The skin, lungs and intestines have an excretory function; only the kidneys fine-tune the excretion of H2O and electrolytes to maintain normal volume and composition of body fluids. Before the use of dialysis, a person in kidney failure excreted urine through the skin, forming malodorous urate crystals. Sweat glands secrete small amounts of nitrogenous compounds, water, and electrolytes. The lungs eliminate CO2 and H2O. Impaired lung function causes the retention of CO2 and acidosis. The intestines excrete digestive wastes (feces).
The kidneys are protected by the lower rib cage. Only the kidneys make urine. The ureters carry the urine to the urinary bladder which temporarily stores it. When the bladder contracts, the urethra carries the urine outside the body.
The renal columns and renal pyramids are concerned with urine formation. The urine drains into the calyces, the renal pelvis, and the ureters. Ask students to identify each of these structures on the cross-sectional illustration of the kidney.
The most important of the kidney’s many tasks are regulating fluid-electrolyte and acid-base balance. Why does a person in kidney failure develop uremia? Uremia refers to the presence in the blood of substance that should be filtered into the urine. When the kidneys fail, these substances stay in the blood. Some of these substances are nitrogenous wastes (azotemia), including elevated levels of creatinine and urea. Measuring levels of creatinine and urea is a diagnostic test for kidney failure. The kidneys also secrete or respond to hormones that regulate blood pressure and RBC production.
The nephron unit makes the urine. Nephrology is the study of the kidney; the term derives from the nephron unit. The nephron unit must be understood in terms of its two partsthe renal tubules and the renal vasculature.
Have students follow the tubular components from Bowman’s capsule to the calyx. The tubular component contains urine. The composition of the urine is adjusted all along the tubular components, including in the collecting duct. Every urinary structure beyond the collecting duct is “plumbing,” meaning that the structures transport urine but make no further adjustment in its composition.
Have students trace the path of the bloodrenal artery, afferent arteriole, glomerulus, efferent arteriole, peritubular capillaries (red to blue), renal vein, inferior vena cava (not shown). The tubular and vascular structures interact at the glomerulus within Bowman’s capsule and within the membranes of the peritubular capillaries that surround the tubular network. Water and dissolved solute exchange between the tubules and the vasculature.
On the diagram, the gray arrows indicate points of exchange between the tubular structures and vasculature.
Filtration is the first step in urine formation. High pressure within the glomerulus filters water and tiny solute into Bowman’s capsule. If glomerular pores are enlarged, they allow the filtration of proteins such as albumin, causing albuminuria. Normal urine is called an ultrafiltrate because it should contain no protein. The appearance of protein in the urine indicates glomerular dysfunction. Not all blood entering the glomerulus is filtered. Whatever is not filtered exits the glomerulus via an efferent arteriole, which then extends as the peritubular capillaries. Components that are filtered include water, Na+, Cl-, glucose, urea, creatinine, and K+ In glomerular filtration, water and solute are moving from the blood (glomerulus) into the tubules.
Blood pressure determines GFR; if BP becomes very low or shocky (80/60 mm Hg), blood flow to the kidney, GFR, and urinary output all decline. A patient with a urinary output < 400 mL/24 hours is oliguric. Oligo means scant. A continuing drop of GFR indicates that a patient is developing renal failure. If a patient’s blood pressure is declining, why should the urinary output be monitored? Oliguria is a symptom of shock and prolonged oliguria can cause further renal damage, such acute tubular necrosis.
Tubular reabsorption is the second step in urine formation. H2O and dissolved solute are reabsorbed back into the blood across the peritubular capillaries. The body filters 180 liters of water/24 hours. However, a person urinates 1.5 liters/24 hours. The tubules reabsorb 178.5 liters/24 hours.
The percentage of reabsorption of solute varies from 0% to 100%. The kidney actively pumps Na+ from the tubules into the peritubular capillaries. H2O and Cl- passively follow the actively pumped Na+. Water follows sodium. Diuretics increase urine flow. Most diuretics block the reabsorption of Na+ in various ways, thus blocking the reabsorption of H2O. A normal glycemic person filters glucose, but reabsorbs all of it. A hyperglycemic person filters a greater amount of glucose but cannot reabsorb 100% of it, and is therefore glucosuric.
The active pumping of Na+ and passive flow of H2O and Cl- are responsible for the reabsorption of a huge amount of water (178.5 liters/24 hours). In case of hypochloremia, the anion that follows Na+ is HCO3-, thereby disturbing acid-base balance. In tubular reabsorption, water and solute move into the peritubular capillaries.
