functions of the blood

Blood is unique; it is the only fluid tissue in the body.

1. Carrier of gases, nutrients, and waste products.

   1. Oxygen enters blood in the lungs and is transported to cells. Carbon dioxide, produced by cells, is transported in the blood to the lungs, from which it is expelled. Ingested nutrients, ions, and water are carried by the blood from the digestive tract to cells, and the waste products of the cells are moved to the kidneys for elimination.

2. Clot formation. Clotting proteins help stem blood loss when a blood vessel is injured.

3. Transport of processed molecules. Most substances are produced in one part of the body and transported in the blood to another part.

4. Protection against foreign substances. Antibodies help protect the body from pathogens.

5. Transport of regulatory molecules. Various hormones and enzymes that regulate body processes are carried from one part of the body to another within the blood.

6. Maintenance of body temperature. Warm blood is transported from the inside to the surface of the body, where heat is released from the blood.

7. pH and osmosis regulation. Albumin is also an important blood buffer and contributes to the osmotic pressure of blood, which acts to keep water in the blood stream.

components of blood

Essentially, blood is a complex connective tissue in which living blood cells, the formed elements, are suspended.

physical characteristics and volume

Blood is a sticky, opaque fluid with a characteristic metallic taste.

• Color. Depending on the amount of oxygen it is carrying, the color of blood varies from scarlet (oxygen-rich) to a dull red (oxygen-poor).

• Weight. Blood is heavier than water and about five times thicker, or more viscous, largely because of its formed elements.

• pH. Blood is slightly alkaline, with a pH between 7.35 and 7.45.

• Temperature. Its temperature (38 degrees Celsius, or 100.4 degrees Fahrenheit) is always slightly higher than body temperature.

plasma

Plasma, which is approximately 90 percent water, is the liquid part of the blood.

• Dissolved substances. Examples of dissolved substances include nutrients, salts (electrolytes), respiratory gases, hormones, plasma proteins, and various wastes and products of cell metabolism.

• Plasma proteins. Plasma proteins are the most abundant solutes in plasma; except for antibodies and protein-based hormones, most plasma proteins are made by the liver.

• Composition. The composition of plasma varies continuously as cells remove or add substances to the blood; assuming a healthy diet, however, the composition of plasma is kept relatively constant by various homeostatic mechanisms of the body.

formed elements

If you observe a stained smear of human blood under a light microscope, you will see disc-shaped red blood cells, a variety of colorfully stained spherical white blood cells, and some scattered platelets that look like debris.

erythrocytes

Erythrocytes, or red blood cells, function primarily to ferry oxygen in blood to all cells of the body.

• Anucleate. RBCs differ from other blood cells because they are anucleate, that is, they lack a nucleus; they also contain a very few organelles.

• Hemoglobin. Hemoglobin, an iron bearing protein, transports the bulk of oxygen that is carried in the blood.

• Microscopic appearance. Erythrocytes are small, flexible cells shaped like biconcave discs- flattened discs with depressed centers on both sides; they look like miniature doughnuts when viewed with a microscope.

• Number of RBCs. There are normally about 5 million cells per cubic millimeter of blood; RBCs outnumber WBCs by about 1000 to 1 and are the major factor contributing to blood viscosity.

• Normal blood. Clinically, normal blood contains 12-18 grams of hemoglobin per 100 milliliters (ml); the hemoglobin content is slightly higher in men (13-18 g/dl) than in women (12-16 g/dl).

leukocytes

Although leukocytes, or white blood cells, are far less numerous than red blood cells, they are crucial to body defense against disease.

• Number of WBCs. On average, there are 4,000 to 11,000 WBC/mm3 , and they account for less than 1 percent of total body volume.

• Body defense. Leukocytes form a protective, movable army that helps defend the body against damage by bacteria, viruses, parasites, and tumor cells.

• Diapedesis. White blood cells are able to slip into and out of the blood vessels- a process called diapedesis.

