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Kidney: Body organ that filters blood for the secretion of Urine and that regulates ion concentrations.

Hormonal Regulation

JoVE 10893

The renin-aldosterone system is an endocrine system which guides the renal absorption of water and electrolytes, thus managing blood pressure and osmoregulation. Activation of the system begins in the kidneys with a small cluster of cells adjacent to the afferent and efferent blood vessels of the renal corpuscle. As the nephrons are filtering blood, juxtaglomerular cells monitor blood pressure. If they detect a decrease in pressure, they release the hormone renin into the bloodstream. Circulating renin interacts with angiotensinogen, a precursor protein synthesized by the liver, to create angiotensin I. A final step cleaves angiotensin I into angiotensin II, a process achieved by angiotensin-converting enzyme, or ACE, which is released by the lungs. Angiotensin II temporarily increases blood pressure by contracting smaller blood vessels. It also induces the release of aldosterone from the adrenal cortex of the kidneys. Aldosterone directly stimulates the reabsorption of sodium and the excretion of potassium by the kidneys to maintain electrolyte balance. Moreover, circulating levels of aldosterone stimulate the release of antidiuretic hormone, or ADH, by the hypothalamus in the brain. Upon reaching the kidneys, ADH upregulates aquaporin channels in the nephrons which increase the water retention in the blood vessels. The combined effects of

 Core: Biology

Filtration

JoVE 10891

The function of the kidneys is to filter, reabsorb, secrete, and excrete. Every day the kidneys filter nearly 180 liters of blood, initially removing water and solutes but ultimately returning nearly all filtrates into circulation with the help of osmoregulatory hormones. This process removes wastes and toxins but is also crucial to maintain water and electrolyte levels. Most of these functions are performed by the tiny but numerous nephrons contained within the kidneys. Blood enters the renal corpuscle of the nephron through a glomerulus of capillaries. The capillaries are surrounded by a structure called the Bowman’s capsule which absorbs water and most solutes from the blood. The blood pressure from capillaries pushes these into the capsules. If the blood pressure is too high, as seen in hypertension, the capillaries can weaken and harden, reducing the ability of the kidney to filter the blood. The filtrate from the corpuscles empty into the proximal convoluted tubules and the descending portions of the Loop of Henle. Here nearly 70% of solutes—salt, glucose, amino acids, and bicarbonates—are reabsorbed into the surrounding capillaries. Circulating blood hormones involved in osmoregulation induce reabsorption of sodium, calcium, or more water if needed to increase or decrease blood pressure and regulate electrolytes. Sec

 Core: Biology

Kidney Structure

JoVE 10890

The kidneys are two large bean-shaped organs located in the upper abdomen. They filter the blood several times a day to remove toxins and rebalance water and electrolytes of the circulatory system via the renal veins. The kidneys receive blood directly from the heart via the renal arteries. These arteries enter the kidney at the hilum, the concave surface of the bean, where they branch and divide into smaller vessels and capillaries. The renal cortex is the thick outer layer of the kidney. It houses renal corpuscles, where capillaries come into close contact with the end of a renal tubule. The end of the tubule, or Bowman’s capsule, surrounds a net of capillaries that looks like a ball, the glomerulus. This unusual arrangement of the capillaries increases the surface area where the end of the renal tubule and the capillaries interact. From the Bowman’s capsule, the convoluted tubules extend into the Loop of Henle that lay in the renal medulla, the tissue beneath the renal cortex. Cortical intrusions structure the medulla into multiple renal pyramids. The apex of each pyramid points towards the hilum area, thus draining the collecting ducts into calyces in the renal pelvis. As the pelvis fills, urine is emptied into the ureter. The ureter connects the kidneys to the bladder, where urine is stored before being eliminated. The renal corp

 Core: Biology

What is the Skeletal System?

