Kidney Anatomy and Physiology (Renal System)
ANATOMY AND PHYSIOLOGY
- Each person has two kidneys; one is attached to the left abdominal wall at the level of the last thoracic and first three lumbar vertebrae and the other is on the right.
- The kidneys are enclosed in the renal capsule.
- The renal cortex is the outer layer of the renal capsule, which contains blood-filtering mechanisms (glomeruli).
- The renal medulla is the inner region, which contains the renal pyramids and renal tubules.
- Together the renal cortex, pyramids, and medulla constitute the parenchyma, or functional unit of the kidneys.
- Located within the parenchyma
- Composed of glomerulus and tubules
- Selectively secretes and reabsorbs ions and filtrates, including fluid, wastes, electrolytes, acids, and bases The nephrons are the functional units of the kidney.
- Each nephron contains tufts of capillaries,which filter large plasma proteins and blood cells.
- Blood flows into the glomerular capillaries from the afferent arteriole and flows out of the glomerular capillaries into the efferent arteriole.
- Thin double-walled capsule that surrounds the glomerulus
- Fluid and particles from the blood such as electrolytes, glucose, amino acids, and metabolic waste (glomerular filtrate) are filtered through the glomerular membrane into a fluid-filled space in Bowman’s capsule (Bowman’s space) and then enters the proximal convoluted tubule (PCT).
- The tubules include the PCT, Henle’s loop, and the distal convoluted tubule (DCT).
- The PCT receives filtrate from the glomerular capsule and reabsorbs water and electrolytes through active and passive transport.
- The descending loop of Henle passively reabsorbs water from the filtrate.
- The ascending loop of Henle passively reabsorbs sodium and chloride from the filtrate and helps maintain osmolality.
- The DCT actively and passively removes sodium and water.
- The filtered fluid is converted to urine in the tubules, and then the urine moves to the pelvis of the kidney.
- The urine flows from the pelvis of the kidneys through the ureters and empties into the bladder.
Functions of kidneys
- Maintain acid-base balance
- Excrete end products of body metabolism
- Control fluid and electrolyte balance
- Excrete bacterial toxins, water-soluble drugs, and drug metabolites
- Secrete renin to regulate the blood pressure and erythropoietin to stimulate the bone marrow to produce red blood cells.
- Synthesize vitamin D for calcium absorption and regulation of the parathyroid hormones.
- As fluid flows through the tubules, water, electrolytes, and solutes are reabsorbed and other solutes such as creatinine, hydrogen ions, and potassium are secreted.
- Water and solutes that are not reabsorbed become urine.
- The process of selective reabsorption determines the amount of water and solutes to be secreted.
Homeostasis of water
- Antidiuretic hormone (ADH) is primarily responsible for the reabsorption of water by the kidneys.
- ADHis produced by the hypothalamus and secreted from the posterior lobe of the pituitary gland.
- Secretion of ADH is stimulated by dehydration or high sodium intake and by a decrease in blood volume.
- ADH makes the distal convoluted tubules and collecting duct permeable to water.
- Water is drawn out of the tubules by osmosis and returns to the blood; concentrated urine remains in the tubule to be excreted.
- When ADH is lacking, the client develops diabetes insipidus (DI).
- Clients with DI produce large amounts of dilute urine; treatment is necessary because the client cannot drink sufficient water to survive.
Homeostasis of sodium
- When the amount of sodium increases, extra water is retained to preserve osmotic pressure.
- An increase in sodium and water produces an increase in blood volume and blood pressure (BP).
- When the BP increases, glomerular filtration increases, and extra water and sodium are lost; blood volume is reduced, returning the BP to normal.
- Reabsorption of sodium in the distal convoluted tubules is controlled by the renin-angiotensin system.
- Renin, an enzyme, is released from the nephron when the BP or fluid concentration in the distal convoluted tubule is low.
- Renin catalyzes the splitting of angiotensin I from angiotensinogen; angiotensin I converts to angiotensin II as blood flows through the lung.
- Angiotensin II, a potent vasoconstrictor, stimulates the secretion of aldosterone.
- Aldosterone stimulates the distal convoluted tubules to reabsorb sodium and secrete potassium.
- The additional sodium increases water reabsorption and increases blood volume and BP, returning the BP to normal; the stimulus for the secretion of renin then is removed.
Homeostasis of potassium
- Increases in the serum potassium level stimulate the secretion of aldosterone.
- Aldosterone stimulates the distal convoluted tubules to secrete potassium; this action returns the serum potassium concentration to normal.
Homeostasis of acidity (pH)
- Blood pH is controlled by maintaining the concentration of buffer systems.
- Carbonic acid and sodium bicarbonate form the most important buffers for neutralizing acids in the plasma.
- The concentration of carbonic acid is controlled by the respiratory system.
- The concentration of sodium bicarbonate is controlled by the kidneys.
- Normal arterial pH is 7.35 to 7.45, maintained by keeping the ratio of concentrations of sodium bicarbonate to carbon dioxide constant at 20:1.
- Strong acids are neutralized by sodium bicarbonate to produce carbonic acid and the sodium salts of the strong acid; this process quickly restores the ratio and thus blood pH.
- The carbonic acid dissociates into carbon dioxide and water; because the concentration of carbon dioxide is maintained at a constant level by the respiratory system, the excess carbonic acid is rapidly excreted.
- Sodium combined with the strong acid is actively reabsorbed in the distal convoluted tubules in exchange for hydrogen or potassium ions. The strong acid is neutralized by ammonia and is excreted as ammonia or potassium salts.
Adrenal glands (see Chapter 54 for information about the adrenal glands)
- One adrenal gland is on top of each kidney.
- The adrenal glands influence blood pressure and sodium and water retention.
- The bladder detrusor muscle, composed of smooth muscle, distends during bladder filling and contracts during bladder emptying.
- The ureterovesical sphincter prevents reflux of urine from the bladder to the ureter.
- The total bladder capacity is 1 L; normal adult urine output is 1500 mL/day.
- The prostate gland surrounds the male urethra.
- The prostate gland contains a duct that opens into the prostatic portion of the urethra and secretes the alkaline portion of seminal fluid, which protects passing sperm.
Risk factors associated with renal disorders