Topic 8 - Homeostasis and Excretion (Unit 2)

Question 1

What is homeostasis?

Answer 1

Homeostasis is the maintenance of a constant internal environment.

Question 2

How is control of water content in our bodies an example of homeostasis?

Answer 2

Drinking is the main way that our bodies gain water, with a little input from the food that we eat and the water formed by cell respiration. The body also loses water, mostly in urine but also smaller volumes in sweat, faeces and exhaled air. Every day, we gain and lose about the same volume of water, so that the total content of water in our bodies remains the same - this is an example of homeostasis.

Question 3

What is the internal environment?

Answer 3

The internal environment refers to the blood and tissue fluids in our bodies.

Question 4

What is tissue fluid?

Answer 4

Tissue fluid is a water solution of salts, glucose and other solutes that surrounds all the cells of the body, forming a pathway for the transfer of nutrients between the blood and the cells.

Question 5

How is tissue fluid formed?

Answer 5

Tissue fluid is formed by leakage from blood capillaries; it is similar in composition to blood plasma, with the only difference being its lack of the plasma proteins.

Question 6

Besides from controlling the water content of the body, what other components of the internal environment are maintained?

Answer 6

Other examples of homeostasis include the level of carbon dioxide in the blood, along with the blood’s pH, the concentration of dissolved glucose and the body temperature.

Question 7

Why is homeostasis important?

Answer 7

Homeostasis is important because cells will only function properly if they are bathed in a tissue fluid which provides them with their optimum conditions. For instance, if the tissue fluid contains too many solutes, the cells will lose water by osmosis and become dehydrated, preventing them from working efficiently and, potentially, causing damage.

Question 8

Why must substances, like urea, be removed from the body?

Answer 8

Substances, such as urea, must be prevented from building up in the blood an tissue fluid, where they would be toxic to cells, causing harm to the body.

Question 9

What are the excretory products of the lungs?

Answer 9

The lungs excrete carbon dioxide as a waste product of aerobic respiration via exhalation.

Question 10

What are the excretory products of the skin?

Answer 10

The skin secretes sweat, which contains small amounts of urea.

Question 11

What are the excretory products of the kidneys?

Answer 11

The kidneys excrete excess water, salts and urea by producing urine via osmoregulation.

Question 12

How much waste products and salts does every litre of urine contain?

Answer 12

On average, every litre of urine contains about 40gg of waste products and salts.

Question 13

What are the main solutes in urine?

Answer 13

Urea, ammonia, sodium chloride (salt), potassium and phosphate are the main solutes in urine. Urine also contains water.

Question 14

Why do all animals have to excrete a nitrogenous waste product?

Answer 14

Carbohydrates and fats only contain the elements carbon, hydrogen and oxygen. However, proteins also contain nitrogen. If the body has too much carbohydrate or fat, these substances can be stored, for example as glycogen in the liver, or as fat under the skin. Excess proteins cannot be stored; the amino acids are first broken down in the liver, where they are converted into carbohydrate, which is stored as glycogen and the main nitrogen-containing waste product, urea.

Question 15

What is excretion?

Answer 15

Excretion is the process by which waste products of metabolism are removed from the body.

Question 16

What is the main nitrogenous excretory substance in humans?

Answer 16

In humans, the main nitrogenous excretory substance is urea.

Question 17

What are the two functions of the kidney?

Answer 17

The kidney is a homeostatic organ, controlling the water and salt (ion) concentration in the body as well as an excretory organ, concentrating nitrogenous waste in the form that can be eliminated.

Question 18

How do plants excrete waste products of metabolism?

Answer 18

Plants also excrete waste products of metabolism. In light, their leaves produce oxygen from photosynthesis. In the dark, they excrete carbon dioxide from respiration. Plants exchange these gases through pores in the leaf called stomata.

Question 19

How is each kidney supplied with blood?

