Renal Review

Renal Anatomy

Two bean shaped organs, one on each side, posterior lateral aspect of abdomen below the diaphragm.  Because of the proximity to diaphragm, the possibility of a pneumothorax exists with renal surgery.

1. Microscopically, the kidney is divided into three parts, the cortex, the medulla, and the pelvis.

2. The microscopic functional unit of the kidney is the nephron.

3. The nephron consists of the glomerulus (capillaries supplied by the afferent arteriole and emptied by the efferent arterioles)

Bowman’s capsule, proximal convoluted tubules, Loop of Henle, and the distal convoluted tubules.

4. Nephrons empty into collecting ducts which drain urine into papilla, into calyxes, into the renal pelvis, into a ureter, into the bladder, into the urethra, and out into the external environment.

(Click above to View Enlarged Image)

Blood Supply

1. The kidneys receive approximately 21% or 1,050 cc of the cardiac output per minute.

2. The renal artery supplies the interlobar arteries which supply the arcuate arteries which supply the interlobular arteries which gives rise to the afferent arterioles.

3. The afferent arterioles supply the glomerular capillaries which drain into the efferent arterioles which supply the peritubular capillaries which then drain into the venous system.  One area of the peritubular capillaries around the Loop of Henle is referred to as the Vasa Recta.  Concentration of urine takes place here.  (countercurrent mechanism)

4. The glomerular capillaries are the only capillaries in the body with an arteriole on both ends.  The fact that the afferent arteriole has a larger diameter than the efferent arteriole leads to higher pressure in the glomerular capillaries (about 50 torr).

5. The glomerular capillaries are 50-1,000 times more permeable than peripheral capillaries.

6. These two factors allow for a glomerular filtrate into Bowman’s capsule of approximately 180 L/day.  The filtrate is normally protein-free as albumin (MW of 69,000), the smallest plasma protein is too large to be permeable.

7. The majority of the water is reabsorbed, and we produce about 1 cc/kg/hour under normal circumstances or approximately 1,600-1,700 cc per day in the 70 kg/man.

8. In the urine, water, nitrogenous waste products, ions such as K and H, water soluble drugs and metabolites, and other waste products are eliminated from the body.  Most Na and Cl is reabsorbed.

9. In the afferent arterioles near the junction with the glomerular capillaries is a group of cells known as the juxtaglomerular apparatus.  When blood flow is decreased, the JGA secretes renin which forms angiotensin I in the plasma which forms Angiotensin II in the pulmonary vasculature under the influence of the converting enzyme.(ACE) Angiotensin II is a potent vasopressor and will increase blood pressure.  Angiotensin converting enzyme inhibitors like Captopril or Vasotec will help decrease blood pressure due to renal disease.

10. There is good autoregulation in MAP’s between 80 and 180 in the normal patient.  Significant changes in RBF and GFR can still occur with BP’s between 80-180 because of ANS and hormonal influence.

11. Innervation by the sympathetic nervous system is prominent to the renal vasculature which is why many vasopressors, while increasing BP, actually decrease RBF.  Dopamine less than 5 mcg/kg and lower doses of Dobutamine increase RBF.

Anesthesia and Normal Renal Function

A. Anesthesia and surgery are associated with a decrease in urine output.  Instead of 1cc/kg/hr, .75cc/kg/hr may be normal.  Should not accept less than .5cc/kg/hr.

B. Anesthesia may cause reduction of RBF and GFR by depressing myocardium or decreasing systemic blood pressure.  Also, autoregulation of RBF is impaired during anesthesia.

C. Too “light” anesthesia will cause increased hormonal effects which decrease urine output.  will have increases in renin, epinephrine, norepinephrine, cortisol, aldosterone, and angiotensin II.

D. Release of ADH which works on the distal tubule and collecting duct increased Na and H20 reabsorption, is stimulated by:

1. Hypovolemia

2. Increase in serum osmolarity ( primarily Na)

3. IPPV(alters Left atrial pressures therefore activates barroreceptors)

4. Pain, trauma

5. Possibly some increase due to narcotics and anesthesia.

Functions of the Kidney

A. Erythropoietic effect-secrets erythropoietin which stimulates bone marrow to produce RBC’s.  In chronic renal failure, this mechanism is depressed and patients frequently develop an anemia.

**Causes right shift in Hgb diss. curve this causes a counter balancing effect.

B. Acid-Base Balance- the kidneys play a major role in acid-base balance of the body by regulating HC03 AND H+ ion concentration in the body fluids.  This is controlled by H+ and HC03 secretion in the urine.  Renal failure patient have a tendency to become acidotic and hyperkalemic.  Diamox, a carbonic anhydrase inhibitor, works primarily in the proximal tubules and promotes the excretion of HC03.  A COPD patient may be unable to adequately buffer CO2 and may become acidotic.  (INHIBITS Na+)

**Angiotensin promotes Aldosterone secretion.

C. Fluid and electrolyte balance-kidneys help maintain osmolarity of the body by Na and H20 excretion or retention.  The kidneys also help control proper levels of chloride, potassium, hydrogen, phosphate, magnesium and calcium ions among others.

1. Patients with chronic renal failure have the following disturbances in fluid and electrolytes.

A. Fluid retention (paradoxically patients may respond to anesthesia as if they are hypovolemic).  Patients may be hypertensive due to fluid overload.

b. Hyperkalemia, hypermagnesemia, hypocalcemia.( pH and K move in opposite directions)

c. Sodium levels are usually normal.

2. Remember, chronic renal disease patients are very prone to infection **Prone to sepsis, most common cause of death in CRF. p.322.

3. Hemodialysis helps correct fluid and electrolyte imbalance–may worsen anemia because of hemolysis.

D. Elimination of nitrogenous waste products such as urea–normal BUN 10-20 mg/dl, normal serum creatinine .5-1.4 mg/dl)  Increases in GFR by 100% will decrease serum creatinine by 50% and vice versa.  Increases in BUN and CREATININE are indications of decreased renal function normally.

1. Oliguric acute renal failure has a higher mortality rate than non-oliguric renal failure.

E. Drug elimination-also of primary concern to us are the effects and elimination of our drugs in patients with compromised renal function.

1. Narcotics–primary metabolism by liver.  Morphine 15% and Fentanyl 10% eliminated unchanged by kidney.  Reduce doses in renal failure patient.

2. Inhalation agents–primarily eliminated by the lungs.  N20, Forane, Ethrane, Halothane are all okay to use.  Penthrane’s use was contraindicated due to possible renal toxicity.

3. Barbiturates–ultra short-acting barbiturates like Pentothal are excreted mainly by the liver.  However, remember patients with renal disease may have low serum proteins; therefore, remember there will be less protein binding.  May not need less but should give Pentothal slowly.

4. Muscle relaxants are of major concern.  Many are excreted in great part unchanged by the kidney.

Acute Renal Failure

A. Three types of acute renal failure

1. Pre-Renal–due to underperfusion or ischemia kidney.  Most common cause and of most concern to us.

2. Post-Renal–due to obstruction of urinary outflow.  Treatment is surgical.

3. Acute Tubular Dysfunction–two types

a. Following ischemia or as a result of trauma or hypoxia.

b. Toxic –can  be caused by antibiotics, transfusion reactions, sepsis toxins, radiographic dyes, myoglobin.