Kidney Anatomy and Urine Production Process
Kidney Anatomy and Structure
Kidneys are paired, bean-shaped organs located on the back of the abdomen, above the waist, and on both sides of the spine. Renal arteries supply blood to the kidneys, renal veins carry blood away, and ureters transport urine to the urinary bladder. From the outside, the kidney shows the following areas:
- Renal Capsule: A fibrous, clear membrane that surrounds and protects the kidney.
- Renal Cortex: The outer, dark red zone, composed of connective tissue.
- Renal Medulla: Lighter in color than the cortex, it is composed of pyramidal structures called Malpighian pyramids. The tip of each pyramid features a renal papilla, which flows into the renal calyx.
- Renal Pelvis: A funnel-shaped structure that collects urine from all calyces.
The Nephron: Kidney’s Functional Unit
The nephron is the functional and structural unit of the kidney. Each kidney contains approximately one million nephrons, distributed between the renal cortex and medulla. These long, coiled, tube-shaped structures have distinct areas, including:
- Bowman’s Capsule: The cup-shaped end of the nephron, enclosing a network of capillaries called the glomerulus. An afferent arteriole supplies blood to the glomerulus, and an efferent arteriole carries blood away. This is where blood filtration into the nephron begins.
- Proximal Convoluted Tubule: A coiled tube located below Bowman’s capsule in the kidney cortex.
- Loop of Henle: A narrowed area of the proximal tubule, with a descending branch that enters the kidney medulla and rotates 180 degrees to return to the cortex via the ascending branch. It is crucial for reabsorption.
- Distal Convoluted Tubule: Located after the loop of Henle, it connects with the collecting duct, which receives tubules from other nephrons. It eventually empties into the renal pelvis.
Components of the Urinary Tract
Ureters
Two tubes that emerge from each kidney and transport urine to the bladder.
Bladder
A hollow, muscular organ that stores urine until it is expelled from the body. Urine is retained in the bladder by two sphincters located at its base.
Urethra
The tube that connects the bladder to the outside of the body.
Urine Formation Process
Urine formation occurs in three main phases and is regulated by two key hormones: ADH (antidiuretic hormone or vasopressin), secreted by the pituitary gland, and aldosterone, secreted by the adrenal cortex.
Glomerular Filtration
This process occurs in Bowman’s capsule, involving the passage of blood from the glomerulus into the nephron’s interior. Filtration allows substances with a sufficiently small size to pass from the blood. Blood cells and most plasma proteins do not pass through. The resulting filtrate has a composition similar to plasma, containing glucose, amino acids, vitamins, minerals, water, urea, uric acid, and creatinine. Approximately 125 mL/min are filtered.
Tubular Reabsorption
This process takes place in the nephron’s tubules (proximal and distal convoluted tubules, and the loop of Henle), from the surrounding capillary network. These capillaries eventually drain into the renal vein, carrying blood cleansed of waste. Reabsorption occurs via passive diffusion or active transport.
- Proximal Convoluted Tubule: Recovers water, glucose, vitamins, urea, amino acids, ions of chloride, sodium, potassium, bicarbonate, phosphate, and any leaked low molecular weight proteins.
- Loop of Henle: Reabsorbs water, sodium, and chloride.
- Distal Convoluted Tubule: Reabsorbs water and sodium.
Of the filtered substances, approximately 124 mL/min are reabsorbed, meaning the kidneys produce about 1 mL of urine per minute.
Tubular Secretion
This process involves the movement of substances from the capillaries into the nephron’s interior. Ions such as potassium, creatinine, and certain drugs are secreted.
Urine Expulsion
The amount of urine excreted by the kidneys is regulated by both hormonal and nervous control. Urine passes from the collecting tubules into the renal pelvis, then through the ureters to reach the bladder. The bladder gradually increases in size to continuously receive urine. When full, it expands, and receptors in its walls detect this expansion, initiating rhythmic contractions. The internal sphincter opens automatically, while the external sphincter is voluntarily controlled, allowing the brain to regulate bladder emptying.