Homeostasis and the Kidney Flashcards
Define homeostasis
Homeostasis is the maintenance of constant steady state conditions within the body
What are the 3 basic features most homeostatic responses have?
1) A control system
2) A corrective mechanism
3) A negative feedback system
What is the function of the control system in homeostatic responses?
-Has sensors/receptors which provide information allowing the monitoring of the factor being controlled
-Usually in the brain
What is the function of the corrective mechanism in homeostatic responses?
-If the receptors show a departure from normal levels for the factor being controlled then the corrective mechanism brings about changes needed to return the factor to normal levels
-Eg. mammals overheating- corrective mechanism can include sweating and vasodilation
What is the function of the negative feedback system in homeostatic responses?
-Causes the corrective mechanism to be turned off and prevents over-correction
-E.g for temperature regulation, the stimulation of sweat glands and the degree of vasodilation is reduced as blood (body) temperature returns to normal
Why is homeostatic control of mammalian body systems essential?
-Providing optimum conditions for enzyme reactions in terms of pH and temperature
-Avoiding osmotic problems in cells and body fluids
What are the two functions of the kidney?
1) Excretion- the removal of toxic waste of metabolism. The main toxic waste product excreted by the kidney is urea. Other products are excreted too e.g. creatine.
2) Osmoregulation- the control of water potential of body fluids. Kidneys regulate water potential of the blood through controlling the volume and concentration of urine produced.
Describe the process of excretion in the urinary (excretory) system of the kidney
-Blood travelling through the aorta and renal artery reaches the kidney at the high pressures required for filtration - the kidney operates as a COMPLEX FILTER, keeping useful products in the blood and eliminating excretory products and excess water.
-Filtered blood leaves the kidney via the renal vein, excretory products and excess water pass into the ureter as urine which takes it to the bladder for storage.
-Sphincter muscles in the base of the bladder control the release of the urine, which exits the body through the urethra.
The kidney contains two main zones (regions) of tissue. What are they?
1) The cortex- outer dark region immediately under the thin covering layer (capsule).
2) The medulla- inner lighter region- subdivided into a number of pyramids where apices extend down into a large central cavity called the pelvis (basin).
State and explain the functional unit of the kidney
-Functional unit of the kidney is the nephron
-Over one million nephrons in each kidney- each operating as an individual filter
-The nephron originates and ends in the cortex, with a long central region (the loop of Henle) extending down into the medulla.
-Many nephrons join with a collecting duct, which also extends down through the medulla
Describe the structure of the nephron
-Originates as a cup-shaped Bowman’s capsule- each Bowman’s capsule is supplied from blood from an afferent arteriole (a branch in the renal artery) and the blood leaves through an efferent arteriole.
-Within the cup of the Bowman’s capsule the arteriole branches form a tightly coiled knot of capillaries called the glomerulus- capillaries which subsequently unite before forming the efferent arteriole.
-After leaving the Bowman’s capsule efferent arteriole branches to form a capillary network (vasa recta) that remains closely associated with the rest of the nephron.
-In the nephron- Bowman’s capsule extends into a coiled tube called the proximal convoluted tubule.
-Proximal convoluted tubule extends into the loop of Henle which dips down into the medulla of the kidney. The descending part of the loop is called the descending limb. The loop of Henle then bends sharply and returns back up through the medulla (the ascending limb) to reach the cortex again.
-Then it becomes the distal convoluted tubule- the distal convoluted tubule (and the others from many other nephrons) joins a collecting duct.
-The collecting ducts converge at the base of the pelvis and empty their contents (urine) into the ureter which takes the urine to the bladder.
What are the two main processes in kidney (and nephron) function?
1) Ultrafiltration- filtration of plasma and substances below a certain size into the Bowman’s capsule
2) Reabsorption- filtered useful products are selectively reabsorbed back into the bloodstream from the nephron after ultrafiltration.
Describe the process of ultrafiltration
-Blood enters the glomerulus with high hydrostatic pressure
-High hydrostatic pressure forces smaller components in the blood (glucose, amino acids, salts, water and urea) out of the capillaries and into the Bowman’s capsule. Larger components like blood cells and plasma proteins too large to pass into Bowman’s capsule pass into the nephron.
Why does the blood entering the glomerulus have high hydrostatic pressure?
-Short distance from the heart that the blood travels down the aorta and into the renal artery before branching into the kidney arterioles.
-The afferent arteriole of each glomerulus is wider than its efferent arteriole- going from a wide lumen to a narrow lumen
-Coiling of the capillaries in the glomerulus restricts blood flow increasing pressure.
How is ultrafiltration aided?
-Aided by the structure of capillary walls of the glomerulus and the lining of the Bowman’s capsule
-Single layer of squamous endothelial cells that form the walls of glomerular capillaries contain small pores
-Bowman’s capsule is lined with podocytes- podocytes have extensions in two planes that allow the filtered material to pass through easily.
-Basement membrane (separating capillaries and podocytes) is the effective filter- determines which components of the blood enter Bowman’s capsule- prevents the blood cells and plasma proteins from leaving the blood.
What are the 3 layers separating the blood in the capillary and inside the Bowman’s capsule and how are they specialised?
1) Capillary endothelium
2) Basement membrane
3) Podocytes
As well as hydrostatic pressure what is the other force in ultrafiltration?
Water potential
How does water potential affect ultrafiltration?
-For filtration to occur, water potential in glomerular capillaries (blood plasma) must exceed water potential in the Bowman’s capsule (glomerular filtrate), i.e. the glomerular filtrate must have a more negative water potential.
What are the 2 components of water potential and how do they affect filtration?
- Pressure potential and solute potential
-Hydrostatic pressure (pressure potential) of the blood is much greater than the hydrostatic pressure (back pressure) created by the filtrate in the nephron.
-The solute potential- represented by plasma proteins, as there are plasma proteins in the blood in glomerular capillaries but not in the filtrate- the filtrate has less negative solute potential than the blood in the glomerulus BUT although the difference in solute potential opposes filtration, the effect is insignificant when compared to the differences in hydrostatic pressure across the basement membrane, a difference that very strongly promotes filtration.
-Consequently the net filtration pressure causes fluid to move from the glomerular capillaries into the Bowman’s capsule.
