Download Anatomy And Physiology Of The kidney

Anatomy and physiology of the kidney
SISTER M. ARTHUR SCHRAMM, CRNA
Yankton, South Dakota
The blood supply
The author presents a thorough review
of the kidney from both an
anatomicaland physiological standpoint.
Her in-depth study encompasses the
kidney's reaction to various diuretic drugs.
The kidneys are two organs, each weighing
125-180 gms (4-6 oz), located retroperitoneally. One is located in each lumbar area,
the right slightly lower than the left, the
upper pole of the right kidney resting on
the 12th rib. The functional unit of the kidney is the nephron. There are approximately
1,200,000 in each kidney. The kidney has
an inner layer, the medulla, the substance
of which forms the 10-15 pyramids and the
outer layer, cortex, in which the base of the
pyramids is located.
The kidney receives its blood supply
from the abdominal aorta; therefore, the
blood flows at a higher pressure when it
reaches the capillary area than in other tissue. Blood flow enters through the renal
artery (Figure 1) at the kidney hilum; it
passes through the interlobar arteries of the
medulla to the junction of the medulla and
cortex where the flow enters the arciform
arteries; the blood then passes through the
interlobular arteries which tranverse the
cortex. Afferent arterioles lead from these
interlobular arteries to the nephrons. The
afferent arteriole sub-divides into capillaries
(the glomerulus), these capillaries converge
to form the efferent arteriole. The efferent
arteriole further sub-divides into the peritubular capillaries and the vasa recta, if the
nephron is a juxtamedullary nephron. The
venous blood follows a similar route of
return to the renal vein and the inferior
vena cava, that is: interlobular, arcuate,
interlobar, vein, and vena cava.
The nephron
Figure 1-GrossAnatomy of the Kidney.
February/1975
The nephron (Figure 2) consists of
three basic parts.
1. The malpighian corpuscle, which is
composed of the glomerulus surrounded by
a blind pouch, Bowman's capsule.
2. The tubule to which the capsule leads,
is composed of three segments: (a) the
proximal convoluted tubule, about 14 mm
in length, which begins in the cortex and
has an outstanding brush border on the
luminal side which increases surface area;
(b) the loop of Ilenle, which narrows as it
dips into the medulla forming descending
and ascending limbs which again widen as
the tubule passes through the outer medullary zone and back into the cortex (about
one out of every eight loops dips deeply
a number of important substances (Figure
3).
The kidney's importance is appreciated
when one compares cardiac output of 5.2
liters/min. for a resting 70 kg man to tissue
size, function, blood flows and oxygen uptake.
Figure 2-The Nephron.
into the medullary area along with its corresponding capillary, the vasa recta, and is
involved in the countercurrent mechanism);
and (c) the distal convoluted tubule, onethird the length of the proximal tubule,
which is located in the cortex. (Its initiation
is associated closely with packed nuclei,
called the macula densa; the brush border
of the tubule is nearly absent.)
3. The collecting duct, which receives
the filtrate from several distal tubules. These
ducts empty into short papillary ducts of
Bellini which empty into the renal calyces
and thus into the renal pelvis.
Figure 3-The magnitude of reabsorption by
the Kidney.
Processes or functions involved in
urine formation
Urine formation is actually a byproduct of the many important functions of
the kidney.
The first process is filtration, which
occurs over the 1.5 square meters of capillary surface in the glomeruli of both kidneys. In comparison, a 70 kg man has a
body surface of 1.73 m2 and the lung capillary surface is 70-90 m2. Yet, for the kidney
size and weight, this is a very large surface.
Approximately one-fourth of the cardiac output (21-25 per cent) perfuses the kidney
or about 1200 ml/min. Considering the
hematocrit and nourishment of the kidney,
the total plasma flow through the glomeruli
is about 650 ml/min.
Filtration is promoted by filtration
pressure differences and permeability, both
of which are increased in the glomerular
capillaries relative to that of skeletal muscle
capillaries, but are decreased in the peritubular capillaries. This filtration per 24
hours in the kidney amounts to 170 liters of
water or 1.5 liters/minute which contains
Figure 4-Diagramof regionalblood flows in basal
state (shaded areas)and at maximum vasodilation
(unshaded areas), scaled for normaladult human
subject, 70kg. Ordinate is blood flow per 100gm
tissue, showing relative vascular resistance of each
organ system. (From Mellander and Johansson)
Tissue size in weight shows that the
kidney is comparable in size to the heart,
less in size than the brain, and very much
smaller than the skeletal muscle or splanchnic areas. Thus, it is not for nutritive purposes that the kidney receives a large blood
supply, but for plasma clearance.