Answer 19

Each kidney is supplied with blood through a short renal artery. This leads from the body’s main artery, the aorta, so the blood entering the kidney is at a high pressure.

Question 20

What happens inside each kidney?

Answer 20

Inside each kidney, the blood is filtered and the ‘cleaned’ blood passes out through each renal vein to the main vein, or vena cava.

Question 21

How does urine leave the kidneys?

Answer 21

Urine leaves the kidneys through two tubes, the ureters, and is stored in a muscular bag called the bladder.

Question 22

What is the urethra?

Answer 22

The urethra is the tube that carries urine from the bladder to the outside of the body.

Question 23

What are the sphincter muscles?

Answer 23

Sphincter muscles are rings of muscle in the wall of an organ, like at the outlet of the bladder, which holds back its contents.

Question 24

What type of control is the lower sphincter muscle under?

Answer 24

The lower sphincter muscle is under conscious control.

Question 25

What type of control is the upper sphincter muscle under?

Answer 25

The upper sphincter muscle is involuntary - it automatically relaxes when the bladder is full to hold back the urine.

Question 26

What is the cortex of the kidney?

Answer 26

The cortex is the outer part of the kidney, containing kidney tubules and blood vessels that branch from the renal artery.

Question 27

What are nephrons?

Answer 27

Nephrons are kidney tubules, which then run down through the middle layer of the kidney.

Question 28

What is the medulla of the kidney?

Answer 28

The medulla is the middle layer, containing blood vessels, loops of Henle and collecting ducts. It has bulges, called ‘pyramids’ that point towards the concave side of the kidney. The tubules in the medulla eventually join up and lead to the tips of these pyramids, where they empty into a funnel-like structure, called the pelvis.

Question 29

What is the pelvis of the kidney?

Answer 29

The pelvis is the funnel-like part of the kidney leading to the ureter, carrying the urine to the bladder.

Question 30

What is the Bowman’s capsule?

Answer 30

The Bowman’s capsule is a structure consisting of a hollow cup of cells at the start of the kidney tubule - it is the site of ultrafiltration.

Question 31

What is the glomerulus?

Answer 31

The glomerulus is a ball of capillaries surrounded by the Bowman’s capsule at the start of a kidney tubule.

Question 32

What is the basement membrane (in the Bowman’s capsule)?

Answer 32

The basement membrane is a membrane in the wall of the Bowman’s capsule that acts as a molecular filter during ultrafiltration in the kidney.

Question 33

What is the glomerular filtrate?

Answer 33

The glomerular filtrate is a fluid that passes through the Bowman’s capsule at the start of the kidney tubule.

Question 34

What is ultrafiltration?

Answer 34

Ultrafiltration is the filtration of the blood, which takes place in the Bowman’s capsule of a kidney tubule, where the filter separates different-sized molecules under pressure.

Question 35

How does ultrafiltration work in the Bowman’s capsule?

Answer 35

At the start of the nephron is a hollow cup of cells, called the Bowman’s capsule. It surrounds a ball of blood capillaries called a glomerulus. It is here that the blood is filtered: blood enters the kidney through the renal artery, which divides into smaller and smaller arteries. The smallest arteries, known as arterioles, supply the capillaries of the glomerulus. A blood vessel with a smaller diameter carries blood away from the glomerulus, leading to capillary networks which surround the other parts of the nephron. Because of the resistance to flow caused by this, the pressure of the blood is very high, forcing fluid from the blood through the walls of the capillaries, into the space in the middle of the capsule. The basement membrane acts like a filter, allowing water, ions and small molecules, like glucose and urea, to pass through, but holding back blood cells and large molecules, such as proteins. The fluid that enters the capsule space is called the glomerular filtrate.

Question 36

What is selective reabsorption?

