Physiology Flashcards
What is osmolarity?
Concentration of osmotically active particles in a solution
What are the units of osmolarity?
mosmol/L
How is osmolarity calculated?
Molar concentration of a solution (e.g. 150mM)
and
number of osmotically active particles present (Na+, Cl- = 2 particles)
150 *2= 300
What is the osmolarity of plasma?
300mosmol/L
What is tonicity?
The effect on the cell that a solution has on it
What is an isotonic solution?
Causes no change to cell volume. Water balance is the same
What is a hypotonic solution?
Water moves from solution to cell since less water in cell.
Causes lysis.
What is a hypertonic solution?
Water concentration is higher inside cell than outside so cell loses water.
What makes up Total Body Water?
Intracellular fluid (66%) Extracellular fluid
What separates ICF and ECF?
Plasma membrane
What is included in ECF?
Lymph
Plasma
Interstitial fluid (80%)
What tracer is used to measure total body water?
3H20
What tracer is used to measure ECF ?
Inulin
What tracer is used to measure plasma?
Labelled albumin
What is included in insensible losses?
Loss of water from skin by passive diffusion and lungs by expiration
In relation to Na+ and Cl- where is most of it found?
Extracellular fluid, bathing the cells
Where is most K+ found?
Intracellular fluid
What is fluid shift?
The movement of water between ICF and ECF in response to an osmotic gradient
What three factors affect fluid homeostasis?
Change in [water]
Change in [NaCl]
Gain or loss of isotonic fluid
Why can small K+ changes cause such big differences?
It is ICF so has bigger impact
What is glomerular filtration?
Bloods enters in afferent arteriole and is filtered through the glomerulus to the Bowman’s capsule
What is tubular reabsorption?
While filtrate is in proximal convoluted tubule it has the opportunity to be reabsorbed back into the efferent arteriole
What is tubular secretion?
While in the efferent arteriole, material may be transferred to PCT
What is rate of filtration?
Mass of a substance filtered into the Bowman’s capsule per unit time
What is the equation for rate of filtration?
Concentration of substance in plasma X GFR
What is GFR?
Glomerular filtration rate
What is the equation of rate of excretion?
Concentration of X in urine X urine flow rate
If the total amount filtered is significantly more than the amount excreted what has happened?
Significant reabsorption has occurred.
What are the 3 barriers to filtration in the glomerulus?
Glomerular capillary endothelium
Basement membrane
Podocytes
What is the basement membrane made of?
Glycoproteins and collagen
How does the basement membrane act as a barrier to filtration?
Has a net negative charge so repels negatively charged plasma proteins
What are the forces called that balance hydrostatic and oncotic pressures?
Starling Forces
In the kidney, what are the 4 forces which contribute to the net filtration pressure?
Glomerular BP
Bowman’s capsule hydrostatic pressure
Capillary oncotic pressure
Bowman’s capsule oncotic pressure
What is oncotic pressure?
Pressure exerted by plasma proteins
Which forces favour filtration?
Glomerular BP
Bowman’s capsule oncotic pressure
Which forces disfavour filtration?
Bowman’s capsule hydrostatic pressure
Capillary oncotic pressure
What is the equation for net filtration?
Forces favouring - forces against
Why is the glomerular capillary blood pressure constant throughout the glomerulus?
Efferent is smaller than afferent so creates pressure in itself
What is the normal GFR?
125
What is the major determinant for GFR?
Glomerular BP
How is GFR regulated? (2)
Extrinsically by sympathetic control by baroreceptors
Intrinsic autoregulation
What effect does vasoconstriction of the afferent arteriole have on the GFR?
Decreased GFR
What is the consequence of a decreased GFR?
Lower urine volume
What is the equation for MAP?
1/3 (systolic-diastolic) +diastolic
What are the two mechanisms of autoregulation of glomerular BP?
Myogenic
Tubuloglomerular feedback - more NaCl leads to constriction
Which cells in the juxtaglomerular apparatus sense NaCl?
Macula densa
What is plasma clearance?
Measures how effectively the kidneys can clean the blood of a particular substance per minute
What is the equation for plasma clearance?
Clearance = rate of excretion / plasma concentration
How does inulin excretion give an exact measurement of GFR?
It is freely filtered at the glomerulus and is neither reabsorbed nor secreted at PCT
Give an example of a substance which is completely reabsorbed
Glucose
Its clearance is 0.
When would clearance be 0?
