Chapter 45:Urinary System
STRUCTURES AND FUNCTIONS
The urinary system consists of two kidneys, two ureters, a urinary bladder, and a urethra.
The bladder provides storage, and the ureters and urethra are the drainage channels for
the urine after it is formed by the kidneys.
The primary functions of the kidneys are (1) to regulate the volume and composition of
extracellular fluid (ECF), and (2) to excrete waste products from the body.
The kidneys function to control blood pressure, produce erythropoietin, activate vitamin
D, and regulate acid-base balance.
The outer layer of the kidney is termed the cortex, and the inner layer is called the
The nephron is the functional unit of the kidney. Each kidney contains 800,000 to 1.2
A nephron is composed of a glomerulus, Bowman’s capsule, and a tubular system. The
tubular system consists of the proximal convoluted tubule, the loop of Henle, the distal
convoluted tubule, and a collecting tubule.
The kidneys receive 20% to 25% of cardiac output.
The primary function of the kidneys is to filter the blood and maintain the body’s internal
Urine formation is the result of a multistep process of filtration, reabsorption, secretion,
and excretion of water, electrolytes, and metabolic waste products.
Blood is filtered in the glomerulus.
The hydrostatic pressure of the blood within the glomerular capillaries causes a portion of
blood to be filtered across the semipermeable membrane into Bowman’s capsule.
The ultrafiltrate is similar in composition to blood except that it lacks blood cells,
platelets, and large plasma proteins.
The amount of blood filtered by the glomeruli in a given time is termed the glomerular
filtration rate (GFR). The normal GFR is about 125 ml/min.
Tubular Function The functions of the tubules and collecting ducts include reabsorption and secretion.
Reabsorption is the passage of a substance from the lumen of the tubules through the
tubule cells and into the capillaries. Tubular secretion is the passage of a substance from
the capillaries through the tubular cells into the lumen of the tubule.
o The loop of Henle is important in conserving water and thus concentrating the
filtrate. In the loop of Henle, reabsorption continues.
o Two important functions of the distal convoluted tubules are final regulation of
water balance and acid-base balance.
Antidiuretic hormone (ADH) is required for water reabsorption in the
Aldosterone acts on the distal tubule to cause reabsorption of sodium ions
+ + +
(Na ) and water. In exchange for Na , potassium ions (K ) are excreted.
o Acid-base regulation involves reabsorbing and conserving most of the bicarbonate
(HCO )3and secreting excess H . +
o Atrial natriuretic peptide (ANP) acts on the kidneys to increase sodium excretion.
o Parathyroid hormone (PTH) acts on renal tubules to increase reabsorption of
Other Functions of the Kidney
The kidneys produce erythropoietin in response to hypoxia and decreased renal blood
flow. Erythropoietin stimulates the production of red blood cells (RBCs) in the bone
Vitamin D is activated in kidneys. Vitamin D is important for calcium balance and bone
Renin, which is produced and secreted by juxtaglomerular cells, is important in the
regulation of blood pressure.
Prostaglandin (PG) synthesis (primarily PGE a2d PGI ) o2curs in the kidney, primarily
in the medulla. Th+se PGs have a vasodilating action, thus increasing renal blood flow
and promoting Na excretion.
The ureters are tubes that carry urine from the renal pelvis to the bladder.
Circular and longitudinal smooth muscle fibers, arranged in a meshlike outer layer,
contract to promote the peristaltic one-way flow of urine.
The urinary bladder is a distensible organ positioned behind the symphysis pubis and
anterior to the vagina and rectum.
Its primary functions are to serve as a reservoir for urine and to help the body eliminate
waste products. Normal adult urine output is approximately 1500 ml/day, which varies with food and
On the average, 200 to 250 ml of urine in the bladder causes moderate distention and the
urge to urinate.
The urethra is a small muscular tube that leads from the bladder neck to the external
The primary function of the urethra is to serve as a conduit for urine from the bladder neck to