Physiology I Flashcards
What is cerebrospinal fluid (CSF)?
Clear and colourless liquid composed mostly of water = produced by secretory epithelium of the choroid plexus
How quickly is CSF produced?
At a rate of 0.4ml/min/gtissue = 500-600ml produced per day
How much CSF is in the central nervous system?
150ml = replaced 3-4 times a day
Where is CSF produced?
Formed in the ventricles of the brain = circulates in subarachnoid space before being absorbed into venous circulation
What does CSF supply tissues with?
Supplies water, amino acids and ions
Removes metabolites
What are the functions of CSF?
Mechanical protection, homeostatic function and circulation
How does CSF protect the brain?
Acts as shock absorbing medium
What does the pH of the CSF have an effect on?
Pulmonary ventilation and cerebral blood flow = due to transport of hormones
Where does the CSF allow exchange of materials between?
Blood and brain tissue
How are samples of CSF for clinical analysis obtained?
Via lumbar puncture = aids diagnosis of brain, meninges and spinal cord
What are the features of normal CSF?
Clear and colourless, contains little protein (15-45mg/dl), has little immunoglobulins (1-5 cells/ml)
What does the developing nervous system consist of at 3 weeks gestation?
Consists of tube = neural canal
What does the cavity of the neural tube give rise to?
The adult brain’s ventricles and the central canal of the spinal cord
Where does the choroid plexus develop from?
Cells in the walls of the ventricles
What is the specialisation of choroid cells?
Secrete cells that produce CSF
What forms the choroid fissure?
Developing arteries invaginate the roof of the ventricle
What forms the choroid plexus?
The involuted ependymal cells along with the vessels enlarge into villi
Where is the choroid plexus located?
In the third, fourth and lateral ventricles of the adult brain
What does CSF secretion involve?
Transport of ions (Na, Cl, HCO3) across the epithelium from blood to CSF
What allows CSF secretion to occur?
The polarised distribution of specific ion transporters in the apical or basolateral membrane in the epithelial cells
What does the secretion of fluid by the choroid plexus depend on?
Active transport of Na across cells into the CSF = electrical gradient pulls Cl and both ions drag water by osmosis
How does CSF compare to blood?
CSF has higher concentrations of Na and Cl, and lower concentrations of K, glucose and protein than blood
Does CSF production depend on arterial blood pressure?
No = is independent process
What connects the lateral and third ventricles?
The interventricular foramina
Where is the foramen of Magendie found?
In the medial aperture = connects fourth ventricle to subarachnoid space through single opening
Where is the foramen of Luschka found?
In the lateral aperture = connects fourth ventricle to subarachnoid space through paired opening
What connects the third and fourth ventricles?
The cerebral aqueduct of Sylvius
Where does the CSF flow in the brain?
Flows into third ventricle through interventricular foramina, flows through aqueduct of Sylvius into fourth ventricle
What happens to the CSF when it is in the third ventricle?
More CSF is added to it by the choroid plexus in the roof of the ventricle
What happens to the CSF when it enters the fourth ventricle?
Another choroid plexus adds more CSF to it
Where does CSF go once it has left the fourth ventricle?
Enters subarachnoid space and circulates in the central canal of the spinal cord
What is the main producer of CSF?
The choroid plexus
Where does the CSF flow through in the subarachnoid space?
Between the pia and the dura mater
How does the CSF return to venous blood?
Through arachnoid granulations into the superior sagittal sinus
What restricts the blood supply to the brain?
The blood brain barrier
What makes up the final portion of the CSF?
Brain interstitial fluid = drains to CSF through the perivascular spaces
What forms the blood brain barrier?
Endothelial cells in brain capillaries = consists of capillary epithelium, its basement membrane and perivascular astrocytes
What prevents paracellular movement of molecules into the brain?
Tight junctions
What parts of the brain have no blood brain barrier?
Circumventricular organs and pineal gland
What is the function of the blood brain barrier?
Protects brain from many bacterial infections and toxins, is determining factor for clinical CSF analysis
What are some common tumours related to CSF?
Colloid cysts = often at interventricular foramen
Ependymomas = arise from ependymal cells
Choroid plexus tumours
What is a ventricular haemorrhage?
An accumulation of blood in the ventricles
What are some common ventricular haemorrhages?
Epidural haematoma = arterial bleed between skull and dura
Subdural haematoma = venous bleed between dura and arachnoid
Subarachnoid haemorrhage
What is hydrocephalus?
Accumulation of CSF in ventricular system or around the brain
What does hydrocephalus cause?
Subsequent enlargement of one or more ventricles and increase in CSF pressure
What is idiopathic intracranial hypertension?
Enigmatic condition = no imaging features of hydrocephalus despite increased CSF pressure
What are the symptoms of idiopathic intracranial hypertension?
Headache and visual disturbances = due to papilloedema
What is papilloedema?
Optic disc swelling due to increased intracranial pressure transmitted to the subarachnoid space surrounding the optic nerve
What are the visual symptoms of papilloedema?
Enlarged blind spot, blurred vision, loss of vision, visual obscurations
What is the aqueous humor?
Specialised fluid that bathes the structures within the eye = provides oxygen and metabolites, contains HCO3
What is the function of the HCO3 in the aqueous humor?
Buffers H produced in the cornea and lens by anaerobic glycolysis
How is the aqueous humor produced?
Via energy dependent process in epithelial layer of ciliary body into the posterior chamber of the eye
Where does the aqueous humor flow to once it has left the posterior chamber of the eye?
Flows into anterior chamber and drains to scleral venous sinus through a trabecular meshwork and the canal of Schlemm
Where is the canal of Schlemm located?
In the angle between the iris and the cornea (iridocorneal angle)
Where does a small amount of the aqueous humour diffuse through?
The vitreous = is then absorbed across the retinal pigment epithelium
What covers the ciliary body and the posterior surface of the iris?
Two juxtaposed layers of epithelial cells = forward continuation of pigment epithelium of retina overlain by an inner non-pigmented epithelial layer
How are HCO3 and H transported into the aqueous humor?
Transported mostly across the basolateral membrane of pigment epithelium into interstitial fluid in exchange for Cl and Na respectively
What produces the HCO3 and H found in the aqueous humor?
Formed in epithelial cells from hydration of CO2 catalysed by carbonic anhydrase
How does Na and Cl enter the cells of the eye?
Diffuse through gap junctions between the pigment epithelium and the non-pigmented epithelium
What transports Na and Cl out of the non-pigmented epithelium into the aqueous humour?
The Na/K/2Cl co-transporter = K leaving the cell is recycled by Na/K pump and some Cl leaves the cell through channels
What accompanies the net movement of Cl and Na from the interstitial fluid to the aqueous humour?
Water = moves through ciliary epithelial cell water channels, aquaporins and through the paracellular pathway down the osmotic gradient created by solute movement
How much aqueous humour is secreted?
1-3 ml/min = flows into the anterior chamber
What is the volume of the anterior and posterior chambers of the eye?
Anterior = 250ml Posterior = 60ml
How long does it take to completely replace the aqueous humour of the posterior and anterior chambers?
Posterior = 30 mins Anterior = 120 mins
What maintains the intra-ocular pressure at 17mmHg?
Balance between secretion and drainage of fluid
What causes raised intra-ocular pressure in glaucoma?
Imbalance between rates of secretion and removal of aqueous humour
What drugs are used to treat glaucoma?
Carbonic anhydrase inhibitors = reduce aqueous humor production
What are some examples of carbonic anhydrase inhibitors?
Dorzolamide = given as eye drops, avoids systemic side effects Acetazolomide = taken orally, targets kidney, causes acidosis
