1. Human urinary system: 2. Kidney, 3. Renal pelvis, 4. Ureter, 5. Urinary bladder, 6. Urethra. (Left side with frontal section)
7. Adrenal gland
Vessels: 8. Renal artery and vein, 9. Inferior vena cava, 10. Abdominal aorta, 11. Common iliac artery and vein
With transparency: 12. Liver, 13. Large intestine, 14. Pelvis
The order of impurities being excreted from the kidneys: Kidneys → Ureters → Urinary Bladder → Urethra
The urinary system, also known as the renal system, consists of the two kidneys, ureters, the bladder, and the urethra. Each kidney consists of millions of functional units called nephrons. The purpose of the renal system is to eliminate wastes from the body, regulate blood volume and pressure, control levels of electrolytes and metabolites, and regulate blood pH. The kidneys have extensive blood supply via the renal arteries which leave the kidneys via the renal vein. Following filtration of blood and further processing, wastes (in the form of urine) exit the kidney via the ureters, tubes made of smooth muscle fibers that propel urine towards the urinary bladder, where it is stored and subsequently expelled from the body by urination (voiding). The female and male urinary system are very similar, differing only in the length of the urethra.
Urine is formed in the kidneys through a filtration of blood. The urine is then passed through the ureters to the bladder, where it is stored. During urination (peeing) the urine is passed from the bladder through the urethra to the outside of the body.
About 1-2 litres of urine are produced every day in a healthy human, although this amount may vary according to circumstances such as fluid intake.
The urinary system refers to structures which conduct urine, formed in the nephrons of the kidney, to the point of its excretion. There are two kidneys in the human body, on the right and the left. Urine begins to be created within a nephron, which is a small unit within the kidney. It travels through the structures of the nephron and into the collecting duct system, which is a system of larger vessels within the kidney. The collecting ducts join together to form minor calyces and ultimately major calyces, larger and larger ducts. These drain into a structure called the pelvis of the kidney, and enter the ureter. The ureter is a tube-like structure which carries the urine from the kidneys to the bladder. The ureters enter the bladder from within the bladder.
Under microscopy, the urinary system is covered in a unique lining called urothelium, a type of transitional epithelium. Unlike the epithelial lining of most organs, transitional epithelium does not have a fixed size. Instead it is capable of flattening and distending. Urothelium covers most of the urinary system, including the renal pelvis and ureters. Because it is a form of epithelium, it rests on a basement membrane.
There are several functions of the Urinary System:
- Removal of waste product from the body (mainly urea and uric acid)
- Regulation of electrolyte balance (e.g. sodium, potassium and calcium)
- Regulation of acid-base homeostasis
- Controlling blood volume and maintaining blood pressure
Average urine production in adult humans is about 1 – 2 litres (L) per day, depending on state of hydration, activity level, environmental factors, weight, and the individual's health. Producing too much or too little urine needs medical attention. Polyuria is a condition of excessive production of urine (> 2.5 L/day), oliguria when < 400 mL (millilitres) are produced, and anuria one of < 100 mL per day.
The first step in urine formation is the filtration of blood in the kidneys. In a healthy human the kidney receives between 12 and 30% of cardiac output, but it averages about 20% or about 1.25 L/min.
The basic structural and functional unit of the kidney is the nephron. Its chief function is to regulate the concentration of water and soluble substances like sodium salts by filtering the blood, reabsorbing what is needed and excreting the rest as urine.
In the first part of the nephron, the renal corpuscle blood is being filtrated from the circulatory system into the nephron. A pressure difference between them forces the filtrate from the blood across the filtration membrane. The filtrate includes water, small molecules and ions that easily pass through the filtration membrane. However larger molecules such as proteins and blood cells are prevented from passing through the filtration membrane. The amount of filtrate produced every minute is called the glomerular filtration rate or GFR and amounts to a staggering 180 litres per day. About 99% of this filtrate is then reabsorbed as it passes through the nephron and the remaining 1% becomes urine.
Regulation of concentration and volume
Antidiuretic hormone (ADH), is a neurohypophysial hormone found in most mammals. Its two primary functions are to retain water in the body and to constrict blood vessels. Vasopressin regulates the body's retention of water by acting to increase water absorption in the collecting ducts of the kidney nephron. Vasopressin increases water permeability of the kidney's collecting duct and distal convoluted tubule by inducing translocation of aquaporin-CD water channels in the kidney nephron collecting duct plasma membrane.
Urination is the ejection of urine from the urinary bladder through the urethra to the outside of the body. In healthy humans (and many other animals), the process of urination is under voluntary control. In infants, some elderly individuals, and those with neurological injury, urination may occur as an involuntary reflex. In other animals, in addition to expelling waste material, urination can mark territory or express submissiveness. Physiologically, micturition involves coordination between the central, autonomic, and somatic nervous systems. Brain centers that regulate urination include the pontine micturition center, periaqueductal gray, and the cerebral cortex. In male placental mammals, urine is ejected through the penis, and in female placental mammals through the vulva.
Urologic disease can involve congenital or acquired dysfunction of the urinary system.
Kidney diseases are normally investigated and treated by nephrologists, while the specialty of urology deals with problems in the other organs. Gynecologists may deal with problems of incontinence in women.
Diseases of other bodily systems also have a direct effect on urogenital function. For instance it has been shown that protein released by the kidneys in diabetes mellitus sensitises the kidney to the damaging effects of hypertension.
Urinary incontinence can result from a weakening of the pelvic floor muscles caused by factors such as pregnancy, childbirth, ageing and being overweight. Pelvic floor exercise known as Kegel exercise is seen to be of help in this condition by strengthening the muscles. There can also be underlying medical reasons for urinary incontinence which are often treatable. In children the condition is called enuresis.
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