Lecture Outline - Chapter 16

16.1. Four Excretory Organs (p. 288)

1. Excretion rids body of metabolic wastes.
2. Kidneys are primary excretory organs but other organs also function in excretion: skin, liver, lungs. (Fig. 16.1)
3. Sweat Glands in Skin Secrete Perspiration
   • a. Sweat is water, salt, some urea.
   • b. Helps rid body of waste products but mainly cools body down by evaporation.
   • c. Sweat gland is coiled tubule in dermis, straightens as exits through epidermis.
   • d. With kidney failure, urea is excreted by sweat glands and forms urea-frost on skin surface.
4. Liver Excretes Bile Pigments
   • a. Bile pigments result from breakdown of hemoglobin.
   • b. Bile is stored in the gall bladder.
   • c. Urochrome is yellow pigment derived from heme breakdown and is excreted by kidneys.
   • d. Liver also excretes cholesterol and excess fat-soluble vitamins.
5. Lungs Remove CO₂
   • a. Expiration (breathing out) removes carbon dioxide from the body.
   • b. Expiration also results in unavoidable loss of water.
6. Kidneys Produce Urine
   • a. Urine contains 95% water plus nitrogenous wastes and inorganic salts.
   • b. Nitrogenous wastes are products of metabolism of amino acids, nucleotides and creatine phosphate.
   • c. Amino acid metabolism produces urea as main nitrogenous end product in humans.
   • d. Urea cycle occurs in liver; two ammonia molecules and a carbon dioxide are converted to water and urea eventually excreted in kidney.
   • e. Creatine phosphate (reservoir of phosphate in muscles) results in end product creatine; breakdown of nucleotides produces insoluble uric acid.
   • f. Too much uric acid in blood precipitates out; if collects in joints, results in painful ailment called gout.

1. Kidneys contribute to homeostasis, producing urine to rid body of nitrogenous wastes and keeping pH and salt/water balance of blood within normal range. (Table 16.1)
2. The Path of Urine
   • a. Kidneys
     • i. Are bean-shaped, reddish brown organs.
     • ii. About size of a fist.
     • iii. Located on either side of vertebral column just below diaphragm.
     • iv. Covered by connective tissue overlaid by adipose tissue.
     • v. Function to produce urine.
   • b. Ureters
     • i. Muscular tubes from the kidneys to the bladder.
     • ii. Transport urine by peristaltic contractions (about five per minute).
   • c. Urinary Bladder
     • i. Hollow muscular organ expands as urine enters.
     • ii. Capacity of up to 600 ml of urine.
     • iii. In male, located in front of rectum, seminal vesicles and vas deferens.
     • iv. In female, located in front of uterus and upper vagina.
     • v. Controlled by two sphincters where urethra joins bladder.
   • d. Urethra
     • i. Duct that transports urine from urinary bladder to external opening.
     • ii. Longer in males (about 20 cm when penis is flaccid); surrounded by prostate gland at base of bladder and connected to reproductive system.
     • iii. Shorter in females (about 4 cm long); no connection to reproductive system.
3. Urination and Nervous System
   • a. Urinary bladder fills with urine.
   • b. Stretch receptors send nerve impulses to spinal cord.
   • c. Nerve impulses from spinal cord cause urinary bladder muscles to contract and sphincters to relax making urination possible.
   • d. Brain controls this reflex in older children and adults.
4. Kidneys: Have Three Regions (Fig. 16.3a) (p. 291)
   • a. On concave side, there is depression where renal blood vessels and ureter enter.
   • b. Renal cortex is outer granulated layer that dips down between the renal medulla.
   • c. Renal medulla has radially striated cone-shaped masses called renal pyramids.
   • d. Renal pelvis is central space or cavity continuous with ureter. (Fig. 16.3b)
5. Nephrons (Fig. 16.3c)
   • a. Nephrons are microscopic, sometimes called renal or kidney tubules.
   • b. Each kidney contains over one million nephrons.
   • c. Several nephrons enter one collecting duct.

