P4 Fluid Balance

P4 – Explain the role of the kidney in the homeostatic control of fluid balance
Homeostasis is how our body works to keep the internal environment constant. The kidneys play a role in the control of fluid balance. For this assignment I will be discussing the gross anatomy of the renal system and the kidney.
Renal System
The renal system is a group of organs that work together to produce, store, and release urine. It consists of 2 kidneys, ureters, the urinary bladder, urethra and renal vessels which include the renal artery and the renal vein. The main blood supply comes from the renal artery which passes from the abdominal aorta into the kidneys. Each artery divides into small branches within the kidney tissues. The blood leaves kidneys via the renal vein.
Kidneys
We have two kidneys of which are bean shaped and dark red in colour. The ureter and renal vein both emerge from the kidney and the renal artery enters the kidney at the hilum. There is a capsule of membrane surrounding the kidney and each is topped by the conical adrenal gland. These are surrounded by adipose tissue. Kidneys are vital in our survival and have a variety of functions that make them so important. They help regulate the amount of water in our body and balance the concentration of mineral ions in our blood. They produce urine which contains waste products, excess mineral ions and excess water. Waste is stored in the bladder before being removed as urine. It keeps useful substances such as glucose and protein. Each day 180 litre of water are filtered through our kidneys however only 1.5 litres of this is passed as urine.
Ureters
There are two ureters in the body, with one end connecting to the kidney and the other to the bladder, which is the organ responsible for collecting, storing, and excreting urine formed by the kidneys.
Urethra
The urethra is the tube that connects the urinary bladder to the outside of the body.
This system filters out excess fluid and other substances from the blood stream and waste. It has several functions including; the removal of metabolic waste products (urea & uric acid), the regulation of electrolyte balance (potassium, sodium, calcium), osmoregulation of acid base homeostasis and blood PH.
Renal system physiology
Maintenance of homeostasis
The kidneys maintain homestasis of a variety of important internal conditions by controlling the excretion of substances out of the body.
Ions – The kidneys are able to control the excretion of potassium, sodium, calcium, magnesium, phosphate and chloride ions into urine. When ions reach higher than normal concentration kidneys increase excretion which helps level return back to normal. When ions are lower than normal the kidneys can conserve these ions by allowing the ions to be reabsorbed into the blood during filtration.
PH – The kidneys monitor and regulate levels of hydrogen ions (H+) and bicarbonate ions in the blood to control blood PH. Kidneys excrete excess H+ into urine and can also conserve bicarbonate ions to act as important PH buffers in blood.
Osmoregulation – Kidneys maintain osmotic balance by controlling the amount of water that is filtered out of blood and excreted into urine. When a person consumes a high amount of water the kidneys decrease reabsorption of water to allow it to be passed as urine. When dehydrated the kidneys reabsorb as much water as possible into the blood to produce highly concentrated urine full of excreted ions and waste. These changes are controlled by the antidiuretic hormone which is produced in the hypothalamus and released by the pituitary gland to help retain water.
Blood pressure – When blood pressure increase the kidneys help reduce it by reducing the volume of blood in the body. They are able to reduce blood volume by decreasing reabsorption of water into the blood. This results in watery/dilute urine. When there is low blood pressure the kidneys produce the enzyme renin which helps constrict blood vessels. This results in more concentrated urine.
Micturition
Micturition is when urine is excreted from the bladder. It involved the contraction of the detrusor muscle and pressure from surrounding structures. Urination involves the relaxation of the external urethral sphincter. Urination is usually stimulated by the distention ofthe bladder as it fills with urine. When the walls of the bladder contract, nerve receptors are stimulated, and the urination reflex is triggered. This relaxation allows the bladder to empty. The bladder can hold up to 600 ml of urine.
Kidneys
The gross anatomy of the kidneys include; Renal hilus, renal vein, renal artery, ureter, interlobular vein, interlobular artery, renal capsule, renal cortex, renal medulla, renal pelvis, kidney nephron, collecting duct (Kidney).
Renal hilus - area of the kidney through which the ureter leaves the kidney and other structures including blood vessels, lymphatic vessels, and nerves enter/leave the kidney.
Renal vein - receives deoxygenated blood from the peritubular veins within the kidney. These merge into the interlobular, arcuate, interlobar and segmental veins which deliver deoxygenated blood to the renal vein, through which it is returned to the systemic blood circulation system.