Blood flow in the kidney is near its
full capacity, 7-8 times greater than the flow
in the heart or the brain, much greater than
skeletal muscle or splanchnic flow. Relative
Journal of the American Association of Nurse Anesthetists
Figure 5-Estimateddistribution of cardiac output
and oxygen consumption in normal subjects
(70kg, surface area 1. 7m 2) at rest under usual
indoor conditions. (From Wade & Bishop)
Figure 6-Distributionof cardiac output in normal
human subjects at rest and during light, strenuous
and maximal exercise that has continued for 10 min.
(From Wade & Bishop)
flows to muscle and heart change during
exercise, and flow is not constant as it is
in the brain.
Oxygen uptake by heart, brain, splanchnic and skeletal muscle is in a greater
amount than the kidney, even at rest. To
clear the plasma does not require as high
oxygen levels as does the nutritive and energy needs of the other organs.
Approximately one-fourth of the cardiac output goes to the kidney, comparable
only to that of skeletal muscle or splanchnic
areas. This allows for adequate plasma clearance of 1200 ml per minute. Yet, under conditions of exercise, other body priorities
will cause sidetracking of this amount.
Glomerular filtration pressure equals
the hydrostatic pressure minus the sum of
colloid osmotic and capsular pressures.
Glomerular hydrostatic pressure is 70 mm
Hg, glomerular colloid osmotic pressure is
32 mm Hg (due to retention of protein),
and capsular tissue pressure is 14 mm Hg,
giving a net filtration pressure of 24 mm
Hg to favor the filtration of 125 ml/min.
The skeletal muscle, in comparison, has
a hydrostatic pressure of 25 mm Hg. The
colloid osmotic pressure is 28 mm Hg. Neg-
February/1975
ative interstitial fluid pressure and colloid
osmotic pressure together exert a pressure
of 11.5 mm Hg, which assist the hydrostatic
pressure to yield a net filtration of 8.5 mm
Hg into the interstitial space in favor of
cellular nutrition. On the venous side, because of a drop in capillary hydrostatic
pressure, there is a net flow back into the
capillary with removal of cellular wastes.
Figure 7-Comparisonof Hydrostatic Pressureof
Kidney to that of other tissues.
The second important process is reabsorption which occurs in the proximal and
distal tubules. In the peritubular capillaries,
the hydrostatic pressure (Figure 7) is less
than that in other tissues. This lowered pressure facilitates net filtration of substances
from the tubule lumen into the capillaries,
thus aiding in the reabsorption process. It
is primarily in the proximal tubule that the
active and passive reabsorption occurs.
Figure8-Processesof the Nephron.
Active reabsorption (Figure 8) which
requires energy and is against a concentration gradient, includes substances such as:
sodium
glucose
amino acids
creatine
phosphate
sulfate
uric acid
potassium
ascorbic acid
ketones as aceto-acetic acid
Beta-hydroxy-butyrate
Passive absorption includes
chloride
and other electrolytes that are filtered, as
well as the osmotically absorbed water and
urea. In the proximal tubule, some of the
substances such as glucose are absorbed
completely if levels are physiological; sodium and water are absorbed to 80 per cent
of the filtered amount, and other substances
to lesser amounts. Therefore, the osmolarity
of tubular fluid in the proximal tubule is
still 285 mOSM, isotonic with that of plasma.
The third process is secretion, (Figure
8) sometimes called tubular excretion. Secretion occurs primarily in the proximal tubule
and involves such compounds as:
PAH
penicillin
sulfonphthaleins
iodinated compounds such as Diodrast®,
conjugated foreign substances
In the distal tubule, sodium, under the
control of aldosterone, is exchanged for
potassium and hydrogen ions. This area is
involved in acid-base balance and electrolyte control as well as fluid balance control.
Regulation of kidney function
The three processes-filtration, reabsorption, and secretion-are regulated by
three mechanisms.
