Endocrine System
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Urine Formation
8.4 Each kidney contains a little over 1 million blood-filtering units called nephrons. Nephrons consist of a tiny tube, or tubule, with a cup-shaped structure at one end, and they produce urine in a two-phase process. First, filtration occurs. Blood coming from the renal artery flows into the capillary network in the cup. Pressure forces water and small molecules, such as urea, out of the capillaries and into the long tubule connected to the cup. Second, reabsorption takes place. About 99 percent of the water and some of the other substances that were filtered out of the blood move into capillaries that surround the tubule. The fluid that remains in the tubule is urine. The tubules join to form larger tubes, which connect with the ureter where it joins the kidney. Urine is a concentrated mixture of water, urea, and various mineral salts. Complex mechanisms regulate urine formation, and the contents of the urine can vary depending on the state of the body. This is why an analysis of the urine, called a urinalysis, is so useful in diagnosing some diseases.
Disorders of the Urinary System
8.5 Urinary-tract infections are fairly common, especially in females. This is because the female’s urethra is much shorter than that of the male. This allows bacteria easier access to the bladder and ureters. It is important to treat urinary-tract infections early. Once an infection is well-established, it can travel up the urinary tract to the kidneys and cause a kidney infection. Acute or repeated kidney infections can cause permanent damage to the nephrons.
8.6 Kidney stones are more common in males than in females. They consist of hardened calcium salts, uric acid, and other by-products that collect inside the kidneys. Approximately 60 percent of kidney stones pass through the urethra and leave the body. If a stone becomes lodged in the ureter, it must be removed. Surgical removal was once the only option, but less invasive methods are now available. For example, fibre-optic laser surgery is used to break the stones into pieces that are small enough to pass through the urethra.
How do People Control when They Urinate?
8.7 Two circular muscles at the base of the bladder control the flow of urine from the body. Adults have voluntary control of the outermost muscle, but young children do not. The bladder can hold up to 600 ml of urine. Stretch receptors in the wall of the bladder sense how full it is, and when it contains about 200 ml, a person usually feels the need to urinate. When the volume approaches 300 ml, the need to urinate becomes urgent.
9. THE muscular System
What is Muscular System?
9.1 Your muscular system enables your body to move. Each muscle consists of muscle cells that can contract or relax. Because muscle cells are usually long and slender, they are called fibres. Skeletal muscles move your bones and help protect your inner organs. Smooth muscles deep within your body help move food, air, and body fluids. Cardiac muscle is a special type of muscle found only in your heart. Your body has more than 600 muscles, which make up about 30 percent of the female body and about 40 percent of the male body.
