The Urinary System Essay Example

retroperitoneal.

The ureters are two muscular tubes that measure approximately 10 inches (25 cm) in length. They transfer urine from the kidneys to the bladder in small spurts through their walls. The bladder, which is a collapsible sac located at the front of the bony pelvis, then temporarily stores this urine. Folds in the bladder mucosa act as valves to prevent backward flow once urine enters from the ureters. A sphincter muscle controls the outlet of the bladder, and when sensory nerves in its wall are stimulated by a full bladder, it relaxes and allows for release of urine.The ability to consciously relax the sphincter is important for allowing urine to enter the urethra, although it is not fully mastered. The urinary bladder, consisting of both muscle and elastic tissue, is situated in the pelvic area above the male prostate gland and its front boundary is defined by the pubic symphysis. In females, its rear boundary includes the vagina, while in males it encompasses the rectum.The urinary bladder can hold up to 17-18 ounces (500-530 ml) of urine, but the feeling of needing to urinate typically arises when it contains only about 150-200 ml. When the bladder is half full, stretch receptors are triggered and nerve impulses travel to the spinal cord, resulting in a reflex nerve impulse that causes relaxation of the muscular valve or sphincter located at the neck of the bladder. This allows urine to flow into the urethra. The Internal Urethral Sphincter cannot be consciously controlled. Both ureters join at the trigone region on the dorsolateral floor of the bladder.

The trigone is triangular and located on the posterior and inferior wall of

the bladder, with its lowest point surrounding the urethra. Insufficient levels of urine in the bladder can cause a drop in body temperature and induce sensations of coldness.

The urethra, which is a muscular tube responsible for expelling urine from the bladder to the outside of the body, varies in length depending on biological sex. Females have an average length of 3.8 cm (1.5 inches), while males measure up to 20 cm (8 inches).

Women have a shorter urethra, measuring about 1-2 inches and located in the vulva between the vaginal and clitoral openings, while men have a longer urethra of approximately 8 inches that exits at the tip of their penis. This disparity in length increases women's vulnerability to urinary tract infections and cystitis compared to men.

The nephron is the fundamental unit of the kidney and consists of two parts - the glomerular (or Bowman's) capsule enclosing a group of capillaries called the glomerulus, forming a renal corpuscle. Attached to the Bowman's capsule is a long and winding renal tubule that comprises four sections: proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct.

The Bowman's capsule and glomerulus facilitate nonselective blood filtration in the renal corpuscle, enabling fluid and tiny particles in the glomerulus to pass into the Bowman's capsule and renal tubules. The resultant liquid within the renal tubules is referred to as filtrate. Arrival of blood to this organ is via the renal arteries, a branch extending from the aorta. Within this network, blood flows through the lobar artery, interlobar artery, arcuate artery, interlobular artery, and afferent arterioles. The term "afferent" pertains to the carrying of blood towards the glomerulus.

The glomerulus

is composed of capillaries that contain fenestrations, enabling the filtration of small particles and water to enter the filtrate. Podocytes surround the glomerulus, creating a barrier with interlocking pedicels to prevent blood cells, platelets, and protein molecules from entering the filtrate. This barrier only permits seven types of matter to pass through: blood plasma, glucose, amino acids, potassium, sodium, chloride, and urea (nitrogenous waste).

To maintain homeostasis, the reabsorption process is crucial since it restores specific materials from the filtrate back into the bloodstream. Following its departure from the glomerulus through the efferent arteriole, which carries blood away from it, reabsorption takes place. This arteriole then creates a peritubular capillary bed that surrounds the renal tubule.

The proximity of the renal tubule to the peritubular capillaries facilitates the reabsorption of important substances like glucose, amino acids, vitamins, water and ions from the filtrate into the bloodstream. After passing through the collecting duct, urine moves through ureters into the urinary bladder for storage. The production of urine is regulated by aldosterone and antidiuretic hormone (ADH). When there is dehydration, ADH is released by pituitary gland to decrease urine volume by increasing water absorption from collecting tubules into bloodstream. Conversely, if excess fluid exists in body, ADH release decreases resulting in dilute urine excretion.

The adrenal glands, situated on both sides of the retroperitoneum above the kidneys, perform crucial tasks in the body. Among these is the promotion of sodium reabsorption through collecting tubules with aldosterone, which concurrently boosts water reabsorption. This leads to a decrease in urine excretion and an increase in blood volume and pressure. The kidneys' endocrine cells create erythropoietin that governs red blood cell

creation.

Each adrenal gland consists of the separate entities of the adrenal cortex and medulla, both generating hormones. The cortex produces cortisol, aldosterone, and androgens while the medulla mainly produces epinephrine and norepinephrine.

The kidneys secrete renin during a drop in blood volume which triggers angiotensin production. This hormone narrows blood vessels thereby increasing blood pressure. Additionally, angiotensin stimulates aldosterone secretion from the adrenal cortex.

The kidneys' tubules reabsorb more sodium and water when aldosterone is released, resulting in increased fluid volume within the body and elevated blood pressure. An overactive renin-angiotensin-aldosterone system may also contribute to high blood pressure. The ADH hormone plays a vital role in maintaining homeostasis by regulating levels of water, glucose, and salt through its effect on kidney tissue permeability. This peptide hormone originates as a preprohormone precursor produced in the hypothalamus and stored in vesicles at the posterior pituitary.

The important role of AVP in regulating the body's water retention involves storing some amounts in the posterior pituitary and releasing it into the bloodstream, while also allowing other amounts to directly enter into the brain. In circumstances where dehydration is present, AVP conserves water by prompting the kidneys to concentrate urine and reduce urine volume. Additionally, high concentrations of AVP lead to moderate vasoconstriction that ultimately raises blood pressure. Conversely, ANF acts as a potent vasodilator when secreted by heart muscle cells and dilates the afferent glomerular arteriole while relaxing mesangial cells along with constricting the efferent glomerular arteriole. This mechanism works towards increasing pressure in glomerular capillaries, enhancing glomerular filtration rate, and leading to increased excretion of sodium and water.

The vasa recta's blood flow is enhanced by ANF, which eliminates solutes such

as NaCl and urea from the medullary interstitium to decrease its osmolarity. This leads to increased excretion and decreased reabsorption of tubular fluid. The phosphorylation of ENaC in the distal convoluted tubule and cortical collecting duct of the nephron via cGMP-dependent means decreases sodium reabsorption by ANF. Additionally, ANF blocks renin secretion, inhibiting both the renin-angiotensin system and aldosterone secretion by the adrenal cortex. Furthermore, EPO, a hormone that stimulates red blood cell formation in bone marrow, is produced by the kidney.

Specialized kidney cells produce EPO, a glycoprotein with a molecular weight of 34,000 units, in response to low oxygen levels. EPO is then released into the bloodstream where it stimulates the bone marrow to increase red blood cell production and enhance the blood's capacity for oxygen transport.

The active form of vitamin D, known as calcitriol, originates from either calciferol or vitamin D3. Calciferol can be produced in the skin when exposed to ultraviolet rays from the sun or obtained through dietary precursors referred to as "vitamin D." To become an active vitamin, calciferol must undergo two conversions: first into 25[OH] vitamin D3 in the liver and then transported while bound to a serum globulin to the kidneys. With assistance from parathyroid hormone (PTH), the final conversion into calcitriol occurs.