General Physiology Flashcards
Structure of cell membrane
- Phospholipid bilayer
- Hydrophobic lipid tails on inside
- Hydrophilic phosphate groups on outside
Contents of cytoplasm
- Water (70-85%)
- Electrolytes - potassium, magnesium, sulphate and bicarbonate
- Proteins
- Lipids
- Carbohydrates
What supports the structure of the cytoplasm
- Actin filaments
- Cytoskeleton of tubulin microtubules
What surrounds the cell nucleus
Double phospholipid membrane which is penetrated by nuclear pores
What protein is DNA wrapped around within the nucleus
Histone
How is mRNA formed
- DNA unwinds from histone when gene is activated
- Two strands separate
- Transcription factor enzyme binds to the promoter region
- Allows RNA polymerase to produce complimentary copies of the gene
How is energy derived from glucose
- Enters cell via facilitated diffusion under control of insulin
- Inside the cell glucose is phosphorylated to glucose-6-phosphate
- Either stored as a polymer (glycogen) or immediately for energy via glycolysis
What maintains the resting potential of a cell
Na/K ATPase
What is the average cell resting potential
-70mV
What causes an action potential
When a stimulus alters the resting potential of the cell by a significant enough amount to cause depolarisation
Outline the physiology of the action potential
- Stimulus alters resting potential of the cell membrane
- Alters the permeability to sodium ions (via voltage-gated sodium channels)
- Sodium influx into the cell
- Membrane potential continues to increase
- Peaks at +50mV
How is the cell repolarised
Depolarisation causes voltage-gated potassium channels which causes potassium to move out of the cell to compensate for the Na influx
What is the refractory period
Time taken for resting potential of the cell to be re-established
What causes the action potential plateau in cardiac and smooth muscle cells
Slow release of calcium ions causes delay to recovery of the resting potential and allows for prolonged contraction
What are the nodes of Ranvier
Bare area that transmits action potentials in myelinated neurons
What is Saltatory conduction
Conduction of action potentials in myelinated nerves
Outline the function of a synapse
- Action potential arrives at synapse
- Stimulates opening of calcium ion channels
- Influx of calcium draws secretory vesicles to the presynaptic membrane
- Vesicles exocytose their contnets and they travel across the synaptic cleft
- Stimulates post-synaptic receptors and alters the post-synaptic membrane to sodium 6. This change in resting potential stimulates post-synaptic action potential
Outline the structure of the neuromuscular junction
- Action potential reaches terminal of the nerve and causes calcium influx
- Triggers the release of secretory vesicles of Acetycholine into the synaptic trough
- Muscle membrane of the trough has multiple Ach receptors which act as gated ion channels
- On binding Ach via nicotinic Ach receptors these channels allow sodium to flood into the cell
- This depolarises the membrane, generating an action potential
How is the action potential transmitted through the muscle fibre
Via T-tubules
Outline the mechanism of skeletal muscle contraction (excitation-contraction coupling)
- Action potential transmitted through muscle via T-tubules
- Depolarisation of T-tubule membrane causes release of calcium from SR in the muscle fibre
- Calcium causes actin and myosin molecule to slide over one another
- Causes muscle contraction
Describe slow-twitch muscle fibres
- Type 1
- Smaller with extensive blood supply
- Contain myoglobin to act as oxygen store
- Contain mitochondira for oxidative phosphorylation
Describe fast-twitch muscle fibres
- Type 2
- Larger
- Extensive sarcoplasmic reticulum for rapid release of calcium ions
- Minimal blood supply
- No myoglobin and so appear white
Difference between actin and myosin contraction in smooth muscle compared to skeletal muscle
Smooth muscle contains calmodulin in the place of troponin
Methods of smooth muscle activation/relaxation
- Nervous impulse (as for skeletal muscle)
- Local tissue factors e.g. hypoxia
- Hormones
Method of cardiac muscle contraction
Striated - contains actin and myosin filaments which contract the same as skeletal muscle
What makes up an actin fibre
- Actin
- Tropomyosin
- Troponin
How do actin and myosin filaments slide over each other on contraction
- Tropomyosin and troponin form a complex that covers/inhibits the active site
- When this complex binds with 4 calcium ions a change occurs which uncovers the active site
- Myosin molecule cross-bridges contain ATPase heads which bind to the actin active site
- ATP is used to walk the myosin along the actin
- Calcium ions are then pumped back into the sarcoplasmic reticulum
List the 3 main types of blood cell
- Erythrocyte (RBC)
- Leucocyte (WBC)
- Thrombocytes (platelets)
What is the haematocrit
Percentage of blood volume made up of erythrocytes
Normal haematocrit
45%
List the 5 types of leucocyte
- Neutrophil
- Eosinophil
- Basophil
- Lymphocyte
- Monocyte
Lifespan of neutrophils
Spend 14 days in the bone marrow but have a half-life of 7 hours in the blood
Role of neutrophils
- Major cell in acute inflammation
- Role against bacteria
List the 3 types of lymphocyte
- B cells
- T cells
- Natural killer cells
Fate and role of B cells
Once activated convert into plasma cells and produce antibodies (the humoral response)