The first, autoregulation (Figure 9)
affects the glomerular filtration rate (GFR)
because of the control on the hydrostatic
pressure of the capillaries by the afferents
and the resistance of the arterioles due to
vasoconstriction or dilation. Sympathetic innervation to the afferent arterioles has been
demonstrated but there seems to exist an
intrinsic control by which the kidney itself
regulates the arteriole pressure so that the
GFR is kept constant.
The second, countercurrent exchange,
(Figure 9) is a mechanism whereby the
isotonic filtrate of the proximal tubule becomes hypertonic in the longer juxtamedullary loops as the filtrate flows down the
descending loop of Henle through an increasingly hypertonic interstitium. Osmo-
Figure 9-Mechanisms affecting Kidney functions.
larity of the filtrate, as sodium passively flows
inward and water passively flows outward
to the interstitium, becomes hypertonic as
the filtrate flows down descending loop. In
the ascending limb, sodium is actively
pumped from this ascending limb in increments back into the interstitium to replace
that lost to the descending loop, this ascending limb wall is impermeable to water. By
the time the filtrate returns to the cortical
area, another 5 per cent of the water has
been lost but relatively more sodium may
have been lost. Thus, the filtrate may be
diluted or reconstituted in osmolarity.
The third, hormonal mechanisms (Figure 9) control sodium and water reabsorption. In the proximal distal tubule, which
originates at the macula densa, the hypotonic filtrate again has the tendency to lose
water and gain sodium from the isotonic
cortical interstitium.
The macula densa are approximate to
another area of cells in the afferent arteriole;
just before their entry into the glomerulus,
these two structures form the juxtaglomerular apparatus. If there is a need for
sodium or the afferent arteriole pressure is
low, this cortical site produces renin which,
indirectly through the renin-angiotensin
mechanism, causes the adrenal cortex to
secrete more aldosterone. This, in turn, increases sodium reabsorption and, thus, water
reabsorption.
In the distal tubule and collecting duct,
another hormone-an antidiuretic hormone
-is liberated from the post pituitary (neurohypophyseal) gland which is innervated by
osmoreceptors in the hypothalamus. This hormone, also secreted by the hypothalamus,
affects the permeability of the tubules to
water as it again passes back through the
Journal of the American Association of Nurse Anesthetists
increasingly hypertonic interstitium to the
ducts of Bellini. Approximately 14 per cent
of the water in the filtrate is reabsorbed in
this area, and the final concentration of urine
occurs here. One per cent of the GFR or 1
ml/min of urine is formed.
Physiological regulation in the body
by the kidney
A review of the many filtrate statistics
and functions of the kidney reveals its great
importance in body homeostasis. Besides the
adjustment of electrolyte or ion concentrations, osmolarity, fluid volumes of extracellular fluid and blood, and removal of
waste products and toxic substances, acidbase balance is one of interest and intrigue.
The long term regulation of the pH to maintain a bicarbonate to carbonic acid ratio of
20:1 in the blood occurs primarily in the
distal tubule. Here is what happens as the
filtrate passes through the distal tubule.
urine and carbon dioxide which is reabsorbed into the blood stream and excreted
by the lungs.
Figure11.
In the event of acidosis, (Figure 12)
more sodium is needed for exchange with
hydrogen. The titratable acid Na 2HPO4 is
utilized. This acid is present in the filtrate
and dissociates into two sodiums and
HP04-. The cellular hydrogen exchanges
for one of the two sodiums. The sodium is
Figure 10.
The sodium of the filtrate (Figure 10)
is exchanged for the hydrogen ion generated by the tubular cell metabolism. In
this metabolism CO2 and H2O are produced;
these form carbonic acid facilitated by the
enzyme carbonic anhydrase. The carbonic
acid then splits into hydrogen and bicarbonate. The hydrogen exchanges with the
tubular sodium. The sodium is actively
pumped into the blood stream and the cellular bicarbonate is passively reabsorbed with
the sodium into the peritubular capillaries.
A common exchange of hydrogen is made
with the dissociated sodium from the filtrate's sodium bicarbonate.
In the tubule, (Figure 11) the exchanged hydrogen and filtrate bicarbonate
form carbonic acid which dissociates into
water. The water, in turn, is excreted as
February/1975
Figure 12.
reabsorbed with the cellular bicarbonate,
and the hydrogen forms NaH 2PO4 which is
excreted.
