Physiology of the Urinary System

The Urinary system consists of the kidneys which form urine and the structures that transport the urine

The Kidneys have 5 main functions:

1. Maintain plasma volume (and therefore blood pressure (BP))
2. Regulate ion and water concentration
3. acid-base balance
4. eliminate wastes, drugs, and hormones
5. endocrine - renin (BP), erythropoietin (RBC production)

Most of these functions are homeostatic

The functional unit of the kidney is the Nephron. It consists of a renal corpuscle and renal tubule. Formation of urine occurs here and involves 3 processes:

1. Glomerular filtration
2. tubular reabsorption
3. tubular secretion

Glomerular filtration is the movement of protein free plasma from blood into Bowman's capsule by bulkflow (due to a pressure gradient) across a filtration membrane. 20% of plasma in the glomerulus is filtered.

The filtration membrane consists of 3 parts:
a) fenestrated endothelium of the glomerulus
b) basement membranes
c) filtration slits between podocyte "fingers" of Bowman's Capsule

The filtrate that is produced is identical to plasma but doesn't have any large proteins. It's pH is about 7.45 and it contains water, glucose, amino acids, vitamins, ions, urea, and some small proteins (in those who consume a large protein diet)

For filtration to occur the protein free plasma had to enter Bowman's capsule using a pressure gradient (bulk flow). This gradient is caused by 4 Glomerular filtration pressures.

1. Glomerular BP = 55mmHg --favours filtration
2. Blood osmotic pressure = 30mmHg -- opposes filtration
3. Capsular hydrostatic pressure = 15 mmHg -- opposes filtration
4. Capsular osmotic pressure = 0 mmHg -- favours filtration

So, the net filtration pressure would be: (55+0) - (30+15) = 10mmHg --> favours filtration. At this average net filtration pressure we have ~ 180 L /day of filtrate produced from both kidneys. We call this the Glomerular filtration rate or GFR. This is the same as saying that the kidneys filter 125mL of blood per minute, so our entire plasma volume is filtered about 65 times every day.

If glomerular blood pressure (BP) changes so does GFR, and this change is directly proportional. This means an increase in Glomerular BP will lead to an increase in GFR

Glomerular Filtration is regulated carefully by intrinsic and extrinsic mechanisms. Intrinsic regulation or autoregulation occurs mainly at the afferent arteriole (brings blood to glomerulus to be filtered). Autoregulation is very important in normal BP ranges (when resting, even moderate exercise).

The first intrinsic method of regulating BP is myogenic stretch of the arteriole. Smooth muscle surrounding arterioles has a tendancy to contract when stretched, preventing blood pressure from reaching very high levels. In the same way, a decrease in BP will lead to less stretch of smooth muscle and a vasodilation of the afferent arteriole thus increasing GFR to normal.

The second method is called Tubuloglomerular feedback and is directed by the macula densa cells of the juxtaglomerular apparatus. These cells respond to NaCl concentration which varies with filtrate flow rate. If GFR is high, then there isn't enough time for NaCl to be reabsorbed, and the macula densa cells release a factor (most likely ATP) that causes the afferent arteriole to constrict and the flow to decrease thus lowering GFR. On the other hand, if flow rate is too slow, reabsorption of NaCl may be too much leading to afferent arteriole vasodilation and an increase of GFR to resting.

Both methods will regulate GFR to prevent it from leaving normal (homeostatic) levels.

Extrinsic regulation of GFR is via the sympathetic nervous system (SNS) which leads to vasoconstriction of the afferent and efferent arterioles. The constriction of the afferent arteriole leads to a decrease in flow entering the glomerulus, and the constriction of the efferent arteriole leads to blood backing up in the glomerulus. So since opposite actions occur, a moderate activation of the SNS leads to no change in the GFR. However, extreme stress (e.g. heavy exercise or a hemmorage) can cause GFR to decrease because so little blood will be filtered that there will be little to no back up of blood in the glomerulus when the efferent arteriole constricts.

When regulation of GFR fails, and net filtration pressure changes --> disease.
For example, blood osmotic pressure (BOP) can increase due to an increase in plasma proteins/ dehydration. So if BOP increases, then net FP will decrease, lowering GFR. Or BOP can decrease due to burns or nephrotic syndrome (glomerulus leaky and proteins get filtered) and therefore net FP increases, increasing GFR.
Also, Capsular hydrostatic pressure (CHP) can increase leading to a decrease in GFR. This may be due to a urinary tract obstruction (urine backs up) which can be caused by kidney stones (typically in ureter), inflammation, and prostate enlargement.

Tubular Reabsorption is the passive and active reabsorption of 99% of the filtrate produced. We filter 180L/day but only produce 1-1.5L/day of urine. So 99% of filtrate is reabsorbed. Na+, ions, glucose, and amino acids are actively reabsorbed (requires energy) and Cl-, water (osmosis) and urea are passively reabsorbed (no energy required). Reabsorption takes place in the:

1. proximal convoluted tubule (PCT)
2. loop of henle (LOH)
3. distal convoluted tubule (DCT)
4. Late distal convoluted tubule & collecting duct (CD)

Here are a few good links:
Quizes-
http://www.execulink.com/~ekimmel/quiz11.htm
http://kidshealth.org/kid/htbw/_bfs_USquizsource.html
http://www.lrn.org/Content/Quizzes/Qurinary.html
http://highered.mcgraw-hill.com/sites/0072907932/student_view0/chapter_19/multiple_choice_quiz.html
This site mcgrawhill includes a very good animation and a quiz
so does this one

Anatomy/physiology sites
http://www.getbodysmart.com/ap/urinarysystem/menu/menu.html
http://www.mhhe.com/biosci/esp/2002_general/Esp/default.htm - works best with ie

animations
Counter current exchange mechanism in loop of henle - here and here