You are probably aware that drinking alcohol can cause visual disturbances, slurred speech, drowsiness, impaired judgment, and loss of coordination because of its effects on the brain. Although it may be less obvious, alcohol also can have serious effects on the functioning of the excretory system.
As you learned in the beginning of the chapter from the conversation between Maeva and Bintou, who were in line for the restroom, consumption of alcohol inhibits a hormone that causes our bodies to retain water. The result is that more water is released in urine, increasing the frequency of restroom trips as well as the risk of dehydration.
Which hormone discussed in this chapter does this? If you answered antidiuretic hormone (ADH; also called vasopressin)—you are correct! ADH is secreted by the hypothalamus of the brain and acts on the kidneys. As you have learned, the kidneys filter the blood, reabsorb needed substances, and produce urine. ADH helps the body conserve water by influencing this process. ADH makes the collecting ducts in the kidneys permeable to water, allowing water molecules to be reabsorbed from the urine back into the blood through osmosis into capillaries.
Alcohol is thought to produce more dilute urine by inhibiting the release of ADH. This causes the collecting ducts to be more impermeable to water, so less water can be reabsorbed and more is excreted in the urine. Because the volume of urine is increased, the bladder fills up more quickly, and the urge to urinate occurs more frequently. This is part of the reason why you often see a long line for the restroom in situations where many people are drinking alcohol. In addition to producing more dilute urine, simply consuming many beverages can also increase urine output.
In most cases, moderate drinking causes only a minor and temporary effect on kidney function. However, when people consume a large quantity of alcohol in a short period of time or abuse alcohol over long time periods, there can be serious effects on the kidney. For example, binge drinking (i.e. consuming about four to five drinks in two hours), can cause a condition called “acute kidney injury,” serious and sudden impairment of kidney function that requires immediate medical attention. As with other cases of kidney failure that you learned about in this chapter, the treatment is to artificially filter the blood using hemodialysis. While normal kidney function may eventually return, acute kidney injury can sometimes cause long-term damage to the kidneys.
In cases where people abuse alcohol, particularly for an extended period of time, there can be many serious effects on the kidneys and other parts of the excretory system. The dehydrating effect of alcohol on the body can impair the function of many organs, including the kidneys themselves. Additionally, because of alcohol’s effect on kidney function and water and ion balance, chronic alcohol consumption can cause abnormalities in blood ion concentration and acid-base balance, which can be very dangerous.
Additionally, drinking more than two alcoholic beverages a day can increase your risk of high blood pressure. As you have learned, high blood pressure is a risk factor for some kidney disorders and a common cause of kidney failure. Therefore, drinking too much alcohol can damage the kidneys by raising blood pressure.
Finally, chronic excessive consumption of alcohol can cause liver disease. As you know, the liver is an important organ of the excretory system that breaks down toxic substances in the blood. The liver and kidneys work together to remove wastes from the bloodstream. For example, as you have learned, the liver transforms ammonia into urea, which is then filtered and excreted by the kidneys. When the liver is not functioning normally, it puts added strain on the kidneys, which can result in kidney dysfunction. This association between alcohol, liver disease, and kidney dysfunction is so strong that most of the patients in the United States with both liver disease and related kidney dysfunction are alcoholics.
As you have learned, the excretory system is essential to remove toxic wastes from the body and regulate homeostasis. Having an occasional drink can temporarily alter these functions, but excessive alcohol exposure can seriously and permanently damage this system in many ways. Limiting alcohol consumption can help preserve the normal functioning of the excretory system so that it can protect your health.
In this chapter, you learned about the excretory system. Specifically, you learned that:
- Excretion is the process of removing wastes and excess water from the body. It is an essential process in all living things and a major way the human body maintains homeostasis.
- Organs of the excretory system include the skin, liver, large intestine, lungs, and kidneys.
- The skin plays a role in excretion through the production of sweat by sweat glands. Sweating eliminates excess water and salts and also a small amount of urea, a byproduct of protein catabolism.
- The liver is a very important organ of excretion. The liver breaks down many substances in the blood, including toxins. The liver also excretes bilirubin, a waste product of hemoglobin catabolism, in bile. Bile then travels to the small intestine and is eventually excreted in feces by the large intestine.
