Nephrology: Glomerular Filtration Rate

The Urinary System

• The urinary system, also called the renal system, includes the kidneys, ureters, bladder, and urethra.
• The purpose of the urinary system is to remove excess fluid, waste products through producing and expelling urine, and performing endocrine functions and balancing water content.

Kidneys

• The kidneys are organs located on the dorsal side of the abdominal cavity, responsible for removing waste products from the blood in the form of urine, regulating the chemical composition of blood, producing hormones, producing the enzyme renin, and maintaining fluid, electrolyte, and acid-base balances.
• Kidneys are highly vascular, containing a great number of blood vessels.
• Each kidney comprises three regions: the renal cortex, the renal medulla, and the renal pelvis.

Renal Cortex

• The renal cortex is the outer layer of the kidney, containing blood-filtering mechanisms and protected by layers of fat and a fibrous capsule.

Renal Medulla

• The renal medulla is the innermost part of the kidney, split into different sections called renal pyramids.

Renal Pyramids

• Renal pyramids are cone-shaped tissues of the kidney that empty into minor calyces.
• Two to three minor calyces come together to form one major calyx.
• Renal pyramids secrete urine.

Renal Pelvis

• The renal pelvis is the innermost layer of the kidney, where urine is discharged before being funneled into the ureter.

The Nephron

• The nephron is the basic structural and functional unit of the kidney and the site of urine formation.
• Nephrons perform two main functions: the secretion and reabsorption of ions and the filtration of fluids, electrolytes, acids, bases, and waste products into the tubular system.

Glomerular Filtration

• The process by which the kidneys filter the blood is called glomerular filtration.
• Glomerular filtration takes place through the semipermeable walls of the glomerular capillaries and Bowman’s capsule.
• The glomerular hydrostatic pressure forces the blood to leave the glomerulus, resulting in filtration of blood.

Tubular Reabsorption and Secretion

• Selective reabsorption occurs in the proximal convoluted tubule, where useful substances including some water, electrolytes, and organic nutrients are reabsorbed from the filtrate back into the blood.
• In the Henle loop, water, sodium, and chloride are reabsorbed, and only 15-20% of the original filtrate reaches the distal convoluted tubule.
• In the collecting ducts, water is reabsorbed according to the body’s needs, and electrolytes are reabsorbed, especially sodium.

Ureters, Bladder, and Urethra

• The ureters are bro-muscular ducts that connect the kidneys to the bladder, transporting urine through peristaltic waves.
• The bladder is a muscular organ that stores urine, with a trigone region containing three openings.
• The urethra is the small duct by which urine passes from the bladder to the outside of the body.