If a substance is not filtered and not secreted e.g. RBC
Give an example of a substance which is partly reabsorbed and not secreted
Urea
Give an example of a substance which is filtered, secreted and not re-absorbed
H+ ions
What is renal plasma flow?
The amount of plasma that runs through the glomerulus in 1 minute
What is the value for RPF?
650
Which substance is used to measure RPF?
Para-amino hippuric acid (PAH)
Why is PAH used to measure RPF?
Freely filtered at the glomerulus, secreted into tubule and not reabsorbed. It is completely cleared.
Why is inulin not commonly used to measure GFR?
Exogenous so needs to be measured.
Creatinine is used instead
What is the filtration fraction?
Fraction of plasma in glomeruli which is filtered into the tubules
How is filtration fraction worked out?
GFR / renal plasma flow
What is glomerular filtrate?
Blood without RBC and plasma proteins
It is therefore iso-osmotic with plasma
What is reabsorbed in the PT to the capillaries?
Sugars
Amino acids
Phosphate
Lactate
What is secreted to the PT from the capillaries?
H+ PAH Uric acid Drugs Toxins
Name the 5 layers the filtrate must pass through from the PT to the capillaries
Apical membrane Cytoplasm Basolateral membrane Interstital fluid Endothelium of capillary
What two ways can the filtrate leave the PT?
Transcellularly
Paracellularly
Name 3 types of carrier-mediated membrane transport than occurs in the PT
Primary active transport
Secondary active transport
Facilitated diffusion
How does primary active transport work?
Energy required to move substrate against concentration gradient
How does secondary active transport work?
Carrier molecule is coupled with another ion movement
What is facilitated diffusion?
Passive carrier-mediated transport down the concentration gradient
Where is a secondary active transporter used at the PCT?
To transport Na out of the tubular cell and into the interstitium. Uses K. Uses ATP
At a Na/K pump, what is the ratio used?
3 Na in
2 K out
Describe Na reabsorption at the PCT
Uses Co-transporters
Uses anteporters
When Na leaves the tubular cells, what is the consequence?
Water follows into the interstitium
How much glucose is reabsorbed into the capillary?
Almost all
How is glucose reabsorption faciltated?
Co-transporters
Facilitated diffusion
At what point will glucose no longer be fully excreted?
10-12 mmol/L
What is glucose in urine called?
Glucosuria
What does the loop of Henle do?
Creates a solute concentration gradient and forms hypertonic urine
Countercurrent multiplier
What does the descending limb of the loop of Henle do?
Absorbs water but no effect on salts
What does the ascending limb do?
Reabsorption of Na and Cl
No water absorption
What is the difference in the thick upper part of the ascending limb and the thinner lower part?
Thick upper part reabsorbs by active transport
Thin lower part is passive reabsorption
Where is the loop of Henle?
In the medulla
Why is there no water absorption in the upper limb?
Tight junctions so no water can be reabsorbed
What transport system for sodium is facilitated in the upper part of the ascending limb?
Triple co-transporter
What happens to the K+ in the co-transporter?
K is recycled and put back to the ascending limb
How does the fluid leaving the ascending limb become concentrated?
Solute is removed from ascending limb and water cannot follow.
Water leaves in the descending limb though so the fluid is concentrated
What happens to the osmolarity as the fluid leaves the DL?
Osmolarity increases
Why does water move passively from the DL?
Because solute is pumped into the interstitial fluid, which increases the osmolality of the interstitial fluid
What is the purpose of countercurrent multiplication?
To concentrate the medullary interstitial fluid
Which hormone is responsible for producing urine of various concentrations and volumes?
Anti-diuretic hormone (ADH)
What role does the vasa recta play?
Equilibrates with the interstitial fluid alongside the loop of Henle
What are the vasa recta permeable to?
NaCl and water
On leaving the loop of Henle and entering the distal tubule, how is the fluid described?
Hypo-osmotic at 100mosmol/L
What does an increased amount of ADH cause?
Increased water reabsorption to blood
If there is more water reabsorption to the blood by ADH what is then urine like?
Small amounts of concentrated urine.
ANTI- diuretic.
What is the distal convoluted tubule divided into?
Early and late
What does the early part of the distal convoluted tubule do?
Reabsorbs NaCl
What does the late part of the distal convoluted tubule do?
Reabsorbs Ca++, Na+ and K+
How is the collecting duct divided?
Early
Late
What part of the collecting duct is particularly influenced by ADH?
Late
Where does ADH come from?
