Renal system summary
The renal system consists of the kidneys producing urine that is then removed from the body through
the urinary tract. Kidneys play a crucial role in homeostasis; by removing excess water from the
blood they maintain water balance and blood pressure. Kidneys regulate levels of electrolytes (e.g.
Na+ and K+), regulate pH, and by removing waste products from the blood prevent toxicity.
Remember the acronym A WET BED to remember all 7 functions.
Kidneys consist of millions of nephrons, their functional units. Each nephron has an afferent
arteriole providing blood to the leaky glomerular capillaries that form a filter, squeezing part of
blood plasma and small molecules out but retaining all the blood cells and plasma proteins. The
resulting filtrate is collected in the Bowman’s capsule and flows as primary urine into the tubule.
The separation of waste products from useful molecules happens by the tubular epithelium, where
sodium is actively transported, dragged against the diffusion gradient, from filtrate back into blood
in the peritubular capillaries surrounding the tubule. This process is called reabsorption, and in a
similar way other useful molecules that the body needs, like electrolytes, glucose and amino acids,
are reabsorbed. Only molecules that have their selective transporters in the tubule will be retained,
the rest will be lost in urine. The result is that the blood in the peritubular capillaries becomes more
concentrated compared to the filtrate in the tubule. This makes water move into the peri-tubular
capillaries by osmosis, a force that moves solvent (water) towards the higher concentration of
solutes. In this way most of the water filtered out in the glomerulus will be retained.
Micturition is the process of releasing urine stored in the bladder through the urethra. The bladder
stretch sends a signal to the micturition centre in the brainstem, where parasympathetic nerves of
the autonomic nervous system cause bladder contraction and release of the internal urethral
sphincter. But luckily we have learned to voluntary control the external urethral sphincter and pee
only when and where appropriate.
The kidneys play an important role in water balance and blood pressure regulation. If there is too
little fluid in the body the blood volume will drop, which decrease the refill of the ventricles during
the diastole, causing decreased stroke volume, which decreases cardiac output and so blood
pressure. Lower-than-normal blood pressure is sensed by the nephrons that respond by releasing
renin that increases the production of angiotensin that increases the release of the hormone
aldosterone. Aldosterone increases the active sodium transport (reabsorption) and with it, increases
the osmotic gradient that drives the reabsorption of more water. The retaining of salty water keeps
blood volume and blood pressure up. This RAA system is important in the long-term regulation of
blood pressure and the response to shock due to blood volume loss.
If you get dehydrated (loosing water without the salt) you need to retain extra water in a way without
increasing sodium transport (otherwise it would not reduce the salt concentration). When the
hypothalamus senses a too-high salt concentration (osmolarity) in the blood, it causes the pituitary
gland to release anti-diuretic hormone (ADH), which increases water reabsorption by increasing
the number of water channels, and so water permeability of the tubular epithelium. The result is that
more water can follow the salts back into the blood and less is lost in urine. The hypothalamus also
gives the sense of thirst that makes you replenish the fluid levels.
The renal system consists of the kidneys producing urine that is then removed from the body through
the urinary tract. Kidneys play a crucial role in homeostasis; by removing excess water from the
blood they maintain water balance and blood pressure. Kidneys regulate levels of electrolytes (e.g.
Na+ and K+), regulate pH, and by removing waste products from the blood prevent toxicity.
Remember the acronym A WET BED to remember all 7 functions.
Kidneys consist of millions of nephrons, their functional units. Each nephron has an afferent
arteriole providing blood to the leaky glomerular capillaries that form a filter, squeezing part of
blood plasma and small molecules out but retaining all the blood cells and plasma proteins. The
resulting filtrate is collected in the Bowman’s capsule and flows as primary urine into the tubule.
The separation of waste products from useful molecules happens by the tubular epithelium, where
sodium is actively transported, dragged against the diffusion gradient, from filtrate back into blood
in the peritubular capillaries surrounding the tubule. This process is called reabsorption, and in a
similar way other useful molecules that the body needs, like electrolytes, glucose and amino acids,
are reabsorbed. Only molecules that have their selective transporters in the tubule will be retained,
the rest will be lost in urine. The result is that the blood in the peritubular capillaries becomes more
concentrated compared to the filtrate in the tubule. This makes water move into the peri-tubular
capillaries by osmosis, a force that moves solvent (water) towards the higher concentration of
solutes. In this way most of the water filtered out in the glomerulus will be retained.
Micturition is the process of releasing urine stored in the bladder through the urethra. The bladder
stretch sends a signal to the micturition centre in the brainstem, where parasympathetic nerves of
the autonomic nervous system cause bladder contraction and release of the internal urethral
sphincter. But luckily we have learned to voluntary control the external urethral sphincter and pee
only when and where appropriate.
The kidneys play an important role in water balance and blood pressure regulation. If there is too
little fluid in the body the blood volume will drop, which decrease the refill of the ventricles during
the diastole, causing decreased stroke volume, which decreases cardiac output and so blood
pressure. Lower-than-normal blood pressure is sensed by the nephrons that respond by releasing
renin that increases the production of angiotensin that increases the release of the hormone
aldosterone. Aldosterone increases the active sodium transport (reabsorption) and with it, increases
the osmotic gradient that drives the reabsorption of more water. The retaining of salty water keeps
blood volume and blood pressure up. This RAA system is important in the long-term regulation of
blood pressure and the response to shock due to blood volume loss.
If you get dehydrated (loosing water without the salt) you need to retain extra water in a way without
increasing sodium transport (otherwise it would not reduce the salt concentration). When the
hypothalamus senses a too-high salt concentration (osmolarity) in the blood, it causes the pituitary
gland to release anti-diuretic hormone (ADH), which increases water reabsorption by increasing
the number of water channels, and so water permeability of the tubular epithelium. The result is that
more water can follow the salts back into the blood and less is lost in urine. The hypothalamus also
gives the sense of thirst that makes you replenish the fluid levels.
