Homeostasis and Human Urinary System, practice questions

B, High hydrostatic pressure at the glomerulus forces water and small solutes into the Bowman capsule; that is ultrafiltration.

C, Glucose is filtered but completely reabsorbed by active transport in the proximal convoluted tubule.

C, Osmoreceptors in the hypothalamus detect changes in blood water potential and regulate ADH release.

B, More ADH increases water reabsorption, giving a small volume of concentrated urine.

B, Insulin promotes glucose uptake and storage as glycogen, lowering blood glucose.

B, Narrowing the arterioles reduces blood flow near the surface, conserving heat.

C, Glucose is filtered into the filtrate but fully reabsorbed, so it is absent from normal urine.

B, The ureter carries urine from the kidney to the bladder; the urethra carries it out of the body.

B, Dialysis fluid has no urea, so urea diffuses out of the blood down its concentration gradient.

B, A negative feedback response counteracts the deviation and returns the condition toward its set point.

• (a) Ultrafiltration.
• (b) Any two of water, glucose, salts, urea.
• (c) Selective reabsorption (by active transport in the tubule).
• (d) Protein molecules are too large to pass through the filtration membrane.

• (a) The blood water potential falls / blood becomes more concentrated.
• (b) ADH (antidiuretic hormone).
• (c) It makes the collecting duct more permeable to water, so more water is reabsorbed.
• (d) A small volume of concentrated urine.

• (a) The pancreas.
• (b) Insulin; it makes cells take up glucose and store it as glycogen, lowering blood glucose.
• (c) Glucagon; it makes the liver break glycogen into glucose, raising blood glucose.
• (d) Negative feedback.

Homeostasis is the maintenance of a constant internal environment, temperature, blood glucose, water content and pH, despite changes outside the body. It keeps conditions stable enough for enzymes and cells to function normally, and is controlled through negative feedback involving the nervous and endocrine systems. Without homeostasis, small outside changes could disrupt enzyme activity and cell metabolism throughout the body.

The urinary system removes urea and excess water and salts from the blood. The kidneys filter the blood continuously; ureters carry the urine they produce to the bladder, which stores it; the urethra then carries urine out of the body during urination. This removal of urea, a waste product of protein breakdown, is why the urinary system is described as an excretory organ system.

The nephron is the kidney's individual filtering and processing unit, made of the glomerulus, Bowman's capsule and a system of tubules. Each kidney contains a very large number of nephrons working in parallel, which is why kidney function can still be adequate even if some nephrons are damaged. This same arrangement explains why a person can stay healthy with only one functioning kidney.

High blood pressure in the glomerulus forces water, glucose, salts and urea out of the blood and into Bowman's capsule, forming the glomerular filtrate. Large plasma proteins and blood cells are too big to pass through the filter and stay in the blood. This high pressure exists because the vessel carrying blood into the glomerulus is wider than the one carrying it out.

As the filtrate flows along the tubule, useful substances are reabsorbed back into the blood: all of the glucose, most of the water, and some of the salts, depending on the body's needs. What remains becomes urine. This process is described as selective because different substances are reabsorbed to different extents rather than all being treated the same way.

Antidiuretic hormone (ADH) adjusts how much water the tubules reabsorb, concentrating or diluting urine to keep blood water content steady. When the kidneys fail, waste and excess water build up in the blood, and treatment is by dialysis or a kidney transplant. More ADH is released when the body is short of water, producing a smaller volume of more concentrated urine.

Negative feedback is the general control pattern behind homeostasis: a receptor detects a change away from the normal level, a control centre triggers a response, and an effector corrects the change so the condition returns toward the set point. Thermoregulation, osmoregulation and blood glucose control all follow this same pattern, which is why learning one example well makes the others easier to explain.

The skin and the hypothalamus in the brain work together to keep body temperature constant. When the body is too warm, skin blood vessels dilate and sweat glands become more active to lose heat; when it is too cold, skin blood vessels constrict and muscles may shiver to generate heat. Hair standing on end (goosebumps) is a related response that traps a thin layer of warm air near the skin.

The pancreas keeps blood glucose within a narrow range using two hormones with opposite effects: insulin lowers blood glucose by increasing its uptake into cells and its storage as glycogen, while glucagon raises blood glucose by breaking glycogen back down. Glycogen is stored mainly in the liver and muscles, acting as a short-term reserve of glucose.

A kidney stone is a hard, crystallised deposit that forms when substances in concentrated urine build up, causing pain as it passes through the urinary tract. A urinary tract infection occurs when bacteria multiply in the bladder or urethra, which can cause pain and a frequent urge to urinate. Drinking enough water so urine stays more dilute is a simple factor linked to lowering the risk of both conditions.