URINARY SYSTEM

Functions

• Excretory: Nitrogenous wastes, ions, toxins
• Regulatory: Renin, Erythropoietin, Vitamin D

Urinary System: Consists of:

1. Kidneys
2. Ureters
3. Urinary Bladder
4. Urethra

Kidney

Housed in fibrous capsule (adipose & fascia surround kidneys)

Sinus - area where ureter, nerves, and vessels collect

Outer renal cortex

Inner renal medulla

Pyramids (base--papilla)

Form lobes (8)

Columns in between pyramids

Kidneys are collection of nephrons -- About 1 million per kidney.

2 Types or Nephrons:

1. cortical: short loops of Henle
2. juxtamedullary: long loops of Henle -- more important for concentrating urine.

Nephrons

• Glomerulus: tuft of capillaries ..
• Glomerular (Bowman's) Capsule...
• Renal Corpuscle = Glomerulus and Glomerular Capsule.
• Tubules: Proximal convoluted tubule, Loop of Henle (descending & ascending loops), Distal convoluted tubule
• Collecting Duct: empties into renal pelvis

Juxtaglomerular Apparatus:

• Distal convoluted tubule interface with afferent arteriole
• Juxtaglomerular cells of arteriole sense blood pressure
• Macula densa cells of tubule are chemo- and osmo-receptors

Urine Formation

Urine formation consists of 3 processes:

1. Glomerular filtration

2. Tubular reabsorption

3. Tubular secretion

1) Glomerular Filtration

• 120-125 mls per minute of plasma filtrate formed
• 180 Liters formed per day
• Kidneys use about 20-25% of body's oxygen at rest
• Relatively non-selective - water and small ions pass through (Size and Electrical Charge Determine Permeability)
• Postively charged molecules pass thru more than negatively charged molecules.
• Water, glucose, amino acids, nitrogenous wastes (urea) pass
• Red blood cells and proteins don't pass through.

Filtration Barrier.

Net (effective) Filtration Pressure (NFP)

• Glomerular Hydrostatic Pressure (CHP)
• Glomerular Osmotic/Oncotic Pressure (GOP)
• Capsular Hydrostatic Pressure (CHP)
• NFP = GHP - (GOP + CHP) = 50 - (25 + 15) = 10 mm Hg

Maintaining blood pressure is important

• Low blood pressure - less filtration pressure
• High blood pressure - more filtration pressure

Arterioles leading to glomerulus can dilate and constrict.

Some autoregulation
 
Renin-angiotensin-aldosterone system RAAS

Low filtration flow --> macula densa cells sense slow flow or low sodium or chloride levels (low osmolarity) --> induce vasodilation. Vasoconstriction if opposite conditions exist.

Renin-angiotensin-aldosterone system. Angiotensin is a vaso-constrictor. Aldosterone leads to sodium retention.

Reduced stretch on JG cells by drop in blood pressure triggers release of renin.

Vasoconstriction and sodium retention increase blood pressure.

Sympathetic NS stimulates renin release

2) Tubular Reabsorption

• Kidney tubules reabsorb most of filtrate
• Active (ATP requiring) and passive processes

Proximal convoluted tubule

• Reabsorbs nutrients, electrolytes, and water.

Loop of Henle

• Descending reabsorbs water by osmosis
• Ascending reabsorbs sodium and chloride

Distal convoluted tubule

• Sodium actively transported (aldosterone dependent)
• Water via osmosis (ADH controls - increases permeability)

Collecting ducts

• Sodium actively transported
• Water moves via osmosis
• Antidiuretic Hormone (ADH) controls water permeability of collecting ducts

Tubular Secretion.

Selective excretion of certain substances: Hydrogen ions, potassium, ammonia, organic acids and bases, penicillin & histamine

Urine is concentrated by Countercurrent Multiplier System

Arrangement of loops of Henle creates an osmotic gradient
Urine produced can be hypo- or hyperosmotic. Osmolarity of plasma 300-330 mOsm. Seawater is about 1000-1500 mOsm. This is about as concentrated as we can make our urine.

Other animals:
Fresh water fish and amphibians live in hypotonic environment. They need to excrete a large amount of water, so they have short loops of Henle and produce dilute urine.
Saltwater fish and desert animals need to retain fluid, so they typically have long loops of Henle and produce very concentrated urine.Aldosterone and antidiuretic hormone regulate volume of urine.

Ureter: tube that transports urine from kidney to bladder

Bladder
Internal folds are called "rugae" (hey, that sounds familiar!). Allows for expansion.
Lining: transitional epithelium.

Urethra: Carries urine from bladder to the outside world.

Males: 3 regions. Urethra also transports semen.

1. Prostatic -- prostatic secretions enter urethra here.
2. Membranous -- passes thru urogenital diaphragm.
3. Penile (spongy)

Females: short, typically less that 4 cm long

Urination (micturition): Emptying of urinary bladder.

• Micturition Reflex is a "stretch reflex" that is mediated by the parasympathetic NS.
• Neurosensory loops involved in micturition reflex.
• Internal sphincter (smooth muscle, involuntary control); external sphincter (skeletal muscle, voluntary control)

Incontinence: inability to control urination

• Stress incontinence: losing urine due to a sudden increase in pressure on the bladder, such as from a cough, sneeze, or running down stairs.
  

There are other forms also. Who has more of a problem with incontinence, men or women?