Urinary System

Urinary System The major function of this system is the elimination of excess water, salts, and waste products. Above 170 bpm, your thinking is reduced to the primitive fight or flight response. Running (or charging) is at its peak performance. The vast majority of people also tend to lose control of their bladder and bowels at this stage because even those functions consume resources that the body prefers to save for fighting or running. Although renal function declines substantially with age, it usually remains sufficient for removing bodily wastes. Nevertheless, reduced renal function decreases the elderly person's ability to respond to various physiological and pathological stresses. In general, aging is associated with an increased incidence of kidney problems. There is a decline in the number of nephrons by about 30-40% between ages 25 and 85. Nephrons are the basic functional units of the kidneys and are made up of a renal corpuscle and renal tubules. There is also a reduction in filtration rate due to cumulative damage to the system and to reduction in blood flow to the kidneys. Urine is formed by the kidneys through three processes: filtration, reabsorption and secretion. The ultimate source of urine is blood, and a large amount of blood is transported to the kidneys for filtration by way of the renal arteries. Renal blood flow progressively decreases from 1200 mL/minute at age 30 to 40 years to 600 mL/minute at age 80. A decrease in

the glomerular filtration rate, or the rate at which blood is forced through and filtered by the renal corpuscle in the kidneys, is the most important functional defect caused by aging. Since the kidneys receive up to 25% of the resting cardiac output through the renal arteries, a large portion of total blood volume is filtered through the renal corpuscle each day. At this rate, it wouldn’t take long to totally deplete the body’s entire blood volume. Since this is obviously not a common problem, the body must have some way to recover a majority of that filtered blood. The process that returns necessary items from the filtrate back into the blood is called reabsorption. Reabsorption is responsible for returning 99% of filtrate back to the circulatory system. Reduced nephron sensitivity which occurs with age, however, results in less reabsorption of water, so urination becomes more frequent while daily fluid requirements increase. At the same time, there is usually a loss of thirst which compounds the problem. The muscles that help regulate the release of urine become weaker which leads to problems with incontinence and often involves slow leakage of urine. This is usually a more common problem in women. In males, urinary retention might develop due to chronic inflammation of the prostate gland. Swelling of the gland prevents the flow of urine as it puts pressure on the urethra. Maintaining Water-Salt Balance

It is the job of the kidneys to maintain the water-salt balance of the blood. They also maintain blood volume as well as blood pressure. Simple examples of ways that this balance can be changed include ingestion of water, dehydration, blood loss and salt ingestion. Reabsorption of water Direct control of water excretion in the kidneys is exercised by the antidiuretic hormone (ADH), released by the posterior lobe of the pituitary gland. ADH causes the insertion of water channels into the membranes of cells lining the collecting ducts, allowing water reabsorption to occur. Without ADH, little water is reabsorbed in the collecting ducts and dilute urine is excreted. There are several factors that influence the secretion of ADH. The first of these happen when the blood plasma gets too concentrated. When this occurs, special receptors in the hypothalamus release ADH. When blood pressure falls, stretch receptors in the aorta and carotid arteries stimulate ADH secretion to increase volume of the blood. Reabsorption of Salt The Kidneys also regulate the salt balance in the blood by controlling the excretion and the reabsorption of various ions. As noted above, ADH plays a role in increasing water reabsorption in the kidneys, thus helping to dilute bodily fluids. The kidneys also have a regulated mechanism for reabsorbing sodium in the distal nephron. This mechanism is controlled by aldosterone, a steroid hormone produced by the adrenal cortex. Aldosterone promotes the

excretion of potassium ions and the reabsorption of sodium ions. The release of Aldosterone is initiated by the kidneys. The juxtaglomerular apparatus is a renal structure consisting of the macula densa, mesangial cells, and juxtaglomerular cells. Juxtaglomerular cells (JG cells, also known as granular cells) are the site of renin secretion. Renin is an enzyme that converts angiotensinogen (a large plasma protein produced by the liver) into Angiotensin I and eventually into Angiotensin II which stimulates the adrenal cortex to produce aldosterone. The reabsorption of sodium ions is followed by the reapsorption of water. This causes blood pressure as well as blood volume to increase. Atrial natriuretic hormone (ANH) is released by the atria of the heart when cardiac cells are streatched due to increased blood volume. ANH inhibits the secretion of renin by the juxtaglomerular apparatus and the secretion of the aldosterone by the adrenal cortex. This promotes the excretion of sodium. When sodium is excreted so is water. This causes blood pressure and volume to decrease.

Stress urinary incontinence (SUI), also known as effort incontinence, is due essentially to insufficient strength of the pelvic floor muscles. It is the loss of small amounts of urine associated with coughing, laughing, sneezing, exercising or other movements that increase intra-abdominal pressure and thus increase pressure on the bladder. The urethra is supported by fascia of

the pelvic floor. If this support is insufficient, the urethra can move downward at times of increased abdominal pressure, allowing urine to pass.

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Above 170 bpm, your thinking is reduced to the primitive fight or flight response. Running (or charging) is at its peak performance. The vast majority of people also tend to lose control of their bladder and bowels at this stage because even those functions consume resources that the body prefers to save for fighting or running.