Biology: Excretion Flashcards
Excretion
The removal of metabolic wastes produced in the body. It is distinguished from elimination, the removal of indigestible material.
Aerobic Respiration
Leads to production of carbon dioxide and water.
Deamination
Deamination of amino acids in the liver leads to the production of nitrogenous wastes such as urea and ammonia.
Principal Organs of Excretion in Humans
The principal organs of excretion in humans are the lungs, liver, skin, and kidneys.
In the lungs, carbon dioxide and water vapor diffuse from the blood and are continually exhaled.
Sweat glands in the skin excrete water and dissolved salts (and a small quantity of urea). Perspiration serves to regulate body temperature, since the evaporation of sweat produces cooling.
The liver processes nitrogenous wastes, hemoglobin, and other chemicals for excretion. Urea is produced by the deamination of amino acids in the liver and diffuses into the blood for ultimate excretion in the kidneys. Bile salts are excreted as bile and pass out with the feces.
The kidneys function to maintain the osmolarity of the blood; excrete numerous waste products and toxic chemicals; and conserve glucose, salt, and water.
Kidneys
They regulate the concentration of salt and water in the blood through the formation and excretion of urine.
The kidneys are bean-shaped and are located behind the stomach and liver. Each kidney is composed of approximately one million units called nephrons.
Kidney Structure
Divided into three regions. The outer cortex, the inner medulla, and the renal pelvis.
A nephron consists of a bulb called Bowman’s capsule, which embraces a special capillary bed called a glomerulus. Bowman’s capsule leads into a long, coiled tubule that is divided into functionally distinct units, the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule, and the collecting duct.
The nephron is positioned such that the loop of Henle runs through the medulla, while the convoluted tubules flows into the pelvis of the kidney, a funnel-like region that opens directly into the ureter.
The ureters from the kidneys empty into the urinary bladder, where urine collects until expelled via the urethra.
Most of the nephron is surrounded by a complex peritubular capillary network to facilitate reabsorption of amino acids, glucose, salts, and water.
Urine Formation
Urine forms by 3 processes. Filtration, secretion, and reabsorption.
Filtration
Blood pressure forces 20% of the blood plasma entering the glomerulus through the capillary walls and into the surrounding Bowman’s capsule.
The fluid and small solutes entering the nephron are called the filtrate. The filtrate is isotonic with blood plasma.
Particles too large to filter through the glomerulus, such as blood cells and albumin, remain in the circulatory system.
Filtration is a process driven by the hydrostatic pressure of the blood.
Secretion
The nephron secretes substances such as acids, bases, and ions like potassium and phosphate from the interstitial fluid into the filtrate by both passive and active transport.
Materials are secreted from the peritubular capillaries into the nephron tubule.
Body Fluid pH
Remains relatively constant at 7.4; this consistency is attained by the removal of carbon dioxide through lungs or hydrogen ions through the kidneys.
Assessment of the pH is measured through the
- Arterial pH
- Partial pressure of the carbon dioxide
- Plasma bicarbonate
There are two types of acid-base disorders.
1) Respiratory - affect the blood acidity by causing changes in the partial pressure of CO2.
2) Metabolic - affect the blood acidity by causing changes in the HCO3-.
Reabsorption
Essential substances (glucose, salts, and amino acids) and water are reabsorbed from the filtrate and returned to the blood.
Reabsorption occurs primarily in the proximal convoluted tubule and is an active process.
Movement of these molecules is accompanied by the passive movement of water. This results in the formation of concentrated urine, which is hypertonic to the blood.
Nephron
Through the selective permeability of its walls and the maintenance of an osmolarity gradient, the nephron reabsorbs nutrients, salts, and water from the filtrate and returns them to the body, thus maintaining the bloodstream’s solute concentration.
The primary function of the nephron is to “clean” the blood plasma of unwanted substances as it passes through the kidney. Because blood plasma contains both wanted and unwanted substances, the nephron will selectively reabsorb wanted substances at selected portions of the nephron back into the plasma, while the remaining substances are excreted in the urine.
The primary site of nutrient reabsorption in the nephron is the proximal convoluted tubule. The ascending loop of Henle, collecting duct, and descending loop of Henle are the primary sites that regulate water, sodium, and potassium loss in the nephron. The distal convoluted tubule is the primary site for secretion of substances into the filtrate.
Aldosterone
A hormone that causes an increased exchange transport of sodium and potassium ions along the distal convoluted tubule, the collecting tubule, and the collecting duct, resulting in a decreased excretion of sodium ions in the urine and an increased potassium ion excretion in the urine. Furthermore, it does not affect renal blood flow.
Osmolarity Gradient
The selective permeability of the tubules establishes an osmolarity gradient in the surrounding interstitial fluid. By exiting and then reentering at different segments of the nephron, solutes create an osmolarity gradient, with tissue osmolarity increasing from cortex to inner medulla.
The solutes that contribute to the maintenance of the gradient are urea and salt (Na+ and Cl-).
The osmolarity of urine (determined by the concentration of dissolved particles) is established in the collecting tubule by means of this counter-current-multiplier system: the anatomic arrangement of the loop of Henle within the kidney permits the reabsorption of 99% of the filtrate in the collecting tubules. Thus, the production of concentrated urine is possible.
Concentration of Urine
The counter-current system causes the medium in the medulla of the kidney to be hyperosmolar with respect to the dilute filtrate flowing in the collecting tubule.
As the filtrate flowing in the collecting tubules passes through this region of the kidney, on its way to the pelvis and ureter, water flows out of the collecting tubules by osmosis. This water is removed by capillaries flowing in the medulla.
The reabsorption of water in this zone of the kidney, which permits the concentration of urine, depends on the permeability of the collecting tubules to water.
Regulation of the permeability of the collecting tubules to water is accomplished by the hormone ADH (vasopressin). ADH increases the permeability of the collecting duct to water, allowing more water to be absorbed and more concentrated urine to be formed.
