The nervous system enables humans to react to their surroundings and coordinate their behaviour. At a junction (called a synapse) between a sensory neurone and a relay neurone in the central nervous system, a chemical is released which causes an impulse to be sent along a relay neurone. Oestrogen is produced by the ovaries and it's target organs are the pituitary gland (blocks production of FSH) and the uterus (causes lining to thicken).
In the body, water moves through semi-permeable membranes of cells and from one compartment of the body to another by a process called osmosis.
Human beings are mostly water, ranging from about 75 percent of body mass in infants to about 50–60 percent in adult men and women, to as low as 45 percent in old age.
Body fluids can be discussed in terms of their specific fluid compartment, a location that is largely separate from another compartment by some form of a physical barrier.
The compositions of the two components of the ECF—plasma and IF—are more similar to each other than either is to the ICF ([link]). Hydrostatic pressure, the force exerted by a fluid against a wall, causes movement of fluid between compartments. Watch this video to see an explanation of the dynamics of fluid in the body’s compartments.
Hydrostatic pressure is especially important in governing the movement of water in the nephrons of the kidneys to ensure proper filtering of the blood to form urine.
The movement of some solutes between compartments is active, which consumes energy and is an active transport process, whereas the movement of other solutes is passive, which does not require energy.
Passive transport of a molecule or ion depends on its ability to pass through the membrane, as well as the existence of a concentration gradient that allows the molecules to diffuse from an area of higher concentration to an area of lower concentration. In pulmonary edema resulting from heart failure, excessive leakage of water occurs because fluids get “backed up” in the pulmonary capillaries of the lungs, when the left ventricle of the heart is unable to pump sufficient blood into the systemic circulation. Mild, transient edema of the feet and legs may be caused by sitting or standing in the same position for long periods of time, as in the work of a toll collector or a supermarket cashier. Medications that can result in edema include vasodilators, calcium channel blockers used to treat hypertension, non-steroidal anti-inflammatory drugs, estrogen therapies, and some diabetes medications.
Solute contributes to the movement of water between cells and the surrounding medium by ________.
Osmosis is basically the diffusion of water from regions of higher concentration to regions of lower concentration, along an osmotic gradient across a semi-permeable membrane. The percent of body water changes with development, because the proportions of the body given over to each organ and to muscles, fat, bone, and other tissues change from infancy to adulthood ([link]).
The intracellular fluid (ICF) compartment is the system that includes all fluid enclosed in cells by their plasma membranes. The ICF makes up about 60 percent of the total water in the human body, and in an average-size adult male, the ICF accounts for about 25 liters (seven gallons) of fluid ([link]). These include the cerebrospinal fluid that bathes the brain and spinal cord, lymph, the synovial fluid in joints, the pleural fluid in the pleural cavities, the pericardial fluid in the cardiac sac, the peritoneal fluid in the peritoneal cavity, and the aqueous humor of the eye. When blood volume decreases due to sweating, from what source is water taken in by the blood? Thus, cations, or positively charged ions, and anions, or negatively charged ions, are balanced in fluids. The hydrostatic pressure of blood is the pressure exerted by blood against the walls of the blood vessels by the pumping action of the heart. As hydrostatic pressure in the kidneys increases, the amount of water leaving the capillaries also increases, and more urine filtrate is formed. Recall that an osmotic gradient is produced by the difference in concentration of all solutes on either side of a semi-permeable membrane.
Sweating depletes your tissues of water and increases the solute concentration in those tissues. Active transport allows cells to move a specific substance against its concentration gradient through a membrane protein, requiring energy in the form of ATP. Some molecules, like gases, lipids, and water itself (which also utilizes water channels in the membrane called aquaporins), slip fairly easily through the cell membrane; others, including polar molecules like glucose, amino acids, and ions do not.


