The human urinary system is an excretory system. It helps to maintain homeostasis in the body.
- Excretion: the getting rid of the waste products of metabolism.
- Homeostasis: the maintenance of a constant internal environment.
Structure of the human urinary system
- Kidneys: filter the blood taking out the waste products of metabolism, e.g. urea.
- Ureters: carry urine from the kidney to the urinary bladder.
- Urinary bladder: stores urine.
- Urethra: carries urine outside of the body.
Kidney structure and functions:
- Excretion: the main function of the kidney is to filter the blood taking out metabolic waste products producing urine.
- Osmoregulation: the kidneys control the amount of water in the body. If there is too much water in the body, the kidneys will excrete the excess water and if there is not enough water in the body, the kidneys will excrete much less water in an effort to conserve the remaining water in the body.
- pH control: the kidneys can control the acidity and alkalinity of the blood by excreting hydrogen ions or conserving hydrogen ions.
- Hormone production: the kidneys produce the hormone erythropoietin (EPO). EPO stimulates the bone marrow to produce red blood cells (erythrocytes).
- Filtration occurs in the cortex of the kidney.
- Blood flows through capillaries of the kidneys and water, salts, urea, glucose and amino acids are filtered out of the blood.
- Red blood cells, white blood cells, platelets and large plasma proteins (such as antibodies) are not filtered through as they are too big.
- The liquid that results after filtration is called the filtrate – it contains wastes as well as useful substances that need to be reabsorbed.
- Reabsorption occurs in both the cortex and medulla.
- Substances in the filtrate that are useful to the body (such as glucose and amino acids) are taken out of the filtrate back into the bloodstream.
- The kidney also transports substances such as drugs and hydrogen ions out of the bloodstream into the tubules of the kidney to contribute to the urine produced by the kidneys.
Urination (Micturition): the passing of urine from the body.
The nephron – the functional unit of the kidney
There are approximately 1 million nephrons in each kidney.
They are composed of four main parts:
- Bowman’s capsule – where filtration occurs.
- Proximal convoluted tubule – where most reabsorption occurs.
- Loop of Henle – where more reabsorption occurs.
- Distal convoluted tubule – where reabsorption of water and secretion of drugs and hydrogen ions occurs.
- The nephron receives blood from the renal arterioles.
- The renal arterioles carry blood to afferent arterioles.
- Each afferent arteriole enter the Bowman’s capsule.
- The ‘ball’ of blood vessels within in the Bowman’s capsule is called the glomerulus.
- The blood is then carried away from the Bowman’s capsule via the efferent arteriole.
- This blood is then circulated around the nephron for reabsorption of useful substances.
- The afferent arteriole is slightly wider than the efferent arteriole – which causes an increased blood pressure within the glomerulus. This increased blood pressure helps with the process of filtration in the Bowman’s capsule.
Urine production – detailed process
- Filtration occurs from the glomerulus into Bowman’s capsule.
- Blood enters the glomerulus from the afferent arteriole.
- Blood is under high pressure in the glomerulus and substances such as water, salts, urea, glucose and amino acids pass through.
- The liquid that passes through is called the glomerular filtrate.
The glomerulus and the Bowman’s capsule are adapted to carry out their functions by having the following characteristics:
- The Bowman’s capsule is cup-shaped to provide maximum surface area for filtration.
- The endothelium of the Bowman’s capsule is only one cell thick.
- The capillary walls of the glomerulus are one cell thick and more leaky than normal capillaries.
- Useful substances (such as glucose and amino acids) in the glomerular filtrate pass back into the bloodstream.
- Water is reabsorbed in the proximal convoluted tubule, descending loop of Henle, the distal convoluted tubule and the collecting duct.
- All the glucose and amino acids are reabsorbed in the proximal convoluted tubule.
- Salts are reabsorbed in the proximal convoluted tubule, the ascending loop of Henle and the distal convoluted tubule.
- Certain substances pass into the tubules of the nephron from the bloodstream by active transport.
- Drugs and poisons are actively transported out of the bloodstream into the proximal and distal convoluted tubules.
- Occasionally the nephrons of the kidney might not work properly and kidney failure may result.
- Patients with total kidney failure have to undergo dialysis.
- Dialysis is where a machine takes blood from the body and removes wastes and excess water from the blood before returning it to the body.
- In the long term, kidney failure patients usually receive a kidney transplant.
Osmoregulation – detailed process:
The kidneys control the amount of water and salts that are excreted.
Too much water in the body:
The brain detects the amount of water in the body. If it is too high, the pituitary stops secreting a hormone called anti-diuretic hormone (ADH). This travels in the bloodstream to the distal convoluted tubules and collecting ducts of the kidney and causes them to become less permeable and therefore, more water is excreted.
Too little water in the body:
The brain detects a reduced amount of water in the body and causes the pituitary to release ADH that travels in the bloodstream to the distal convoluted tubules and collecting ducts and causes them to become more permeable. Water is then reabsorbed into the blood and less is excreted in the urine.