Excretory System

Excretion rids the body of metabolic wastes

4 Excretory Organs: 1. Skin–excretes perspiration (water, salt & some urea) not to get rid of wastes as much as to cool us down

2. Liver excretes bile pigments, cholesterol and excess fat-soluble vitamins

3. Lungs excrete CO2

4. Kidneys produce urine (water, nitrogenous wastes, inorganic salts) urea is the primary nitrogenous waste in humans (produced by liver but excreted by kidneys) 4. Kidneys

Urea Production (by liver) Overall reaction of the urea cycle: O \\ 2 NH3 + CO2 --- H2N-C-NH2 + H2O ammonia urea (ammonia comes from the breakdown of amino acids)

Anatomy

Urinary System – Gross Anatomy a. Kidneys size of a fist produce urine

Kidney transplant in a cat

b. Ureters - muscular tubes - carry urine to bladder by peristalsis

c. Urinary Bladder - can hold up to 600 mL hollow, muscular, expandable organ contracts when emptying contains stretch receptors, telling you it is full

Bladder Cancer

d. Urethra passes urine to the outside short in females (4 cm); longer in males (20 cm) two sphincters control the flow of urine from the bladder

e. Renal Cortex outer layer surrounding renal medulla Site of filtration, tubular re-absorption and secretion

f. Renal Medulla cone-shaped tissue masses called renal pyramids Site of water re-absorption

g. Renal Pelvis central cavity that connects with the ureter

h. Nephrons (renal or kidney tubules) - over 1,000,000 / kidney several will be connected to the same collecting duct (enter the renal pelvis)

Blood Supply to the Nephron:

Blood Supply to the Nephron: The renal artery supplies the kidney from the renal artery, an afferent arteriole leads to the glomerulus (a knot of capillaries inside the glomerular capsule) (Bowman’s Capsule)

blood leaves the glomerulus via the efferent arteriole which joins the peritubular capillaries (surrounds the rest of the nephron)

Glomerular (Bowan’s) capsule Glomerlus Afferent arteriole Efferent arteriole Proximal convoluted tubule Ascending limb Decending limb Loop of Henle Peritubular capillary Distal convoluted tubule Collecting duct Renal medulla

Urine formation: Four steps: 1. Glomerular Filtration (in cortex) Blood enters the Glomerular capsule (Bowman’s capsule)

“filterable blood components” (water, nitrogenous wastes, nutrients, salts) leave the glomerulus and enter the capsule. (= glomerular filtrate)

Water and dissolved nutrients, salts and urea enter the capsule due to the glomerular blood pressure (about 60mmHg) large molecules and formed elements cannot enter the glomerulus (too big) and therefore, they leave the glomerulus via the efferent arteriole

- approximately 180 litres of water are filtered per day - only 1.8 liters of water are excreted

2. Tubular Re-absorption (in cortex) - molecules and ions are passively and actively reabsorbed from the nephron to the peritubular capillary

- Na+ is actively reabsorbed while Cl- follows passively - water will follow the NaCl (passively) to balance the osmolarity of the blood (approximately 60-70% of Na+ are reabsorbed at the proximal convoluted tubule)

- specific carrier molecules actively reabsorb specific nutrients (glucose, amino acids, vitamins) ex. Glucose - there are MANY glucose carrier molecules to ensure that glucose is not lost

patients with uncontrolled diabetes mellitus have too much glucose in the blood, therefore, there will not be enough carrier molecules to reabsorb it all.

too much glucose left in the filtrate results in increased osmolarity in the filtrate compared to the blood, therefore, H2O will stay in the filtrate to “dilute” it. this accounts for the frequent urination and constant thirst of patients that have not controlled their diabetes

therefore, the proximal convoluted tubule reabsorbs most water, nutrients and required salts (ions) - things that are NOT reabsorbed are: some water, much nitrogenous waste, and excess salts (ions)

3. Tubular Secretion (in cortex) a second way that substances are removed from the blood and added to the tubular fluids

- H+, ammonium ions, creatinine, and drugs (ex - H+, ammonium ions, creatinine, and drugs (ex. Penicillin) move (actively) from the blood in the peritubular capillary to the distal convoluted tubule

4. Re-absorption of Water: - dependent mostly on the loop of Henle and the collecting duct (in the MEDULLA of the kidney)

- The loop of Henle sets up an osmotic gradient by actively pumping NaCl into the extracellular fluid surrounding the loop in the MEDULLA

- the concentration gradient gets greater as the loop travels deeper into the medulla - as the filtrate travels down the descending loop, water diffuses out because of the artificial gradient in the medulla

- the descending loop is impermeable to salts, therefore the concentration of salts begins to build up inside the tubule as you travel down the descending loop

- the thin portion of the ascending loop is permeable to salts, but impermeable to water and urea. Therefore, salt will diffuse out of the tubule into the extracellular space.

- this contributes to the artificial concentration gradient of salts in the medulla - urea also diffuses into the extracellular space near the end of the loop, but it comes from the collecting duct that runs parallel to the loop

(The extra salt and urea at this end of the tubule are what caused the water to diffuse out from the descending loop)

- as the filtrate reaches the large portion of the ascending loop, salt must be actively pumped out.

salt is also actively pumped out as the ascending loop goes through the cortex, and becomes the distal tubule. Water will follow the salt out of the distal tubule because now, the distal tubule is permeable to water.

[ at the distal tubule, ammonia, H+, K+, and drugs are actively pumped INTO the tubule from the efferent arteriole (= tubular secretion)]

Tubular secretion of H+ is important in maintaining control of the pH of the blood * When the pH of the blood starts to drop, more hydrogen ions are secreted. * If the blood should become too alkaline, secretion of H+ is reduced.

Finally, the last bit of water can be reabsorbed if necessary as the collecting duct passes through the region of the medulla that is very concentrated in salts.

Regulatory Functions of the Kidney: 1. Antidiuretic hormone (ADH) (secreted from the posterior pituitary) controls how much (or how little) water is reabsorbed at the distal convoluted tubule and the collecting duct.

If the blood solute concentration needs to be decreased, ADH is produced; this makes the collecting duct and the distal tubule VERY permeable to water. (increases blood volume)

If the blood solute concentration needs to be increased, ADH is not produced; the absence of ADH makes the collecting duct and the distal tubule less permeable to water. (decreases blood volume)

2. Renin-Angiotensin-Aldosterone Homeostatic System When blood volume (and therefore blood pressure) is too low to promote glomerular filtration, the “juxtaglomerular apparatus” (in the kidneys) secretes renin.

renin (an enzyme) changes angiotensinogen (produced by the liver) into angiotensin I. angiotensin I gets converted into angiotensin II angiotensin II is a strong vasoconstrictor and also stimulates the adrenal cortex to release aldosterone.

Aldosterone (a hormone) promotes the reabsorption of sodium ions, which causes more water to reabsorbed. Therefore, aldosterone (from the adrenal cortex) acts to increase blood volume and blood pressure.

3. Atrial natriuretic hormone (ANH) - a hormone secreted by the heart when stretch receptors in the atria indicate that the blood volume is too large. - ANH inhibits the secretion of renin and aldosterone 3. Atrial natriuretic hormone (ANH)

Therefore, ANH promotes the excretion of sodium ions, (which means that water will follow). (= decreasing blood volume and blood pressure.)

Diuretics - agents that increase the flow of urine Examples: Alcohol - inhibits the secretion of ADH Caffeine - increases glomerular filtration rate and decreases tubular re-absorption of sodium ions.

Some diuretic drugs prescribed to fight high blood pressure, inhibit the re-absorption of sodium ions, which decreases the blood pressure.