- The main excretory function of the large intestine is to eliminate solid waste that remains after food is digested and water is extracted from the indigestible matter. The large intestine also collects and excretes wastes from throughout the body.
- The lungs are responsible for the excretion of gaseous wastes, primarily carbon dioxide from cellular respiration in cells throughout the body. Exhaled air also contains water vapor and trace levels of some other waste gases.
- The paired kidneys are often considered to be the main organs of excretion. Their primary function is the elimination of excess water and wastes from the bloodstream by the production of urine. The kidneys filter many substances out of blood, allow the blood to reabsorb needed materials, and use the remaining materials to form urine.
- The two bean-shaped kidneys are located high in the back of the abdominal cavity on either side of the spine. A renal artery connects each kidney with the aorta and transports unfiltered blood to the kidney. A renal vein connects each kidney with the inferior vena cava and transports filtered blood back to the circulation.
- The kidney has two main layers involved in the filtration of blood and formation of urine: the outer cortex and inner medulla. At least a million nephrons, which are the tiny functional units of the kidney, span the cortex and medulla. The entire kidney is surrounded by a fibrous capsule and protective fat layers.
- As blood flows through a nephron, many materials are filtered out of the blood, needed materials are returned to the blood, and the remaining materials are used to form urine.
- In each nephron, the glomerulus and the surrounding Bowman’s capsule form the unit that filters blood. From Bowman’s capsule, the material filtered from the blood, called filtrate, passes through the long renal tubule. As it does, some substances are reabsorbed into the blood and other substances are secreted from the blood into the filtrate, finally forming urine. The urine empties into collecting ducts, where more water may be reabsorbed.
- The kidneys are part of the urinary system, which also includes the ureters, urinary bladder, and urethra. The main function of the urinary system is to eliminate the waste products of metabolism from the body by forming and excreting urine. After urine forms in the kidneys, it is transported through the ureters to the bladder. The bladder stores the urine until urination, when urine is transported by the urethra to be excreted outside the body.
- Besides the elimination of waste products such as urea, uric acid, excess water, and mineral ions, the urinary system has other vital functions. These include maintaining homeostasis of mineral ions in extracellular fluid, regulating acid-base balance in the blood, regulating the volume of extracellular fluids, and controlling blood pressure.
- The formation of urine must be closely regulated to maintain body-wide homeostasis. Several endocrine hormones help control this function of the urinary system, including antidiuretic hormone from the hypothalamus, parathyroid hormone from the parathyroid glands, and aldosterone from the adrenal glands. For example, the kidneys are part of the renin-angiotensin-aldosterone system that regulates the concentration of sodium in the blood to control blood pressure. In this system, the enzyme renin secreted by the kidneys works with hormones from the liver and adrenal gland to stimulate nephrons to reabsorb more sodium and water from urine.
- The kidneys also secrete endocrine hormones, including calcitriol, which helps control the level of calcium in the blood; and erythropoietin, which stimulates the bone marrow to produce red blood cells.
- The process of urination is controlled by both the autonomic and the somatic nervous systems. The autonomic system causes the detrusor muscle in the bladder wall to relax as the bladder fills with urine, but the conscious contraction of the detrusor muscle expels urine from the bladder during urination.
- Ureters are tube-like structures that connect the kidneys with the urinary bladder. Each ureter arises at the renal pelvis of a kidney and travels down through the abdomen to the urinary bladder. The walls of the ureter contain smooth muscle that can contract to push urine through the ureter by peristalsis. The walls are lined with transitional epithelium that can expand and stretch.
- The urinary bladder is a hollow, muscular organ that rests on the pelvic floor. It is also lined with transitional epithelium. The function of the bladder is to collect and store urine from the kidneys before the urine is eliminated through urination. Filling of the bladder triggers the autonomic nervous system to stimulate the detrusor muscle in the bladder wall to contract. This forces urine out of the bladder and into the urethra.
- The urethra is a tube that connects the urinary bladder to the external urethral orifice. Somatic nerves control the sphincter at the distal end of the urethra. This allows the opening of the sphincter for urination to be under voluntary control.
