Critical Care Nursing

Renal Outline: Physiology

Critical Care Nursing, The University of Iowa
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III. Physiology

A. Excretory functions

1. Filtration - the process of removing fluids and small particles from the blood.

-the glomerulus, which lies between two arterioles, allows for high-pressure filtration system.

Capillary filtration pressure in the glomerulus is 2-3 times as high as that of other capillary beds in the body. The filtration pressure and the glomerular filtration rate (GFR) are regulated by the constriction or relaxation of the afferent and efferent arterioles. During strong sympathetic stimulation, which causes marked constriction of the afferent arteriole, the filtration pressure is reduced to the point where GFR drops to almost 0.

Capillary membrane of the glomerulus is composed of 3 layers:

1. endothelial layer of the capillary
2. basement membrane
3. a layer of epithelial cells that line Bowman's capsule

Glomerular capillary permeability is 100-1000 times as great as capillaries elsewhere in the body. All 3 layers allow water and dissolved particles, such as electrolytes, to leave the blood and pass rapidly into Bowman's capsule. Blood cells and plasma proteins are too large to pass through the glomerular membrane of a healthy kidney.

Glomerular filtration rate (GFR) is normally about 125 ml per minute. GFR can provide a measure to assess renal function, and can be measured clinically by collecting timed samples of blood and urine.

Creatinine - product of creatine metabolism by the muscle. Is filtered by the kidney, but not absorbed in the renal tubule.

Formula for creatinine clearance: C=UV P

C = clearance rate
U = urine concentration
V = urine volume
P = plasma concentration

Normal creatinine clearance is 115-125 ml/min (corrected for body surface area) Usually 24 hour collection with blood drawn when urine collection is completed.

2. Tubular reabsorption and secretion

The filtrate from the glomerulus passes through:

1. proximal tubule
2. loop of Henle
3. distal tubule
4. collecting duct

Then it reaches the pelvis and kidney

Reabsorption: water, sodium, and other substances leave the lumen of the tubule and enter the blood.

Secretion: substances from the blood enter lumen of the tubule.

Glucose and amino acids - completely reabsorbed

Filtered water - 99% reabsorbed
Urea - about 50% reabsorbed
Creatinine - none
Electrolytes - determined by need

3. Urine concentrating ability of the kidney: 2 mechanisms (SLIDE)

1. increased solute concentration in the medullary area surrounding the collecting tubules. Loop on Henle and peritubular capillary (vasa recta) descending into the renal medulla. Here a countercurrent mechanism controls water ans solute flow. As a result, water is kept out of the peritubular area surrounding the tubules, and sodium and urea are retained.
2. selective permeability of collecting tubules (controlled by ADH) During dehydration, the kidney plays a major role in maintaining water balance. Osmoreceptors in the hypothalamus sense the increase in extracellular osmolality and stimulate release of ADH from posterior pituitary. Collecting tubules (under influence of ADH) become permeable to water. In the absence of ADH, the renal tubules remain impermeable to water and a dilute urine is formed. Specific gravity (osmolality) of urine varies with its concentration of solutes. Specific gravity provides index of hydration status and functional ability of the kidneys. Concentrated urine: 1.030 - 1.040 (SLIDE) Marked hyration or dilute: 1.000

4. Sodium and potassium regulation: (SLIDE) glomerular filtrate reabsorbed in proximal tubule. Na and KC1 pumped (requires energy) into intercellular spaces, and absorbed into peritubular capillaries. Water movement accompanies the movement of these particles. Na reabsorption in distal tubule is variable and dependent on aldosterone. In the presence of aldosterone, almost all of sodium is reabsorbed and urine becomes almost sodium free.

Potassium regulation - aldosterone mediated secretion of K into tubular fluid. (Can be reabsorbed in distal and collecting tubules, but since dietary intake far exceeds need, secretion usually exceeds reabsorption.)

B. Endocrine fuctions (SLIDE)

1. Renin - released by special cells located near the glomerulus (juxtaglomerular cells) in response to:
1. reduction in GFR
2. sympathetic stimulation
Combines with angiontensinogen, a plasma protein that circulates in the blood to form angiotensin I, then converted to angiotensin II (potent vasoconstrictor and stimulator of aldosterone release).

2. Erythropoietin - released in response to hypoxia. Acts on bone marrow to stimulate production and release of RBCs. Persons with chronic hypoxia often have increased RBCs (polycythemia) due to increased erythropoietin levels. Examples: congestive heart failure, chronic lung disease, living at high altitude.

3. Vitamin D - activated and converted in kidney. Affects calcium metabolism.

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Last Modified: January 22, 1997