1. Urinary System and Excretion
Organs Urine Formation Homeostatic Mechanisms

2. Path of Urine through the Urinary System
Kidneys
produce urine
Ureters transport urine
Urinary bladder stores urine
Urethra passes urine to outside

3. Functions of the Urinary System
Excretion: removal of metabolic wastes
Urea: breakdown product of amino acids
Creatinine: breakdown product of creatine phosphate, a high energy molecule in muscles
Uric acid: breakdown product of nucleotides
Ammonium from amino acids used for energy

4. Structure of the Kidney
Renal Cortex: outer layer
Renal Medulla: inner layer
Renal Pelvis: central space
Nephrons are microscopic tubules that produce urine

5. Structure of the Nephron
Proximal convoluted tubule
Distal convoluted tubule
Glomerular capsule
Peritubular capillary network
Glomerulus (capillaries)
Collecting Duct
Descending limb
Ascending limb
Loop of the nephron

6. Urine Formation: Glomerular Filtration
Cells, cell fragments and proteins remain in the bloodstream
Formation of a filtrate containing some components from whole blood
Water and small molecules enter glomerular capsule from the glomerulus
Proteins, cell fragments and cells are excluded from the filtrate
Water, nutrients, salts, nitrogenous wastes enter nephron

7. Urine Formation: Tubular Reabsorption
Water, nutrients, salt reenter the bloodstream
Molecules and ions are reabsorbed into the blood, either actively or passively
Reabsorption occurs from the proximal convoluted tubule into the peritubular capillary network
Water, nutrients and salts are reabsorbed into the bloodstream
Nitrogenous waste, excess salts, some water remain in nephron
Sodium ions are actively reabsorbed, chloride ions follow passively. This increase in NaCl increases the salt concentration of the blood, so water moves passively into the blood.
Nutrients such as glucose and amino acids return to the blood by a selective process, they must be recognized by carrier molecules in order to be actively absorbed. Glucose is ordinarily completely reabsorbed due to a large quantity of carrier molecules. In diabetes, the kidneys cannot absorb the excess levels of glucose in the blood so glucose appears in the urine.
Caffeine increases glomerular filtration rate and decreases tubular reabsorption of Na.
Peritubular capillary network

8. Urine Formation: Tubular Secretion
H+, creatinine, drugs enter the nephron
Substances are actively transported from the blood (peritubular capillary network) into the kidney tubule
Involves transport of hydrogen ions, creatinine, penicillin and other drugs
Peritubular capillary network
In the end, urine contains substances that have undergone glomerular filtration but have not been reabsorbed, and substances that have undergone tubular secretion.

9. Urine Formation: Water Reabsorption
Water is reabsorbed into the bloodstream from all parts of the nephron, and the collecting duct
Occurs by osmosis through aquaporins, membrane channel proteins
Controlled by
Reabsorption of salt
Gradient of NaCl in medulla tissue surrounding the loop
ADH, antidiuretic hormone, released from pituitary when water intake is low, stimulates aquaporin activity in distal convoluted tubule and collecting duct
release is inhibited by alcohol
Descending limb: permeable to water but less permeable to Na and Cl
Thick ascending limb: actively transports Cl, with Na following passively, from tubule into the surrounding tissue fluid. Not permeable to water. NaCL concentration increases in tissue fluid
Thin ascending limb: allows Na and Cl to move passively into surrounding tissue fluid; not permeable to water
ADH is released by posterior lobe of pituitary and increases the permeability of the collecting duct.
In presence of ADH, more water is reabsorbed at collecting duct and urine volume decreases.
If not drinking much, ADH released to retain water. If drinking excess water, ADH is not released and a greater volume of urine is produced.
Alcohol inhibits the secretion of ADH.
Diuretic drugs inhibit active transport of Na+ in the loop of the nephron. These are used to counteract high blood pressure.
Concentration Gradient formed by diffusion and active transport of NaCl in ascending limb
Descending limb and collecting duct are permeable to water
Reabsorption causes increased concentration of urine compared to blood plasma
Urine has a lower water concentration than blood

