2.89 Describe the role of the skin in temperature regulation, with reference to sweating, vasoconstriction and vasodilation

Stimulus: body temperature
Receptor: hypothalamus (brain)
Effector: skin
Response: warming up / cooling down

It's important to keep our body temperature constant because most of our enzymes work best at about 37ºC. Keeping internal conditions within acceptable limits is known as homeostasis.

Too cold

• Shivering: generates heat in the muscles + from increased respiration
• Hairs stand on end: traps an insulating layer on the surface of your skin
• Vasoconstriction: capillaries near the surface of the skin narrow so they carry less blood and more heat is kept in
• Reduced sweating: because sweat cools you down and you don't want that!

Too hot

• Vasodilatation: capillaries widened and pushed towards the surface of the skin by this so more heat is lost through radiation and conduction
• Sweating: produced by sweat glands in your skin and it is heated by the body. When it evaporates, it takes the heat with it.
• Hairs lie flat: less air is trapped

2.88 Understand the function of the eye in focusing near and distant objects, and in responding to changes in light intensity

Distant object
Ciliary muscles contract, suspensory ligaments relax so the lens becomes fat

Near object
Ciliary muscles relax, suspensory ligaments contract so the lens becomes thin

Source: BBC Bitesize
Figure 1

Low light
Circular muscles in the iris relax and radial muscles contract

Bright light
Circular muscles contract and radial muscles relax

2.87 Describe the structure and function of the eye as a receptor

Structure of the eye is shown above.

Each part and its function is below:

• Sclera: the tough outer coat of the eye which protects it from mechanical damage
• Cornea: a transparent 'window' that refracts light directly into the eye and helps focus the image on the retina
• Iris: the front of the lens that controls the amount of light entering the eye
• Pupil: an opening that lets light through
• Retina: Inside a choroid layer, it's a light sensitive membrane with receptors called cones and rods (see below for more info on those)
• Optic nerve: a nerve that transmits electrical impulses with information to the brain to be interpreted
• Vitreous humor: A jelly-like substance that helps the eye keep its shape and helps refract light onto the retina
• Conjunctiva: a transparent membrane that keeps the eye moist by secreting mucus
• Lens: A biconvex structure that can change shape according to whether the ciliary muscle and suspensory ligaments relax/contract
• Fovea: Where the majority of rods and cones are found (not sure about this one)
• Suspensory ligaments: keep the lens in place and help control its shape so light can be refracted onto the retina appropriately
• Ciliary muscles: contract and relax to help control the lens shape

The eye has cones and rods in the retina. Cones detect COLORS and have high sensitivity to detail. There are red, green and blue cones. Rods are responsible for vision with low light levels and detect black and white only.

When objects are far away, ciliary muscles relax and the suspensory ligaments contract so the lens is thin. When objects are near, ciliary muscles contract and suspensory ligaments relax so the lens is fat.

Distant
Is
Thin

Near
Is
Fat

2.86 Describe the structure and functioning of a simple reflex arc illustrated by the withdrawal of a finger from a hot object

DEFINITION
Reflex: an automatic reaction - an action performed without conscious thought in response to a stimulus

DOWN TO BUSINESS NOW

1. Sense organ detects stimulus (let's say a hot object)
2. Sensory neurone carries electrical impulse to CNS (central nervous system)
3. Relay neurone carries impulse through CNS where a response is decided to the motor neurone
4. Motor neurone carries impulse to effector (eg muscle)
5. Response is executed (eg you pull your hand away from a hot object)

HERE'S A LINK TO THE NERVE SONG 

http://scicast.org.uk/films/2011/06/the-nerve-song.html

2.85 Understand that stimulation of receptors in the sense organs sends electrical impulses along nerves into and out of the central nervous system, resulting in rapid responses

Electrical impulses are sent from receptors in sense organs when there is a stimulus to the CNS. They are then sent straight back (after a response has been decided) through nerves to the effector. This has a sort of reflex-arc effect. 

(I wrote this in purple because I felt like it)

2.84 Understand that the central nervous system consists of the brain and spinal cord and is linked to sense organs by nerves

The central nervous system, known as the CNS, consists of the brain and spinal cord, linked by nerves which send electrical impulses. The CNS is responsible for deciding a response to a stimulus. The receptors in sense organs (like the tongue, eyes, mouth, etc.) send electrical impulses to the CNS through nerves to your brain/spinal cord which will send electrical impulses back to the effector.

2.79 Understand that a coordinated response requires a stimulus, a receptor and an effector

To carry out a response, 3 main things are needed:

• A stimulus: A change in environment
• A receptor: Something that detects the change 
• An effector: To carry out a response

Example:

• Stimulus: you touch a hot object
• Receptor: nerves in your finger
• Effector: muscles that pull your hand away from the potentially dangerous hot object

2.78 Understand that homeostasis is the maintenance of a constant internal environment and that body water content and body temperature are both examples of homeostasis

Homeostasis is the control of internal conditions basically. Examples of this include:

• Osmoregulation: control of water
• Thermoregulation: control of temperature

Thermoregulation is essential because (at least for humans), 37°C is about the optimum temperature for your body to function. Osmoregulation is also important so that the plasma in the blood isn't too dilute and isn't too concentrated (more on that in urinary system posts)

Here's a random GIF because why not

2.77 Understand that organisms are able to respond to changes in their environment

So yes, we know that organisms can respond to changes to their environment. Changes are known as stimuli. They can be light, pressure, chemicals, temperature, etc. Organisms must have receptors and, in order to respond, must have effectors (such as muscles or glands). The response determines whether an organism is able to survive the change in environment. 

