Homeostasis and the Kidney Flashcards
What is homeostasis?
The maintenance of a constant internal environment
What is mammalian tissue made up of?
A collection of cells bathed in tissue fluid
What features of the blood must be kept within narrow limits?
-Blood glucose concentration
-Temperature
-pH
-Water potential
Conc. of ions
-Oxygen levels
What is the mechanism of delivering homeostasis?
Negative feedback
What is negative feedback?
Type of control in which the conditions being regulated are returned to set values as soon as theyre detected to have deviated
What are the components of a negative feedback system?
- A detector device that measures the value of the variable (sensor)
- A control unit to which this information is transmitted
- When the value is below or above required level, the control unit activates an effector device
- Once it returns to normal levels, the control unit switches off the response (to avoid over correction, but does normally contain some level of overshoot)
What does hypotonic mean?
Having a lower osmotic pressure (Hypo - under/below)
What does hypertonic mean?
Having a higher osmotic pressure (more dissolved particles) (hyper - over/above)
What does isotonic mean?
?Having the same osmotic pressure
What is osmosis?
Net movement of molecules from a region of high water potential to a region of low water potential through a selectively permeable membrane
What is simple diffusion?
The movement of molecules through a cell membrane without aid
What is facilitated diffusion?
Passive movement of substances across a membrane with a carrier molecule
What is active transport?
The movement of dissolved molecules into or out of a cell from a region of lower concentration to a region of lower concentration, requiring energy as ATP from respiration
What is pinocytosis?
A type of endocytosis where the cell takes in fluids and dissolved particles
What are the roles of the kidney in mammals?
Excretion and osmoregulation
What is excretion?
The removal of toxic waste products of metabolism
What is osmoregulation?
The control of the water potential of body fluids
How do kidneys receive blood?
Through the renal artery
How does blood leave the kidneys?
Drained via the renal vein
How does urine leave the body
Flows to ureter and pushed to bladder by peristalsis, where it is stored and later released by relaxing the sphincter muscles at base of bladder allowing urine to pass to urethra and out
What does the kidney consist of?
- A thin outer covering (renal capsule)
- An outer cortex
- Inner medulla
- millions of nephrons
- A large central pelvis
How does a nephron purify blood
ultrafiltration, selective reabsorption, and concentration
What are the regions of a nephron?
- Renal corpuscle (glomerulus and Bowman’s Capsule)
- Proximal convoluted tubule
- Descending limb of loop of Henle
- Ascending limb of loop of Henle
- Distal convoluted tubule
- Collecting duct
Describe the glomerulus
A dense network of capillaries held in an invagination of the Bowman’s Capsule. Blood enters from the wider Afferent arteriole and leaves through narrow Efferent arteriole - difference in diameter creates high blood pressure
Describe the Bowman’s capsule
Cup shaped capsule containing the glomerulus. Between inner and outer linings there is a capsular space. Inner layer is composed of cells called Podocytes which dont fit closely together. They have foot-like projections in contact with basement membrane and filtrate flows between these gaps. This arrangement results in filter preventing passage of cells and large molecules
What is the effective filter in the kidney?
The basement membrane - determines what components enter bowman’s capsule
What is ultrafiltration?
The filtration of plasma and substances below a certain size into Bowman’s capsule
Why is blood in glomerular kidneys under high hydrostatic pressure?
- Diameter of efferent arteriole s=narrower than afferent arteriole
- Short distance from heart blood travels
- Coiling of capillaries in the glomerulus further restricts blood flow increasing pressure
What happens at the proximal convoluted tubule?
Site of selective reabsorption. Water is reabsorbed in the PCT to avoid dehydration. Inner layer covered by microvilli. Surface area of the base of epithelium is increased due to infoldings of the plasma membrane and numerous mitochondria lie between the infoldings. Close association of many capillaries, with thin squamous epithelium and the flow of blood within the capillaries and the filtrate in the tubules helps maintain the conc. gradients for movement o molecules
What is reabsorbed in the PCT?
Over 80% of filtrate, all food substances, most hormones, and 85% of salt and water
What happens to amino acids and glucose in PCT?
move by facilitated diffusion into cells and actively transported into intercellular space where they diffuse into surrounding capillaries
How does water leave the PCT?
solute uptake makes the water potential of the blood more negative so 70% of water in filtrate follows by osmosis
How does small proteins leave PCT?
Reabsorbed by pinocytosis (Selective reabsorption)
What happens to urea in PCT?
Not reabsorbed
What does the Loop of Henle allow?
The production of hypertonic urine in mammals
Describe the ascending limb of LOH
- Wider
- Walls lined with cuboidal epithelium, contain mitochondria, impermeable to water
- Secretes Na+ and Cl- ions into medulla via active transport
- NaCl builds up in interstitial fluid of medulla, creating v. neg. solute potential.
- Filtrate becomes progressively dilute as it ascends the loops due to active loss of ions, so is hypotonic at top of limb
Describe descending limb of LOH
- Narrow
- Highly permeable to water
- Water is osmotically removed into interstitial fluid, enters capillary network and removed from medulla
- Filtrate becomes more and more concentrated as it descends
What happens in the distal convoluted tubule?
Further regulation of blood composition. pH and ionic composition of blood in capillaries surrounding the tubule are adjusted and toxic substances secreted from blood to filtrate for disposal
What happens in the collecting duct?
osmoregulation. Most water is reabsorbed in PCT and LOH, the progress is passive and the exact amount of water reabsorbed back into blood cannot be controlled. Reabsorption of water from collecting duct and DCT can be controlled by ADH
What is ADH?
Hormone controlling permeability of collecting duct walls. Produced by hypothalamus, secreted into posterior lobe of pituitary body and stored. Increases permeability of the walls of CD and DCT by promoting opening of aquaporins in the walls
What happens when there is a rise in blood concentration? (Less water)
- Detected by osmoreceptors in hypothalamus
- Osmoreceptors send impulses to the posterior lobe of pituitary body
- Pituitary releases more ADH
- Increases permeability of walls by opening more aquaporins in walls of ducts, letting more water leave the tubules to pass into the medulla, so smaller volume of highly conc. urine produced
What happens with a fall in blood concentration?
- Detected by osmoreceptors in hypothalamus
- Osmoreceptors send impulses to the posterior lobe of pituitary body
- Pituitary releases less ADH
- Walls of CD and DCT become impermeable to water by closing the aquaporin channels in the walls of these ducts. Less water leaves the tubules to pass into medulla and more remains in tubules, larger volume of urine produced
Why could blood become too concentrated?
sweating, not drinking enough, salty meal, diarrhoea
When could blood become too dilute?
Drinking excess water or when loss of water by sweating is minimal
