Biology 115 Lecture 8:
T. Irving 01/07/97; revised A.Howard 02/07/99
Urinary System and Excretion

Objectives

Students should be able to:

1. Name the four excretory organs and state the functions of each
2. State the contents of urine
3. Trace the path of urine giving the structure and function of organs along its path
4. Describe the macroscopic anatomy of the kidney
5. Identify the parts of a nephron, relating these to the macroscopic anatomy
6. Identify the path of blood about the nephron
7. Relate the three steps in urine formation to the parts of a nephron
8. Describe how the loop of Henle contributes to water reabsorption
9. Describe the role of the kidney in maintaining pH and salt balance of the blood
10. Describe role of 2 major hormones in maintaining blood volume.
11. Describe how a kidney machine works.

Excretion

• Defecation is the elimination of feces, which consists primarily of undigested food products
• Excretion is the elimination of the waste products of metabolism

Excretory organs

• Skin: excretes perspiration ("dilute urine")
• Liver: excretes bile pigments
• Lungs: expel carbon dioxide and water
• Kidneys: produce urine
  Consist of three regions: the cortex, medulla, and the renal pelvis

Path of Urine

• Urine passes from the kidneys to the ureters to the bladder.
• During urination, urine is expelled from the body via the urethra
• Stretch receptors in the bladder send nerve impulse to the spinal cord causing sphincters to relax to allow urination
• Reflex can be consciously over-ridden

Nephrons

• The kidney consists of many suborgans called nephrons where the formation and collection of urine takes place

• Components of a nephron:
  • Bowman's Capsule; lies within renal cortex
  • Proximal convoluted tubule; lies within renal cortex
  • Loop of Henle; penetrates into medulla
  • Distal Convoluted Tubule; lies within renal cortext
  • Collecting ducts; primarily in medulla

• Each nephron has its own blood supply:
  Afferent arteriole ends in the capillaries of the glomerulus and the peritubular capillary network

Urine Formation

• Pressure filtration
  Small molecules (water, AA’s, glucose, salts, urea, etc.) leave capillaries in the glomerulus into Bowman’s Capsule leaving formed elements and plasma proteins behind, producing "glomerular filtrate"
• Selective Reabsorption
  
  • Water returns to the blood by osmosis beginning in the proximal convoluted tubule and continuing along as the filtrate passes through the nephron
  • Na⁺ is actively transported back in along with nutrients (glucose etc.)
  • anions (negatively charged ions) follow passively
• Tubular Excretion
H⁺, NH₄⁺, creatinine, and many drugs move by active transport from the peritubular capillaries into the distal convoluted tubule

Reabsorbing water

• Concentration of the urine occurs primarily on the loop of Henle and the collection duct
• Water diffuses out of the permeable descending limb of the Loop of Henle
• The ascending limb is impermeable to water but actively transports salts out.
• Solute concentration in the tissue fluid increases on going from the cortex to the inner medulla
• The solute in the tissue in the innermost part of the medulla is primarily urea
• Fluid entering the collecting duct is isotonic with blood
• As it travels down the duct, it encounters higher and higher solute concentrations in the tissue causing water to leave the duct.

Regulatory Functions of the Kidney

pH

• Excess H⁺, NH4⁺, is excreted
• HCO₃^- and Na⁺ may be reabsorbed
• The amounts absorbed and excreted are adjusted to keep pH near 7.4
• Likewise for important ions such as K⁺, Mg²⁺.

Hormonal Control of pH, [salt], and blood volume

• Antidiuretic Hormone (ADH)
  • Secreted posterior pituitary
  • Primarily responsible for maintaining blood volume
  • Increased ADH increases permeability of collecting duct to water concentrating the urine
  • Decreased ADH decreases permeability of collecting duct to water increasing volume of urine

• Diuretic Drugs

• Alcohol inhibits secretion of ADH
• Caffeine blocks ADH at the tubules
• Other diuretics may be prescribed for high blood pressure, pulmonary edema, and congestive heart failure
• Aldosterone
  • Secreted by the adrenal cortex
  • Maintains Na⁺ and K⁺ balance
  • Causes reabsorption of Na⁺ and excretion of K⁺ in the distal convoluted tubule.
  • Water intake caused by increased absorption of Na⁺ increases blood volume and blood pressure

Juxtoglomerular Apparatus

• This is a location near the glomerulus where cells on the surface of the afferent arteriole and the distal convoluted tubule are in very close contact
• If blood pressure is too low for pressure filtration the afferent arterial cells secrete renin which enzymatically changes angiotensinogen to angiotensin I
• Angiotensin I travels to the lungs where it is enzymatically converted to angiotensin II
• Angiotensin II is a vasoconstrictor - blood pressure rises
• Also stimulates adrenal cortex to produce aldosterone - blood pressure rises

Kidney disorders

Bacterial infections:

• Urethritus - infections of the urethra
• Cystitis - infections of the bladder
• Pyelonephritis- kidneys themselves
• Infectious agent usually e. coli

Kidney failure

• Involves decreases in glomerular filtration with decreased urine output
• Kidney stones, glomerulo-nephritus often appear
• Detected when albumin, blood cells appear in the urine
• Urea build-up, water and salt retention, edema
• Often the best answer: a transplant
  • Kidney transplants have high success rates
  • Only about a quarter of a kidney required to keep you healthy - organ donations frequently from relatives
  • Artificial kidneys can keep people alive until a transplant in available