• Positive chemotaxis. In addition, WBCs can locate areas of tissue damage and infection in the body by responding to certain chemicals that diffuse from the damaged cells; this capability is called positive chemotaxis.

• Ameboid motion. Once they have “caught the scent”, the WBCs move through the tissue spaces by ameboid motion (they form flowing cytoplasmic extensions that help move them along).

• Leukocytosis. A total WBC count above 11, 000 cells/mm3 is referred to as leukocytosis.

• Leukopenia. The opposite condition, leukopenia, is an abnormally low WBC count.

• Granulocytes. Granulocytes are granule-containing WBCs; they have lobed nuclei, which typically consist of several rounded nuclear areas connected by thin strands of nuclear material, and includes neutrophils, eosinophils, and basophils.

• Neutrophils. Neutrophil are the most numerous of the WBCs; they have a multilobed granules and very fine granules that respond to acidic and basic stains; neutrophils are avid phagocytes at sites of acute infection, and are particularly partial to bacteria and fungi.

• Eosinophils. Eosinophils have blue red nucleus that resembles an old-fashioned telephone receiver and sport coarse, lysosome-like, brick-red cytoplasmic granules; their number increases rapidly during allergies and infections by parasitic worms or entering via the skin.

• Basophils. Basophils, the rarest of the WBCs, contain large, histamine-containing granules that stain dark blue; histamine is an inflammatory chemical that makes blood vessels leaky and attracts other WBCs to the inflammatory site.

• Agranulocytes. The second group of WBCs, the agranulocytes, lack visible cytoplasmic granules; their nuclei are closer to the norm- that is, they are spherical; they are spherical, oval, or kidney-shaped; and they include lymphocytes and monocytes.

• Lymphocytes. Lymphocytes have a large, dark purple nucleus that occupies most of the cell volume; they tend to take up residence in lymphatic tissues, where they play an important role in the immune response.

• Monocytes. Monocytes are the largest of the WBCs; when they migrate into the tissues, they transform into macrophages with huge appetites; macrophages are very important in fighting chronic infections.

• Platelets. Platelets are not cells in the strict sense; they are fragments of bizarre multinucleate cells called megakaryocytes, which pinch off thousands of anucleate platelet “pieces” that quickly seal themselves off from surrounding fluids; platelets are needed for the clotting process that occurs in plasma when blood vessels are ruptured or broken.

hematopoiesis

Blood cell formation, or hematopoiesis, occurs in red bone marrow, or myeloid tissue.

• Hemocytoblast. All the formed elements arise from a common type of stem cell, the hemocytoblast.

• Descendants of hemocytoblasts. The hemocytoblast forms two types of descendants- the lymphoid stem cell, which produces lymphocytes, and the myeloid stem cell, which can produce all other classes of formed elements.

formation of red blood cells

Because they are anucleate, RBCs are unable to synthesize proteins, grow, or divide.

• Life span. As they age, RBCs become more rigid and begin to fragment, or fall apart, in 100 to 120 days.

• Lost RBCs. Lost cells are replaced more or less continuously by the division of hemocytoblasts in the red bone marrow.

• Immature RBCs. Developing RBCs divide many times and then begin synthesizing huge amounts of hemoglobin.

• Reticulocyte. Suddenly, when enough hemoglobin has been accumulated, the nucleus and most organelles are ejected and the cell collapses inward; the result is the young RBC, called a reticulocyte because it still contains some rough endoplasmic reticulum (ER).

• Mature erythrocytes. Within 2 days of release, they have rejected the remaining ER and have become fully functioning erythrocytes; the entire developmental process from hemocytoblast to mature RBC takes 3 to 5 days.

• Erythropoietin. The rate of erythrocyte production is controlled by a hormone called erythropoietin; normally a small amount of erythropoietin circulates in the blood at all times, and red blood cells are formed at a fairly constant rate.

