Human Body Excretory System and Kidneys
Kidneys are the body’s main excretory organs and form part of the urinary system. They purify the blood by forming urine and eliminating waste products like uric acid and urea, which are produced by cell activity. The kidneys are located in the abdomen.
Lungs are organs that form part of the respiratory system. They eliminate carbon dioxide from the blood, which is produced by chemical reactions in the cells.
Organs of the digestive system eliminate the residue from food digestion through defecation. Additionally, the liver transforms toxic substances into less toxic ones, which are then excreted in feces or urine.
Sweat glands are found in the skin and are exocrine glands that produce sweat. Sweat has a similar composition to urine but is more diluted. Sweat glands regulate body temperature and are minimally involved in eliminating waste.
Common Urinary System Disorders
| Cystitis | Renal Calculi | Renal Insufficiency |
|---|---|---|
Acute or chronic inflammation of the bladder, most commonly caused by infections. Symptoms include a burning sensation, frequent urination, or a continuous desire to urinate. | Also called kidney stones, these are small crystals that form in the kidney from deposits of salts. Their passage through the urinary tract causes intense pain called renal colic. | Decreased or interrupted kidney function, affecting urine flow. This disease has various causes and can be acute or chronic. |
Nephrons are the microscopic functional units of the kidneys that produce urine. Each kidney contains about a million nephrons, each consisting of a glomerulus and a renal tubule with different segments.
Urine Formation
Glomerular filtration: Water and most substances in the blood, except cells and proteins, are filtered from the glomerulus capillaries into the glomerular capsule.
Tubular reabsorption: Most of the filtered water and substances, like glucose, amino acids, vitamins, and ions, are returned to the blood through the peritubular capillaries.
Tubular secretion: Certain substances not needed by the body, such as creatinine and medications, are secreted from the blood into the renal tubule.
Cardiac Cycle
The heart pumps blood through two coordinated and rhythmic movements: contraction (systole) and relaxation (diastole). Each heartbeat comprises a full cardiac cycle, including atrial and ventricular systole and diastole.
• Atrial systole: Both atria contract, pushing blood into the ventricles.
• Ventricular systole: The ventricles contract, closing the atrioventricular valves to prevent blood from returning to the atria. The pulmonary artery and aorta valves open, pushing blood towards the lungs and the rest of the body.
• Atrial and ventricular diastole: The aortic and pulmonary valves close, and the atria and ventricles relax. The atria fill with blood, the atrioventricular valves open, and the ventricles begin to fill with blood.
Blood Vessels
| Name | Internal Structure | Function | Location |
|---|---|---|---|
| Artery | Thick, strong, and elastic wall | Carry blood from the heart to organs | Body |
| Capillary | One layer of cells, extremely thin endothelium | Connect arteries and venules | Tissue |
| Vein | Thinner wall and less elastic than arteries | Carry blood from organs to the heart | Body |
The Urinary System
Ureters are narrow tubes that collect urine produced in each kidney and carry it to the bladder.
Bladder is an expandable muscular bag where urine collects before being expelled. Located in the center of the pelvis, it has a capacity of 250-500 mL and expands as urine collects.
Urethra is a tube that carries urine from the bladder to outside the body. The involuntary sphincter keeps the urethra closed when urine is not passing through, while the second sphincter is made of skeletal muscle and can be controlled. When the bladder is full, involuntary contraction of the bladder and relaxation of the sphincters facilitate urination.
Kidneys
These two organs are 10 to 12 cm long and located behind the abdomen, above the waist, and beside the spine on each side of the body.
Renal capsule: A sheet of connective tissue that protects the kidney.
Renal medulla: The inner part of the kidney, divided into conical structures called renal pyramids.
Renal cortex: The outer part of the inside of the kidney.
Renal pelvis: A funnel-shaped cavity that collects urine from the renal pyramids and passes it into the ureters.
Blood containing waste enters the kidneys through the renal arteries.
Nephrons
Glomerular or Bowman’s capsule: A cup-shaped dilation at the beginning of the renal tubule that surrounds the glomerulus.
Glomerulus: A ball-shaped arrangement of blood capillaries through which blood carrying waste circulates.
Collecting tubule: A tube shared by various nephrons that receives newly formed urine and carries it to the renal pelvis.
Renal tubule: A long tube surrounded by peritubular capillaries through which the forming urine flows.
Filtered, waste-free blood leaves the kidneys through the renal veins.