Physiology of Renal System

Question 1

Function of renal system

Answer 1

1. Excretion
   Get rid of waste products - urine production occurs in the kidneys; metabolic wastes and toxic molecules are excreted in urine
2. Regulate blood volume and blood pressure - Control extracellular volume by producing large amount of dilute urine, small amounts of concentrated urine
3. Solute (Cl, Na, urea) concentration in the blood, extracellular pH, red blood cell synthesis, vitamin D synthesis

Question 2

Urine production

Answer 2

Kidneys - regulate body fluid composition, sort chemicals in the blood for removal/return into the blood
Nephrons - structural component that sorts blood
Urine production - 3 stages; filtration, tubular reabsorption, tubular secretion

Question 3

Filtration

Answer 3

Movement of fluid across filtration membrane; nonspecific process - filtered based on size and charge
Filtrate - water, small molecules, ions that can pass through membrane; no red blood cells, proteins, large molecules
Renal fraction - part of total cardiac output that passes through the kidneys; varies from 12-30% in a healthy resting adult
Glomerular filtration rate (GFR) - amount of filtrate produced each minute; 125 ml/minute, 180 L/day
Average urine output - 1-2L
Most filtrate (99%) must be reabsorbed
Removes toxins quickly from blood

Question 4

Juxtaglomerular apparatus

Answer 4

smooth muscle cells from the wall of afferent arteriole (juxtaglomerular cells) and specialised tubule cells from DCT (mucula densa)
Secrete renin (enzyme) - filtrate formation and blood pressure

Question 5

Tubular reabsorption

Answer 5

The return of water, small molecules and ions to blood
Occurs as filtrate flows through the lumens of renal tubules
Substances are reabsorbed into interstistial fluid and into circulation via peritubular capillaries
Amino acids, glucose, fructose, sodium, potassium, calcium, chlorine, hydrochloric acid

Question 6

Tubular reabsorption - proximal tubule

Answer 6

reduced filtrate down to ~35%; water (osmosis)
Active transport of Na+ across basal membrane from cytoplasm to interstitial fluid linked to reabsorption of most solutes
Concentration of Na+ is low inside cell - moves into nephron from filtrate through apical membrane
Other substances move by symport (glucose, amino acids, chlorine)
Enters cell with sodium; glucose builds up (high conc); crosses basal membrane via facilitated diffusion (high to low) into interstitial fluid, enters peritubular capillaries and reenters circulation

Question 7

Tubular reabsorption - Loop of Henle

Answer 7

further reabsoption of water and some solutes (osmosis- high concentration of solutes in medulla)
Reduced filtrate down to 20%
Ions move via diffusion, active transport & symport

Question 8

Tubular reabsorption - DCT, Collecting duct

Answer 8

under hormonal control (ADH)
Makes tubule more permeable to water - small concentrated urine
Some reabsorption of water and ions (sodium, chlorine, hydrogen) - active transport and symport

Question 9

Tubular secretion

Answer 9

The movement of non-filtered substances, toxic by-products of metabolism, drugs or molecules not normally produced by the body, into nephron for excrection
Occurs mainly in distal convoluted tubules
Can be active or passive
Ammonia - toxic byproduct of protein metabolism
H+, K+, penicillin - actively secreted into nephron

Question 10

Micturition reflex

Answer 10

Full bladder activates stretch receptors - sends message to CNS
Brain voluntarily controls external urinary sphincter
External sphincter relaxes - urination

Question 11

Micturition

Answer 11

elimination of urine froom bladder