• Control of RBC production. An important point to remember is that it is not the relative number of RBCS in the blood that controls RBC production; control is based on their ability to transport enough oxygen to meet the body’s demands.

formation of white blood cells and platelets

Like erythrocyte production, the formation of leukocytes and platelets is stimulated by hormones.

• Colony stimulating factors and interleukins. These colony stimulating factors and interleukins not only prompt red bone marrow to turn out leukocytes, but also marshal up an army of WBCs to ward off attacks by enhancing the ability of mature leukocytes to protect the body.

• Thrombopoietin. The hormone thrombopoietin accelerates the production of platelets, but little is known about how that process is regulated.

hemostasis

The multistep process of hemostasis begins when a blood vessel is damaged and connective tissue in the vessel wall is exposed to blood.

• Vascular spasms occur. The immediate response to blood vessel injury is vasoconstriction, which causes that blood vessel to go into spasms; the spasms narrow the blood vessel, decreasing blood loss until clotting can occur.

• Platelet plug forms. Injury to the lining of vessels exposes collage fibers; platelets adhere to the damaged site and platelet plug forms.

• Coagulation events occur. At the same time, the injured tissues are releasing tissue factor (TF), a substance that plays an important role in clotting; PF3, a phospholipid that coats the surfaces of the platelets, interacts with TF, vitamin K, and other blood clotting factors; this prothrombin activator converts prothrombin, present in the plasma, to thrombin, an enzyme; thrombin then joins soluble fibrinogen proteins into long, hairlike molecules of insoluble fibrin, which forms the meshwork that traps RBCs and forms the basis of the clot; within the hour, the clot begins to retract, squeezing serum from the mass and pulling the ruptured edges of the blood vessel closer together.

blood groups and transfusions

As we have seen, blood is vital for transporting substances through the body; when blood is lost, the blood vessels constrict and the bone marrow steps up blood cell formation in an attempt to keep the circulation going.

human blood groups

Although whole blood transfusions can save lives, people have different blood groups, and transfusing incompatible or mismatched blood can be fatal.

• Antigen. An antigen is a substance that the body recognizes as foreign; it stimulates the immune system to release antibodies or use other means to mount a defense against it.

• Antibodies. One person’s RBC proteins will be recognized as foreign if transfused into another person with different RBC antigens; the “recognizers” are antibodies present in the plasma that attach to RBCs bearing surface antigens different from those on the patient’s (blood recipient’s) RBCs.

• Agglutination. Binding of the antibodies causes the foreign RBCs to clump, a phenomenon called agglutination, which leads to the clogging of small blood vessels throughout the body.

• ABO blood groups. The ABO blood groups are based on which of two antigens, type A or type B, a person inherits; absence of both antigens results in type O blood, presence of both antigens leads to type AB, and the presence of either A or B antigen yields type A or B blood.

• Rh blood groups. The Rh blood groups are so named because one of the eight Rh antigens (agglutinogen D) was originally identified in Rhesus monkeys; later the same antigen was discovered in human beings; most Americans are Rh+ (Rh positive), meaning that their RBCs carry the Rh antigen.

• Anti-Rh antibodies. Unlike the antibodies of the ABO system, anti-Rh antibodies are not automatically formed and present in the blood of Rh- (Rh-negative) individuals.

• Hemolysis. Hemolysis (rupture of RBCs) does not occur with the first transfusion because it takes time for the body to react and start making antibodies.

blood typing

The importance of determining the blood group of both the donor and the recipient before blood is transfused is glaringly obvious.

• Blood typing of ABO blood groups. When serum containing anti-A or anti-B antibodies is added to a blood sample diluted with saline, agglutination will occur between the antibody and the corresponding antigen.

• Cross matching. Cross matching involves testing for agglutination of donor RBCs by the recipient’s serum and of the recipient’s RBCs by the donor serum;

• Blood typing for Rh factors. Typing for the Rh factors is done in the same manner as ABO blood typing.

functions of the heart

The functions of the heart are as follows:

1. Managing blood supply. Variations in the rate and force of heart contraction match blood flow to the changing metabolic needs of the tissues during rest, exercise, and changes in body position.