People with pulmonary edema likely will experience difficulty breathing, and they may experience chest pain. Because the left side of the heart is unable to pump out its normal volume of blood, the blood in the pulmonary circulation gets “backed up,” starting with the left atrium, then into the pulmonary veins, and then into pulmonary capillaries. A decrease in the normal levels of plasma proteins results in a decrease of colloid osmotic pressure (which counterbalances the hydrostatic pressure) in the capillaries.
This is because deep veins in the lower limbs rely on skeletal muscle contractions to push on the veins and thus “pump” blood back to the heart. Underlying medical conditions that can contribute to edema include congestive heart failure, kidney damage and kidney disease, disorders that affect the veins of the legs, and cirrhosis and other liver disorders. Activities that can reduce the effects of the condition include appropriate exercises to keep the blood and lymph flowing through the affected areas. Body fluids are aqueous solutions with differing concentrations of materials, called solutes.
How can this be if individual ions of sodium and chloride exactly balance each other out, and plasma is electrically neutral?
The osmotic pressure results from differences in solute concentrations across cell membranes.
In the human body, solutes vary in different parts of the body, but may include proteins—including those that transport lipids, carbohydrates, and, very importantly, electrolytes. As a result, water will move into and out of cells and tissues, depending on the relative concentrations of the water and solutes found there.
Your brain and kidneys have the highest proportions of water, which composes 80–85 percent of their masses. This fluid volume tends to be very stable, because the amount of water in living cells is closely regulated.
Plasma travels through the body in blood vessels and transports a range of materials, including blood cells, proteins (including clotting factors and antibodies), electrolytes, nutrients, gases, and wastes. Because these fluids are outside of cells, these fluids are also considered components of the ECF compartment. The IF has high concentrations of sodium, chloride, and bicarbonate, but a relatively lower concentration of protein. As seen in the previous graph, sodium (Na+) ions and chloride (Cl-) ions are concentrated in the ECF of the body, whereas potassium (K+) ions are concentrated inside cells. In capillaries, hydrostatic pressure (also known as capillary blood pressure) is higher than the opposing “colloid osmotic pressure” in blood—a “constant” pressure primarily produced by circulating albumin—at the arteriolar end of the capillary ([link]). If hydrostatic pressure in the kidneys drops too low, as can happen in dehydration, the functions of the kidneys will be impaired, and less nitrogenous wastes will be removed from the bloodstream.
The magnitude of the osmotic gradient is proportional to the difference in the concentration of solutes on one side of the cell membrane to that on the other side. As this happens, water diffuses from your blood into sweat glands and surrounding skin tissues that have become dehydrated because of the osmotic gradient.
For example, the sodium-potassium pump employs active transport to pump sodium out of cells and potassium into cells, with both substances moving against their concentration gradients.
Some of these molecules enter and leave cells using facilitated transport, whereby the molecules move down a concentration gradient through specific protein channels in the membrane. Pulmonary edema can be life threatening, because it compromises gas exchange in the lungs, and anyone having symptoms should immediately seek medical care.
The resulting increased hydrostatic pressure within pulmonary capillaries, as blood is still coming in from the pulmonary arteries, causes fluid to be pushed out of them and into lung tissues. This process causes loss of water from the blood to the surrounding tissues, resulting in edema. Otherwise, the venous blood pools in the lower limbs and can leak into surrounding tissues. Other therapies include elevation of the affected part to assist drainage, massage and compression of the areas to move the fluid out of the tissues, and decreased salt intake to decrease sodium and water retention.
An appropriate balance of water and solute concentrations must be maintained to ensure cellular functions.


Hydrostatic pressure results from the pressure of blood as it enters a capillary system, forcing some fluid out of the vessel into the surrounding tissues. Often in medicine, a mineral dissociated from a salt that carries an electrical charge (an ion) is called and electrolyte. An appropriate balance of solutes inside and outside of cells must be maintained to ensure normal function.
Extracellular fluid has two primary constituents: the fluid component of the blood (called plasma) and the interstitial fluid (IF) that surrounds all cells not in the blood ([link]). If the amount of water inside a cell falls to a value that is too low, the cytosol becomes too concentrated with solutes to carry on normal cellular activities; if too much water enters a cell, the cell may burst and be destroyed. Although sodium and potassium can “leak” through “pores” into and out of cells, respectively, the high levels of potassium and low levels of sodium in the ICF are maintained by sodium-potassium pumps in the cell membranes. This pressure forces plasma and nutrients out of the capillaries and into surrounding tissues. Water will move by osmosis from the side where its concentration is high (and the concentration of solute is low) to the side of the membrane where its concentration is low (and the concentration of solute is high). Additionally, as water leaves the blood, it is replaced by the water in other tissues throughout your body that are not dehydrated.
Edema is almost always caused by an underlying medical condition, by the use of certain therapeutic drugs, by pregnancy, by localized injury, or by an allergic reaction. For instance, sodium ions (Na+) and chloride ions (Cl-) are often referred to as electrolytes. Cells are separated from the IF by a selectively permeable cell membrane that helps regulate the passage of materials between the IF and the interior of the cell.
These pumps use the energy supplied by ATP to pump sodium out of the cell and potassium into the cell ([link]). Fluid and the cellular wastes in the tissues enter the capillaries at the venule end, where the hydrostatic pressure is less than the osmotic pressure in the vessel. In the body, water moves by osmosis from plasma to the IF (and the reverse) and from the IF to the ICF (and the reverse). For example, glucose is transferred into cells by glucose transporters that use facilitated transport ([link]). In the limbs, the symptoms of edema include swelling of the subcutaneous tissues, an increase in the normal size of the limb, and stretched, tight skin.
If the cytosol becomes too dilute due to water intake by cells, cell membranes can be damaged, and the cell can burst.
Filtration pressure squeezes fluid from the plasma in the blood to the IF surrounding the tissue cells.
In the body, water moves constantly into and out of fluid compartments as conditions change in different parts of the body.
When a dehydrated person drinks water and rehydrates, the water is redistributed by the same gradient, but in the opposite direction, replenishing water in all of the tissues.
One quick way to check for subcutaneous edema localized in a limb is to press a finger into the suspected area.
Hydrostatic pressure is the force exerted by a fluid against a wall and causes movement of fluid between compartments. The surplus fluid in the interstitial space that is not returned directly back to the capillaries is drained from tissues by the lymphatic system, and then re-enters the vascular system at the subclavian veins. Edema is likely if the depression persists for several seconds after the finger is removed (which is called “pitting”).
Active transport processes require ATP to move some solutes against their concentration gradients between compartments.
Passive transport of a molecule or ion depends on its ability to pass easily through the membrane, as well as the existence of a high to low concentration gradient.



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