and are joined by tight cellular junctions formed by the transmembrane proteins the occludins and the claudins.

These junctions form a physical barrier are impermeable to proteins and also restrict the passage of non-lipid soluble molecules. Oxygen and carbon dioxide can diffuse across the endothelial cells. Within the BBB, most molecules use carrier systems to cross the endothelial cells as the tight junctions prevent molecules from crossing via the paracellular route (between cells).

Larger hydrophilic molecules can be transported across the endothelial cells from the lumen of the blood vessels into the interstitial fluid by a variety of ways such as the specific receptor mediated endocytosis or transcytosis or the less specific adsorptive mediated transcytosis. The endothelial cells possess transporters for glucose, amino acids, purine bases, nucleosides, choline and other substances.

The properties of the BBB are induced and probably maintained by molecules secreted by astrocytes. The astrocyte endfeet (perivascular endfeet) surround the endothelial cells and have K+ channels and aquaporins and so are likely to be involved in ion and water volume regulation within the neural environment. As well as the transporters mentioned, there are also efflux pumps present, such as p-glycoprotein, responsible for transporting potentially toxic molecules back into the lumen of the blood vessels.

Along with this ‘physical barrier’, a ‘metabolic barrier’ also exists which involves the presence of intracellular and extracellular enzymes. Peptidases and nucleotidases are capable of metabolizing peptides and ATP where as the intracellular enzymes such monoamine oxidase and cytochrome P450 can break down neuroactive and potentially toxic compounds.

The BBB prevents circulating antibodies reaching the central nervous system (CNS) and thus is a component of the immunological privilege manifest by the CNS. The BBB is not considered to affect the movement of inflammatory cells into the CNS; activated lymphocytes can enter the normal CNS.

Inflammatory responses in the CNS are different to elsewhere in the body. Injections of lipopolysaccaride, which would result in massive influx of granulocytes (polymorphonuclear leukocytes (PML)) in peripheral tissue result in no influx into the brain substance, however influx into CSF does occur.

The blood brain barrier is not well developed and the endothelium is ‘leaky’ in the regions of the circumventricular organs such as the pars nervosa of the pituitary gland (posterior pituitary) and the hypothalamus, as these regions may be involved in homeostatic regulation, which requires the detection of concentrations of certain blood constituents.

Defects of the Blood Brain Barrier

Vasogenic Oedema

This BBB defect causes the BBB to break down in many pathological situations, e.g around brain tumours and when there is inflammation in the CNS. When this happens protein rich fluid spreads through the extracellular space of the brain, and is termed vasogenic oedema and since there are no lymphatic channels in the CNS, the excess fluid flows down pressure gradients, making its way to white matter and eventually to the ventricles.

The presence of protein rich fluid causes astrocytes and microglial cells to react, causing astrocyte hypertrophy, swelling and upregulate many molecules. Microglia express MHC-2 molecules and take up plasma proteins. The presence of extra fluid in the extracellular space results in an increase in brain volume.

Cytotoxic Oedema

It is important to distinguish the brain swelling which results from the presence of plasma in the extracellular space from the brain swelling that occurs as a consequence of cell swelling. Accumulation of fluid within cells, cytotoxic oedema is a common manifestation of many neurotoxic situations. It usually affects specific cells, or even parts of cells with the location being a reflection of the metabolic process being blocked by the neurotoxin:

  • Hexochlorophene and tri-ethyl tin cause fluid to accumulate in myelin sheaths

  • Cyanide caused swelling of axons

  • Interfering with energy generation causes swelling of astrocytes.

The distinction between cytotoxic and vasogenic oedema is important clinically when one is trying to decrease brain volume using hyperosmolar agents since one can only decrease brain volume with these agents when the BBB is intact.

Cerebrospinal Fluid

  • It is formed by the choroid plexus of the lateral, third and fourth ventricles of the brain.

  • CSF is periodically absorbed by the arachnoid villi of the subarachnoid space. This structure is quite large in man and horse, but generally small or microscopic in other domestic animals.

  • The piamater covering the surface of the brain and spinal cord also contributes small quantities of CSF. This fluid circulates throughout the sub arachnoid space between the piamater and arachnoid membrane, ventricles of the brain and central canal of spinal cord.

  • CSF contains a very small quantity of protein. The concentration of sodium and chloride are higher and potassium, urea and glucose are lower in CSF than plasma. The pH is same as blood. It does not contain cellular elements except very few lymphocytes.

  • CSF secretion is an active process and is not affected by the blood pressure or CSF pressure. The choroid plexus provides some selectivity of permeability for certain substances of the blood and the CSF is not identical with the blood plasma.

    It contains lower concentration of proteins, K+ and glucose than plasma, but has higher concentration of Na+ ions. Thus, the choroid plexus functions as blood-cerebrospinal fluid barrier which protects the CNS from the influence of a variety of substances.

  • CSF is absorbed by the arachnoid villi of the arachnoid membrane, which project through the venous sinuses of the duramater.

  • The CSF serves partly as a nutritive medium for the brain and spinal cord as well as cushioning these structures against shock. It also aids in the transport of some peptide hormones and other substances of the brain into the circulation.

Avian Circulatory System

Birds have very efficient cardiovascular systems that permit them to meet the metabolic demands of flight (and running, swimming, or diving). The cardiovascular system not only delivers oxygen to body cells (and removes metabolic wastes) but also plays an important role in maintaining a bird’s body temperature.The avian circulatory system consists of a heart plus vessels that transport:

  • nutrients
  • oxygen and carbon dioxide
  • waste products
  • hormones
  • heat

Birds, like mammals, have a 4-chambered heart (2 atria & 2 ventricles), with complete separation of oxygenated and de-oxygenated blood. The right ventricle pumps blood to the lungs, while the left ventricle pumps blood to the rest of the body. Because the left ventricle must generate greater pressure to pump blood throughout the body (in contrast to the right ventricle that pumps blood to the lungs), the walls of the left ventricle are much thicker & more muscular.

Drawing of a bird's heart
Cross-section through the ventricles of a chicken heart

Birds tend to have larger hearts than mammals (relative to body size and mass). The relatively large hearts of birds may be necessary to meet the high metabolic demands of flight. Among birds, smaller birds have relatively larger hearts (again relative to body mass) than larger birds. Hummingbirds have the largest hearts (relative to body mass) of all birds, probably because hovering takes so much energy.

Avian hearts also tend to pump more blood per unit time than mammalian hearts. In other words, cardiac output (amount of blood pumped per minute) for birds is typically greater than that for mammals of the same body mass. Cardiac output is influenced by both heart rate (beats per minute) and stroke volume (blood pumped with each beat). ‘Active’ birds increase cardiac output primarily by increasing heart rate.

In general, bird hearts ‘beat’ at somewhat lower rates than mammals of the same size but pump more blood per ‘beat.’ Among birds, heart rate varies with size:

SpeciesResting heart rate‘Active’ heart rate
Herring Gull130625
American Robin570
Blue-throated Hummingbird1260

Blood pumped by the avian heart enters the blood vessels. The main types are:

  • arteries – carry blood away from the heart & toward the body cells
  • arterioles – ‘distribute’ blood (that is, direct blood where needed with more going to active tissues & organs & less to less active tissues & organs) by vasodilating & vasoconstricting
  • capillaries – exchange of nutrients, gases, & waste products between the blood & the body cells
  • venules (small veins) & veins– conduct blood back to the heart
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