intro to kidneys and body fluids Flashcards
describe body water and fluid compartments in humans
- Body composition is approximately 60% water in males, 50% in females
- 70kg male: 0.6 x 70 = 42 L body water
- 60 kg female 0.5 x 60 = 30 L body water
- This water is distributed in two main fluid compartments, ICF and ECF.
- Two thirds of the body water is intracellular, one third is extracellular
- 70kg male: 2/3 x 42 = 28 L ICF vol, 1/3 x 42 = 14 L ECF vol
- 60 kg female 2/3 x 30 = 20 L ICF vol, 1/3 x 30 = 10 L ECF vol
what is osmoregulation ?
process that maintains osmotic equilibrium between ICF and ECF
* The cell membrane is semipermeable
* Permeable to water (via water channels: aquaporins)
* Impermeable to most solutes
- Change in solute concentration in either ICF or ECF will generate osmotic gradient, resulting in shifts of water between compartments
what is volume regulation ?
regulation of ECF volume in order to ensure effective circulating volume
* ECF compartment is subdivided
* Interstitial (or extravascular) compartment (about 75% of ECF)
* Plasma (or vascular) compartment (about 25% of ECF)
what does the maintenance of salt and water volume require ?
- integration of osmoregulation and volume regulation
- Osmoregulation: control of salt concentration 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 is central to both processes
what is the central function of the kidney
- homeostasis
- The production of urine is a by-product of this central function
what are the specific renal functions ?
- Osmoregulation
- Volume regulation
- Acid-base balance
- Regulation of electrolyte balance (eg potassium, calcium, phosphate)
- 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 the kidney
- The nephron consists of special blood vessels and elaborate tubules
- Microscopic structures, 1.25 million per kidney
describe the structural organisation of the nephron
- Bowman’s capsule
- Proximal tubule (proximal convoluted tubule, PCT)
- Loop of Henle
- Distal tubule (DCT)
- Collecting duct (CD)
- CDs join and ultimately drain into ureter
what are the blood vessels of the nephron ?
- Afferent arteriole
- Glomerulus
- Efferent arteriole
- Peritubular 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 glomerular filtration
- Balance of Starling forces drive water and solute across the capillary membrane
- Small molecules pass readily – large ones (proteins) and cells cannot pass
- This leads to a plasma ultra filtrate in the Bowman’s capsule, the first step in the production of urine
Glomerular filtration rate (GFR):
* The amount of filtrate produced by the kidneys each minute
* Averages 125 ml/min (approx 20% of renal plasma flow)
* Very important clinical indicator (reduced in renal failure)
* Plasma creatinine can be used as an index of GFR
describe tubular reabsorption
- Many substances are filtered and then reabsorbed from the tubular lumen into the peritubular capillaries
- About 70% of filtered salt and water reabsorbed from proximal tubule
- 20-25% from loop of Henle
- Variable fraction of remaining 5-10% is reabsorbed from distal tubule and collecting duct
describe tubular secretion
- Tubular secretion is important for disposing of substances beyond their level in the filtrate
- Eliminating toxins and metabolic by-products
Important in
* Potassium balance: excess K+ secreted in DT and CD
* Acid-base balance relies on H+ secretion in DT and CD
describe the 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 (1) - amount reabsorbed (2) + amount secreted (3)
- If (2) and (3) are both zero, then for that substance
amount excreted = amount filtered. - Can be used to estimate GFR
- e.g., creatinine is filtered but not reabsorbed; can estimate GFR from plasma concentration
what happens when you add water to the ECF ?
- kidney increases renal water excretion after this process down below this will reduce ECF volume
what does osmoregulation depend on ?
- monitoring the osmolality of the ECF
how is total body water balance calculated ?
net balance should be 0
what are the physiological responses to water restriction ?
Loss of water (skin, lungs)
* Plasma osmolality rises
Response is
* Increased thirst
* Increased secretion of hormone, ADH (antidiuretic hormone, also known as vasopressin)
ADH increases renal water reabsorption
* Decreased urine volume
* Increased urine osmolality
what are the physiological responses to water intake ?
Increase in water absorption through GIT
* Plasma osmolality falls
Response is
* Decreased thirst
* Reduced secretion of ADH
Results in
* Urine volume increases
* Urine osmolality decreases
what is ECF volume determined by ?
- determined by the amount of sodium in this compartment
- Sodium intake and excretion must be balanced to maintain constant ECF volume
- A fall in blood volume is opposed by hormonal signals promoting sodium retention; water follows osmotically, restoring volume
- Main volume sensors are in the cardiovascular system
what happens when you add extra NaCl and water to ECF ?
- The Na ions cannot cross the cell membrane and would be retained in the ECF.
- However, the saline was isotonic, meaning an osmotically equivalent amount of water was added with the salt.
- So there’s no change in concentration, no change in osmolarity, so no osmotic gradient. Both the salt and the water are retained in the ECF, expanding its volume, but without changing its concentration.
- This is how renal Na retention works: because of the osmotic coupling between Na and water, increasing renal Na reabsorption is the equivalent of adding isotonic saline.
what is needed to maintain ECF volume ?
sodium balance
what are the hormonal systems(effectors) involved in sodium balance ?
Renin-angiotensin-aldosterone system (RAAS)
* Increases renal Na reabsorption
* Increases ECF volume
Cardiac natriuretic peptides (ANP)
* Decreases renal Na reabsorption
* Decreases ECF volume