JoVE 10863

The adult human skeleton comprises 206 bones that are connected through cartilage, tendons, and ligaments. The skeleton provides a rigid framework for the human body, protects internal organs, and enables movement and locomotion. The human skeletal system consists of the axial and appendicular skeletons. Bone tissue is continuously built up and chewed away by specialized bone cells which are essential to overall health. Dysregulated bone cells and incorrect levels of chemical compounds in the blood lead to bone diseases. The axial skeleton consists of 80 bones and is divided into three regions: the skull, the vertebral column, and the rib cage. The upper portion of the skull—the cranium—consists of eight bones that enclose the brain, while the lower part consists of 14 bones. The vertebral column consists of 33 vertebrae: seven cervical, 12 thoracic, five lumbar, five fused sacral vertebrae, and four fused coccygeal vertebrae. The rib cage adds stability to the vertebral column and also protects the lungs and heart. It consists of 12 pairs of ribs, which attach to the thoracic vertebra via the costovertebral joint. The anterior portion of the rib cage attaches to the sternum—the flat bone at the center of the front of the chest—via the costal cartilages. The first seven ribs on each side are known as true ribs, as their cartilages

 Core: Biology

What Are Osmoregulation and Excretion?

JoVE 11001

Organisms must keep bodily fluids at a constant temperature and pH while maintaining specific solute concentrations in order to support life functions. Osmoregulation is the process that balances solute and water levels.

Osmosis is the tendency of water to move from solutions with lower ion concentrations, or osmolarities, to those with higher ion concentrations. Osmosis occurs in response to differences in the molecular concentrations of solutions separated by a semipermeable membrane. Bodily fluids, which are separated by such membranes, contain water, non-electrolytes, and electrolytes—solutes that dissolve into ions in water. Both electrolytes and non-electrolytes influence osmotic balance. However, since the more important factor to osmosis is solute number, rather than size, the contribution of electrolytes is more significant. Unlike water, electrolytes cannot diffuse passively through membranes but rely on facilitated diffusion and active transport. In facilitated diffusion, protein-based channels move solutes across membranes. Conversely, energy is used to move ions against concentration gradients in active transport. When animals ingest food, material that cannot be used is excreted from the body. Excretory systems in nature involve tradeoffs between conserving energy and water. Nitrogen is among the most signific

 Core: Biology

Urea Cycle

JoVE 10892

The urea cycle describes how liver cells convert ammonia to urea. Ammonia is a toxic waste product of protein catabolism. Land animals must convert ammonia into the less toxic urea which can be safely eliminated by the kidneys through urine. Marine animals excrete ammonia directly, and the surrounding water dilutes the ammonia to safe levels.

There are five basic steps in the urea cycle: the conversion of ammonia (NH3) to carbamoyl phosphate the introduction of ornithine in the transformation of carbamoyl phosphate to citrulline the transformation of citrulline into arginosuccinate involving aspartate and chemical energy (ATP) the conversion of arginosuccinate into arginine with fumarate as a by-product the formation of urea and ornithine from arginine Notice that ornithine is used in the second step and is regenerated in the last step. Since ornithine is recycled, the urea cycle is sometimes referred to as the ornithine cycle. Elevated levels of blood ammonia, or hyperammonemia, results from an interruption of the urea cycle. This can occur at the organ level where scar tissue blocks the blood supply to the liver. Scar tissue, or cirrhosis, can result from chronic alcohol abuse, hepatitis B, or hepatitis C infection. Within the liver cells, disruption of the urea c

 Core: Biology

Hypothalamic-Pituitary Axis

JoVE 10879

The response to stress—be it physical or psychological, acute or chronic—involves activation of the Hypothalamic-Pituitary-Adrenal (HPA) axis. The HPA axis is part of the neuroendocrine system because it involves both neuronal and hormonal communication. Its function is to regulate homeostatic systems—metabolic, cardiovascular, and immune—providing the necessary means to respond to a stressor. In response to stress, the neurons in the hypothalamus release corticotropin-releasing hormone, or CRH, into the bloodstream. CRH takes a short journey to the pituitary gland where it stimulates the release of adrenocorticotropic hormone, or ACTH. The site of action for ACTH are the adrenal glands which lay just on the surface of the kidneys. When stimulated, the adrenal glands release two types of stress messages. Neural stimulation initiates the first message—the release of epinephrine and norepinephrine from the adrenal medulla. This activates the sympathetic nervous system resulting in elevated heart rate, blood flow, and respiration—processes designed to activate states of alertness and arousal. These two chemicals are also referred to as adrenaline and noradrenaline, respectively. ACTH initiates the second message—the release of glucocorticoids by the adrenal cortex. In humans, cortisol is the primary hormone

