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Structures of the Renal System
- The kidneys are paired structures lying bilaterally between the twelfth thoracic and third lumbar vertebrae.
- The kidney is composed of an outer cortex and an inner medulla.
- The calyces join to form the renal pelvis, which is continuous with the upper end of the ureter.
- The nephron is the urine-forming unit of the kidney and is composed of the glomerulus, proximal tubule, hairpin loops of Henle, distal tubule, and collecting duct.
- The glomerulus contains loops of capillaries. The capillary walls serve as a filtration membrane for the formation of the primary urine.
- The proximal tubule is lined with microvilli to increase surface area and enhance reabsorption.
- The hairpin loops of Henle transport solutes and water, contributing to the hypertonic state of the medulla.
- The distal tubule adjusts acid-base balance by excreting acid into the urine and forming new bicarbonate ions.
- The ureters extend from the renal pelvis to the posterior wall of the bladder. Urine flows through the ureters by means of peristaltic contraction of the ureteral muscles.
- The bladder is a bag composed of the detrusor and trigone muscles and innervated by parasympathetic fibers. When accumulation of urine reaches 250 to 400 ml, mechanoreceptors, which respond to stretching of tissue, stimulate the micturition reflex.
Renal Blood Flow
- Renal blood flows at about 1000 to 1200 ml/min, or 20% to 25% of the cardiac output.
- Blood flow through the glomerular capillaries is maintained at a constant rate in spite of a wide range of arterial pressures.
- The glomerular filtration rate (GFR) is the filtration of plasma per unit of time and is directly related to the perfusion pressure of renal blood flow.
- Autoregulation of renal blood flow and neural regulation of vasoconstriction maintain a constant GFR.
- Renin is an enzyme secreted from the juxtaglomerular apparatus and causes the generation of angiotensin, a potent vasoconstrictor. The renin-angiotensin system is thus a regulator of renal blood flow.
- The major function of the nephron is urine formation, which involves the processes of glomerular filtration, tubular reabsorption, and tubular secretion and excretion.
- Glomerular filtration is favored by capillary hydrostatic pressure and opposed by oncotic pressure in the capillary and hydrostatic pressure in Bowman capsule. The balance of favoring and opposing filtration forces is known as net filtration pressure (NFP).
- The GFR is approximately 120 ml/min, and 99% of the filtrate is reabsorbed.
- The proximal tubule reabsorbs about 60% to 70% of the filtered sodium and water and 90% of other electrolytes.
- Because most molecules are reabsorbed by active transport, the carrier mechanism can become saturated at a point known as the transport maximum (Tm). Molecules not reabsorbed are excreted with the urine.
- The distal tubules actively reabsorb sodium and secrete potassium and hydrogen for the regulation of electrolyte and acid-base balance.
- The concentration of the final urine is a function of the level of antidiuretic hormone (ADH) that simulated the distal tubules and collecting ducts to reabsorb water. The countercurrent exchange system of the long loops of Henle and their accompanying capillaries establishes a concentration gradient within the renal medulla to facilitate the reabsorption of water from the collecting duct.
- The distal nephron regulates acid-base balance by excreting hydrogen ions and forming new bicarbonate.
- The kidney secretes or activates a number of hormones that have systemic effects, including 1,25-dihydroxy-vitamin D3, erythropoietin, and natriuretic hormone.
- Creatinine, a substance produced by muscle, is measure in both plasma and urine to calculate a commonly used clinical measurement of GFR.
- Both the plasma creatinine concentration and the blood urea nitrogen (BUN) levels indicate glomerular function. Plasma creatine is measure to monitor progressive renal dysfunction; BUN is an indicator of hydration status.
Pediatrics & Renal Function
- Infants and children have more dilute urine than do adults because of higher blood flow and shorter loops of Henle.
- Children are more affected than adults by fluid imbalances resulting from diarrhea, infection, or improper feeding because of their limited ability to quickly regulate changes in pH or osmotic pressure.
Aging & Renal Function
- Older adults have a decreased ability to concentrate urine and are less able to tolerate dehydration or water loads because they have fewer nephrons.
- Response to acid-base changes and reabsorption of glucose are delayed in older adults.
- In older adults, drugs eliminated by the kidney can accumulate in the plasma, causing toxic reactions.