Kidney Function, Osmoregulation, and Adaptations for Water Conservation
Kidney Function and Osmoregulation
Ultrafiltration
The Bowman’s capsule, glomerulus, proximal convoluted tubule (PCT), and distal convoluted tubule (DCT) are located in the cortex of the kidney. The loop of Henle and collecting duct are located in the medulla. Kidneys are located in the abdominal cavity and receive oxygenated blood from the renal artery. Amino acids can’t be stored in the body and must be deaminated in the liver to urea via the ornithine cycle. Urea then diffuses into the blood and is removed by the kidneys.
Ultrafiltration is the removal of small filtrates, resulting in the glomerular filtrate. The capillary walls of the glomerulus are highly permeable due to fenestrations made of a single layer of endothelial cells. A basement membrane surrounding the capillaries acts as a molecular filter.
Water and solutes are forced into the Bowman’s capsule. Blood cells and large solutes, such as plasma proteins, are prevented from leaving the blood.
High hydrostatic pressure in glomerular blood is due to:
- Contraction of the left ventricle in the heart
- The efferent (leaving) arteriole lumen being narrower than that of the afferent arteriole
This pressure forces filtrate through the endothelium and basement membrane, through spaces within the podocytes, and into the Bowman’s capsule, now called ultrafiltrate.
Selective Reabsorption
The PCT consists of a single layer of epithelial cells and possesses microvilli to provide a large surface area for reabsorption via carrier proteins. Many mitochondria provide ATP for the active transport of solutes.
Renal capillaries are close to the PCT, creating a short diffusion pathway.
Eighty percent of the filtrate is glucose, amino acids, and Na+. Fifty percent of urea passes back into the blood via facilitated diffusion.
Reabsorption of these solutes back into the blood lowers water potential (WP), forcing water to move in via osmosis from the PCT.
Solutes, such as Na+, are actively transported out of the epithelial cells of the PCT and into the intercellular spaces.
The concentration of solutes (Na+) is increased within the intercellular spaces. Solutes are transported away via the renal vein.
The now lower concentration of solutes in the epithelial cells causes solutes to move out of the filtrate in the PCT via facilitated diffusion.
Loop of Henle
The loop of Henle conserves water to produce urine with a high solute concentration and lower WP than blood.
- Descending limb: permeable to water, impermeable to ions
- Ascending limb: permeable to ions, impermeable to water
The ascending limb creates a WP gradient between the filtrate and tissue fluid within the medulla.
Na+ and Cl– are actively pumped out of the filtrate into the medulla, increasing its ion concentration and lowering its WP.
Water moves out of the descending limb into the medulla by osmosis. The impermeability of the ascending limb prevents water from moving back in.
Water is reabsorbed into the capillaries so the tissue fluid WP is not progressively increased. As filtrate moves down the descending limb, water is lost, creating a lower WP and a higher ion concentration. As filtrate moves up the ascending limb, ions are lost, creating a lower ion concentration and a higher WP. The apex of the loop is where the ion concentration is at its maximum.
Counter-Current Multiplier
– Filtrate flows in opposite directions within the loop/multiplied generation of a solute/WP gradient in the medulla due to ions being pumped out of the ascending limb & replaced as the filtrate flows from the PCT to the descending limb
Reabsorption of h2o-Filtrate entering the DCT has a higher WP than the medullar tissue/as filtrate moves down the collecting duct, it meets medullar tissue with an even lower WP due to the ascending limb pumping out ions/h2o therefore moves out of the filtrate by osmosis along the entire collecting duct & is reabsorbed into the blood creating a conc urine (with low WP)
Homeostasis – mechanisms which maintain a constant internal environment
Insures conditions inside cells remain optimum for function despite changes in external environment
Using negative feedback, a change in norm will bring about a corrective mechanism to restore it, an efficient one will minimise fluctuation around a set-point
The permeability of the DCT and collecting duct are controlled by the hormone ADH
This allows the control of the volume of h2o re-entering the blood to maintain its WP/This is osmoregulation under the endocrine system control
It maintains h2o & solute content of the blood, h2o can be lost by sweating, urine, faeces or evaporation from lungs
A decrease in WP of blood is detected by osmoreceptors within the hypothalamus
Nerve impulses are generated & passed to the posterior pituitary gland which responds by secreting mor ADH
ADH travels within the bloodstream & increases the permeability of the DCT & collecting duct
More h2o is reabsorbed into the blood from the filtrate & a small volume of (hypertonic) conc urine is produced
A high WP of blood = fear nervous impulses to posterior pituitary gland so less ADH released into blood = collecting duct & DCT less permeable to h2o so less h2o is reabsorbed into the blood from the filtrate by osmosis, a large vol of dilute urine produced
Desert animals- create metabolic h2o from the breakdown of food reserves during cellular respiration
They also live in cool/humid areas e.g underground to reduce h2o loss
Ammonia- small soluble & highly toxic must be excreted as it can’t be stored in body requiring a large vol of h2o to dilute to non-toxic levels to excrete safely
Freshwater fish– have body fluids with lower WP then surrounding & absorb large vol of h2o by osmosis via permeable gills & excrete ammonia as they must remove large vols of excess h2o some via kidney but most diffuse out via gills as ammonia is highly soluble
Urea– less toxic than ammonia so can be stored in body tissue for longer so requires less h2o to dilute to a safer level/high energy cost to produce but is an adaptation to land mammals to prevent dehydration
Uric acid– low toxicity & can be stored for long period, very little h2o needed to store & safely excrete (as white paste)/can also accumulate inside eggs of birds & reptiles without damaging embryo/also high energy cost but allows survival in arid environments & reduces mass= flight advantage