- Diabetic nephropathy is a progressive kidney disease caused by damage to the capillaries in the glomeruli of the kidneys due to long-standing diabetes mellitus. Years of capillary damage may occur before symptoms first appear.
- Polycystic kidney disease (PKD) is a genetic disorder (autosomal dominant or recessive) in which multiple abnormal cysts grow in the kidneys.
- Diabetic nephropathy, PKD, or chronic hypertension may lead to kidney failure, in which the kidneys are no longer able to adequately filter metabolic wastes from the blood. Kidneys may fail to such a degree that kidney transplantation or repeated, frequent hemodialysis is needed to support life. In hemodialysis, the patient’s blood is filtered artificially through a machine and then returned to the patient’s circulation.
- A kidney stone is a solid crystal that forms in a kidney from minerals in the urine. A small stone may pass undetected through the ureters and the rest of the urinary tract. A larger stone may cause pain when it passes or be too large to pass so it blocks a ureter. Large kidney stones may be shattered with high-intensity ultrasound into pieces small enough to pass through the urinary tract, or they may be removed surgically.
- A bladder infection is generally caused by bacteria that reach the bladder from the GI tract and multiply. Bladder infections are much more common in females than males because the female urethra is much shorter and closer to the anus. Treatment generally includes antibiotic drugs.
- Urinary incontinence is a chronic problem of uncontrolled leakage of urine. It is very common, especially at older ages and in women. In men, urinary incontinence is usually caused by an enlarged prostate gland. In women, it is usually caused by stretching of pelvic floor muscles during childbirth (stress incontinence) or by an “overactive bladder” that empties without warning (urge incontinence).
You have learned that the excretory system protects your body through the removal of toxic wastes and the maintenance of homeostasis. But how does your body protect itself against pathogens and other threats? Read the next chapter on the immune system to find out.
Chapter Summary Review
- Match each organ of the excretory system with the description that best fits it. Each organ is used only once. Organs: lungs, skin, kidneys, liver, large intestine
- Excretes solid wastes from food
- Cools the body while it eliminates water and other wastes
- Part of the urinary system
- Produces bile and breaks down toxins in the blood
- Eliminates waste gases
- In what ways can the alveoli of the lungs be considered analogous to the nephrons of the kidney?
- What is urea? Where is urea produced and what is it produced from? How is urea excreted from the body?
- True or False. The lungs help excrete excess water, in addition to carbon dioxide.
- True or False. There are two kidneys in the human body, and both are the same size and shape.
- True or False. The renal pelvis is a bone that surrounds the kidneys.
- If a person has a large kidney stone that prevents urine that has left the kidney from reaching the bladder, where do you think this kidney stone is located? Explain your answer.
- Match each of the following structures with the description that best fits it. Each structure is used only once. Structures: urethra; nephron; ureters; urinary bladder
- Produces urine
- Stores relatively large quantities of urine
- Releases urine to the outside of the body
- Transports urine between where it is produced and where it is stored
- Explain what is meant by “Excretion = Filtration – Reabsorption + Secretion” regarding the production of urine.
- Which disease discussed in the chapter specifically affects the glomerular capillaries of the kidneys? Where are the glomerular capillaries located within the kidneys, and what is their function?
- Describe one way in which the excretory system helps maintain homeostasis in the body.
- Secretion in a nephron occurs:
- before filtration, and goes from the filtrate into the blood
- after filtration, and goes from the filtrate into the blood
- before filtration, and goes from the blood into the filtrate
- after filtration, and goes from the blood into the filtrate
- High blood pressure can both contribute to the development of kidney disorders and be a symptom of kidney disorders. What is a kidney disorder that can be caused by high blood pressure? What is a kidney disorder that has high blood pressure as a symptom? How does blood pressure generally relate to the function of the kidney?
- If the body is dehydrated, what do the kidneys do? What does this do to the appearance of the urine produced?
- Which of the following contain filtered blood in the kidney?
- B and C
- True or False. Hormones influence the functions of the kidney, but the kidney itself does not produce hormones.
- True or False. Both the lungs and the kidneys help maintain the pH of the blood.
- Identify three risk factors for the development of kidney stones.