Answer 36

Selective reabsorption is the process by which select parts of the filtrate move back into the blood. All glucose is reabsorbed from the filtrate into the bloodstream. All amino acids must also be absorbed - any excess has already been turned into urea. This happens by both diffusion and then active transport. We need a balance of both water and mineral ions; most of the water is reabsorbed by osmosis and some mineral ions are absorbed by a process of diffusion.

Question 37

Where does selective reabsorption take place?

Answer 37

Selective reabsorption takes place in the proximal convoluted tubule.

Question 38

What is the loop of Henle?

Answer 38

The loop of Henle is the u-shaped part of the nephron, involved in concentrating the fluid in the tubule.

Question 39

What is osmoregulation?

Answer 39

Osmoregulation keeps the water concentration of the blood steady. The volume and concentration of water in the urine will differ. The hypothalamus of the brain detects the concentration of the blood; the pituitary gland produces hormones, called the anti-diuretic hormone (ADH), which controls this. The collecting duct of the nephron has specific receptors for ADH, which affects permeability and, thus, controls osmoregulation.

Question 40

Where does osmoregulation take place?

Answer 40

Osmoregulation takes place in the collecting duct.

Question 41

What is the collecting duct?

Answer 41

The collecting duct is the last part of a kidney tubule, where water is reabsorbed before the final urine is produced.

Question 42

What is the loop of Henle’s role?

Answer 42

The loop of Henle is involved with concentrating the fluid in the tubule by causing more water to be reabsorbed into the blood.

Question 43

What happens in the kidney nephron?

Answer 43

Part of the plasma leaves the blood in the Bowman’s capsule and enters the nephron. The filtrate consists of water and small molecules. As the fluid passes along the nephron, all the glucose is absorbed back into the blood in the first coiled part of the tubule, along with most of the sodium and chloride ions. In the rest of the tubule, more water and ions are reabsorbed, and some solutes, like ammonium ions are secreted into the tubule. The final urine contains urea at a much higher concentration than in the blood. It also contains controlled quantities of water and ions.

Question 44

What is antidiuretic hormone?

Answer 44

Antidiuretic hormone (ADH) is a hormone released from the pituitary gland; it controls the water content of the blood by increasing reabsorption of water from the collecting ducts of the kidney into the blood.

Question 45

When does ADH start to work?

Answer 45

ADH starts to work when your body loses too much water; for example, if you are sweating heavily and not replacing lost water by drinking.

Question 46

What happens when there is too little water in the blood?

Answer 46

When there is too little water in the blood, the body must conserve water so it tells the hypothalamus tells the collecting duct to reabsorb more water from the filtrate into the blood. Lots of ADH is produced by the pituitary gland, which makes the collecting duct more permeable. Little, concentrated urine (of a dark colour) is produced.

Question 47

What are the causes of too little water being in the blood?

Answer 47

No drinking, sweating and vomiting are causes of water loss in the blood.

Question 48

What happens when there is too much water in the blood?

Answer 48

When there is too much water in the blood, the body needs to remove all excess water so the collecting duct needs to be less permeable. The pituitary gland releases less/no ADH, meaning that less water is put back into the blood from the filtrate. Lots of dilute, paler urine is now produced.

Question 49

What are the causes of too much water in the blood?

Answer 49

Intake of too much fluid causes excess water in the blood.

Question 50

Why is osmoregulation important in the blood?

Answer 50

If there is too little water, cells will lose water and begin to crenate, whilst, if there is too much water, the cells will expand and will burst - this is known as lysis. Cells must be held in a solution of the same concentration in order to regulate osmosis and maintain equilibrium.

Question 51

What is the advantage of a human maintaining a constant body temperature?

Answer 51

It means that all the chemical reactions taking place in the cells of the body can go on at a steady, predictable rate. It is also important that the body does not become too hot, in order to avoid enzymes being destroyed by denaturing.

Question 52

What is a homeotherm?

Answer 52

A homeotherm is an animal that maintains a constant body temperature by physiological means.

Question 53

What is an endotherm?

