Solid food and fluids are expected to be excreted after a while and thus, the importance of a urinary system. The system consists of the kidneys, urethra, and the urinary bladder. These organs making up the urinary system are covered with a number of different tissues whose main function is to filter blood and create urine which needs to be eliminated from the body.
Contents
The Kidney
The kidney is a bean shaped organ found just above the muscular wall of the abdomen, it is found as a pair whose main function is filtering and disposal of waste products from the body. The kidneys are surrounded by three layers of tissues; renal fascia , adipose tissues and renal capsule.
The renal fascia is a thin, outer layer of fibrous connective which surrounds each kidney and fastens it to the structures surrounding it like the adrenal glands. There is a thin layer of fibrous connective tissue which forms the renal capsule on each kidney. This capsule provides a stiff outer shell to maintain the shape of the soft inner tissues. It also prevents entry of infections to the kidneys. The kidney has an adipose capsule which is a middle layer of adipose tissue which enables cushioning of the kidneys since it surrounds the kidneys and offers protection.
The Urinary bladder
The urinary bladder is tube-like in appearance. The organ is easily flexible and acts like a storage tank for urine before it is passed out. It is located in the pelvic cavity, just above the reproductive organs. The female urinary bladder is reduced in size compared to that of male since it shares the limited space of the pelvic cavity within the uterus. The urinary bladder has three separate tissues layers; mucosa layer which is the innermost layer, sub mucosa and the outermost layer being the visceral muscles.
The mucosa layer is the innermost layer covering the urinary bladder; it has a transitional epithelial tissue lining which enables it to stretch and thus allowing room for large volumes of urine. Transitional epithelium also provides protection of the underlying tissues from acidic or basic nature of urine. Beneath the mucosal lays the sub mucosal layer a connective tissue which has blood vessels and nervous tissues that supports and controls the surrounding tissue layers. The muscularis has visceral muscles surrounding the submucosa, it also enables the urinary bladder to expand and contract during urination. The tissues also form a ring muscle around the opening of the urethra called sphincter muscle; it relaxes during urination thus allowing flow of urine.
The Urethra
The urethra is tube-like and connects the urinary bladder to the external opening of the urethra whose main function is transportation of urine from the bladder to the external opening in the perineum. The male and female urethra differ in function as well as size. The male urethra is 20-22cm long whereas that of the female is 4cm. Moreover, the female urethra function is urination while the male urethra is not limited to urination only but also holds an essential function in passing out of semen in male reproductive system. The urethra is composed of an inner epithelial lining, a spongy sub mucosa, middle smooth muscle layer and an outer fibroelastic connective-tissue layer.
The spongy sub mucosa has a rich vascular plexus that provides adequate pressure to prevent any leakage of urine. It is composed of stratified squamous cells which become transitional folds when it approaches the bladder. The urethral smooth muscle is made of fibroelastic connective tissues that are arranged in a longitudinal manner. They are adequately supplied with nerves which contribute to opening and closing of the urethra. The urethra is lined by stratified columnar epithelium which is protected by mucus secreting glands which produces mucus enhancing enabling prevention of corrosion by urine.
The Ureter
These are tube-like in appearance. Ureter connects the urinary bladder with each pair of the kidney. Each ureter arises from the pelvis of a kidney and passes down the side of a kidney. They are about 25 and 30cm in length. The walls of ureters consist of numerous types of tissues; transitional epithelium, loose connective tissue and smooth muscles
Transitional tissues layer is a special type of epithelium which is the innermost layer. It has capabilities of flattening and distending thus allowing the ureter to stretch and expand as urine fills it or as it passes through it. Beneath it is a loose connective tissues layer which contains elastic fibres, nerves, blood and lymphatic vessels. This layer as well allows expansion and contraction of the ureter as urine passes. The smooth muscle layer consists of longitudinal and circular layers thus enabling it to contract in waves of peristalsis motion thus enabling propelling urine down the ureter from the kidneys to the urinary bladder.
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