Neurohormone produced by supraoptic and paraventricular nuclei in hypothalamus
Where is ADH stored?
Posterior pituitary
What receptor on the basolateral membrane of the collecting tube is sensitive to ADH?
Type 2 vasopressin receptor
What kind of receptor is the type 2 vasopressin receptor?
G protein coupled
What does the activation of the type 2 vasopressin receptor do?
Causes ATP to release cAMP which causes increased expression of aquaporins at the apical membrane
What is the result of more aquaporins at the apical membrane?
Increased permeability for water to move from the collecting tube to the interstitial fluid and then to the capillary.
If ADH is low then what is the urine like?
High volume of dilute urine
True or False
More ADH causes more salt to be excreted
False.
Salt excretion is constant, regardless of ADH
What hormone causes arterial vasoconstriction?
ADH
If blood pressure is low, what happens to ADH secretion?
Increases to reduce water lost in urine to maintain blood volume
Which substance inhibits ADH release?
Alcohol.
Low ADH –> more urine
If ADH is low, how does this affect glomerular flow?
Increases flow to 20ml/min
If ADH secretion is maximal, how does this affect glomerular flow rate?
Reduces flow to 0.2ml/min
Why does osmolarity increase in the collecting duct if ADH secretion is high?
High ADH means less water in tubule so a higher proportion of salt, raising the osmolarity
Where is aldosterone secreted from?
Adrenal cortex
What does aldosterone do?
Activates RAAS to stimulate Na+ reabsorption and secrete K+
If there was no aldosterone what effect would this have on sodium?
Sodium would be excreted
What does an increase in plasma potassium cause?
Stimulation of adrenal cortex to make aldosterone
Which organs makes angiotensinogen?
Liver
Which organ makes renin?
Kidney
Which organ makes angiotensin converting enzyme?
Lungs
What does ACE do?
Converts angiotensin 1 to angiotensin 2
What does angiotensin 2 do?
Stimulates adrenal gland to make more aldosterone
Increases thirst
Increase ADH
Which cells in the juxtaglomerular apparatus sense a difference in Na?
Macula densa
What is the relationship between renin and Na?
More renin means more sodium will be reabsorbed
How does aldosterone cause Na reabsorption?
Acts on sodium channels to open them
What is ANP?
Atrial natiuretic peptide
What does ANP do?
Released from atrial muscle cells when they are stretched due to increased plasma volume.
Promotes Na excretion to reduce BP
What is the micturition reflex?
Involuntary emptying of the bladder by simultaneous bladder contraction and opening of the internal and external urethral sphincter
What is the equation for pH?
pH = log 1/ H+
What is the pH of arterial blood?
7.45
What is the pH of venous blood?
7.35
What effect does an increase in H+ ions have on pH?
Reduces pH
What effect does an acidic environment have on the body?
Denatures enzymes
Depresses CNS
Changes K+ levels
What is the dissociation constant?
K = [H+] [A-} / [HA]
What is the Henderson Hasselbach equation?
pH = pK + log [A-] / [HA]
What systems controls bicarbonate levels?
Kidneys
Lungs
What enzyme is needed to balance carbon dioxide and water?
Carbonic anhydrase
Where is HCO3- reabsorbed?
Proximal tubule
How does kidney make new HCO3-?
CO2 in interstitial fluid forms with water.
What substance produced in the liver forms ammonia?
Glutamine
What should HCO3 in plasma be?
25
What is respiratory acidosis?
Retention of CO2, reduces pH
How does kidney compensate for respiratory acidosis?
H+ secretion is stimulated, raising pH
What is respiratory alkalosis?
Removal of CO2 from body
High pH
What causes respiratory alkalosis?
Hyperventilation
Low pO2
How does kidney compensate for respiratory alkalosis?
No new HCO3 is produced.
Maintains/lowers pH
What is metabolic acidosis?
Excess H+ not from CO2
pH low
What causes metabolic acidosis?
Loss of base e.g. diarrhoea
Ingestion of acids
Lactic acid production
How is metabolic acidosis compensated?
Ventilation increased to blow off CO2 to reduce acid
Why does metabolic acidosis compensated by kidney?
Acid load is excreted.
Takes a long time
What is metabolic alkalosis?
Excessive loss of H+ from body
pH high
What causes metabolic alkalosis?
Loss of HCl from vomiting
Ingestion of alkali
Aldosterone hypersecretion
How does the respiratory system compensate for metabolic alkalosis?
CO2 retention
