Gross Anatomy of the Urinary System

The proximal convoluted tubule (PCT) is the site where the vast majority of reabsorption occurs in the nephron. It reabsorbs approximately 65% of the water, sodium, and potassium from the filtrate, along with virtually 100% of glucose and amino acids.

Filtrate is formed in the glomerular (Bowman's) capsule and then flows sequentially through the proximal convoluted tubule (PCT), down and up the loop of Henle, and finally through the distal convoluted tubule (DCT) before entering the collecting duct.

The kidneys are located retroperitoneally in the superior lumbar region. They are protected from physical impact and mechanical trauma by the lower ribs of the ribcage and thick surrounding layers of adipose tissue (fat).

Urine is produced in the kidneys, travels down the ureters to be stored in the urinary bladder, and is then expelled through the urethra and out of the body via the urethral opening.

The kidneys are located retroperitoneally, which means they lie behind the parietal peritoneum against the posterior (dorsal) abdominal wall in the superior lumbar region.

Micturition (urination) requires the relaxation of both the internal urethral sphincter (which is involuntary and made of smooth muscle) and the external urethral sphincter (which is voluntary and made of skeletal muscle).

The urinary bladder features three openings that outline a triangular region called the trigone. These include two openings where the ureters enter from the kidneys, and one opening where the urethra exits to drain the bladder.

The primary function of the urinary bladder is to serve as a temporary storage reservoir for urine before it is eliminated from the body. It does not regulate the kidneys' output or recycle salts back into the blood.

Nephrons are the microscopic structural and functional units of the kidneys. Each kidney contains about one million nephrons, which perform the essential tasks of filtration, reabsorption, and secretion to form urine.

The right kidney is positioned slightly lower than the left kidney because the large, heavy liver occupies a significant amount of space in the upper right quadrant of the abdominal cavity, pushing the right kidney downward.

The outer region of the kidney is called the renal cortex. The inner region is the renal medulla, which contains the renal pyramids, and the renal pelvis is the flat, funnel-shaped tube continuous with the ureter.

The ureters are muscular tubes that transport urine from the renal pelvis of each kidney to the urinary bladder. The urethra, on the other hand, connects the bladder to the outside of the body.

An adrenal (or suprarenal) gland sits on top of each kidney. Although anatomically situated on the kidneys, these glands are functionally unrelated to excretion and belong to the endocrine system.

The renal arteries branch directly from the abdominal aorta to supply the kidneys with oxygenated blood, while the renal veins drain the filtered, deoxygenated blood back into the inferior vena cava.

The primary role of the kidneys is excretion, specifically filtering the blood to remove metabolic wastes, excess ions, and water-soluble toxins in the form of urine. While the liver processes toxins, it does not directly excrete urine.