 Core: Biology

Inflammation

JoVE 10902

In response to tissue injury and infection, mast cells initiate inflammation. Mast cells release chemicals that increase the permeability of adjacent blood capillaries and attract additional immune cells to the wound or site of infection. Neutrophils are phagocytic leukocytes that exit the bloodstream and engulf invading microbes. Blood clotting platelets seal the wound and fibers create a scaffold for wound healing. Macrophages engulf aging neutrophils to end the acute inflammatory response. Tissue injury and infection are the primary causes of acute inflammation. Inflammation protects the body by eliminating the cause of tissue injury and initiating the removal of cell debris resulting from the initial damage and related immune cell activity. Inflammation involves mediators of both the innate and adaptive immune system. Proper regulation of inflammation is crucial to clear the pathogen and remove cell debris without overly damaging healthy tissue in the process. If inflammatory processes are not properly regulated, chronic inflammation can arise that is often fatal. Mast cells are the first to respond to tissue injury, as they are primarily located in areas that have contact with the exterior: the skin, gut, and airways. Mast cells have an arsenal of receptors on their cell surface and can hence be activated by a wide variety of stimuli, such as mi

 Core: Biology

Protein Digestion

JoVE 10833

Protein digestion begins in the stomach, where the highly acidic environment can easily disrupt protein structure by exposing the peptide bonds of polypeptide chains. After polypeptide chains are broken into individual amino acids by a series of digestive enzymes, the amino acids are transported to the liver via the bloodstream to produce energy.

Pepsin is a protease, or protein-digesting enzyme, that is produced in the stomach and is one of the main digestive enzymes in the human digestive system. Working in conjunction with chymotrypsin and trypsin released in the small intestine, pepsin severs the links between specific types of amino acids to form shorter polypeptide chains. Other enzymes, called peptidases, then split off one amino acid at a time from the ends of these polypeptide chains. The small intestine can easily absorb the resulting amino acids. The liver plays an essential role in the metabolism of proteins. Liver cells alter digested amino acids from the small intestine so that they can be used to produce energy or make carbohydrates and fats. A byproduct of this process is a toxic substance called ammonia, which the liver then converts into a much less toxic substance called urea. Urea is then released into the blood and transported to the kidneys, which excrete urea out of the body through urine.

 Core: Biology

What is Glycolysis?

JoVE 10737

Cells make energy by breaking down macromolecules. Cellular respiration is the biochemical process that converts “food energy” (from the chemical bonds of macromolecules) into chemical energy in the form of adenosine triphosphate (ATP). The first step of this tightly regulated and intricate process is glycolysis. The word glycolysis originates from Latin glyco (sugar) and lysis (breakdown). Glycolysis serves two main intracellular functions: generate ATP and intermediate metabolites to feed into other pathways. The glycolytic pathway converts one hexose (six-carbon carbohydrate such as glucose), into two triose molecules (three-carbon carbohydrate) such as pyruvate, and a net of two molecules of ATP (four produced, two consumed) and two molecules of nicotinamide adenine dinucleotide (NADH). Did you know that glycolysis was the first biochemical pathway discovered? In the mid-1800s, Louis Pasteur determined that microorganisms cause the breakdown of glucose in the absence of oxygen (fermentation). In 1897, Eduard Buchner found that fermentation reactions can still be carried out in cell-free yeast extracts, achieved by breaking open the cell and collecting the cytoplasm which contains the soluble molecules and organelles. Shortly thereafter in 1905, Arthur Harden and William Young discovered that the rate of fermentation decreases wit

 Core: Biology
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