Urinary System

The urinary system consists of two kidneys, two ureters, a bladder, and a urethra.
The kidneys are located below the diaphragm at the top of the abdomen.
The kidney consists of three main regions:
- Cortex: The outer layer.
- Medulla: The middle region.
- Pelvis: The inner region that collects urine for the ureter.
Renal Arteries: Branch from the aorta to carry oxygenated blood rich in waste into the kidneys.
Renal Vein: Carries purified blood (waste-free) away from the kidneys.

Excretion: Removal of metabolic waste products from the body.
Osmoregulation: Controlling the balance of water and salts in bodily fluids.
pH Control: Regulating blood pH to maintain a normal value of $7.4$ .

Urine Typical Composition: $96\%$ water, $2.5\%$ nitrogenous waste (primarily urea), and $1.5\%$ salts.
Urea Production: Produced in the liver through the deamination of excess proteins.
Urine Flow: Pelvis $→$ Ureters $→$ Bladder (storage controlled by two sphincter muscles) $→$ Urethra.
Formation involves two main processes: Filtration (water and small molecules entering the kidney under high pressure) and Reabsorption (useful substances returning to the blood).

The nephron is the functional unit of the kidney, measuring approximately $3\,cm$ in length.
Bowman’s Capsule: Located in the cortex; contains the Glomerulus (a cluster of capillaries).
Filtration Process: Occurs in the glomerulus. High pressure forces water, glucose, amino acids, vitamins, urea, and salts into the capsule to form glomerular filtrate.
Exclusions: Red blood cells, white blood cells, platelets, and large proteins (antibodies, clotting proteins) remain in the blood because they are too large to pass through the capillary walls.

Approximately $99\%$ of the glomerular filtrate is reabsorbed.
Proximal Convoluted Tubule: Reabsorbs most water (osmosis), glucose, amino acids, and vitamins (active transport and diffusion).
Loop of Henle:
- Descending Limb: Reabsorbs a small amount of water.
- Ascending Limb: Impermeable to water but reabsorbs salts.
Distal Convoluted Tubule and Collecting Duct: Final reabsorption of water (via osmosis) and salts.

Anti-diuretic Hormone (ADH): Produced/stored in the pituitary gland; controls the volume of urine produced.
High Plasma Concentration (Dehydration/high salt): Pituitary releases ADH $→$ increased permeability of the distal tubules and collecting ducts $→$ more water reabsorbed $→$ low volume of concentrated urine.
Low Plasma Concentration (High water intake): ADH is not released $→$ tubules remain relatively impermeable to water $→$ less water reabsorbed $→$ large volume of dilute urine.

What organ in the body forms urea? Urea is produced in the liver when excess proteins are broken down (deaminated).
How does urine travel from the kidneys to the bladder? Urine is carried by the two ureters to the bladder.
Is it normal for urine to contain protein? No, proteins are too large to be filtered into Bowman’s capsule.
Is it normal for urine to contain red blood cells, white blood cells or platelets? No, these are too large to be filtered.
Is it normal for urine to contain glucose or amino acids? No, because they are normally reabsorbed back into the blood.
How is the glomerulus adapted for filtration? High pressure, large surface area of capillaries, porous capillary walls, and thin capsule walls.
How is the proximal convoluted tubule adapted for reabsorption? It is thin (one cell thick), long with many infoldings, and has many mitochondria for active transport.