Anatomy & Physiology Lecture Outline: Week #3a

• The Urinary System

LSc 107 Anatomy and Physiology Spring 1999

Thibodeau Ch 28

Primary organs: Kidneys

Accessory organs: ureters, urinary bladder, urethra

What do the kidneys do for the body?

They REGULATE the excretion of certain substances, and because they control what leaves the body through them, are able to make large contributions to homeostasis, esp of plasma (regulate plasma water, ions, and pH balance)

Excretory functions

• excretion of water
• excretion of H⁺ ions
• excretion of electrolytes
• excretion of the products of protein metabolism (urea, uric acid)
• excretion of drugs, or breakdown products of drugs

Other organs of excretion? skin, lungs, intestines (Box 28-1)

Non-excretory functions

• renin production
• erythropoeitin production
• activation of Vit D

Renal Anatomy

Kidneys (Fig. 28-2)

• position on abdominal wall (retroperitoneal)
• size, shape (L larger and higher than R)
• structures of the hilum (concave depression where blood vessels & ureter enter and exit kidney)
• cortex (outer layer)
• medulla (renal pyramids) 6-18 / kidney
• renal calyx (1^st urine collecting tube), pelvis (becomes ureters)

Ureters

• structure – 3 layers (muscular walls propel urine by peristalsis)
• valves – prevent backflow

Bladder (Fig.28-5)

• location - Fig. 28-1 (below peritoneum)
• size
• structure – muscular walls with rugae when empty, distends considerably
  • transitional epithelium
  • detrusor muscle
• emptying mechanism – requires relaxation of external bladder sphincter & contraction of bladder muscle
  • reflex of sacral spinal cord
  • voluntary suppression by conscious brain

Urethra

• length depends on sex (3 cm in females, 20 cm in males)
• site in females - anterior to vagina
• site in males - runs down through prostate gland, joined by the ejaculatory ducts (from reproductive system) before running through length of penis

Microscopic structure of kidney

• nephron - Fig. 28-4, functional unit of the kidney - comprise almost entire kidney

glomerulus – vascular component 28-7

Bowman’s capsule – tubular component

• glomerular tuft (capillaries)
• Bowman's capsule
• proximal convoluted tubule
• loop of Henle
• distal convoluted tubule
• collecting duct
• Juxtaglomerular apparatus (regulation of BP in kidney)
• filtration membrane (the structures which substances leaving the blood and entering Bowman's capsule pass through)
  • capillary endothelium - more "leaky" than other capillaries
  • basement membrane
  • epithelium of Bowman's capsule

 

Arrangement of nephrons in the kidney - Fig. 28-9

Blood flow through the kidney Fig. 28-3 - (25% of CO)

• renal artery
• interlobar arteries
• interlobular arteries
• afferent arterioles
• glomerular tuft (capillaries)
• efferent arterioles
• capillaries
• venules
• veins (renal)

Function of the Kidney Summary of Nephron Function Table 28-1

See excretory and non-excretory functions above

How does the kidney accomplish all this?

• filtration
• reabsorption
• secretion

Filtration

• every blood component except large proteins and blood cells passes through
• filtration pressure - caused mainly by blood pressure in glomerulus 10 mm Hg
• about 180 liters/day formed (GFR)
• factor influencing the GFR: rise in systemic blood pressure (but note that kidney minimizes this by autoregulation of its own blood pressure)

Reabsorption

• selective reabsorption controls what leaves in the urine
• reabsorption in proximal tubule (most of the water and solute!)
  • adaptations for reabsorption - brush border, single cell thickness
  • sodium is actively absorbed, and water and Cl- follows (out of tubule)
  • nutrient substances (glucose and amino acids out of tubule)
  • electrolytes
  • about half of the urea filtered is reabsorbed
• reabsorption in the loop of Henle (countercurrent flow)
  • NB - ascending limb and descending limb have different permeabilities
  • selective active reabsorption of Na and Cl by the ascending limb causes the tissue fluid outside the tubule to become more concentrated. Water is prevented from following. This effect is stronger deeper in the medulla As the filtrate in the tubule passes through the concentrated area water passes out of the descending limb, and urea passes in to equalize the osmotic pressure
  • overall effect:
    • reabsorption of water
    • excretion of some urea
    • low osmolality of filtrate leaving loop
    • concentrated NaCl deep in pyramid

Why do the blood vessels (vasa recta) not wash away the NaCl which makes the medulla concentrated? sluggish blood flow

• Events of the distal tubule
  • small amount of Na⁺ (reabsorbed throughout the nephron) - under influence of aldosterone - hormone from adrenal cortex which reabsorbs salt and water
  • acid base balance - HCO₃- reabsorbed, and H⁺ excreted as necessary
  • excretion of ammonia
• Events of collecting duct
  dilute or concentrated urine: Filtrate from loop is dilute. If it passes through duct unchanged, urine will be dilute. If the walls of the collecting duct are permeable as they pass through the concentrated medulla - Fig. 28-20 again then water will be reabsorbed by osmosis, and urine will be more concentrated

What determines whether the walls of the collecting duct are permeable?

ADH (antidiuretic hormone) - hormone from pituitary

ADH present - water reabsorbed, concentrated urine; ADH absent - dilute urine

Regulation of urine volume

• excretion of osmotic substances - more solute, more urine
• effect of ADH
  • water load (conserve water in blood, more concentrated urine)
  • dehydration
  • effect of sweating
  • effect of alcohol, caffeine
• effect of ANH atrial natrurietic hormone - opposes Na and water reabsorption (more dilute urine)
• effect of aldosterone (conserves water in blood, more conc. urine)
• effect of arterial blood pressure - autoregulation
• diuretic drugs

Fig. 28-20 for effect of ADH and aldosterone on urine and plasma volume

Blood pressure and the kidney

JGA - juxta glomerular apparatus consists of a component from the afferent arteriole of the glomerulus and a component from the distal tubule.

JGA is sensitive to Na loss from kidney, and to blood pressure in arteriole.

If BP falls, secretes renin an enzyme which converts angiotensin I to II -causing vasoconstriction, aldosterone production, increased Na and water retention

Renin-angiotensin-aldosterone mechanism important in long term control of BP

Urine composition

p. 841, Table 28-2

water (95%), pH 4.6-8.0, N-wastes, electrolytes, toxins, pigments, hormones

 

[ WNJ Home Page | A & P Home | Lecture Outlines | Syllabus | Announcements ]