10. Urine Formation: Salt Reabsorption
Sodium: >99% reabsorbed
reabsorption in PCT, DCT, collecting duct
active transport in ascending limb of loop of nephron
Other reabsorbed or excreted ions
potassium ions
bicarbonate ions
magnesium ions

11. Hormonal Control of Sodium Reabsorption
If blood pressure decreases
Angiotensinogen
Angiotensin I
Renin
secretes
Angiotensin II
Aldosterone (from adrenal glands)
stimulates secretion of
constricts blood vessels
Juxtaglomerular apparatus is a region of contact between the afferent arteriole and the distal convoluted tubule. When blood volume, and therefore blood pressure, is not sufficient to promote glomerular filtration, the JGA secretes renin. Renin is an enzyme that changes angiotensinogen (a large plasma protein produced by the liver) into angiotensin I. Later, angiotensin I is converted to antiotensin II, a powerful vasoconstrictor that also stimulates the adrenal cortex to release aldosterone. Aldosterone promotes the excretion of potassium ions and the reabsorption of sodium ions. The reabsorption of sodium ions is followed by the reabsorption of water. Therefore, blood volume and blood pressure increase.
Stimulates sodium reabsorption from DCT,
water follows sodium

12. Hormonal Control of Sodium Excretion
If blood pressure increases
prevents renin secretion
Angiotensinogen
Angiotensin I X
Renin
secretes
inhibition of secretion X
Angiotensin II
Heart atrial cells
Atrial Natriuretic
Hormone (ANH)
secrete
Juxtaglomerular apparatus is a region of contact between the afferent arteriole and the distal convoluted tubule. When blood volume, and therefore blood pressure, is not sufficient to promote glomerular filtration, the JGA secretes renin. Renin is an enzyme that changes angiotensinogen (a large plasma protein produced by the liver) into angiotensin I. Later, angiotensin I is converted to antiotensin II, a powerful vasoconstrictor that also stimulates the adrenal cortex to release aldosterone. Aldosterone promotes the excretion of potassium ions and the reabsorption of sodium ions. The reabsorption of sodium ions is followed by the reabsorption of water. Therefore, blood volume and blood pressure increase.
Sodium is excreted in urine, water follows
Aldosterone (from adrenal glands)

13. Functions of the Urinary System
Maintenance of Water-Salt Balance important for blood pressure and blood volume
Water reabsorption
Loop of the Nephron
Collecting Duct
Sodium reabsorption or excretion
Proximal convoluted tubule
Distal convoluted tubule

14. Functions of the Urinary System
Maintenance of Acid-Base Balance
Blood pH is controlled by the bicarbonate buffering system
Buffer: substance that resists pH change
When lungs release carbon dioxide, H+ has been used to produce water, so pH remains about neutral (7.4)
bicarbonate ion
hydrogen ion
carbonic acid
carbon dioxide
water
When blood pH decreases, chemoreceptors in the carotid bodies (in carotid arteries) and in aortic bodies (aorta) stimulate the respiratory center and the rate and depth of breathing increase. When blood pH begins to rise, the respiratory center is depressed and the level of bricarbonate ions increases in the blood.

15. Functions of the Urinary System
Maintenance of Acid-Base Balance
Kidney
excretes excess hydrogen ions (H+)
reabsorbs bicarbonate ions (HCO3-)

16. Functions of the Urinary System
Assistance with Hormone Release and Secretion of Hormones
Renin (enzyme) influences secretion of aldosterone from adrenal glands located at top of kidneys
Aldosterone regulates water/salt balance of blood by influencing ion exchange at the distal convoluted tubule
Erythropoietin: stimulates red blood cell production

17. Functions of the Urinary System
Assisting other body systems
Kidneys regulate the level of calcium ions in blood
Convert Vitamin D to a form that assists with calcium absorption
Regulate the excretion of calcium
Kidneys regulate the sodium and potassium ions in blood, important for nerve conduction, heart and skeletal muscle contraction