Plant examples: they grow towards the light, roots grow towards water and towards gravitational pull.
Animal example: a gazelle will run away from a lion

2.76 Understand that urine contains water, urea and salts.

Again, incredibly self explanatory. Urine is made up of mainly water, urea and salts that the body no longer needs. (also has organic and inorganic compounds)

2.75 Describe the role of ADH in regulating the water content of the blood

THIS FINALLY MAKES SENSE.

Excess or lack of water is detected in the brain by the hypothalamus. It causes the pituitary gland to produce ADH (anti-diuretic hormone). It travels through the blood stream and into the kidneys - specifically, into the collecting duct. According to the water levels in the body, it will give a signal to either make the membrane (walls) more porous. Increased levels of ADH are typically secreted when there is lack of water in the body, because it makes the membrane walls more porous.

The more porous the membrane is, the more water is absorbed. 

It causes urine to be more concentrated and of less quantity. It is also typically a yellowish color. If there is too much water, the levels of ADH will lower and the opposite effects will happen; the membrane becomes less porous, less water is absorbed and urine is clear and less concentrated (and more in quantity)

FOR STUDYING
A ridiculously long and rather unhelpful acronym:

Brain detects water levels
Hypothalamus (does this)
Pituitary gland produces ADH
(B)lood stream carries hormone
(K)idneys receive hormone
(C)ollecting duct specifically
(M)membrane 
Porous (refers to membrane)
Water
(A)bsorbed
Returned to blood stream

BHPBKCMPWAR (long version)
BHPPWR (simplified)

A very simplified acronym of the process:

Detect
Produce ADH
Carry
Recieve
Collecting duct
Porous (membrane)
Absorbed
Returned (to blood stream)

DPCRCPAR
(Daddy Paced Carol's Room Cindy Passed Away Restfully)

For the super long acronym, here is a suggestion for remembering it (refers to Harry Potter)
Brave
Harry
Painted
Blue
Kittens
Cursing
Merrily
Patently
Wearing 
Red

2.74 Understand that selective reabsorption of glucose occurs at the proximal convoluted tubule

Pretty self explanatory. Basically, glucose is re-absorbed in the proximal convoluted tubule (aka the first twisty tube). This is because glucose is needed in the body for respiration. It is removed in the nephron (which is the entire yellow structure shown in the diagram above) and taken back into the blood stream.

2.73 Understand that water is reabsorbed into the blood from the collecting duct

Water is re-absorbed in the collecting duct one last time into the medulla because it is so salty. 

Following the rules of osmosis, water travels from a high concentration to a low concentration through a semi permeable membrane - that is exactly what happens, because the concentration of water in the medulla is low compared with the filtrate. However, it depends on how permeable the membrane(s) are/is: the more permeable, the more water is absorbed. I.e. if your body lacks water, the membrane will be permeable so there is no unnecessary water loss.

More about this on post 2.71

2.72 Describe ultrafiltration in the Bowman’s capsule and the composition of the glomerular filtrate

In your kidneys, there are thousands of little structures called nephrons. These are responsible for filtering the blood. It begins in the glomerulus.

The pressure in the glomerulus is high enough to squeeze some fluid out of the blood. This enters the Bowman's capsule and is known as the filtrate. It is made up of water, urea and smaller molecules and ions. But how exactly does this happen?

The blood comes in through the afferent arteriole at high pressure. It branches and becomes much smaller and the pressure increases further. The high pressure forces the plasma (liquid) into the space inside the bowman's capsule. It is now called filtrate.

Glomerular filtrate is composed of mainly water, ions (potassium, calcium, chlorine, etc.) and organic molecules.

2.71 Describe the structure of a nephron, to include Bowman’s capsule and glomerulus, convoluted tubules, loop of Henlé and collecting duct

Figure 1

Figure 1 shows the structure of a nephron. Its main features are:

• the glomerulus
• the bowman's capsule
• convoluted tubules
• the loop of Henle
• the collecting duct

Blood enters the nephron through the glomerulus. Here, some liquid is 'squeezed' out of the blood due to the high pressure. This is known as the filtrate. Large particles / cells such as erythrocytes (red blood cells) and large amino acids are not filtered. Instead, they continue in small blood vessels. Glucose is also re-absorbed immediately into the capillaries. The filtrate goes into the bowman's capsule and that is where the filtering process begins.