2. Producing blood pressure. Contractions of the heart produce blood pressure, which is needed for blood flow through the blood vessels.

3. Securing one-way blood flow. The valves of the heart secure a one-way blood flow through the heart and blood vessels.

4. Transmitting blood. The heart separates the pulmonary and systemic circulations, which ensures the flow of oxygenated blood to tissues.

anatomy of the heart

The cardiovascular system can be compared to a muscular pump equipped with one-way valves and a system of large and small plumbing tubes within which the blood travels.

heart structure and functions

The modest size and weight of the heart give few hints of its incredible strength.

• Weight. Approximately the size of a person’s fist, the hollow, cone-shaped heart weighs less than a pound.

• Mediastinum. Snugly enclosed within the inferior mediastinum, the medial cavity of the thorax, the heart is flanked on each side by the lungs.

• Apex. Its more pointed apex is directed toward the left hip and rests on the diaphragm, approximately at the level of the fifth intercostal space.

• Base. Its broad posterosuperior aspect, or base, from which the great vessels of the body emerge, points toward the right shoulder and lies beneath the second rib.

• Pericardium. The heart is enclosed in a double- walled sac called the pericardium which is the outermost layer of the heart.

• Fibrous pericardium. The loosely fitting superficial part of this sac is referred to as the fibrous pericardium, which helps protect the heart and anchors it to surrounding structures such as the diaphragm and sternum.

• Serous pericardium. Deep to the fibrous pericardium is the slippery, two-layer serous pericardium, where its parietal layer lines the interior of the fibrous pericardium.

layers of the heart

The heart muscle has three layers and they are as follows:

• Epicardium. The epicardium or the visceral and outermost layer is actually a part of the heart wall.

• Myocardium. The myocardium consists of thick bundles of cardiac muscle twisted and whirled into ringlike arrangements and it is the layer that actually contracts.

• Endocardium. The endocardium is the innermost layer of the heart and is a thin, glistening sheet of endothelium hat lines the heart chambers.

chambers of the heart

The heart has four hollow chambers, or cavities: two atria and two ventricles.

• Receiving chambers. The two superior atria are primarily the receiving chambers, they play a lighter role in the pumping activity of the heart.

• Discharging chambers. The two inferior, thick-walled ventricles are the discharging chambers, or actual pumps of the heart wherein when they contract, blood is propelled out of the heart and into circulation.

• Septum. The septum that divides the heart longitudinally is referred to as either the interventricular septum or the interatrial septum, depending on which chamber it separates.

associated great vessels

The great blood vessels provide a pathway for the entire cardiac circulation to proceed.

• Superior and inferior vena cava. The heart receives relatively oxygen-poor blood from the veins of the body through the large superior and inferior vena cava and pumps it through the pulmonary trunk.

• Pulmonary arteries. The pulmonary trunk splits into the right and left pulmonary arteries, which carry blood to the lungs, where oxygen is picked up and carbon dioxide is unloaded.

• Pulmonary veins. Oxygen-rich blood drains from the lungs and is returned to the left side of the heart through the four pulmonary veins.

• Aorta. Blood returned to the left side of the heart is pumped out of the heart into the aorta from which the systemic arteries branch to supply essentially all body tissues.

heart valves

The heart is equipped with four valves, which allow blood to flow in only one direction through the heart chambers.

• Atrioventricular valves. Atrioventricular or AV valves are located between the atrial and ventricular chambers on each side, and they prevent backflow into the atria when the ventricles contract.

• Bicuspid valves. The left AV valve- the bicuspid or mitral valve, consists of two flaps, or cusps, of the endocardium.

• Tricuspid valve. The right AV valve, the tricuspid valve, has three flaps.