Tubular secretion is the third and last step in urine formation. Tubular secretion fine-tunes the ionic concentrations, as well as secretes substances such as uric acid and drugs. The secretion of H+, which is usually accompanied by the reabsorption of HCO3-, allows the kidneys to help regulate acid-base balance. The kidney is the primary organ of drug excretion. Patients with impaired renal function require adjustments in drug dosages.
The distal tube is the site of aldosterone’s action. Why can a diuretic that blocks aldosterone cause hyperkalemia? When aldosterone is blocked, Na+ and H2O are not reabsorbed, but K+ is reabsorbed, causing hyperkalemia. In acute adrenal cortical insufficiency, why is the patient hyperkalemic with a shocky blood pressure? The lack of aldosterone from the adrenal cortex prevents the reabsorption of N+ and H2O, thereby lowering blood volume and BP. K+ is excreted in urine.
Activation of the renin-angiotensin-aldosterone system (RAAS) releases aldosterone into the blood. The most common triggers activating the system are low BP or diminished blood volume. The RAAS is verbally described on the next slide; however, it may be useful to return to this slide and ask students to trace the system on the illustration.
If a person has a low-volume shock, activation of the renin-angiotensin system causes the release of aldosterone, which expands blood volume, therefore elevating BP. The activated system also produces angiotensin II, a potent vasoconstrictor, which elevates PB. Why would a drug that blocks the converting enzyme lower BP? These drugs are called angiotensin converting enyzme (ACE) inhibitors; they block the release of aldosterone and the formation of angiotensin II, both of which elevate blood pressure.
Ask students to show on the slide where ADH works. ADH increases the permeability of the collecting duct membrane to H2O, allowing more H2O to flow into the tissue space. The H2O in the tissue space is reabsorbed into the peritubular capillaries and returned to the general circulation. What happens with an ADH deficiency? Because ADH controls the permeability of the collecting duct to H2O, a deficiency of ADH also called diabetes insipidus) prevents H2O from leaving the collecting duct and allowing excess H2O in the urine (polyuria).This will cause hypovolemia. Excess ADH, often seen in traumatic head injuries, causes hypervolemia.
The pH of urine varies with diet, and can range from alkaline to acid. Why does a urinary high specific gravity indicate dehydration? Specific gravity is a ratio of solid components to volume. In dehydration, the volume decreases and the specific gravity increases.
A urine sample provides a large amount of diagnostic information. For example, glucosuria is indicative of hyperglycemia, most often caused by diabetes mellitus. Why does albuminuria pinpoint the problem structure in the nephron unit? Albuminuria only occurs when the size of the glomerular pores increase, allowing the filtration of albumin, a large protein. Why might a patient with cystitis or pyelonephritis have pyuria? The WBCs are present because of a urinary tract infection.
In both urinary retention and renal suppression, the patient does not urinate. Why? In urinary retention the kidneys produce urine but the bladder won’t empty. In renal suppression, the kidneys do not produce urine. Urinary retention is often caused by anticholinergic drugs such as atropine, often used as perioperative medications.
The concepts of diffusion and osmosis help explain dialysis. In the artificial kidney (hemodialysis), waste and water diffuse out of the tube containing the patient’s blood into a dialysate (a commercially prepared solution of water and electrolytes that is similar to the body’s water and electrolyte content). In peritoneal dialysis, the dialysate is run into the patient’s abdominal cavity and the patient’s peritoneal membranes act as the dialyzing membranes. After a period of time, the dialysate containing the waste is removed. Dialysis takes time, often 4 hours every 3rd day, and so it can be disruptive to patient’s lives.
Ask students to say “Hi,” to Joe the plumber. All of the structures described on this slide are “plumbing,” but are not involved in the formation of urine. The trigone of the bladder is formed by the entrance of the two ureters and the exit of the urethra. The detrusor muscle, a smooth muscle, causes bladder contraction and relaxation for expelling urine. The detrusor muscle forms the internal sphincter of the bladder. The bladder’s external sphincter is composed of skeletal muscle and is under voluntary control. How could a urologist access a stone in the right ureter? A tube could be inserted into the urethra, bladder, and right ureter; a grasping device can then extract the stone.
Whereas the internal sphincter is involuntary and relaxes as part of the micturition reflex, the external sphincter is composed of skeletal muscle and is under voluntary, allowing you to “hold it.” During micturition, the detrusor muscle contracts and the two sphincters relax. What urinary structure is brought under control during potty training of a toddler? The external sphincter is brought under voluntary control.
The Human Body in Health and
Illness, 4th edition