Human Urinary System: Anatomy and Urine Formation Explained

The Human Urinary System

The urinary system consists of two kidneys, two ureters, the bladder, and the urethra.

Kidney Anatomy

• Cortex: The outer zone of the kidney. It has a pale red, granular appearance because it contains the Malpighian corpuscles and the distal nephron tubules.
• Medulla: The inner, dark red area of the kidney, shaped into fluted structures called Pyramids of Malpighi.

The Nephron

The nephron is the functional anatomical unit of the kidney. Nephrons are small tubes distributed throughout the cortical and medullary areas, which are highly vascularized.

Glomerulus

The glomerulus is a specific structure responsible for filtration. It consists of a ball of capillaries surrounded by the first segment of the renal tubule, known as Bowman's capsule. This set of capillaries and Bowman's capsule is called a Malpighian corpuscle.

Renal Tubules

The cavity of Bowman's capsule continues into a long, winding tubule, divided into:

• Proximal convoluted tubule: A continuation of Bowman's capsule that follows a tortuous path in the cortex before entering the medulla.
• Loop of Henle: The segment between the proximal and distal tubules, consisting of a descending and an ascending limb. The descending limb travels into the medulla, while the ascending limb returns to the cortex to connect with the distal tubule.
• Distal convoluted tubule and collecting duct: The distal tubule is located in the cortex and follows a tortuous path, finally leading to the collecting tubule. The latter gathers filtrate from several nephrons and leads to the renal papilla.

Formation of Urine

The system facilitates the elimination of metabolic wastes such as uric acid, urea, and creatinine.

Glomerular Filtration

Approximately 1250 cc of blood passes through the kidneys every minute, representing a quarter of the cardiac output. This blood reaches the glomerulus via the afferent arteriole, branches into capillaries, and exits through the efferent arteriole. Due to high resistance at the glomerular level, blood is filtered, allowing substances like urea, glucose, amino acids, small proteins, salts, and water to pass into Bowman's capsule. This glomerular filtration rate is an entirely passive process.

Tubular Reabsorption

During filtration, the blood is cleansed of toxic waste but also stripped of essential nutrients. Tubular reabsorption is the vital process that recovers these useful substances through active transport or passive diffusion.

Tubular Secretion

Substances that were not reabsorbed are sent from the bloodstream into the renal tubule for excretion.

Lower Urinary Tract

• Ureters: Two muscular tubes that connect the renal pelvis to the bladder.
• Bladder: A smooth muscular organ that stores urine, adapting to changes in volume.
• Urethra: A duct extending from the underside of the bladder that carries urine to the outside of the body. The external opening is called the urethral meatus.