Intro to Kidneys & Body Fluid Flashcards
How much of our body weight is made up of water?
Total body water: 60% body weight
How much of our body weight is composed of intracellular water?
Intracellular water: 40% body weight (ICF compartment)
How much of our body weight contributes to ECF?
Extracellular water: 20% body weight (ECF compartment)
How does water move between ICF and ECF?
Osmosis determines movement of water between ICF and ECF
Describe the water content of a 70kg average man
A 70kg man,
60% water by weight: total body water is 42 L
40% body weight will be intracellular (ICFV = 28L)
20% extracellular (ECFV = 14L)
What are the 2 components of ECFV?
The ECFV consists of two sub-compartments:
- Plasma (3L)
- Interstitial fluid (11L)
How do substances move between plasma and Interstitial fluid?
Starling forces from capillary filtration determine fluid and solute movement between plasma and interstitial fluid
What is osmolarity?
Total concentration of osmotically active solutes
How is movement between ICF and ECF regulated?
Solute composition of ECF and ICF differs, but osmolarity must be kept the same to avoid excessive shifts of water between ECF and ICF
What determines ECF osmolarity?
Sodium is the principal electrolyte of the ECF, therefore sodium (with associated anions) is the major determinant of ECF osmolarity
What is the significance of body fluids?
Cell structure + function
- Large shifts between ECFV / ICFV disrupts tissue
structure + function
Tissue perfusion
- Depends on balance between circulating volume
(plasma) + interstitial volume
What 2 key processes determine salt and water balance?
Osmoregulation and volume regulation
What is osmoregulation?
The maintainance of osmotic equilibrium between ICFV and ECFV
What is meant by volume regulation?
Maintaining adequate ECFV to support plasma volume
How is plasma osmolarity estimated?
2[Na] + 2[K] + [glucose] + [urea] (all in mmol L-1)
How is osmolarity of a solution changed?
- Add/remove solute
2. Add/remove water
How does the body achieve osmoregulation?
The body accomplishes osmoregulation by adding or removing water not sodium
Explain the effect of increasing plasma osmolarity
Plasma osmolarity rises – more water needed
Kidneys respond by producing small volume of concentrated urine (water retention)
What is the effect of decreasing plasma osmolarity?
Plasma osmolarity falls – too much water
Kidneys respond by producing large volume of dilute urine (water excretion)
What is volume regulation referring to?
Refers specifically to the control of the circulating (plasma) volume
How does the body achieve volume regulation?
Changes detected by stretch / pressure receptors in CVS
A fall in blood volume is opposed by Na retention; water follows osmotically, restoring volume
What is the main function of the kidneys?
Kidney function is homeostasis
Urine is a by-product of kidney function
What is the significance of the urinary tract?
The urinary tract is important for temporary storage and then to remove the urine from the body
List the functions of the kidneys
- Osmoregulation
- Volume regulation
- Acid-base balance
- Regulating electrolyte balance (eg k+, Ca2+, phosphate)
- Removal of metabolic waste products from blood
- Removal of foreign chemicals in the blood (e.g. drugs)
- Regulation of rbc production (erythropoietin)
What is a nephron?
Basic unit of structure in the kidney
Outline the structural features of nephrons
Consists of special blood vessels and elaborate tubules (tiny tubes)
Microscopic, structures
1.25 million per kidney
Where urine production begins
What major structures does each nephron consist of?
- Blood vessels
- The glomerulus
- Bowman capsule
- The renal tubule
Which major vessels supply the kidneys?
Common Iliac artery
Inferior vena cava
Outline the blood vessels allowing perfusion through kidneys
Renal. A ↓ Segmental. A ↓ Interlobular. A ↓ Arcuate . A ↓ Interlobular. A ↓ Afferent arterioles ↓ Nephrons ↓ Venules ↓ Interlobular. V ↓ Arcuate. V ↓ Interlobular. V ↓ Renal Vein
How is the kidney supplied with blood?
The whole kidney is supplied by the renal artery which subdivides many times into fine arterioles supplying each nephron, the afferent arteriole
How is renal blood drainage carried out?
Drained by the renal vein, which is supplied by venules coming away from each nephron, joining together into larger veins which merge into the renal vein
What is between the arterioles and venules?
Complex capillary networks
What is the process of urine formation
- Glomerular Filtration
- Tubular Reabsorption
- Tubular Secretion
- Excretion of water + solutes in urine
What is glomerular filtration?
(GFR) - first step in urine production
=> amount of filtrate kidneys produce each minute (125 ml/min)
What affect does renal failure have on GFR?
GFR is reduced in renal failure
Explain how glomerular filtration occurs
Hydrostatic pressure forces fluids + solutes through glomerular capillary membrane
Small molecules pass readily- large (proteins) and cells can’t pass
=> leads to plasma ultrafiltrate in Bowman’s capsule
What is reabsorption?
Many substances are filtered and then reabsorbed from the tubular lumen into the peritubular capillaries
What is the significance of Tubular secretion?
- Disposing substances not already in filtrate
- Eliminating undesirable substances e.g. urea + uric acid
- Ridding the body of excess K+ ions
- Controlling blood pH
How is excretion of fluid and solutes regulated?
[ ] of any solute present in urine is given by the amount filtered out of plasma - amount reabsorbed + amount secreted
How is water and sodium balance maintained?
- Changes in water intake/excretion
- Changes in body fluid osmolarity
- Sensors (e.g. sensory cells in hypothalamus) detect
new ECF osmolarity - Effectors (kidneys) adjust amount of water excreted /
retained - Change in renal water excretion
=> Intake = output
What is the effect of intake / output mismatch?
Mismatch of intake and output will cause a change in osmolarity
Shift of water between ICFV and ECFV
What can cause water restriction?
Loss of water (sweat, breathing). Thirsty, but no water is available to drink
What is the physiological response to water restriction?
Plasma osmolality rises
Response is increased secretion of hormone, ADH
What is the effect of ADH on water restriction?
Results in
- Decreased urine volume
- Increased urine osmolality
What is the effect of increased water intake?
Increase in water absorption through GIT
- Plasma osmolality falls
- Response is reduced secretion of ADH
What does reduced ADH cause?
Results in
- Urine volume increases
- Urine osmolality decreases
What is the normal plasma osmolality?
Plasma osmolality is maintained (~280 - 300 mOsmol/Kg) in face of changes in water intake by retaining or by excreting water
How is plasma osmolality maintained?
Done by ADH system, kidneys & behavioural thirst response
What determines ECF volume?
ECF volume is determined by the amount of sodium in this compartment
How is ECFV maintained?
Sodium intake and excretion must be balanced to maintain constant ECF volume
Main volume sensors are in the cardiovascular system
What is the physiological response to low blood volume?
A fall in blood volume is opposed by hormonal signals promoting sodium retention; water follows osmotically, restoring volume
How is sodium balance maintained?
The maintenance of sodium balance is based (mainly) on the control of the ECFV
Intake ~ output
What hormone systems contribute to Na balance?
Sodium retaining:
- RAAS
Sodium Eliminating:
- ANP
Explain how the hormone systems respond to a decreased ECFV
- Low ECFV / ↓Na excretion
- ↓ANP
- ↑RAAS
