Essentials of Anatomy and Physiology

tubule that encloses the glomerulus; inner layer

is made of podocytes, has pores, and is very per-

meable; contains renal filtrate (potential urine).

2. Renal tubule—consists of the proximal convoluted
   tubule, loop of Henle, distal convoluted tubule,
   and collecting tubule. Collecting tubules unite to
   form papillary ducts that empty urine into the
   calyces of the renal pelvis.

• Peritubular capillaries—arise from the efferent
  arteriole and surround all parts of the renal
  tubule.

Blood Vessels of the Kidney (see Figs. 18–1,
18–2, and 18–3)

1. Pathway: abdominal aorta →renal artery →small
   arteries in the kidney → afferent arterioles →
   glomeruli →efferent arterioles →peritubular cap-
   illaries →small veins in the kidney →renal vein →
   inferior vena cava.


2. Two sets of capillaries provide for two sites of
   exchanges between the blood and tissues in the
   process of urine formation.

Formation of Urine (see Fig. 18–4)

1. Glomerular filtration—takes place from the
   glomerulus to Bowman’s capsule. High blood pres-
   sure (60 mmHg) in the glomerulus forces plasma,
   dissolved materials, and small proteins out of the
   blood and into Bowman’s capsule. The fluid is now
   called filtrate. Filtration is selective only in terms of
   size; blood cells and large proteins remain in the
   blood.


2. GFR is 100 to 125 mL per minute. Increased blood
   flow to the kidney increases GFR; decreased blood
   flow decreases GFR.


3. Tubular reabsorption—takes place from the filtrate
   in the renal tubule to the blood in the peritubular
   capillaries; 99% of the filtrate is reabsorbed; only
   1% becomes urine.

• Active transport—reabsorption of glucose,
  amino acids, vitamins, and positive ions; thresh-
  old level is a limit to the quantity that can be
  reabsorbed.


• Passive transport—most negative ions follow the
  reabsorption of positive ions.


• Osmosis—water follows the reabsorption of
  minerals, especially sodium.


• Pinocytosis—small proteins are engulfed by
  proximal tubule cells.

4. Tubular secretion—takes place from the blood in
   the peritubular capillaries to the filtrate in the renal
   tubule; creatinine and other waste products may be
   secreted into the filtrate to be excreted in urine;
   secretion of Hions helps maintain pH of blood.


5. Hormones that affect reabsorption—aldosterone,
   atrial natriuretic peptide, antidiuretic hormone,
   and parathyroid hormone—see Table 18–1 and
   Fig. 18–5.

The Kidneys and Acid–Base Balance

1. The kidneys have the greatest capacity to compen-
   sate for normal and abnormal pH changes.


2. If the body fluids are becoming too acidic, the kid-
   neys excrete Hions and return HCO 3 ions to
   the blood (see Fig. 18–6).


3. If the body fluids are becoming too alkaline, the
   kidneys return Hions to the blood and excrete
   HCO 3 ions.

Other Functions of the Kidneys

1. Secretion of renin by juxtaglomerular cells when
   blood pressure decreases (see Table 18–2). Angio-
   tensin II causes vasoconstriction and increases
   secretion of aldosterone.


2. Secretion of erythropoietin in response to hypoxia;
   stimulates red bone marrow to increase rate of
   RBC production.


3. Activation of vitamin D—conversion of inactive
   forms to the active form.

Elimination of Urine—the function of the

ureters, urinary bladder, and urethra

Ureters (see Figs. 18–1 and 18–7)

1. Each extends from the hilus of a kidney to the
   lower posterior side of the urinary bladder.


2. Peristalsis of smooth muscle layer propels urine
   toward bladder.

Urinary Bladder (see Figs. 18–1 and 18–7)

1. A muscular sac below the peritoneum and behind
   the pubic bones; in women, below the uterus; in
   men, above the prostate gland.


2. Mucosa—transitional epithelial tissue folded into
   rugae; permit expansion without tearing.


3. Trigone—triangular area on bladder floor; no
   rugae, does not expand; bounded by openings of
   ureters and urethra.

436 The Urinary System