cocentrating abilities of the kidneys is complex through countercurrent mechanism operting at the...

Physiology 441

The Renal System, Chp. 14Text: Human Physiology (Sherwood), 6th Ed.

Julie Balch Samora, MPA, MPHjbsamora@hsc.wvu.edu293-3412, Room 3145

Medullary vertical osmotic gradient

• The medullary vertical osmotic gradient and ADH allow the kidney to excrete urine of varying concentrations

• Without this osmotic gradient, we could not produce a concentrated urine, and would be unable to conserve water

• We would likewise be unable to rid the body of excess water w/o this system

Proximaltubule

Distaltubule

Distaltubule Glomerulus

Bowman’scapsule

Proximaltubule

Loop of Henle

Other nephrons emptying intothe same collecting duct

Collectingduct

Ascendinglimb ofloop ofHenle

To renalpelvis

Vasa recta

Descendinglimb ofloop ofHenle

Medulla

Cortex

Fig. 14-5, p. 505

Countercurrent Multiplication

• The loops of Henle of the juxtamedullary nephrons are responsible for establishing the concentration gradient in the renal medulla

• Descending limb is freely permeable to H2O, but NOT NaCl – so H2O goes into IF

• Ascending Limb IMPERMEABLE to water• The ascending limb actively transports out

NaCl

Countercurrent

Renal Disease

• Acute – sudden shutdown of kidneys; may be reversible

• Chronic – progressive, slow, insidious, irreversible loss of kidney fxn. Up to 75% of kidney tissue can be destroyed before loss of kidney fxn is even noticeable

• End stage renal failure – when 90% of kidney fxn has been lost

Consequences of Renal Failure

• Uremic toxicity – retention of waste products – body overwhelmed by toxins

• Metabolic acidosis – results from the inability of the kidneys to adequately secrete H+ - leads to CNS depression

• Potassium retention – from inadequate tubular K+ secretion – altered cardiac excitability

Consequences of Renal Failure

• Sodium imbalances – from inability of kidneys to vary Na+ reabsorption

• Phosphate and calcium imbalances – from impaired reabsorption – leads to skeletal disturbances

• Loss of plasma proteins – from leakiness of glomerular membrane – leads to edema

• Inability to vary urine concentrations – from impairment of countercurrent system

Consequences of Renal Failure

• Hypertension – from salt and fluid retention and excess angiotensin II

• Anemia – deficiency of erythropoietin• Depression of immune system – toxic

effect on immune system – ↑ infxns

Micturition

• Transmission of urine to the bladder• Urine forced along ureters by peristalsis• Ureters penetrate bladder obliquely,

preventing reflux• As bladder fills, the ureteral openings are

compressed• Even though urine can still enter bladder

through compressed openings, urine cannot escape back up ureters

Micturition

• Storage of urine• The bladder wall, made of smooth muscle,

can stretch without a concomitant build-up of intrabladder pressure

• Once bladder fills beyond 200-400 ml, the pressure rises, stretching receptors

• Internal urethral sphincter – at end of bladder, closes off

• Bladder supplied by PNS

Micturition

• Role of the external urethral sphincter• Farther down the urethra is a circular layer

of skeletal muscle• This sphincter is supplied by a motor

neuron• Unless the motor neuron is inhibited, it

fires continuously, keeping the external sphincter contracted, thereby preventing urine from leaving bladder

Micturition Reflex

• When stretch receptors within the bladder wall are stimulated, the micturition reflex is initiated

• Afferent fibers from the stretch receptors stimulate the PNS to inhibit the motor neuron supplying the external urethral sphincter

• In response to PNS stimulation of the bladder, it contracts.

Micturition Reflex

• As the bladder contracts, the internal urethral sphincter is pulled open mechanically

• Inhibition of the motor neuron supplying the external sphincter allows for relaxation

• Both sphincters are relaxed, bladder is contracting, and urine flows freely from the bladder through the urethra

• Entirely a spinal reflex!

Voluntary Control of Micturition

• Must be learned• Can override the micturition reflex• One can delay micturition despite synaptic

input from bladder stretch receptors• Although internal sphincter is open, can

keep the external sphincter contracted so no urine is expelled

Review Questions

• What gives rise to the unique appearance of the kidney?

• What is the average GFR?

Review Questions

• What is the role of the vasa recta?

• What is the importance of the vertical osmotic gradient?

Review Questions

• What is the major force favoring glomerular filtration?

• A decrease in afferent arteriolar resistance will do what to the GFR?

Review Questions (T/F)

• The kidney eliminates toxins such as urea, uric acid, glucose and creatinine

• The kidney plays a role in the body’s vitamin D concentration

• The kidney produces renin

Review Questions

• What are the types of autoregulation in the kidney?

• Where is sodium reabsorbed?

Review Questions (T/F)

• Most of the filtered plasma is excreted

• Glomerular filtration is a highly discriminating process

• The higher the BUN, the better function the kidneys have

Review Questions

• What is an example of a substance that is both actively secreted and actively reabsorbed?

• What is the formula for FF?

Review Questions• If a gentleman’s avg. GFR is 125 ml/min, a

substance that is being reabsorbed would have a clearance rate that is ↑, ↓, or ↔ compared with his avg. GFR.

• What establishes the medullary vertical osmotic gradient via the countercurrent multiplication?

Review Questions (T/F)

• The ascending limb of the loop of Henle of a juxtamedullary nephron is permeable to water.

• The glomerular capillary pressure is lower than capillary pressure elsewhere in the body

Review Questions (T/F)

• Water moves by osmosis from an area of lower osmolarity to an area of higher osmolarity

• ADH is secreted during water excess

Review Questions (T/F)

• The amount of glucose filtered is directly proportional to the plasma glucose concentration

• Upon entering the distal tubule, the tubular fluid is hypertonic

Review Questions (T/F)

• Potassium is filtered and both actively reabsorbed and actively secreted

• Urea is filtered and actively reabsorbed

• What substance is filtered, and completely reabsorbed (under nrml conditions)?