The Renal System.

• Kidney.
  -Produces urine.
• Ureter.
  -Transports urine toward the urinary bladder.
• Urinary bladder.
  -Temporarily stores urine prior to elimination.
• Urethra.
  -Conducts urine to exterior.

• Anatomy of the kidney.
  -Kidneys are retroperitoneal (left kidney superior to right).
  -Fibrous capsule surrounds the kidney.
  -renal artery and renal nerve enter at hilum.
  -Renal vein and ureter exit at hilum.
• Internal anatomy of the kidney.
  -Nephrons.
    -Urine produced by nephrons.
    -About a million nephrons per kidney.
  -Renal pelvis.
    -Urine collects here from calyces.
    -Input from two major calyces. Each major calyx is fed by four to five minor calyces.
  -Urine leaves pelvis to ureter.

The urinary stystem removes most of the physiological wastes from the body and produces urine. The goal of urine production is to maintain homeostasis by regulating the volume and composition of blood.

• Excretion.
  -The removal of organic waste products from body fluids. Metabolic wastes, urea, creatinine and uric acid.
• Homeostatic regulation of blood plasma.
  -Regulating blood volume and pressure (adjust water loss).
  -Regulating plasma ion concentrations.
  -Stabilizing blood pH.
  -Conserving nutrients.
• Elimination.
  -The discharge of waste products into the envrionment.
• Produces hormones.
  -Erythropoietin.
  -Calcitrol.
  -Detoxify superoxides, free radicals and drugs.
  -Produces the enzyme renin.

The nephron is the functional unit of the kidney where all the functions of the system take place. Each nephronis made up of a renal capsule consisting of the glomerular (Bowman's) capsule and a capillary network known as glomerulus. It aslo has a renal tubule known as the glomerulus. It is a long tubular structure that may be 50mm in length.There are two types of nephron:

• Cortical nephrons.
  -~85% of all nephrons.
  -Located in the cortex.
• Juxtamedullary nephrons.
  -Closer to the renal medulla.
  -Loops of Henle extend deep into renal pyramids.

Nephron functions include:

• Production of filtrate.
• Reabsorption of organic nutrients, water and ions.
• Secretion of waste products into tubular fluid.

• Glomerulus.
  -Ball of capillaries involved in filtration of blood- keeps large particles (blood cells, large proteins) out of the filtrate (filtarte becomes urine in the collecting tubule).
• Renal tubules.
  -Involved in reabsorption (putting needed substances back into the blood) and secretion (movinf wastes from blood to the filtrate to be excreted in urine.

Glomerular filtration.

• Occurs as fluids move across the glomerulus.
• Blood pressure at the glomerulus forces water and solutes out of the blood stream and into the capsular space.
• Filtration pressure in the glomerulus is higher than capillary blood pressure because of differences in diameter of afferent and efferent arterioles.
• Blood pressure forces fluid and dissolved substances across the endothelial wall of glomerular capillaries into the capsular space.
• Glomerular filtration rate (GFR) depends on blood pressure and any change in blood pressure effects filtration.

Roughly 180 litres of filtrate is produced at the glomeruli each day. That represents 70 times the total plasma volume. Almost all of that fluid volume must be reabsorbed to avoid fatal dehydration.

• Renin release.
  -Decline in filtration pressure (BP) triggers renin release (released by juxtaglomerular apparatus).
  -Renin leads to increased blood volume and blood pressure.
  -GFR returns to normal.
  -Restores homeostasis.

Reabsorption:

• The removal of water and solutes from the filtrate.
• Most of the reabsorbed material are nutriantsthe body can use. The reabsorbtion substances eventually reenter the blood. Reabsorption can take place by simple diffusion or by activity of carrier proteins. Reabsorption of water takes place through osmosis.

Secretion:

The blood entering the peritubular capillaries still contains some undesirable substances that did not cross the filtration membrane at the glomerulus. If the concentration of these substances becomes too high, the tubular cells will absorb them from the peritubular fluid and secret them onto the tubular fluid. Some substances that are secreted are K+, H+, creatinine and ammonia.

PCT:

• Reaborbs critical substances from filtrate e.g.ions, proteins, nutriants (glucose,amino acids).
• Releases them into surrounding interstitial fluid.
• Enter peritubular capillaries and return to blood stream.

LOH:

• Reclaims water, ions from filtrate.
• Ascending limb pumps out sodium and chloride ions, descending limbs absorbs water.
• Interstitial osmotic pressure pulls water from tubular fluid.
• Urea concentration rises as fluid is lost,

DCT:

• Perfoms final adjustment
  -Secretes and reabsorbs ions, reabsorbs sodium in exchange for secreted potassium and     hydrogen ions. Aldosterone increases sodium reabsorption and potassium loss,
• Juxtaglomerular apparatus- at start of DCT (releases renin, erythropoietin).

ADH:

• Regulates water loss.
  -DCT, collecting tubule, collecting duct made permeable to water by ADH.
• Water then exits to interstitial fluid and remains in the body.
• Urine becomes concentrated (low in water, high in solute).
• The higher the ADH level, the more concentrated the urine.
• Concentration of urine.
  -ADH enhances the reabsorption of water in collecting ducts.
  -Triggered when blood pressure and volume are low.
  -Makes urin very concentrated. Urine is dilute when ADH is not present.

Aldosterone:

• Released from the adrenal glands on top of the kidneys.
• Leads to conservation of sodium (and sometimes water) and the excretion of potassium.
• Raises blood pressure by increasing blood volume.

• Renin- enzyme secreted by juxtaglomerular cells in the kidneys in response to low blood pressure/volume.
• Leads to cascade of reactions that results in production of angiotensin II, a vasoconstrictor.
• Angiotensin II stiumulates aldosterone production from adrenal glands, increasing blood pressure.

System;

• Low blood pressure causes renin release.
• This causes the production of angiotensinogen, which then turns into Angiotensin I and Angiotensin II (this production can be stopped by Angiotensin Converting Enzyme).
• Angiotensin II is a vasoconstrictor that also stimulates Aldosterone production from the adrenal glands which helps salt retention.
• This then increases blood pressure.

• Filtrate modification and urine production end when fluid enters the renal pelvis.
• The ureters, urinary bladder and urethra are responsible for the transport, storage and elimination of urine.
• The ureters are a pair of muscular tubes that conduct urine from the kidneys to the urinary bladder.
• The urinary bladder is a hollow, muscular organ that stores urine prior to micturation.
• The middle layer of the bladder wall is made up of inner and outer layers of longitudenal smooth muscle with a circular layer in between. The three layers of smooth muscle for the powerful detrusor muscle of the bladder which contracts on urination to expel urine from the bladder.

• Holds urine (made of smooth muscle).
• Stretch receptors in the wall of the bladder trigger the urge to urinate (occurs after about 200ml of urine collect). The maximum capacity is about 800ml-1000ml.
• Micturition (urination) occurs when it is convenient to go.