Volume And Distribution Of Body Fluids (Physiology) Flashcards
Explain the relationship between fat content and body water as a percentage of body weight
- For men total body water (TBW) = 0.6 x body weight
- For women TBW = 0.5 x body weight
- Total body water constitutes 60% of body weight in men and 50% of body weight in women. This is because women have more fat than men and water is not stored with fat, therefore women have less body water.
Describe the way in which water is distributed between intracellular, transcellular, and extracellular (interstitial and blood) compartments.
TBW = 1/3 extracellular fluid (ECF) and 2/3 intracellular fluid (ICF) ECF = 3/4 interstitial fluid, 1/4 plasma, and 0.5L transcellular fluid
Describe how fluids move between the different compartments
- Fluid is continually exchanged between compartments via capillary walls and plasma membranes.
- Water moves by osmosis from the digestive tract to the bloodstream.
- Capillary filtration moves water from the blood to the tissue fluid.
- From the tissue fluid, water may be reabsorbed by the capillaries, osmotically absorbed into cells, or taken up by the lymphatic system, which returns it to the bloodstream.
- Osmosis from one fluid compartment to another is determined by the relative concentration of solutes in each compartment.
Explain how the body maintains a normal water balance
- A person is in a state of fluid balance when daily gains and losses of fluid are equal.
Increasing body water
- Plasma osmolality increases or the circulating volume of fluid decreases (osmoreceptors detect change in concentration of body fluids).
- Thirst increases which leads to increased water ingestion, helping to retain water.
- Antidiuretic hormone (ADH) secretion increases which decreases water excretion, also helping to retain water.
- As a result, plasma osmolality decreases and the circulating volume of fluid increases.
- ADH secretion is decreased via negative feedback and thirst also decreases.
Decreasing body water
- Osmoreceptors in the hypothalamus detect a decrease in osmolality which causes a decrease in ADH secretion.
- Decrease in ADH secretion causes an increases in urine output.
- Once fluid osmolality has returned to normal, ADH secretion increases again via negative feedback.
Describe how water is lost from the body at rest and during heavy exercise
At rest
- 1500ml/day is excreted as urine (sensible water loss - we are aware of it).
- 200ml/day is eliminated in the faeces.
- 300ml/day is lost in expired breath (insensible water loss - we are not aware of it).
- 100ml/day of sweat is secreted by a resting adult at an ambient air temperature of 20 degrees celsius.
- 400ml/day is lost as cutaneous transpiration which is water that diffuses through the epidermis and evaporates (insensible water loss).
During heavy exercise
- Sweating during intensive exercise can produce water losses of up to 12L/day (sensible water loss).
Explain the difference in the body’s response to the ingestion of one litre of water and one litre of isotonic saline
Water
- Osmolality of the ECF decreases.
- Causes water to move into the cells by osmosis as the ECF is hypotonic compared to the ICF.
- As a result, both the ECF and ICF volumes increase.
Isotonic saline
- The saline is isotonic with the ICF.
- Water does not move by osmosis into cells.
- ECF volume increases and ICF volume remains the same.
Describe the composition of blood
- 45% red blood cells.
- 55% plasma.
- <1% white blood cells and platelets (buffy coat).
- Plasma = 92% water, 7% plasma proteins (albumins = 60%, globulins = 35%, fibrinogen = 4%, regulatory proteins = <1%), and 1% other solutes (elecrtolytes, organic nutrients, organic wastes).
Describe the proportion of bodyweight blood volume represents in males and females
- Male = 42-52%
- Female = 37-47%
- Testosterone increases red blood cell production while women lose red blood cells due to menstruation.
Describe how blood is distributed throughout the body
- 25% = venules and medium-sized veins.
- 21% = large venous networks (liver, bone marrow, skin).
- 18% = large veins.
- 13% = systemic arterial system (aorta, elastic arteries, muscular arteries, arterioles).
- 9% = pulmonary circuit (pulmonary arteries, pulmonary capillaries, pulmonary veins).
- 7% = systemic capillaries.
- 7% = heart.
Describe the composition of plasma, listing the main electrolytes and proteins
- A complex mixture of water, proteins, nutrients, electrolytes, nitrogenous wastes, hormones, and gases.
- Plasma has the same ionic composition as the rest of the ECF.
- Unlike the rest of the ECF plasma contains plasma proteins.
- Main electrolytes: sodium ions, chloride ions, and bicarbonate ions.
- Main proteins: albumin, globulin, and fibrinogen.
Explain how plasma proteins are separated into albumin, fibrinogen, and alpha, beta and gamma globulins
Albumin
- 60% of total plasma proteins.
- Smallest and most abundant plasma protein.
- Transports fats, amino acids, enzymes, and drugs.
- Buffers the pH of blood plasma.
- Exerts plasma oncotic pressure (pulls water into blood circulation).
Globulins
- 35% of total plasma proteins.
- Divided into three subclasses: alpha, beta, and gamma globulins.
- Transports ions, hormone, and vitamins. Different classes transport different substances.
- Gamma globulins come from plasma cells and are therefore antibodies.
Fibrinogen
- 4% of total plasma proteins.
- It’s a soluble precursor of fibrin, a sticky protein that forms the framework of a blood clot.
Explain the terms haematocrit, anaemia, and polycythaemia
Haematocrit
- Also referred to as the packed cell volume.
- The percentage of the volume of the packed red blood cells to the total blood volume.
Anaemia - Refers to a deficiency of haemoglobin which therefore reduces the oxygen carrying capacity of blood. - Caused by an iron deficiency. - Symptoms include: fatigue, lethargy, shortness of breath, palpitations, headache, and dizziness. - Anaemia can be classified as: Microcytic, hypochromic Normocytic, normochromic Macrocytic
Polycythaemia
- Refers to excess red blood cells in the blood.
- Blood thickens so heart has to work harder.
- Red blood cells can clump together to form clots, leading to stroke or heart attack.
Explain the process of erythropoiesis and its control
- Refers to erythrocyte production.
- In foetus - liver, spleen, and lymph nodes.
- After birth - bone marrow.
- In adult - marrow of membranous bones e.g. vertebrae, ribs, and pelvis.
Process
Proerythroblast - basophilic erythroblast - polychromatophilic erythroblast - orthchromatophilic erythroblast - nucleus is ejected to form a reticulocyte - reticulin is lost to form a mature red blood cell.
Control of erythropoiesis
- A stimulus disrupts homeostasis by decreasing oxygen delivery to kidneys (and other tissues).
- Receptors in the kidneys detect low oxygen level which causes an increases of erythropoietin secretion into the blood.
- Proerythroblasts in red bone marrow mature more quickly into reticulocytes and therefore more reticulocytes enter the circulating blood.
- Large number of red blood cells in circulation causes increased oxygen delivery to tissue.
- Homeostasis is returned when oxygen delivery to kidneys increases to normal.
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What is macrocytic anaemia caused by
Folate deficiency
What vitamins are important for normal erythropoiesis
Iron, vitamin B12, and folate
