Urinary System

Urinary System Function

• The function of the urinary system is to help maintain the appropriate balance of water and solutes in the bodies fluids

Urinary System:General Anatomy

• The urinary system is composed of the

1. Kidneys involved in chemical balance

2. Ureters that channels urine to the bladder

3. Urinary Bladder that stores urine

4. Urethra that channels urine to the body surface

Kidney Anatomy

• Each kidney has a generous blood supply distributed to both the external cortex and internal medulla where its concentrations are manipulated in nephrons, the resulting urine filling the renal pelvis

Nephron Anatomy

• The kidney is composed of over 1 million separate nephrons

• The nephron is composed of a Bowman’s capsule, proximal tubule, loop of Henle, distal tubule, and collecting duct

• A blood capillary forms a bulbous glomerulus just inside Bowman’s capsule and then descends into the peritubular capillaries that surround the nephron tubules

The Nephron

Nephron Physiology: General

• The nephron is the site of 1. Filtration in Bowman’s capsule2. Secretion and re-absorption in the

proximal tubule3. Changes in NaCl and Water

permeability in the Loop of Henle4. Secretion in the distal tubule5. Variable re-absorption of water in the

collecting duct

Filtration

• Each minute 120 ml of water and solutes are filtered from the glomerular capillaries into the Bowman’s capsules

• The filtrate does not contain large molecules like proteins but can contain amino acids and glucose.

• Salt and Water are filtered (important in osmolarity)

• Urea and H+ are also filtered (important to Nitrogen waste and pH balance)

Proximal Tubule

• Important organic molecules are reabsorbed.

• Na+ are actively transported and Cl- and water follow by diffusion and osmosis.

• Bicarbonate is re-absorbed to aid in pH buffering.

• Continues with a concentrated filtrate of H+, urea, and ammonia.

Proximal Tubule - Descending

• As the filtrate descends it re-absorption of water continues.

• The nephron is impermeable to NaCl though.

• So, the filtrate becomes more concentrated with solutes (increase in osmolarity)

Loop of Henle and Ascending Tubule

• In the Loop of Henle the nephron becomes impermeable to water but permeable to NaCl.

• This helps in developing the hyperosmolar concentration of the interstitial fluid of the medulla.

• In the Distal Tubule the NaCl is actively transported in the interstitial fluid.

• The filtrates osmolarity decreases as salt is removed and it become relatively dilute because water can’t leave.

Distal Tubule & Collecting Duct

• In the distal tubule, K+ ions are variably secreted while NaCl variably re-absorbed.

• The secretion of H+ and re-absorption of bicarbonate occurs here too.

• In the collecting duct the NaCl is re-absorbed, while water’s permeability is controlled by hormones.

Antidiuretic Hormone

• When extracellular volume decreases (osmolarity increases)

1. The hypothalamus triggers the secretion of ADH from the post. pituitary

2. ADH increases the permeability of the distal tubules to water (so re-absorbs more)

3. Urine becomes more concentrated.

Aldosterone

• When extracellular volume decreases resulting in a lowering in blood pressure.

1. Sensors in the heart and blood vessels of the kidney signal cells in the glomerulus which secretes renin.

2. Renin activates the plasma protein Angiotensin II which triggers the adrenal cortex to secrete Aldosterone.

3. Aldosterone acts on distal tubule so that they reabsorb Na+ and promote the reabsorption of water.

4. This cause the blood pressure to increase.