Role of T-cells
Produce the cell-mediated immune response
Role of T-helper cells
Activate macrophages and B cells
Role of Basophils
Initiate immediate hypersensitivity via histamine release
How are thrombocytes (platelets) produced
Produced from megakaryocytes via cytoplasmic fragmentation
Causes of primary polycythaemia
Polycythaemia rubra vera:
- Excess erythrocyte production despite low EPO
- Due to proliferation of pluripotent stem cells
- Unknown cause
Diagnostic Hb in polycythaemia rubra vera (male and females)
- Male Hb >180
- Female Hb >160
Secondary polycythaemia - causes with appropriate rise in EPO in response to hypoxia
- Altitude
- Cardiac disease
- Pulmonary disease
- Smoking
- Haemoglobinopathy
Risks of polycythaemia
- Increase in blood viscosity
- Increase risk of clots
List the causes of lymphocytosis
- Viral infection
- Chronic infection e.g. TB, toxoplasmosis
- CLL, lymphoma
- Acute transient response to stress (24 hours only)
Causes of platelet production failure
- Aplastic anaemia
- Cytotoxic drugs
- Alcohol
- EBV, CMV
- Leukaemia, myelofibrosis, myeloma
Hereditary thrombocytopenia
Causes of reduced platelet survival
- Idiopathic thrombocytopenic purpura
- Heparin, penicillamine, gold
- Meningococci
- Subacute bacterial endocarditis
- Thrombotic thrombocytopenic purpura
- DIC
- Dilutional from blood transfusion
- HUS
- Extracorporeal bypass
What occurs in DIC
Simultaneous activation of coagulation and fibrinolytic systems:
- Widespread microvascular thrombosis
- Fibrin deposition
- Bleeding due to consumption of clotting factors
- Fibrinolysis
How long should aspirin and clopidogrel ideally be stopped prior to operating
7 days
When and why does sickle cell disease manifest itself
6 months of age - when fetal Hb levels fall to be replaced by adult Hb
Diagnostic investigations of sickle cell disease
- FBC
- Blood film
- Sickle solubility test
- Confirmed with Hb electrophoresis
Sickle cell inheritance pattern
Autosomal recessive
Clinical features of sickle cell
- Haemolytic anaemia
- Pigment gallstone formation
- Vaso-occlusive crisis
- Ischaemic pain in fingers, chest, kidney, liver and penis
Sites of haemopoeisis in adults
Red marrow remains only in the axial skeleton, ribs, skull, proximal ends of femur and humerus
Sites of haemopoeisis in the fetus
- Bone marrow
- Spleen
- Liver
How are RBCs removed from the circulation
- Removed by macrophages in the spleen
- Broken down into haem and globin
- Haem releases iron that attaches to transferrin
- Remaining haem is converted to bilirubin
When does the proportion of reticulocytes in the blood stream increase
When bone marrow production of erythrocytes increases e.g. after haemorrhage
Laboratory evidence of haemolysis
- Increased unconjugated bilirubin
- Reduced serum haptoglobin
- Morphological evidence of damage e.g. spherocytes
Cause of osteomyelitis in sickle cell anaemia
Salmonella
Treatment of hereditary spherocytosis
Splenectomy delayed until aged 10
Pathophysiology of hereditary spherocytosis
Defect in red cell membrane
Platelet survival time
8-10 days
List the four components of haemostasis
(1*Vascular injury with exposure of subendothelial tissue factor and collagen)
- Vasoconstriction
- Platelet activation
- Coagulation mechanism
- Fibrinolytic system
What mediates vasoconstriction
- Local reflexes
- Thromboxane A2 released from activated platelets
- Serotonin released from activated platelets
Describe the process of platelet adherence
- Vessel injury causes loss of endothelium and exposure of collagen
- Platelets adhere to damaged area and there is activation of the intrinsic pathway via thromboplastim
- Damaged endothelial cells release VWf which is necessary for platelet adhesion
- Platelet granules release ADP which is needed for platelet aggregation
Describe the process of platelet aggregation
- Platelet phospholipids release arachidonic acid
- Thromboxane A2 is produced by the arachidonic acid
- Thromboxane A2 induces further platelet aggregation
What forms the platelet plug
- Platelets
- Thrombin
- Fibrin
What clotting factor is activated as the result of the enzyme reactions in the intrinsic and extrinsic pathway
Factor X
What is required for the initiation of the extrinsic pathway
Tissue thromboplastin and factor 7
Outline the intrinsic coag pathway
- Subendothelial damage
- Formation of primary complex on collagen by kininogen, prekallikrein, factor 12
- Prekallikrein is converted to kallikrein and factor 12 is activated
- Factor 12 activates factor 11
- Factor 11 activates factor 9
- Activated factor 9 forms tenase complex with factor 8a
- Activates factor 10
Outline the extrinsic coag pathway
- Tissue damage
- Factor 7 binds to tissue factor
- Activates factor 9
- Activated factor 9 and factor 7 activated factor 10
Outline the common coag pathway
- Activated factor 10 causes conversion of prothrombin to thrombin
- Thrombin hydrolyses fibrinogen bonds to form fibrin
- Thrombin also activates factor 13 to form cross-links between fibrin molecules
How is fibrin removed
Via the fibrinolytic system during the repair process in blood vessels and healing wounds
Outline the fibrinolytic system
- Tissue plasminogen activator is released by endothelial cells
(regulated by plasminogen-activator inhibitor 1 from endothelial cells) - Permits conversion of plasminogen to plasmin
- Plasmin breaks down fibrin into fibrin degradation products