The beginning is in the renal cortex and consists of the glomerulus and the bowman's capsule which surrounds it. The bowman's capsule is a cup-shaped structure that takes in the filtrate while the glomerulus is a 'knot' of capillaries in the middle. The bowman's capsule is the starting point for the filtrate.

The glomerulus is directly connected to the proximal tubule, which is the first convoluted tubule. It's basically a windy tube where mainly organic solutes are re-absorbed, along with water and things like glucose, amino acids, sodium and potassium among others.

After the Loop of Henlé, it reaches the distal tubule, which is the farther-away-curly-tube, aka the second convoluted tubule. Here, the levels of potassium, calcium and sodium are regulated. This is done by pumps and hormones. Once it is done, all of the wanted particles, water, salt, etc. have been taken out, which leaves excess water, urea and other types of metabolic waste.

The Loop of Henlé dips down into the renal medulla and is a hairpin shaped tubule. The upper section is in the outer medulla and the lower section in the inner medulla. It extracts mainly water as it travels down into the medulla. On its way up, it pumps out the salts that the body needs. This causes the medulla to become really really salty and creates a concentration gradient (medulla becomes hypertonic). The further down you go, the saltier it is. In the descending end, the membrane is highly permeable to water. Not really to salt or anything else. When the filtrate gets to the bottom of the loop (because the inner medulla is so so salty) it's highly concentrated. In the ascending end of the loop, the process happens in reverse. Here, the membrane is not-so-permeable to water and instead is lined with channels that transport ions like potassium, sodium and chlorine.

The collecting duct spans both the cortex and the medulla. In the bottom section (in the medulla), because the outside is hypertonic, even more water can be extracted (follows rules of osmosis). Hormones tell the collecting ducts how porous to make their membranes. If a membrane is made very porous, more water is absorbed into the medulla and the urine becomes even more concentrated. If the membrane is made not-very-porous, less or no water is absorbed so the urine is less concentrated.

2.70 Describe the structure of the urinary system, including the kidneys, ureters, bladder and urethra

Source: CrashCourse, YoutubeFigure 1

The urinary system is made up of:

• the kidneys
• the bladder
• the ureter
• the urethra

Looking at figure 1, you can see where each organ / part is located in the body. The functions of each part are below:

• Kidney: filters out blood to prevent dehydration and manage the soluble substances in your blood and turns unwanted substances into urine
• Bladder: stores urine
• Ureter: carries urine from kidneys to bladder
• Urethra: carries urine from the bladder to outside the body

Click here to see a video on the urinary system (seriously, it's worth it)

2.69 Understand how the kidney carries out its roles of excretion and osmoregulation

Nephrons in the kidneys (contains millions of nephrons) are in charge of excretion and osmoregulation.

Excretion
Excretion is filtering the bad stuff from the body and keeping the good bits we need. The kidneys excrete water, salts and urea from the body.
The liver converts amino acids (which contain nitrogen, which is toxic to the body) into urea. The kidneys filter urea from the blood stream and combine it with water to create urine which then moves into the bladder via the urethra.

Osmoregulation
Osmoregulation is the balancing of water levels in the body (remember OSMO-sis? Therefore water-regulation)

The kidneys react to ADH hormone released by the pituitary gland. 

Basically, in the nephron, there is a collecting duct. If more ADH is released, the walls of the duct will become more permeable and more water will be re-absorbed. If less, duct will become more impermeable.

2.68 Recall that the lungs, kidneys and skin are organs of excretion

Lungs
What? The lungs excrete carbon dioxide, which is a waste product of respiration. It diffuses into the lungs and is breathed out. 

Why? Carbon dioxide diffuses into blood plasma and makes it more acidic. It needs to be excreted so it doesn't reach toxic levels.As blood flows through the lungs, the carbon dioxide diffuses from the red blood cell into the alveoli and is breathed out.

Higher tier
If the levels of carbon dioxide are too high, the brain will detect this. It sends signals to increase the breathing rate so carbon dioxide can be removed from the blood more quickly, and the levels return back to normal.

Figure 1

Kidneys
What? The kidneys excrete excess water, salt and urea through urine. 

Why? (urea) Proteins are broken down into amino acids. Excess amino acids are broken down to form ammonia in the liver. It's then converted into urea (still in the liver). Although it is less toxic than ammonia, it still needs to be excreted from the body. 

(water) Water is a waste product of respiration and is also taken in by food or drink. Excess water could cause the plasma to be too dilute and cause blood cells to swell and burst :( while too little water could cause the plasma to be too concentrated and the blood cells to shrivel up / shrink and stop working. It's important to keep the water concentration in the blood plasma constant.

Skin
What? The skin excretes urea, water, salt and ammonia. This is released through sweat, which is a combination of urea, salts and water and is produced by the sweat glands.

Why? Things like ammonia and urea could be toxic if left in the body for too long. (see why water needs to be excreted by the kidneys)

Structure of the kidney: see 'pages' menu on right hand side

Source: BBC Bitesize http://www.bbc.co.uk/schools/gcsebitesize/science/triple_ocr_gateway/the_living_body/waste_disposal/revision/4/