Urinary System

Exercises 40 and 41

Organs of the Urinary System

• 2 kidneys

• Urinary bladder

• 2 ureters

• Urethra

• Primary function is to filter blood, and reabsorb water and needed substances back into blood. Remaining water and wastes are removed in urine.

• Regulate blood volume and composition

• Regulate blood pH

• Regulate blood pressure

• Contribute to metabolism– Calcitriol– Erythropoietin– Gluconeogenesis

Location of kidneys

• Between T12 and L3

• Partially protected by 11th and 12th ribs

• Right kidney is lower than left

• Kidneys and ureters are retroperitoneal

• Protected by three layers:– Renal capsule– Adipose capsule– Renal fascia

Anatomy

• Renal hilum – ureter, blood and lymphatic vessels

• Renal sinus

• Renal cortex

• Renal medulla– Renal pyramids– Renal papilla

• Renal columns

Nephron

• Functional unit of kidney

• Number constant from birth

• Can function on 2/3 of one kidney

• Forms the urine

Duct system

• Minor calyces

• Major calyces

• Renal pelvis

• Ureter

• Urine is not changed after it leaves the collecting duct

• Kidneys = 1% of body mass

• Receive 15-30 % of blood flow

• Blood supplied to kidney by renal artery

• Afferent arteriole divides into ball shaped capillary network called the glomerulus.

• Capillaries rejoin to form efferent arteriole, then peritubular capillaries

• Vasa recta

Functions of the Nephron

• Filtration

• Tubular reabsorption

• Tubular secretion

Structure of a nephron

• Renal corpuscle– Glomerulus– Glomerular capsule or Bowman’s capsule

• Renal tubule– Proximal convoluted tubule– Nephron loop or loop of Henle– Distal convoluted tubule– Collecting duct

Bowman’s capsule

• Outer layer - simple squamous epithelium

• Inner layer – modified simple squamous epi.– Cells called podocytes– Projections are pedicels

• Endothelium of capillary and podocytes form the endothelial-capsular (filtration) membrane.

Filtration membrane

1. Fenestrated endothelium – prevents the passage of cells

2. Basement membrane of glomerulus – prevents passage of large proteins

3. Slit membrane (between pedicels) – prevents the passage of medium sized proteins

• Wall of renal tubules made up of single layer of epithelial cells and a basement membrane.

• Proximal convoluted tubule – cells are cuboidal and have microvilli.

• About 65 % of water and 100 % of some solutes are reabsorbed here.

Renal Physiology

• Glomerular filtration – most substances easily filtered

• Filtrate = 180 L/ day but urine = 1-2 L /day

• Filter is thin

• Glomerular capillaries are long

• Capillary blood pressure is high(afferent arteriole diameter > efferent arteriole)

• Filtration fraction is the percentage of plasma that becomes filtrate, usually 16-20 %.

• Glomerular Filtration Rate (GFR) is the amount of filtrate formed in both kidneys per minute; normally about 125 ml / min.

Renal autoregulation

• JGA senses Na+, Cl- and water

• When these ↓, JGA dilates afferent arteriole, ↑ blood pressure in glomerulus

Hormonal regulation

• When BP ↓, JGA secretes renin which converts angiotensinogen to angiotensin I, angiotensin I is converted to angiotensin II in the lungs. Angiotensin II is a potent vasoconstrictor. It also stimulates the release of aldosterone, and stimulates the thirst center in the hypothalamus and release of ADH from the posterior pituitary gland.

Tubular reabsorption

• Volume of fluid that enters the proximal convoluted tubule in ½ an hour > total plasma volume.

• Reabsorb water, glucose, amino acids, urea and ions, esp. Na+, Cl- and K+

• Na+ diffuses into cells, but is actively pumped out into interstitial fluid

• Promotes reabsorption of water

DCT and collecting ducts• By end of DCT 90 % of water and solutes

reabsorbed.

• Aldosterone causes principal cells to synthesize sodium pumps. Also causes potassium to be secreted into urine.

• ADH stimulates cells to put water channels into membranes. ADH is responsible for facultative (responding to need) water reabsorption.

Tubular secretion

• Two main functions:

• Secretion of substances to eliminate them from body – ions, nitrogen-containing waste (urea, creatinine) and certain drugs

• Secretion of H+ helps to maintain blood pH

Urinary bladder

• Posterior to pubic symphysis

• Stores urine prior to micturition

• Trigone – between openings of ureters and internal urethral orifice – mucosa is firmly bound to muscularis.

• Muscosa, submucosa, muscularis (detrusor muscle), and serous coat.

Evaluation of kidney function

• Blood Urea Nitrogen (BUN) – urea is a breakdown product of proteins– Influenced by diet

• Creatinine – end product of muscle metabolism– Normally constant– Can determine creatinine clearance

Normal urine

• Transparent yellow

• Contains ions : Na+, Cl-, K+ ; nitrogenous wastes: ammonia, creatinine, urea and uric acid; and foreign chemicals: drugs, pesticides, food additives, etc.

• Suspended solids in trace amounts: bacteria, blood cells, casts.

Not normally present

• Albumin

• Glucose, Acetone– diabetes mellitus

• Bile – liver disease, obstruction of bile ducts, hemolytic disease

• pH 4.6 -8.0

Specific gravity• 1.001 -1.035

• Damage to tubules , s.g. fixed at 1.010 = filtrate

• Diabetes insipidus (lack of ADH) - – 1.003

• Diabetes mellitus – “sugar diabetes”– 1.030 osmotic diuresis

• Emesis or fever –– 1.040