Renal artery - The renal artery delivers oxygenated blood to the kidney. This main artery divides into many smaller branches as it enters the kidney via the renal hilus. These eventually separate into arterioles, one of which serves each nephron in the kidney.
Ureter – where urine is passed from kidney into the bladder.
Interlobular artery - delivers oxygenated blood at high pressure to the glomerular capillaries.
Renal capsule – This surrounds, encloses, and protects the kidney. Each kidney has its own renal capsule which helps to maintain the shape of the kidney as well as protecting it from damage.
Renal cortex - outer part of the kidney and has a reddish colour
Renal medulla – inner part of kidney
Renal Pelvis - receives the urine drained from the kidney nephrons via the collecting ducts and then the (larger) papillary ducts.
Kidney nephrons – Perform kidneys main functions. Approximately a million nephrons within each kidney.
Collecting duct - part of the kidney nephron. Collecting ducts unite to drain urine extracted by the kidney into papillary ducts, then into a minor calyx, then the major calyx then finally into the ureter through which the urine leaves the kidney.
Structure and functions of the kidney nephrons
The kidney nephrons are tiny filtering structures in the kidney, they consist of a twisted tubule closed at one end and open at the other with a network of blood vessels. Each kidney contains around one million nephrons. Each nephron has a length of about 3 cm.
They help to clean blood by removing excess water, waste and other substances. They also return substances such as sodium, potassium, or phosphorus whenever any of these are low in the body. The nephron is composed of two main structures; the glomerulus and the kidney tubule. The glomerulus is a tiny blood vessel/capillary which resembles a ball of yarn. It filters the blood by acting as a sieve. This helps to keep normal proteins and cells in the blood stream and allows waste to pass. The kidney tubules are tiny tubes where wastes and excess fluid pass through. They are located at the end of the nephron. They are important as they contain renal filtrate which is a filtrate that eventually becomes urine. After the tubular fluid leaves the renal tubules, it passes on to the collecting duct system, which connects the nephron to the ureter, through which urine is excreted.
Ultrafiltration
Ultrafiltration occurs at the barrier between the blood and the filtrate in the renal capsule or Bowman's capsule in the kidneys. It is the process of urea being extracted from the blood into the kidneys. Blood passing the top of the nephron is under high pressure so fluid is forced through the sieve-like capillaries and into the capsule. This is called filtrate. It doesn’t contain blood cells or larger proteins as they are to big to pass out of the capillaries and into the capsule.
Selective reabsorption
Selective reabsorption is when the kidneys reabsorb much need molecules whilst allowing the un needed molecule to be passed out in urine. The kidneys selectively reabsorb only those molecules which the body needs, back into the blood stream. Reabsorbed molecules include; glucose which was originally filtered out, as much water as the body needs to maintain a constant water level in the blood plasma and as many ions as the body needs to maintain a constant balance of water and mineral ions in the blood plasma
Counter current mechanism
Filtrate from the proximal tubule flows into the descending limb of the loop of Henle, which is permeable to water but does not transport ions. As the loop dips into the medulla, water moves by osmosis from the descending limb into the progressively more concentrated interstitial fluid. As a result, solutes are left behind in the proximal tubule, and the filtrate also becomes more concentrated. When the fluid flow reverses direction, the properties of the tubule epithelium change: the tubule becomes impermeable to water, but actively transports Na+, K+, and Cl- out of the tubule and into the interstitial fluid. The osmolarity of the filtrate is about 10X greater at the bottom of the loop than at the top. The purpose of this counter current exchange system is to produce hyperosmotic interstitial fluid in the medulla and hyposmotic filtrate leaving the loop of Henle (the water that leaves the descending loop of Henle is absorbed into the vasa recta, which prevents dilution of the interstitial fluid).
Sources of information http://www.abpischools.org.uk/page/modules/homeostasis_kidneys/index.cfm?coSiteNavigation_allTopic=1 https://www.boundless.com/physiology/textbooks/boundless-anatomy-and-physiology-textbook/the-urinary-system-25/overview-of-the-urinary-system-238/overview-of-the-urinary-system-1166-24/ http://www.innerbody.com/image/urinov.html http://medical.tpub.com/14295/css/14295_67.htm http://www.kidneyhealthcare.com/2010/12/nephron-structure-function-nephron.html http://www.s-cool.co.uk/a-level/biology/homeostasis/revise-it/water-levels-and-the-kidney