1. Blood Flow Through Nephron (Fig. 16.4)
   • a. Renal artery brings blood in.
   • b. Afferent arteriole leads to glomerulus, a tuft of capillaries.
   • c. This first capillary region is inside glomerular capsule.
   • d. Efferent arteriole leaves glomerulus.
   • e. Peritubular capillary surrounds rest of nephron.
   • f. Blood collects in venules that join the renal vein.
2. Nephron Has Several Parts
   • a. Glomerular (Bowman's) capsule is cup-like structure that encloses glomerulus.
     • i. Outer layer of capsule is squamous epithelial cells.
     • ii. Inner layer of capsule is made of podocytes with long cytoplasmic processes.
     • iii. Podocytes cling to capillary walls leaving pores allowing easy passage for small molecules.
     • iv. Process is glomerular filtration.
     • v. Glomerular capsule lies within renal cortex.
   • b. Proximal Convoluted Tubule
     • i. Named because it is nearest glomerular capsule.
     • ii. Has cuboidal cells with inner brush border or microvilli to increase surface area for tubular reabsorption.
     • iii. Cells have many mitochondria to supply energy for active transport. (Fig. 16.5)
     • iv. Convoluted tubules lie within renal cortex.
   • c. Loop of the Nephron (Loop of Henle)
     • i. Tubule narrows and forms U-shaped turn with simple squamous epithelium.
     • ii. Descending limb allows water to leave; ascending section is impervious to water.
     • iii. Distal convoluted tubule has cuboidal cells with many mitochondria but no microvilli.
     • iv. Active in tubular secretion for moving molecules from blood into tubule.
     • v. Collecting ducts are in renal medulla carry urine to renal pelvis.

1. The three steps involved in urine formation are: glomerular filtration, tubular reabsorption, and tubular secretion.
2. Glomerular Filtration
   • a. Occurs when whole blood enters afferent arteriole and glomerulus.
   • b. Glomerular blood pressure (about 60 mm Hg) pushes water and small molecules across to glomerular capsule; includes water, nitrogenous wastes, nutrients, and salts.
   • c. Large molecules and formed elements are too large to pass across capillary wall; includes blood cells, platelets, and proteins.
   • d. Glomerular filtrate dissolved molecules are about same concentration as plasma.
   • e. Efferent arteriole carries away remaining thicker blood.
   • f. 180 liters of water with dissolved salts and nutrients pass across per day. (Table 16.2)
   • g. Failure to reclaim some of this would result in dehydration, starvation, and low blood pressure leading to death.
3. Tubular Reabsorption is Both Passive and Active
   • a. Molecules are reabsorbed from tubule into surrounding peritubular capillaries.
   • b. When sodium ions (Na⁺) are actively reabsorbed, chloride ions (Cl^-) follow passively; about 60 - 70% of Na⁺ ions lost are reabsorbed in proximal convoluted tubule.
   • c. Reabsorption of salt (Na⁺Cl^-) increases blood osmolarity; water then moves passively from tubule to blood.
   • d. Carrier molecules select specific molecules for active reabsorption; glucose is completely reabsorbed.
   • e. If the maximum rate of transport is exceeded, excess molecules appear in urine; as glucose levels reach 400 mg/100 ml plasma, glucose appears in urine.
   • f. In patients with diabetes mellitus, liver and muscles fail to store glucose; thus high glucose levels prevent kidneys from reabsorbing all sugar in filtrate.
   • g. Molecules not reabsorbed pass to loop of Henle to become part of urine.
4. Tubular Secretion Adds Substances
   • a. Hydrogen and ammonium ions, creatinine, penicillin, etc., move from blood into distal convoluted tubules.
   • b. Urine contains substances from glomerular filtration and tubular secretion.
5. Reabsorbing Water (Fig. 16.7)
   • a. In loop of the nephron (loop of Henle), salt leaves ascending thin limb of loop by passive transport and thick portion by active transport.
   • b. Along with urea loss from lower collecting duct, an osmotic gradient is created within renal medulla, concentration of salt is greater in direction of inner medulla.
   • c. Ascending limb is impermeable to water; water can still leave descending limb.
   • d. Countercurrent mechanism results when increasing solute concentration encounters decreasing number of water molecules in descending limb; water continues to leave descending limb from top to bottom.
   • e. Isotonic fluid in distal convoluted tubule enters collecting duct and passes through osmotic gradient in medulla.
   • f. Water diffuses out of tubule into surrounding capillaries; fluid in collecting tubule becomes hypertonic relative to blood plasma.
   • g. Urine composition (Table 16.1) includes non-reabsorbed molecules as well as tubular secretions; excreted in collecting ducts.

