The Urinary System

The urinary system includes two kidneys, two ureters, the urinary bladder, and the urethra. Together, these organs remove wastes from the blood and help keep body conditions stable. The kidneys are bean-shaped and sit in the retroperitoneal space on either side of the vertebral column, between T12 and L3. They are partly protected by the rib cage and cushioned by perirenal fat.

The kidneys make urine and also help control blood pressure, electrolyte levels, and hormone production. Each ureter...

The kidneys are a pair of bean-shaped organs that help keep the body healthy. They filter waste from the blood, help regulate blood pressure, maintain electrolyte balance, and support red blood cell production.

The kidneys sit in the retroperitoneal space, which means behind the abdominal lining, on either side of the vertebral column. The 11th and 12th ribs protect them from behind. The right kidney lies slightly lower than the left because the liver is above it. In adults, the kidneys are...

The kidney is a vital organ that helps maintain homeostasis and overall health. It sits against the posterior abdominal wall, behind the abdominal cavity on either side of the spine. The kidneys are retroperitoneal organs. They are found roughly between the twelfth thoracic vertebra and the third lumbar vertebra, and each one is usually 10-12 cm long, 5-6 cm wide, 3-4 cm thick, and about 150 grams.

The outer region of the kidney is the renal cortex. It is a light-colored layer that surrounds...

The kidneys rely on a strong network of blood vessels and nerves to filter blood and keep body fluids balanced. They remove waste, regulate electrolytes, and depend on steady circulation to do this work. Their nerve supply also helps control filtration, urine flow, and blood pressure.

Blood reaches each kidney through the renal arteries, which branch off the abdominal aorta. These arteries enter the kidney at the hilum, the notch on the medial side of each kidney. Inside the kidney, each renal...

Kidneys contain millions of nephrons, the working units that filter blood and form urine. Each nephron has two main parts. The renal corpuscle filters blood plasma, and the renal tubule processes the glomerular filtrate.

The renal corpuscle is made of the glomerulus and Bowman's capsule. The glomerulus is a capillary network, and Bowman's capsule is a double-walled epithelial structure around it. Blood plasma is filtered in Bowman's capsule, and the filtered fluid then...

The renal corpuscle begins kidney filtration in the nephron. It includes the glomerulus and Bowman’s capsule, two microscopic structures that work together to help make urine. The glomerulus filters blood, and Bowman’s capsule collects the fluid that passes through.

The glomerulus is a tiny tuft of capillaries at the start of the nephron. It sits inside Bowman’s capsule and gets blood from an afferent arteriole. Blood leaves through an efferent arteriole. This setup is unusual because the...

The renal tubule reclaims useful substances and helps the body manage water, salts, and pH. It has three main parts: the proximal convoluted tubule, the Loop of Henle, and the distal convoluted tubule. These segments work with the collecting duct to form urine and keep internal conditions stable.

The proximal convoluted tubule, or PCT, is the first segment and extends from Bowman’s capsule around the glomerulus. Its coiled shape and microvilli, which are tiny membrane folds, increase surface...

The kidneys make urine through a careful process that helps keep body fluids stable. This process supports homeostasis, which means the body keeps its internal conditions balanced. The kidneys remove wastes and extra substances from the blood and turn them into urine.

The first stage is glomerular filtration. Each kidney has about 1 million nephrons, which are the functional units that filter blood. At the center of each nephron is the glomerulus, a network of tiny capillaries where filtration...

The selective kidney filtration membrane filters blood in the renal system. It is made of glomerular capillaries and podocytes. Together, these structures form a barrier that lets water and small solutes pass while keeping most plasma proteins and blood cells in the bloodstream.

The filtration process depends on three layers. The first is the glomerular endothelial cells, which line the capillaries. These cells have large pores called fenestrations. The fenestrations let blood plasma solutes...

Kidney filtration in the glomerulus depends on the balance of three pressures. These forces control how much blood plasma moves from the glomerulus into the capsular space. The process is driven by glomerular blood hydrostatic pressure, or GBHP, and opposed by capsular hydrostatic pressure, or CHP, and blood colloid osmotic pressure, or BCOP.

GBHP is the pressure of blood inside the glomerular capillaries. It averages about 55 mmHg and pushes water and dissolved solutes through the filtration...

Glomerular filtration rate, or GFR, is a key measure of kidney function. It shows how much filtrate the kidneys form each minute. Average GFR is about 125 mL/min in males and 105 mL/min in females.

Keeping GFR steady helps the kidneys control body fluid balance and maintain extracellular stability. The body uses intrinsic controls first. These include the myogenic mechanism and tubuloglomerular feedback.

The myogenic mechanism responds to changes in systemic blood pressure. When blood...

Kidney tubule transport in the nephron changes the fluid that leaves the glomerulus. Once this fluid enters the proximal convoluted tubule, it is called filtrate. Its composition shifts as the tubule reabsorbs useful substances and secretes unwanted ones.

Tubular reabsorption is a selective process that begins in the proximal tubules. Substances can move by the transcellular route, which passes through the tubule cell and into the peritubular capillaries, or by the paracellular route, which...

The proximal convoluted tubule, or PCT, is a major site where filtered water, ions, glucose, and amino acids are moved back into the body. It works in the kidney’s filtration system after the glomeruli produce the filtered fluid. Most of the filtered water and many solutes are reabsorbed here.

