Week 3 Flashcards
What are the functions of the kidney?
Removing metabolic waste from extracellular fluid, controlling the volume of extracellular fluid and maintaining optimal concentrations of vital solutes in the extracellular fluid.
Where is extracellular fluid?
In intravascular spaces, or interstitial space
What is extraceullar fluid composition and volume dependent on?
Salt intake (hunger, food availability), water intake (thirst, water availability) and salt and water losses (sweat and gut)
What are the three basic processes of the nephron?
Glomerular filtration, tubular reabsorption and tubular secretion
What is glomerular filtration?
Filtering of blood into tubule forming the primitive urine (glomerular filtrate)
What is tubular reabsorption?
Selective absorption of substances from tubule to blood
What is tubular secretion?
Secretion of substances from blood to tubular fluid
How is selective reabsorption achieved?
Unique renal tubular cell membrane composition:
What percentage of cardiac output do kidneys receive?
Approximately 20%
How is selective reabsorption achieved?
Unique renal tubular cell membrane composition
What percentage of total body weight is extracellular water?
20%
What is the most common cation in the extracellular fluid?
Sodium
What is the most common cation in the intracellular fluid?
Potassium
What is the most common anion in the extracellular fluid?
Chloride
What is the most common anion in the intracellular fluid?
Protein
What happens in the fluid compartments of the body if one litre of water is added?
Number of particles in each compartment does not change, volume changes very slightly (water is distributed evenly in each compartment) and concentration in each compartment falls
What happens in the fluid compartments of the body if 300 meq of sodium is added?
Particles increase the IV and the IT compartments, but not in the IC compartment. Because of water redistribution, all compartments change in size, and concentration (increases)
What happens in the fluid compartments of the body if 300 meq of sodium, and 1 litre of water is added?
Isotonic addition of salt and water leads to expansion of the IV and IS compartments, but no change in the IC compartment
Describe some of the mechanisms of selective reabsorption:
The basolateral surface of the tubular cells maintains a negative charge inside the cells (by Na/K ATPase), which facilitates net movement of a sodium ion, along with an anion, from the lumen of the tubule and into the interstitial space
What features of the proximal tubule enable selective reabsorption?
Cellular projections into the lumen, many mitochondria at the basolateral surface and infoldings of the basolateral membrane for ATPase
How does movement of solutes and water across membranes occur?
Diffusion, or by specific membrane channels (energy dependent, down a concentration gradient, down an electrical gradient, down an osmotic gradient)
How is movement of solutes and water across membranes controlled?
Presence and activity of membrane channels controlled by systemic and local (paracrine) mediators e.g. angiotensin II, ADH, aldosterone, PTH
Where is the majority of the sodium chloride reabsorbed?
70% is reabsorbed in the proximal tubule, as well as nearly all amino acids and glucose
How does the loop of henle facilitate countercurrent exchange?
Thick ascending limb is impermeable to water, but actively transports sodium, potassium and chloride.
Thick ascending limb provided the concentration gradient to promote water reabsorption from the thin DLH
Thin descending limb is freely permeable to salt and water
Vasa recta does not wash away the gradient by using the countercurrent exchange
What does the juxta-glomerular apparatus do?
Maintains GFR in face of increases or decreases in blood flow to the kidney. The macula densa senses tubular flow
What happens when there is increased tubular flow to the kidney?
Sensed by the macula densa, macula densa produces adenosine, which causes afferent arteriolar constriction
What happens when there is reduced tubular flow to the kidney?
Sensed by macula densa, granular cells produce renin, causing secretion of angiotensin II which increases reabsorption of sodium chloride and water
What is the kidney response to reduced sensed volume?
Efferent glomerular arteriolar constriction preserves waste excretion
Avid tubular sodium and water reabsorption preserves extraceullar fluid volume
