Chapter 13: The urinary system Flashcards
Urinary system consists of?
The urinary system is the main excretory system and consists of the following structures:
- 2 kidneys, which secrete urine
- 2 ureters that convey the urine from the kidneys to the urinary bladder
- the urinary bladder, which collects and stores urine
- the urethra through which urine leaves the body
Urinary system
•The urinary system plays a vital part in maintaining homeostasis of water and electrolytes within the body. The kidneys produce urine that contains metabolic waste products, including the nitrogenous compounds urea and uric acid, excess ions and some drugs. The main functions of the kidneys are:
-formation of urine, maintaining water, electrolyte and acid–base balance
-excretion of waste products
-production and secretion of erythropoietin, the hormone that stimulates formation of red blood cells
-production and secretion of renin, an important enzyme in the control of blood pressure.
•Urine is stored in the bladder and excreted by the process of micturition.
•The first sections of this chapter explore the structures and functions of the organs of the urinary system and the impact of ageing on kidney function. In the final section the consequences of abnormal functioning of the various parts of the urinary system on body function are considered.
Kidneys
- The kidneys lie on the posterior abdominal wall, one on each side of the vertebral column, behind the peritoneum, and below the diaphragm. They extend from the level of the 12th thoracic vertebra to the 3rd lumbar vertebra, receiving some protection from the lower rib cage. The right kidney is usually slightly lower than the left, probably because of the considerable space occupied by the liver.
- Kidneys are bean-shaped organs, about 11 cm long, 6 cm wide, 3 cm thick and weigh 150 g. They are embedded in and held in position by, a mass of fat. A sheath of fibrous connective tissue, the renal fascia, encloses the kidney and the renal fat.
Organs associated with the kidneys
As the kidneys lie on either side of the vertebral column, each is associated with different structures.
Right kidney
- Superiorly – the right adrenal gland
- Anteriorly – the right lobe of the liver, the duodenum, and the hepatic flexure of the colon
- Posteriorly – the diaphragm, and muscles of the posterior abdominal wall.
Left kidney
- Superiorly – the left adrenal gland
- Anteriorly – the spleen, stomach, pancreas, jejunum, and splenic flexure of the colon
- Posteriorly – the diaphragm and muscles of the posterior abdominal wall.
Gross structure of the kidney
•There are three areas of tissue that can be distinguished when a longitudinal section of the kidney is viewed with the naked eye:
-an outer fibrous capsule, surrounding the kidney
-the cortex, a reddish-brown layer of tissue immediately below the capsule and outside the renal pyramids
-the medulla, the innermost layer, consisting of pale conical-shaped striations, the renal pyramids.
•The hilum is the concave medial border of the kidney where the renal blood and lymph vessels, the ureter, and nerves enter.
•Urine formed within the kidney passes through a renal papilla at the apex of a pyramid into a minor calyx
The microscopic structure of the kidney
The kidney contains about 1–2 million functional units, the nephrons, and a much smaller number of collecting ducts. The collecting ducts transport urine through the pyramids to the calyces, giving the pyramids their striped appearance. The collecting ducts are supported by connective tissue, containing blood vessels, nerves, and lymph vessels.
The nephron
Nephron, the functional unit of the kidney, is the structure that produces urine in the process of removing waste and excess substances from the blood. There are about 1,000,000 nephrons in each human kidney.
Formation of urine
The kidneys form urine, which passes to the bladder for storage prior to excretion. The composition of urine reflects the exchange of substances between the nephron and the blood in the renal capillaries. Waste products of protein metabolism are excreted, water and electrolyte levels are controlled and pH (acid-base balance) is maintained by the excretion of hydrogen ions. There are three processes involved in the formation of urine:
- filtration
- selective reabsorption
- secretion
Filtration
- This takes place through the semipermeable walls of the glomerulus and glomerular capsule. Water and other small molecules readily pass through, although some are reabsorbed later. Blood cells, plasma proteins, and other large molecules are too large to filter through and therefore remain in the capillaries. The filtrate in the glomerulus is very similar in composition to plasma with the important exceptions of plasma proteins and blood cells.
- Filtration takes place because there is a difference between the blood pressure in the glomerulus and the pressure of the filtrate in the glomerular capsule.
Autoregulation
- Renal blood flow, and therefore glomerular filtration, is protected by a mechanism called autoregulation, whereby renal blood flow is maintained at a constant pressure across a wide range of systolic blood pressures (from around 80–200 mmHg). Autoregulation operates independently of nervous control, i.e., if the nerve supply to the renal blood vessels is interrupted, autoregulation continues to operate. It is therefore a property inherent in renal blood vessels; it may be stimulated by changes in blood pressure in the renal arteries or by fluctuating levels of certain metabolites, e.g., prostaglandins.
- In severe shock, when the systolic blood pressure falls below 80 mmHg, autoregulation fails and renal blood flow and the hydrostatic pressure decrease, impairing filtration within the glomeruli.
Selective reabsorption
- Most reabsorption from the filtrate back into the blood takes place in the proximal convoluted tubule, whose walls are lined with microvilli to increase surface area for absorption. Many substances are reabsorbed here, including some water, electrolytes, and organic nutrients such as glucose. Some reabsorption is passive, but some substances, e.g., glucose, are actively transported.
- Active transport takes place at carrier sites in the epithelial membrane, using chemical energy to transport substances against their concentration gradients
- Some ions, e.g., sodium and chloride, can be absorbed by both active and passive mechanisms depending on the site in the nephron.
- Some constituents of glomerular filtrate (e.g., glucose, amino acids) do not normally appear in urine because they are completely reabsorbed unless blood levels are excessive.