Sodium ion transport drives much of this reabsorption. The PCT uses symport, which moves substances in the same direction, and antiport, which moves substances in opposite directions. One key example is...

The loop of Henle controls ion movement in the nephron. In the thick ascending limb, cells use Na+–K+–2Cl− symporters on the apical membrane to take up one sodium ion, one potassium ion, and two chloride ions from the tubular fluid.

After entry into the cell, sodium ions are pumped into the interstitial fluid at the base and sides of the cell. They then diffuse into the vasa recta, the tiny blood vessels that help carry substances away. Chloride ions leave through leakage channels in the...

The early distal convoluted tubule, or DCT, reabsorbs water and salts as filtrate moves through the kidney. It handles about 10 to 15% of filtered water and about 5 to 10% of filtered sodium and chloride. This part of the tubule uses Na+-Cl- symporters on the apical membrane, sodium-potassium pumps, and chloride leakage channels on the basolateral membrane.

The early DCT is also a place where parathyroid hormone, or PTH, can increase calcium reabsorption when the body needs it. Farther along,...

Urine is mostly water, but its color, smell, pH, and specific gravity can reveal useful health clues. It is about 95% water and 5% solutes. The main solute is urea, which forms when amino acids are broken down. Other important waste products include uric acid, from nucleic acid metabolism, and creatinine, from creatine phosphate breakdown in skeletal muscle tissue.

Urine also contains many dissolved ions and small molecules. These include sodium, chloride, potassium, phosphate, sulfate,...

The kidneys excrete excess water by making dilute urine, especially during high fluid intake. This process depends mainly on juxtamedullary nephrons, which help the body keep fluid balance by adjusting how much water and ions stay in the filtrate.

As filtrate moves through the proximal convoluted tubule (PCT), its osmolarity stays isosmotic with the renal cortex. Osmolarity means the concentration of dissolved particles in a fluid. In the PCT, water and solutes are reabsorbed together, so the...

Kidney loop mechanisms help the body conserve water by making urine more concentrated. A steep solute gradient forms in the kidney medulla, the inner region of the kidney. This medullary osmotic gradient runs from the renal cortex down through the medulla.

Juxtamedullary nephrons build and maintain this gradient. Their loops extend deep into the medulla, where countercurrent mechanisms work. Countercurrent multiplication happens when the descending and ascending limbs of the nephron loop...

Kidney function can be measured by looking at glomerular filtration rate, or GFR. GFR shows how efficiently the glomeruli filter blood. Renal clearance is one common way to estimate this rate using substances such as inulin or creatinine.

Renal clearance means the volume of plasma that is cleared of a specific substance in a set time. To measure it, urine is collected during each bladder voiding over a 24-hour period. A single blood sample is taken at the end of that interval. The clearance...

The ureters are retroperitoneal tubes that carry urine from each kidney to the urinary bladder. They lie on either side of the vertebral column. These tubes have thick walls and are about 25 to 30 cm long.

At the renal pelvis, the ureter is about 10 mm wide. It then narrows to about 1 mm as it enters the back wall of the bladder at an angle. The ureteric orifices are slit-like, and this shape helps prevent urine from flowing backward toward the kidneys when the bladder contracts.

Each ureter...

The urinary bladder is a hollow, muscular sac that stores urine before it leaves the body. It can hold about 600 mL of urine before micturition, or urination. The bladder sits behind the pubic symphysis in the pelvic floor and is retroperitoneal, meaning it lies behind the lining of the abdominal cavity.

Its position changes slightly by sex. In males, the bladder lies in front of the rectum. In females, it is located anterior to the vagina and uterus.

The bottom of the bladder has a...

Urination depends on the bladder, its detrusor muscle, and two sphincters working together. The detrusor muscle is the smooth muscle in the bladder wall. The two sphincters help keep urine in the bladder until it is time to empty it.

As urine builds up, the bladder wall stretches. That stretch activates the urine storage reflex. This reflex is controlled by the sacral spinal segments and the pontine storage center. During storage, sympathetic signals make the detrusor muscle relax and the...

The urethra is the tube that carries urine out of the body. It runs from the bladder to the external opening, where urine leaves the urinary system. Its main job is to release urine, but its structure and role are not the same in males and females.

In females, the urethra is short. It measures about 3–4 cm and opens in front of the vaginal opening. In males, the urethra is longer and passes through the penis. It has two jobs in males. It carries urine and also helps eject semen.

The male...

Dialysis helps support people with renal failure when the kidneys can no longer filter waste from the blood well. Renal failure can be acute renal failure (ARF), which begins suddenly, or chronic renal failure (CRF), which develops slowly over time. Acute kidney injury may come from pre-renal causes such as hypovolemia or shock, intrinsic renal causes such as acute tubular necrosis, or post-renal causes such as urinary obstruction. Chronic renal failure is often linked to diabetes mellitus and...

The urinary system removes waste and extra fluid from the body. Several urinary tract problems can disrupt this process. These problems may result from infections, stress, age, congenital abnormalities, or lifestyle factors.

Urinary tract infections, or UTIs, are among the most common disorders. Bacteria enter the urethra and may spread to the bladder, causing cystitis. If the infection moves up to the renal pelvis, it becomes pyelonephritis. UTIs are usually treated with antibiotics, but...