1. Maintaining Blood Volume
   • a. Juxtaglomerular apparatus is region of contact between afferent arteriole and distal convoluted tubule. (Fig. 16.8)
   • b. When blood pressure is insufficient for glomerular filtration, afferent arteriole cells secrete renin, and enzyme that changes angiotensinogen into angiotensin I.
   • c. Angiotensin I is then converted into angiotensin II in lungs via angiotensin- converting enzyme.
   • d. Angiotensin II is a vasoconstrictor which stimulates adrenal cortex, the outer portion of adrenals atop kidneys, to release aldosterone.
   • e. Hormone aldosterone causes distal convoluted tubule to reabsorb sodium (Na⁺) leading to more water reabsorption, increasing blood volume and pressure.
   • f. Renin-angiotensin-aldosterone system is always active in people with hypertension (high blood pressure); diuretics counteract hypertension by inhibiting reabsorption of Na⁺ so less water is reabsorbed.
   • g. Antidiuretic hormone (ADH) is hormone of posterior pituitary lobe; triggered when solutes in blood become concentrated due to lack of water intake when Na+ has been reabsorbed. (Fig. 16.9)
   • h. Diuresis is increased urine flow; antidiuresis is decrease in urine.
   • i. ADH results in more water reabsorbed and less urine formed.
     • i. If individual drinks less water, pituitary releases ADH causing reabsorption and blood volume is maintained.
     • ii. If individual drinks more water, pituitary doesn't release ADH causing water excretion and blood volume is maintained.
   • j. Alcohol inhibits secretion of ADH from posterior pituitary causing more urine volume or diuresis.
   • k. Action of aldosterone and ADH are opposed by hormone ANH (atrial natriuretic hormone).
     • i. ANH is released by cardiac cells when atria of heart are stretched by blood volume.
     • ii. ANH inhibits secretion of renin by juxtaglomerular apparatus and aldosterone by adrenal cortex; causes excretion of Na⁺ (natriuresis).
     • iii. When Na⁺ is excreted, so is water; blood volume and pressure decrease.
2. Maintaining Blood pH
   • a. Kidneys help maintain pH level of blood around 7.4.
   • b. If blood is acidic, hydrogen ions are excreted with ammonia; sodium and bicarbonate ions are reabsorbed (sodium bicarbonate is a base).
   • c. If the blood is basic, fewer hydrogen ions are excreted; fewer sodium and bicarbonate ions are reabsorbed. (p. 299)
   • d. Bicarbonate, potassium, and magnesium ions are also regulated.
   • e. This keeps blood pH in narrow range.

1. Since kidneys are critical in fluid homeostasis, renal diseases can be life-threatening.
2. Illnesses include progressive renal diseases and total renal failure.
3. Urethritis is a urinary tract infection more common in women (shorter urethra) than men.
4. Cystitis is infection of bladder.
5. Pyelonephritis involves infection of kidneys.
6. Glomerular damage may make glomeruli too permeable; albumin, white, and red blood cells may be found in urine; test is called urinalysis.
7. Trace amounts of protein in urine are not serious.
8. If over two-thirds of nephrons are inoperative, wastes and urea accumulate in blood; called uremia, it can lead to edema or retention of water and salts in body tissues.
9. Replacing the Kidneys (p. 301)
   • a. Patients with renal failure may undergo kidney transplant operations.
   • b. One-year survival rate is 97% if it is received from a relative and 90% if from a non-relative, due to organ rejection.
10. Without a transplant, dialysis of blood is performed on a kidney machine or by continuous ambulatory peritoneal dialysis (CAPD). (Fig. 16.10)
11. Hemodialysis passes the patient's blood through semipermeable membrane tubes across from a balanced salt (dialysis) solution.
    • a. Waste substances more concentrated in blood pass across to dialysate.
    • b. If blood is acidic, bicarbonate ions or drugs can be diffused into blood.
    • c. Six-hour hemodialysis removes 50 - 250 grams of urea; needed twice a week and confines patient to machine.
12. CAPD uses a permanently implanted bag to introduce dialysate into abdominal cavity.
    • a. Fluid is collected each four to eight hours.
    • b. Patient can pursue normal activity during CAPD.