With increasing acidosis, (Figure 13)
ammonium is formed from glutamine. Sodium chloride, which is in the filtrate, dissociates into sodium and chloride. The sodium is exchanged for the cellular hydrogen
ion and is reabsorbed with the cellular bicarbonate. The hydrogen ion along with the
amine NHg forms the compound ammonium
(NH4 + ). Ammonium along with the chloride forms ammonium chloride which is
excreted in the urine.
In the state of alkalosis, the hydrogen
ion may be sparse. Cellular potassium may
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exchange with sodium instead of the cellular
hydrogen so as to conserve the hydrogen
and allow bicarbonate excretion.
Figure 13.
Alteration of functions
Pathology of the kidney could alter any
of these mechanisms, whether it be glomerular or tubular disease. Blood pressure extremes could affect the GFR by overriding
autoregulation.
Drugs affecting GFR, hormones, osmolarity or electrolytes also could alter the
response of these functions.
An example of this alteration is found
in diuretics. They can affect kidney function
in five ways (Figure 14) by:
1. Increasing the blood supply to the
kidney such that (a) aminophyllin causes
vasodilation, (b) digitalis increases cardiac
output, and (c) water intake increases plasma volume.
2. Preventing tubular reabsorption of
water by use of osmotic urea, glucose, sucrose, or mannitol.
3. Preventing tubular reabsorption of.
sodium or inhibiting carbonic anhydrase
which interfere with chloride, bicarbonate,
potassium, and water absorption and excretion.
4. Decreasing hormonal control, that
is, ADH and aldosterone.
5. Decreasing production of aldosterone
and cortisone by the adrenal cortex by use
of drugs as metyrapone.
Figure 14-Effect of Diuretics on Renal Function.
Figure 15.
Thus, various diuretics may have effects on fluid and electrolyte balance as well
as on acid base balance. (See Table I.)
Osmolarity will have some relationship
to specific gravity, but the two are not directly interchangeable. Specific gravity depends on concentration and the nature of
the solutes. (See Table II.)
REFERENCES
(1) Brobeck, J. R., ed., 1973. Best & Taylor's
Physiological Basis of Medical Practice. Baltimore,
Md.: Williams & Wilkins Co.
Table II
Correlation of specific gravity with mOSM of solute in urine
SG.
mOSM
1.005
1.010
1.015
1.020
1.025
1.030
1.035
1.040
200
400
600
800
1000
1200
1400
1600
Journal of the American Association of Nurse Anesthetists
(2) Goodman, L. S. and Gilman, A., 1970. The
Pharmacological Basis of Therapeutics. 4th ed.
New York, N. Y.: Macmillan Co.
(3) Goth, A., 1972. Medical Pharmacology. 6th ed.
St. Louis, Mo.: C. V. Mosby Co.
(4) Guyton, A. C., 1971. Textbook of Medical
Physiology. Philadelphia, Pa.: Saunders.
(5) Melmon, K. L. and Morrelli, H. F., 1972.
Clinical Pharmacology. New York, N. Y.: Macmillian Co.
(6) Mountcastle, V. B., ed., 1974. Medical Physiology. Vol. 2, 13th ed. St. Louis: C. V. Mosby Co.
AUTHOR. Sister M. Arthur Schramm, CRNA, is
a graduate of Sacred Heart Hospital School of
Nursing and School of Anesthesia. She holds a BA
degree from Mount Marty College, Yankton, South
Dakota and is studying for a PhD in physiology at
the University of South Dakota. She has served as
instructor and director of the Sacred Heart Hospital School of Nurse Anesthesia. She is presently
director of Mount Marty College School of Nurse
Anesthesia in Yankton and head of the Department
of Respiratory Sciences. This paper was presented
at the American Association of Nurse Anesthetists
41st Annual Meeting and Clinical Session held in
Chicago in August, 1974.
February/1975
(7) Netter, F. H., 1973. The CIBA Collection of
Medical Illustrations. CIBA Pharmaceutical Company, Division of CIBA-Geigy Corporation.
(8) Pitts, R. F., 1974. Physiology of the Kidney
and Bold Fluids. 3rd ed. Chicago, Ill.: Year Book
Publishers, Inc.
(9) Reidenberg, M., 1971. Renal Function and
Drug Action. Philadelphia, Pa.: Saunders.
(10) Wood-Smith, F. G., Stewart, H. C., and Vickers, M. D., 1968. Drugs in Anesthetic Practice.3rd.
ed. New York, N. Y.: Appleton-Century-Crafts.