Introduction to kidneys and body fluids Flashcards
What are the 2 major fluid compartments?
Intracellular fluìd - 2/3 body water
Extracellular fluid - 1/3 body
What does ECF consist of?
- Interstitial (extravascular) compartment, approx 75% of ECF
- Plasma (vascular) compartment, approx 25% of ECF
What is the main electrolyte in intracellular fluid?
K +
What is the main electrolyte in extracellular fluid?
Na +
Why is the cell membrane semipermeable?
- It’s permeable to water (via water channels: aquaporins)
- Impermeable to most solutes
What does a change in solute concentration in either the ICF or ECF generate?
- Osmotic gradient, resulting in shifts of water between compartment- This gradient is what allows the movement of fluid between the 2 compartments
- This is because the ICF and ECF must be in osmotic equilibrium
Why is the osmolarity of the ECF regulated?
What is the normal range?
- To avoid osmotic shifts of water between ICF and ECF volumes
- Normal range 280-300 mOsm/L
- Large shifts must be avoided to prevent changes in cell volume
- Most serious complications are neurological
Osmoregulation = maintains constant ECF osmolarity
Why is regulation of ECF volume important?
- ECF compartment is subdivided
- Interstitial (extravascular) compartment, approx 75% of ECF
- Plasma (vascular) compartment, approx 25% of ECF
Volume regulation is control of the ECF volume to ensure appropriate plasma volume and blood pressure
How are salt and water balance maintained?
Osmoregulation - control of salt concentration in the ECF by adjusting the amount of pure water in the body
Volume regulation - control of the amount of salt and water in the ECF and hence, ECF volume
Many organs and systems involved, but kidney central to both processes
How is ECF volume determined?
- By the amount of sodium in the compartment
- Main volume sensors are in the CVS
- A fall in blood volume is opposed by hormonal signals promoting sodium retention; water follows osmotically, restoring volume
- Sodium intake and excretion must be balanced to maintain constant ECF volume
What are the major functions of the kidneys?
- Homeostasis - central function of kidney
- Osmoregulation
- Acid-base balance
- Regulation of electrolyte balance (eg potassium, calcium, phosphate)
- Production of urine - by-product of homeostasis
- Removal of metabolic waste products from blood
- Removal of foreign chemicals in the blood (e.g. drugs)
- Regulation of red blood cell production (erythropoietin)
What is the nephron?
- Functional unit of kidney
- Nephron consists of special blood vessels and elaborate tubules
Describe the structural organisation of the nephron
- Bowman’s capsule
- Proximal convoluted tubule (PCT)
- Loop of Henle
- Distal convoluted tubule (DCT)
- Collecting duct (CD)
- Collecting ducts join and ultimately drain into ureter
What are the blood vessels that are connected to the nephron?
- Afferent arteriole
- Glomerulus
- Efferent arteriole
- Pertibular capillaries
- Vasa recta
What are the 4 basic processes of renal function?
- Glomerular filtration
- Tubular reabsorption
- Tubular secretion
- Excretion of water and solutes in the urine
Describe the process of glomerular filtration
- Balance of Starling forces drive water and solute across capillary membrane
- Small molecules pass readily - large ones such as proteins and cells cannot pass
- Leads to a plasma ultra filtrate in the Bowman’s capsule, first step in urine production
What is the glomerular filtration rate?
- Amount of filtrate produced by kidneys each minute
- Average 125ml/min (approx 205 renal plasma flow)
- Reduced in renal failure
- Plasma creatine can be used as index of GFR
Describe the process of tubular reabsorption
- Many substances filtered then reabsorbed form tubular lumen into peritubular capillaries
- About 70% filtered salt and water reabsorbed from proximal tubule
- 20-25% from loop of Henle
Outline tubular secretion
- Important for disposing of substances beyond their level in the filtrate
- Eliminates toxins and metabolic by-products
Important in: - Potassium balance : excess K+ secreted in Distal convoluted tubule and collecting duct
- Acid-base balance relies on H+ secretion in distal convoluted tubule and collecting duct
Outline excretion of water and solutes in the urine
- The tubular fluid remaining after filtration, reabsorption and secretion is excreted as urine
- Amount excreted = (amount filtered - amount reabsorbed) + amount secreted
- If the amount reabsorbed ad amount secreted are both 0, then for that substance amount excreted = amount filtered
- This can be used to estimate GFR, e.g. creatine is filtered but not reabsorbed; can estimate GFR from plasma concentration
What does osmoregulation depend on monitoring?
- Osmolality of ECF
Sensor: Plasma osmolality monitored by osmoreceptors in hypothalamus
Effector: ADH acts on collecting duct to decrease (High ADH) or increase (Low ADH) water excretion. It does this by binding to receptors on the collecting duct, and creating water channels (aquaporins) so if water levels are low, more water can be reabsorbed
What does ADH do to regulate water balance?
If H2O levels low, osmoreceptors in hypothalamus detect this, increased ADH secreted, binds to collecting duct receptors, creates aquaporins, increases water reabsorption
Where is ADH secreted from?
Posterior pituitary gland
What are the physiological responses to water restriction?
- Plasma osmolality rises
- Increased thirst
- Increases secretion of hormone, vasopressin (ADH)
- ADH increases renal water reabsorption- decreases urine volume and increases urine osmolality
What are the physiological responses to an increase in water intake?
- Increase in water absorption through Gastrointestinal tract
- Plasma osmolality falls
- Response is
- Decreased thirst
- Reduced secretion of ADH
- Results in
- Increased urine volume
- Decreased urine osmolality
What are the hormonal systems in sodium balance?
- Renin-angiotensin-aldosterone system (RAAS) - this doesn’t detect Na volume, but infers it from the ECF volume, which is sensed via the cardiovascular system:
- Increase renal Na reabsorption
- Increases ECF volume
- Cardiac natriuretic peptides: (CNP)
- Decreases renal Na reabsorption
- Decreases ECF volume
What does a high plasma osmolarity mean?
- High solute:water ratio
What does a low plasma osmolarity mean?
- Low solute:water ratio
