The Urinary System Flashcards
EXCRETION
Removal of wastes from the body; following organs serve this purpose
- Skin (sweat) - water, some metabolic wastes and salts
- Lungs- water vapour, carbon dioxide, other volatile metabolites
- GIS (faeces) ; bile metabolites, indigestible materials, small amount of water
- Urinary system (urine) water, salts, some nitrogenous wastes
NITROGENOUS WASTE
- Several nitrogen containing metabolites can’t accumulate in body→ excreted as nitrogenous waste in urine
- Accumulation of ammonia (amine group) has toxic effects→ amine converted to less toxic urea
- Creatinine formed from breakdown of creatine; creatine provides small reserve of ATP
- If components of DNA and RNA are degraded; the nitrogen containing subunit metabolised to uric acid
FUNCTIONS OF THE URINARY SYSTEM
- Principal means of removing foreign compounds from the body; by urine
- Maintenance of body fluids in a state of homeostasis; achieved by regulating physical and chemical properties of the blood (e.g. pH, volume/pressure, ion concentrations)
- Regulation of rate of production of red blood cells
- Vitamin D activation
THE NEPHRON
- Kidney functions achieved by combined response of several million units called nephrons
- Under normal conditions; kidney receives ⅕ of cardiac output
- Renal artery gives rise to a network of arteries→ branches to form arterioles network
Capillary network of the nephron is unusual in that there 2 capillary beds
- Afferent arteriole gives rise to multi- looped capillary bed (glomeruli)
- Blood drains for glomerulus into efferent arteriole
- From efferent arteriole, blood flows through the peritubular capillaries
- From capillaries blood drains through venous network of the kidney and out of kidney via renal vein
Additional components; several components to each nephron
- Each nephron has a renal tubule
- Bulbous opening; bowman’s capsule folds around and surrounds the glomerulus
THE STAGES OF URINE FORMATION (3)
- Filtration
- Reabsorption
- Secretion
URINE FORMATION: FILTRATION
- Walls of glomerulus are really leaky
- Inner of Bowman’s Capsule has many slits in it
- Blood flowing through glomerulus is under higher pressure than in other capillary beds
Bowman’s capsule and glomerulus structure→ means as blood passes through the glomerulus; most passes out of the glomerulus and into the peritubular capillaries
- But a fraction of it leaves the circulation and enters the capsular space
- This filtrate is what enters tubule of each nephron
THE FILTRATE
- ‘Plasma-like’
- Any molecule 8 billionths of a meter or smaller will pass into the capsular space (e.g. glucose, salts, urea)
- Albumin, larger proteins and blood cells largely remain in the blood as it exits the glomerulus
URINE FORMATION: REABSORPTION
- This is the process by which vital components are returned to the blood
- The sequence for any vital component being
- Filtrate→ transport across cells of tubular wall→ into space between tubules and peritubular capillaries→ across endothelium of the capillaries→ back into blood
PROXIMAL PARTS OF TUBULE
- Most of reabsorption occurs in proximal convoluted tubule (e.g. most of water and ions and vital nutrients)
DISTAL PARTS OF TUBULE
- Most of nutrient, ion and water reabsorbed occurs before filtrate reaches distal part of tubules but there remains substantial; amount of filtrate processing that occurs in this region
- The processing in this region is regulated by hormones
- Homeostasis; kidneys can ‘fine-tune’ the composition of the blood (e.g. person is dehydrated; increase water reabsorption)
URINE FORMATION: TUBULAR SECRETION
- Waste materials (e.g. drugs) or excess of other components travel along a different path (tubular secretion)
- Blood→ across endothelium→ into the space between tubules and peritubular capillaries→ transport into cells of tubular wall→ into the filtrate
LEAVING THE NEPHRONS
- Collecting tubules from many nephrons drain into each collecting duct
- Several collecting ducts form a papillary duct
URINE TRANSPORT
- Papillary ducts drain into a calyces (‘cups”)
- The calyces combine to form renal pelvis which is the funnel-shaped opening of the ureter
- Each ureter is a muscular tube that extends from each renal pelvis to the bladder
- Urine produced by each kidney drains out through a ureter→ ureters drain into bladder
- Urine drains out of the body via urethra
THE BLADDER
- Bladder designed to be stretched
- Moderately full bladder can hold 500mL but the capacity is between 700-800mL
- Contractions of the detrussor muscle lead to expulsion of the urine
- As bladder starts to fill it will stretch; stretching leads to signals being sent back to CNS
- Sympathetic motor signals lead to detrussor relaxation and contraction of an internal urethral sphincter. Somatic motor signals keep an external urethral sphincter closed→ bladder can fill with no risk of leakage
MICTURITION/URINATION
As bladder fills; sensory signals also reach micturition centre in pons→ integrates with info from higher brain centres leading to;
- Parasympathetic neurons stimulate the detrussor to contract and relax the internal urethral sphincter
When time is right→ cortical signals can inhibit the motor neurons that control the external sphincter
- Urination can occur if you are in an appropriate place to do that