Answer 53

An endotherm uses heat from the chemical reactions in its cells to warm its body. It then controls its heat loss by regulating processes like sweating and blood flow through the sin. Endotherms use behavioural way sot control their temperature too.

Question 54

What is the thermoregulatory centre?

Answer 54

The thermoregulatory centre is the part of the brain that monitors core body temperature.

Question 55

Where is the thermoregulatory centre located in the brain?

Answer 55

The thermoregulatory centre is located in the brain’s hypothalamus - it acts as the body’s ‘thermostat’.

Question 56

What happens if a person goes into a warm or cold environment?

Answer 56

If a person goes into a warm or cold environment, the first thing that happens is that temperature receptors in the skin send electrical impulses to the hypothalamus, which stimulates the brain to alter our behaviour. We start to feel hot or cold and, usually, do something about it, such as finding shade or putting on more clothes.

Question 57

What happens if changes to our behaviour are not enough to keep our body temperature constant?

Answer 57

If this is the case, the thermoregulatory centre in the hypothalamus detects a change in the temperature of the blood flowing through it. It then sends signals via nerves to other organs of the body, which regulate the temperature by physiological means.

Question 58

What are the functions of the skin?

Answer 58

The skin forms a tough outer layer able to resist mechanical damage, acts as a barrier to the entry of pathogens, forms an impermeable surface, preventing loss of water, acts as a sense organ for touch and temperature changes and controls the loss of heat through the body surface.

Question 59

What are the three layers of the skin?

Answer 59

The three layers of the skin are: the epidermis, dermis and hypodermis.

Question 60

What is the function of the outer epidermis?

Answer 60

The outer epidermis consists of dead cells that stop water loss and protect the body against invasion by microorganisms such as bacteria.

Question 61

What is the function of the hypodermis?

Answer 61

The hypodermis contains fatty tissue, which insulates the body against heat loss and is a store of energy.

Question 62

What is the function of the dermis?

Answer 62

The middle layer, the dermis, contains many sensory receptors. It is also the location of sweat glands and many small blood vessels, as well as hair follicles. These last three structures are involved in temperature control.

Question 63

How do the sweat glands help to reduce body temperature?

Answer 63

The sweat glands produce greater amounts of sweat. This liquid is secreted onto the surface of the skin. When a liquid evaporates, it turns into a gas. This change needs energy, called the latent heat of vaporisation. When sweat evaporates, the energy is supplied by the body’s heat, cooling the body. In very humid atmospheres, the sweat stays on the skin and does not evaporate, having very little cooling effect.

Question 64

How do hairs help to maintain a constant body temperature?

Answer 64

Hairs on the surface of the skin lie flat against the skin’s surface. This happens because of the relaxation of tiny muscles, called hair erector muscles, attached to the base of each hair. In cold conditions, these contract and the hairs are pulled upright. The hairs trap a layer of air next to the skin and, since the air is a poor conductor of heat, this acts as insulation. In warm temperatures, the thinner layer of trapper air means that more heat will be lost - this is not very effective in humans because the hair over most of our body do not grow very long.

Question 65

What is vasodilation?

Answer 65

Blood flows through capillary loops, radiating heat to the outside and cooling the body down. If the body is too hot, arterioles leading to the capillary loops dilate. This increases the blood flow to the skin’s surface and is called vasodilation.

Question 66

What is vasoconstriction?

Answer 66

In cold conditions, the arterioles leading to the surface capillary loops constrict and blood flow to the surface of the skin is reduced, so that less heat is lost.

Question 67

How does shivering warm the body?

Answer 67

Shivering, where the muscles contract and relax rapidly, requires more energy, which increases respiration and the amount of heat produced, thus warming the body.

Question 68

How can the liver produce heat?

Answer 68

In cold conditions, the body’s metabolism speeds up, generating more heat. The liver, a large organ, can produce a lot of metabolic heat, in this way. The hormone, adrenaline, stimulates the increase in metabolism.