- Reabsorption of nitrogenous waste products, such as urea, uric acid, and creatinine is very limited.
parathyroid hormone
This is secreted by the parathyroid glands and together with calcitonin from the thyroid gland regulates the reabsorption of calcium and phosphate from the distal collecting tubules, so that normal blood levels are maintained. Parathyroid hormone increases the blood calcium level and calcitonin lowers it.
Antidiuretic hormone (ADH)
This is secreted by the posterior pituitary. It increases the permeability of the distal convoluted tubules and collecting tubules, increasing water reabsorption. Secretion of ADH is controlled by a negative feedback system
Aldosterone
Secreted by the adrenal cortex, this hormone increases the reabsorption of sodium and water and the excretion of potassium. Secretion is regulated through a negative feedback system.
Atrial natriuretic peptide (ANP)
This hormone is secreted by the atria of the heart in response to the stretching of the atrial wall when blood volume is increased. It decreases the reabsorption of sodium and water from the proximal convoluted tubules and collecting ducts. Secretion of ANP is also regulated by a negative feedback system.
Tubular secretion
Filtration occurs as blood flows through the glomerulus. Substances are not required and foreign materials, e.g., drugs including penicillin and aspirin, may not be entirely filtered out of the blood because of the short time it remains in the glomerulus. Such substances are cleared by secretion from the peritubular capillaries into the filtrate within the convoluted tubules. Tubular secretion of hydrogen ions (H+) is important in maintaining normal blood ph.
Summary of urine formation
The three processes involved – filtration, selective reabsorption, and tubular secretion.
Composition of urine
Urine is clear and amber in color due to the presence of urobilin, a bile pigment altered in the intestine, reabsorbed then excreted by the kidneys. The specific gravity is between 1020 and 1030, and the pH is around 6 (normal range 4.5–8). A healthy adult pass from 1000 to 1500 mL per day. The volume of urine produced, and the specific gravity varies according to fluid intake and the amount of solute excreted.
Water balance and urine output
- The source of most body water is dietary food and fluid although a small amount (called ‘metabolic water’) is formed by cellular metabolism. Water is excreted as the main constituent of urine, in expired air, feces, and through the skin as sweat. The amount lost in expired air and feces is constant; the amount of sweat produced is associated with environmental and body temperatures.
- Sensory nerve cells in the hypothalamus (osmoreceptors) detect changes in the osmotic pressure of the blood. Nerve impulses from the osmoreceptors stimulate the posterior pituitary to release ADH. When the osmotic pressure is raised, i.e. the blood is becoming more concentrated, ADH output is increased and as a result, water reabsorption by the distal convoluted tubules and collecting ducts is increased, reducing the blood osmotic pressure and ADH output. This negative feedback mechanism maintains the blood osmotic pressure (and therefore sodium and water concentrations) within normal limits
- When blood volume is increased, stretch receptors in the atria of the heart are stimulated and cardiac muscle cells release atrial natriuretic hormone (ANP). This reduces the reabsorption of sodium and water by the proximal convoluted tubules and collecting ducts, meaning that more sodium and water are excreted. In turn, this lowers blood volume and reduces atrial stretching, and through the negative feedback mechanism, ANP secretion is switched off. Raised ANP levels also inhibit the secretion of ADH and aldosterone, further promoting the loss of sodium and water.
Electrolyte balance
•Changes in the concentration of electrolytes in the body fluids may be due to changes in:
-the body water content, or
-electrolyte levels.
•Several mechanisms maintain the balance between water and electrolyte concentration.
Sodium and potassium balance
- Sodium is the most common cation (positively charged ion) in extracellular fluid and potassium is the most common intracellular cation.
- Sodium is a constituent of almost all foods and, furthermore, salt is often added to food during cooking. This means that intake is usually in excess of the body’s needs. It is excreted mainly in urine and sweat.
- The amount of sodium excreted in sweat is usually insignificant unless sweating is excessive. This may occur when there is pyrexia (fever), a high environmental temperature, or sustained physical exercise. Normally the renin-angiotensin-aldosterone mechanism (described below) maintains the concentration of sodium and potassium within physiological limits. When excessive sweating is sustained, e.g., living in a hot climate or working in a hot environment, acclimatization occurs in approximately 7 to 10 days and electrolyte secretion lost in sweat is reduced.
- Sodium and potassium occur in high concentrations in digestive juices – sodium in gastric juice and potassium in pancreatic and intestinal juice. Normally these ions are reabsorbed by the colon but following acute and prolonged diarrhea they may be excreted in large quantities causing electrolyte imbalance.
Renin-angiotensin-aldosterone system
Sodium is a normal constituent of urine, and its excretion is regulated by the hormone aldosterone, secreted by the adrenal cortex. Cells in the afferent arteriole of the nephron release the enzyme renin in response to sympathetic stimulation, low blood volume, or low arterial blood pressure. Renin converts the plasma protein angiotensinogen, produced by the liver, to angiotensin 1. Angiotensin converting enzyme (ACE), formed in small quantities in the lungs, proximal convoluted tubules, and other tissues, converts angiotensin 1 into angiotensin 2, which is a very potent vasoconstrictor and increases blood pressure. Renin and raised blood potassium levels also stimulate the adrenal gland to secrete aldosterone. Water is reabsorbed with sodium and together they increase the blood volume, which reduces renin secretion through the negative feedback mechanism. When sodium reabsorption is increased potassium excretion is increased, indirectly reducing intracellular potassium. Profound diuresis may lead to hypokalemia (low blood potassium levels).