A Level CIE Biology: 14 Homeostasis

homeostasis

the process of maintaining constant internal body conditions

in order to function properly and efficiently, organisms…

have diff control systems that ensure their internal conditions are kept relatively constant

why is homeostasis important

ensures the maintenance of optimal conditions for enzyme action and cell function

sensory cells

detect information about the conditions inside and outside of the body

6 physiological factors controlled by homeostasis in mammals:

• core body temp

• metabolic waste e.g. co2 and urea

• blood pH

• conc of glucose in blood

• water potential of blood

• conc of respiratory gases (co2 and o2) in blood

how is homeostatic balance (keep factors within certain limits) maintained 

majority of homeostatic control mechanisms in organisms use negative feedback

3 things negative feedback control loops involve

• receptor

• coordination system

• effector

receptor/sensor

detects stimulus that is involved with condition/physiological factorc

coordination system (nervous and endocrine)

transfer information between different parts of body

effector (muscles and glands)

carry out a response

3 outcomes of negative feedback loop:

• factor/stimulus is continuously monitored

• if increase in factor, body responds to make factor decrease

• if decrease in factor, body responds to make factor increase

what 2 coordination systems does homeostasis in mammals rely on to transfer info between diff parts of body

• nervous - info transmitted as electrical impulses that travel along neurones

• endocrine - info transmitted as chemical messengers called hormones that travel in blood

what do metabolic reactions within body produceq

waste products

excretion

removal of waste products (e.g. co2 and urea)

where is urea produced

liver

why is urea produced

excess amino acids, if more protein eaten than required, excess can’t be stored in body.

why does deamination occur

amino acids within protein provides useful energy so amino group must be removed from each amino acid to make energy accessible

deamination 3

• the amino group (-NH2) of an aa is removed, together with an extra hydrogen atom

• these combine to form ammonia (NH3)

• the remaining keto acid may enter the krebs cycle to be respired, be converted to glucose, or converted to glycogen/fat for storage

ammonia features 3

• very soluble

• highly toxic compound

• produced during deamination

why is ammonia damaging if allowed to build up in blood 3

• dissolves in blood to form alkaline ammonium hydroxide, disrupting blood pH

• impacts reactions of cell metabolism e.g. respiration

• interferes with cell signalling processes

why is ammonia converted to urea

urea is less soluble and less toxic than ammonia

how is ammonia converted to urea

combined with co2

kidney 2 functions

• osmoregulatory organ - regulate water content of blood (vital for maintaining blood pressure)

• excretory organ - excrete toxic waste products of metabolism (e.g. urea) and substances in excess of requirements (e.g. salts)

humans have_ kidneys

2

excretory system diagram

structures of excretory system

• renal artery

• renal vein

• kidney

• ureter

• bladder

• urethra

renal artery function

carries oxygenated blood (containing urea and salts) to the kidneys

renal vein function

carries deoxygenated blood (that has had urea and excess salts removed) away from the kidneys

kidney function

regulates water content of blood and filters blood

ureter function

carries urine from the kidneys to the bladder

bladder function

stores urine (temporarily)

urethra function

releases urine outside of the body

fibrous capsule

fairly tough outer layer that surrounds the kidney

3 main kidney parts beneath the fibrous capsule

• the cortex (contains glomerulus as well as Bowman’s capsule, proximal convoluted tubule, and distal convoluted tubule of the nephrons)

• the medulla (contains loop of Henle and collecting duct of the nephrons)

• the renal pelvis (where the ureter joins the kidney)

nephron 3 features

• thousands of tiny tubes in each kidney

• functional unit of the kidney

• responsible for urine formation

network of blood vessels associated with each nephron:

• within bowmans, each nephron = glomerulus

• each glomerulus is supplied w blood by an afferent arterial (which carries blood from renal artery)

• capillaries of the glomerulus rejoin to form efferent arteriole

• blood flows from efferent arteriole into network of caps that run closely alongside rest of nephron

• blood from caps eventually flows into renal vein

2 stages of urine formation in the kidneys

• ultrafiltration

• selective reabsorption

where does ultrafiltration occur

bowman’s capsule

where does selective reabsorption occur

proximal convoluted tubule

ultrafiltration:

small molecules (including aas, water, glucose, urea and inorganic ions) are filtered out of blood caps of glomerulus and into bowman’s capsule to form filtrate known as glomerular filtrate

selective reabsorption

useful molecules are taken back/reabsorbed from the filtrate and returned to the blood as the filtrate flows along the nephron

what happens after selective reabsorption (2)

• after the necessary reabsorption of aa, water, glucose and inorganic ions is complete (even some urea is reabsorbed), the filtrate eventually leaves the nephron and is now referred to as urine

• urine flows out of kidneys, along the ureters and into the bladder, where it is temporarily stored

ultrafiltration: where and why do arterioles branch off 

they branch off the renal artery and lead to each nephron, where they form a knot of capillaries (glomerulus) sitting inside the cup-shaped Bowman’s capsule

ultrafiltration: why is pressure increasing further into the glomerulus

capillaries get narrower and increases pressure on blood moving through which is already at high pressure bc directly from renal artery connected to aorta

ultrafiltration: what is the effect of increasing pressure in glomerulus

smaller molecules being carried in the blood is forced out of caps into bowman’s capsule, where they form filtrate

ultrafiltration: how is the glomerular capillaries separated from the lumen on Bowman’s

by 2 cell layers with a basement membrane:

• first cell layer: endothelium of the capillary - each capillary endothelial cell is perforated by thousands of tiny membrane-lined circular holes

• next layer: basement membrane - made up of network of collagen and glycoproteins

• second cell layer: epithelium of bowman’s capsule - these epithelial cells have many tiny finger-like projections with gaps in between them known as podocytes

ultrafiltration: what happens as blood passes through the glomerular capillaries

the holes in cap endothelial cells and the gaps between podocytes allow substances dissolved in blood plasma to pass into bowman’s capsule

• glomerular filtrate

glomerular filtrate

the fluid that filters through from the blood into bowman’s with main substances passing out:

• amino acids

• water

• glucose

• urea

• inorganic ions (mainly Na+, K+, Cl-)

ultrafiltration: why do red and white blood cells and platelets remain in the blood

too large to pass through the holes in capillary endothelial cells

ultrafiltration: basement membrane

acts as filter as it stops large protein molecules from getting through

ultrafiltration diagram

why does ultrafiltration occur

due to differences in water potential between plasma in the glomerular caps and the filtrate in the bowmans (water moves down water potential gradient, from region of high wp to lower wp. wp increased by high pressure, decreased by solutes)

2 factors affecting water potential

• pressure

• solute concentration

how pressure affects wp in glomerulus and bowmans 

• as afferent arteriole is wider than efferent, bp high in glom caps

• wp of blood plasma in glom caps is raised above wp of filtarte in bowmans

• water moves down wp gradient from blood plasma in glom caps into bowmans

how solute concentration affects wp in glom and bowmans

• basement membs allow most solutes within blood plasma to filter into bowmans, plasma protein mols are too big to get thru and stay in blood

• solute conc in blood plasma in glom caps higher than the filtrate in bowmans capsule

• wp of blood plasma lower than that of filtrate in bowmans

• water moves down wp gradient from bowmans cap into blood plasma in glom caps

the effect of pressure gradient outweighs…

effect of solute gradient, therefore wp of blood plasma in glom > wp of filtrate in bowmans so blood flows thru glom, overall movement of water down wp gradient from blood into the bowmans capsule

seelective reba