Urinary system

Urinary system organs

The organs of the urinary system are the kidneys, ureters, bladder and urethra. The kidneys perform the filtration functions of the urinary system and create urine, while the remaining organs act as transport tubes or provide temporary urine storage. The anatomy of the urinary system can be seen here in the urinary system diagram.

Kidneys

The kidneys are paired retroperitoneal organs located anterolateral to the spinal cord, near the posterior body wall. They are roughly 12 cm in length and 6 cm wide, spanning from T12 to L3 vertebral levels. The bean-shaped kidneys are somewhat protected by the lower rib cage, and by a protective layer of perirenal fat. On their concave medial surface is the renal hilum, which is the location where the ureters, blood vessels and nerves enter or exit the organ. The most important blood vessels entering and exiting the kidneys are the renal artery and renal vein. The adrenal glands sit on the superior pole of each kidney.

Internally, the kidneys consist of 2 layers; a highly vascularized outer renal cortex and an inner renal medulla. Spanning across these two layers are millions of the kidneys’ functional units, the nephrons.

Nephron

Nephrons are the filtering structure of the kidney. They have two main parts, the renal corpuscle and the renal tubule. The renal corpuscle is a small ball of capillaries (glomerulus) surrounded by a double layered epithelial capsule (glomerular or Bowman's capsule) similar to pushing your hand into a balloon.

The renal tubule is formed by small convoluted tubes which are closely associated with blood vessels. Corpuscles are situated in the renal cortex, while tubules dive into the medulla. Water, electrolytes and waste products are exchanged between the blood vessels and the capsule/tubes of the nephrons by the process of both passive and active transport.

The products to be removed from the blood, or returned to the blood are very carefully selected by these cells. Any liquid or chemical that remains in the nephrons’ tubes becomes urine. The filtrate, or urine, gets carried out of the nephrons by collecting ducts, traveling through various parts of the kidney before reaching the ureters.

Bladder

The urinary bladder is a muscular sac whose function is to temporarily store urine. It is located in the pelvis, just posterior to the pubic symphysis. Its walls consist of smooth muscle which allows the bladder to stretch, permitting the bladder to store an increasing amount of urine.

When distended with urine, the bladder sends a signal to the nervous system to initiate the process of micturition (urination).

In males, the neck of the bladder is surrounded by the prostate gland.

Urethra

The urethra is a single thin walled tube that carries urine from the bladder to outside of the body. It is associated with two urethral sphincters, an internal smooth muscle (involuntary) urethral sphincter and an external skeletal muscle (voluntary) urethral sphincter.

The urethra differs between sexes, with the female urethra being much shorter in length and the opening located anterior to the vagina. The male urethra is longer as it travels the length of the penis, with the opening located at the tip of the glans penis.

The male urethra has the double function of both urine and semen transportation.

Urinary system function

The main function of the urinary system is to remove waste products from the body and maintain water, electrolyte and acid-base (pH) balance in the blood. It does this through the formation of urine. Waste products are filtered from the blood into the urine filtrate by nephrons. Water, electrolyte and acid-base balance is a finely controlled process of filtration, reabsorption and secretion, also occurring through nephrons.

The urinary system has a homeostatic relationship with many other body systems, by which one or both rely on the other for effective functioning. For example the endocrine system produces hormones such as antidiuretic hormone and aldosterone, which help to control the kidney’s reabsorption of water and salts. This same water and salt balance is vital for the accurate function of the muscular system, cardiovascular system and nervous system. Without appropriate levels of sodium, potassium and calcium contraction of skeletal and cardiac muscle and the function of nerves would be disrupted. Further, water and electrolyte balance in the body is necessary for maintaining blood volume and blood pressure. While systemic blood pressure of the cardiovascular system is responsible for driving the passive filtration of water and substances out of the glomerulus into the glomerular capsule. 

The urinary system has other endocrine functions too, with the kidney producing the enzyme renin, the hormone erythropoietin and acting in the conversion of vitamin D. Our bodies derive vitamin D either through diet or by skin synthesis when exposed to ultraviolet light. It is then processed, firstly by the liver then by the kidneys, to transform it from its initial inactive prohormone state into the active product of 1,25 vitamin D (calcitriol). Activated Vitamin D can then be used by the body in various processes, including the absorption of calcium from the gastrointestinal tract and the deposition of calcium into bone. Erythropoietin is produced by specialized kidney cells, called interstitial cells, in response to tissue hypoxia. Low oxygen situations such as anemia, living at high altitude or certain cardiac and pulmonary diseases will result in tissue hypoxia. Erythropoietin acts on stem cells in bone marrow to increase red blood cell production, this in turn results in more red blood cells circulating in our blood, carrying more oxygen around the body and raising oxygenation of tissues. Renin is a hormone that plays a very important role in blood pressure control, via the renin-angiotensin-aldosterone mechanism. Low blood pressure, low sodium levels, or specific sympathetic nervous system activity stimulates juxtaglomerular cells in the kidneys to produce renin.

Renin then acts like an enzyme on the plasma protein angiotensinogen, converting it to angiotensin I. Angiotensin I has some vasoconstrictor properties but for the most part it leaves the kidneys and enters the circulatory system to travel to the lungs. Here angiotensin I is converted to angiotensin II by angiotensin converting enzyme. Angiotensin II has a number of functions; it is a strong vasoconstrictor, it acts on the kidneys to decrease salt and water excretion and it stimulates the production of aldosterone from the adrenal glands (which also decreases salt and water excretion). All these actions individually result in increased blood pressure, acting together gives an enhanced effect.

Neurovascular supply

The kidneys are supplied by the renal artery and drained by the renal vein. Neural supply is by the renal plexus.

The ureters receive blood supply from a number of different sources, including the ureteric branches of renal artery, ovarian/testicular artery, ureteric branches of the abdominal aorta and common iliac arteries, ureteric branches of the superior and inferior vesical and uterine arteries Nervous innervation is by the renal plexus and ganglia, aortic plexus, ureteric branches of intermesenteric plexus, pelvic splanchnic nerves, superior and inferior hypogastric plexuses.

The urinary bladder receives arterial supply from the superior and inferior vesical arteries, venous drainage is by the vesical venous plexus (vesical veins) and innervation is provided by the vesical plexus, and inferior hypogastric plexus.

The urethra is supplied by the internal pudendal artery, vaginal artery, vesical venous plexus (vesical veins), vesical nerve plexus and pudendal nerve, in the case of the female and by the inferior vesical artery, artery of bulb of penis, prostatic venous plexus, prostatic nerve plexus and pudendal nerve in males.

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