Urinary System: Functions, Components, and Health
**Functions of the Urinary System**
- Removes metabolic wastes from the blood
- Maintains proper pH in blood (approximately 7.4)
- Controls the amount of electrolytes (mineral salts)
- Regulates water balance in the body
- Produces some hormones, such as erythropoietin (for red blood cell production)
**Urinary System Components**
- Kidneys
- Bladder
- Ureters
- Urethra
**Kidneys**
Kidneys are paired, bean-shaped organs. Humans can live with one, even with 90% of its function lost, without needing a transplant. They are located on the dorsal side of the spine. The kidneys are covered by tissue called “renal fascia” (fibrous and protective). The renal pelvis holds urine. Each kidney is composed of nephrons (functional units, renal cells).
**Kidney Disease**
Renal impairment is a type of bacterial infection that can be treated with antibiotics. It is caused by repeated infections. Pyelonephritis is a permanent infection (kidney tissue does not regenerate). Kidney stones are formed by calcium and uric acid.
**Ureters**
Ureters are paired structures that join each kidney. They communicate down to the urinary bladder, making peristaltic movements (carrying urine to the bladder).
**Bladder**
The bladder is located in the pelvic cavity. Its walls are muscular, with a maximum capacity of 750 ml, but the need to urinate is generated at 350 ml.
**Malpighian Glomerulus**
The Malpighian glomerulus is a capillary with two branches, labeled arteries.
**Stages of Urine Formation**
**Filtration**
Filtration is the process that occurs between the capillary and Bowman’s capsule. There is a filtration membrane (structure shown in size) formed by mesangial cells.
The vast majority of the substances in the blood can pass.
Filtered substances: Almost all the water, mineral salts (Na, K, Cl), vitamins, amino acids, glucose, small proteins, and bicarbonates.
Unfiltered Substances: Red blood cells, white blood cells, platelets, and proteins of high molecular weight.
180 liters/day are filtered, 125 ml/min.
Urea: Waste that gives color to the urine (derived from the destruction of proteins from the cell’s own metabolism).
Proteins —-> Ammonia (NH3) – going to -> liver-processing> urea (less toxic)
**Proximal Convoluted Tubule**
Subsequently located, Bowman’s capsule has many folds (outlined) and is close to the capillary (proximal). Here, the filtered substance is received, and the second phase of urine formation begins.
**Tubular Reabsorption**
Tubular reabsorption is the process of removing useful substances that were filtered to avoid getting lost in the urine. This process occurs mainly in the proximal tubule and the loop of Henle.
**Characteristics of the Proximal Tubule**
Its cells have microvilli that increase absorption, and cells with many mitochondria provide a lot of ATP to use in the recovery of substances.
Note: Approximately 80% of the filtrate is reabsorbed in the proximal tubule because of these characteristics (water is reabsorbed by osmosis and does not waste energy).
Amino acids, glucose —> active transport (energy spent)
**Loop of Henle**
Continuous tubular reabsorption occurs here (removing useful substances and sending them to the blood).
**Distal Convoluted Tubule**
In this structure, the last stage of urine formation occurs. Stage 3 is tubular secretion, where more hazardous substances are sent from the capillaries to the distal tubule to leave through the urine. (The process of secretion mainly uses active transport).
**Secreted Substances**
- Creatinine
- Hydrogen ions (H+) that help maintain blood pH at 7.4
- Ions (K+), although K+ is useful, too much potassium can cause cardiac arrest
- Drugs that were previously chemically marked by our liver
Creatinine: Metabolic waste resulting from the metabolism of proteins (but only in the muscles). It is highly toxic and is equivalent to urea.
**Collecting Tubules**
When the fluid reaches the collecting tubule, urine is ready to be removed from the kidney and sent to the renal pelvis.
Antidiuretic hormone (ADH) or arginine vasopressin (AVP) is a hormone released primarily in response to changes in osmolarity or blood serum volume. Also known as argipressin, it causes the kidneys to conserve water by concentrating urine and reducing its volume by stimulating the reabsorption of water. It is named after this important function as a homeostatic regulator of fluids. It also has functions in the brain and blood vessels.