Structure and Function of Kidneys Flashcards
Structure of a nephron and glomeruli
nephrons - the functional units of the kidney. each nephron has a glomerlus that filters the blood and a system of tubular structures.
the kidney has no ability to regenerate nephrons.
each nephron consists of capillary structures (glomerulus & peritubular capillaries), a proximal convoluted tubule, a loop of Henle, a distal convoluted tubule, and a collecting duct.
glomerulus consists of a compact tuft of capillaries encased in a thin, double-walled capsule called Bowman capsule. blood flows into the glomerular capillaries from the afferent arteriole and flows out of the glomerular capillaries into efferent arteriole; leads into peritubular capillaries.
fluid & solutes from blood filtered through capillary membrane into fluid-filled space in Bowman capsule; called Bowman space.
**the portion of blood that is filtered into the capsule space is called the filtrate
the glomerular capillary membrane is composed of 3 layers:
capillary endothelial layer
basement membrane
single-cells capsular epithelial layer
Tubular filtration and GFR
the urine filtrate flows through the tubular component of the nephron. as filtrate flows through the tubule, the concentration of water and electrolytes in the filtrate changes due to; reabsorption of water and solutes by tubular cells into the peritubular capillary blood and secretion from the blood into the tubular lumen.
the nephron tubule is divided into four segments: a highly coiled segment called the proximal convoluted tubule, which drains the Bowman capsule. A thin, looped structure called the loop of Henle. A distal coiled portion called the distal convolute tubule. A collecting tubule (or collecting duct), which joins w/ several tubules to collect the filtrate. The filtrate passes through each of these segments before reaching the pelvis of the kidney.
urine formation involves (a) the filtration of blood through the glomerulus to form an ultrafiltrate of urine and (b) the tubular reabsorption of electrolytes and nutrients needed to maintain the constancy of the internal environment while eliminating waste materials.
the glomerular filtrate has a chemical composition similar to plasma, but it contains almost no proteins bc large molecules do not readily cross the glomerular wall. approximately 125mL of filtrate is formed each minute; called the glomerular filtration rate (GFR).
Regulation of urine concentration (ADH, Aldosterone and sympathetic nervous system)
ADH assists in maintenance of the extracellular fluid volume by controlling the permeability of the medullary collecting tubules. osmoreceptors in the hypothalamus sense an increase in osmolality of extracellular fluids and stimulate the release of ADH from the posterior pituitary gland. in exerting its effect, ADH, also known as vesopressin, producing a marked increase in water permeability.
aldosterone exerts its action on Na+ reabsorption and K+ secretion & elimination.
the kidney is richly innervated by the sympathetic nervous system. increased sympathetic activity causes constriction of the afferent and efferent arterioles and thus a decrease in RBF.
The Juxtaglomerular complex and RAAS
juxtaglomerular complex is thought to represent a feedback control system that links changes in the GFR with renal blood flow. bc of its location between the afferent and efferent arterioles, the juxtaglomerular complex plays essential feedback role in linking the level of arterial blood pressure and renal blood flor to the GFR and the composition of the distal tubular fluid.
distal tubular site nearest to the glomerulus is characterized by densely nucleated cells called the macula densa. these cells contain granules of inactive renin, an enzyme that functions in the conversion of angiotensinogen to angiotensin.
renin functions by means of angiotensin II to produce vasoconstriction of the efferent arterioles to prevent lung decreases in GFR. angiotensin II also increases sodium reabsorption indirectly by stimulating aldosterone secretion from the adrenal gland and directly by increasing sodium reabsorption by proximal tubule cells.
Elimination functions of the kidney
the primary functions of the kidneys are elimination of water, waste products, excess electrolytes, and unwanted substances from the blood.
renal clearance is the volume of plasma that is completely cleared each minute of any substance that finds its way into the urine.
elimination of electrolytes, particularly Na+ and K+, is regulated by the GFR and by humoral agents that control their reabsorption. aldosterone functions in the regulation of Na+ and K+ elimination. in the presence of aldosterone, almost all the sodium is reabsorbed, and the urine essentially becomes sodium free. in the absence of aldosterone, no sodium is reabsorbed.
uric acid (an organic anion) is a product of purine metabolism. excessively high blood levels (i.e., hyperuricemia) can cause gout, and high levels of uric acid in the urine can cause kidney stones. uric acid is freely filtered in the glomerulus.
urea is formed in the liver as a by-product of protein metabolism and is eliminated entirely by the kidneys. during periods of dehydration, the blood volume and GFR drop, causing BUN levels to rise.
many drugs are eliminated in urine. only drugs that are not bound to plasma proteins are filtered in the glomerulus and therefore are able to be eliminated by the kidneys.
Tests of renal function (urinalysis, BUN, and creatinine)
urinalysis - urine dipstick. a random urine, clean catch (have them clean external urethral meatus; midstream), PCR testing
BUN - measures urea nitrogen. the BUN-to-creatinine ratio provides useful diagnostic info. ratio is normally approx 10:1. ratios >15:1 represent prerenal conditions (such as CHF and upper GI tract bleeding) that produce an increase in BUN, not in creatinine
creatinine is produced by muscles as a product of metabolism of a molecule called creatine. the formation and release of creatinine are relatively constant and proportional to the amount of muscle mass present. creatinine is freely filtered in the glomeruli, is not reabsorbed from the tubules into the blood, and is only minimally secreted into the tubules from the blood. therefore, its blood values depend closely on the GFR.
in addition to calculating GFR, the serum creatinine level is used in estimating the function capacity of kidneys. if value doubles, GFR (and renal function) probably has fallen to one half of its normal state. a rise in the serum creatinine level to 3x normal value suggests 75% loss of renal function.