• Semilunar valve. The second set of valves, the semilunar valves, guards the bases of the two large arteries leaving the ventricular chambers, thus they are known as the pulmonary and aortic semilunar valves.

cardiac circulation vessels

Although the heart chambers are bathed with blood almost continuously, the blood contained in the heart does not nourish the myocardium.

• Coronary arteries. The coronary arteries branch from the base of the aorta and encircle the heart in the coronary sulcus (atrioventricular groove) at the junction of the atria and ventricles, and these arteries are compressed when the ventricles are contract and fill when the heart is relaxed.

• Cardiac veins. The myocardium is drained by several cardiac veins, which empty into an enlarged vessel on the posterior of the heart called the coronary sinus.

blood vessels

Blood circulates inside the blood vessels, which form a closed transport system, the so-called vascular system.

• Arteries. As the heart beats, blood is propelled into large arteries leaving the heart.

• Arterioles. It then moves into successively smaller and smaller arteries and then into arterioles, which feed the capillary beds in the tissues.

• Veins. Capillary beds are drained by venules, which in turn empty into veins that finally empty into the great veins entering the heart.

tunics

Except for the microscopic capillaries, the walls of the blood vessels have three coats or tunics.

• Tunica intima. The tunica intima, which lines the lumen, or interior, of the vessels, is a thin layer of endothelium resting on a basement membrane and decreases friction as blood flows through the vessel lumen.

• Tunica media. The tunica media is the bulky middle coat which mostly consists of smooth muscle and elastic fibers that constrict or dilate, making the blood pressure increase or decrease.

• Tunica externa. The tunica externa is the outermost tunic composed largely of fibrous connective tissue, and its function is basically to support and protect the vessels.

physiology of the heart

As the heart beats or contracts, the blood makes continuous round trips- into and out of the heart, through the rest of the body, and then back to the heart- only to be sent out again.

intrinsic conduction system of the heart

The spontaneous contractions of the cardiac muscle cells occurs in a regular and continuous way, giving rhythm to the heart.

• Cardiac muscle cells. Cardiac muscle cells can and do contract spontaneously and independently, even if all nervous connections are severed.

• Rhythms. Although cardiac muscles can beat independently, the muscle cells in the different areas of the heart have different rhythms.

• Intrinsic conduction system. The intrinsic conduction system, or the nodal system, that is built into the heart tissue sets the basic rhythm.

• Composition. The intrinsic conduction system is composed of a special tissue found nowhere else in the body; it is much like a cross between a muscle and nervous tissue.

• Function. This system causes heart muscle depolarization in only one direction- from the atria to the ventricles; it enforces a contraction rate of approximately 75 beats per minute on the heart, thus the heart beats as a coordinated unit.

• Sinoatrial (SA) node. The SA node has the highest rate of depolarization in the whole system, so it can start the beat and set the pace for the whole heart; thus the term “pacemaker“.

• Atrial contraction. From the SA node, the impulse spread through the atria to the AV node, and then the atria contract.

• Ventricular contraction. It then passes through the AV bundle, the bundle branches, and the Purkinje fibers, resulting in a “wringing” contraction of the ventricles that begins at the heart apex and moves toward the atria.

• Ejection. This contraction effectively ejects blood superiorly into the large arteries leaving the heart.

the pathway of the conduction system

The conduction system occurs systematically through:

• SA node. The depolarization wave is initiated by the sinoatrial node.

• Atrial myocardium. The wave then successively passes through the atrial myocardium.

• Atrioventricular node. The depolarization wave then spreads to the AV node, and then the atria contract.

• AV bundle. It then passes rapidly through the AV bundle.

• Bundle branches and Purkinje fibers. The wave then continues on through the right and left bundle branches, and then to the Purkinje fibers in the ventricular walls, resulting in a contraction that ejects blood, leaving the heart.

cardiac cycle and heart sounds

In a healthy heart, the atria contract simultaneously, then, as they start to relax, contraction of the ventricles begins.

• Systole. Systole means heart contraction.

• Diastole. Diastole means heart relaxation.

• Cardiac cycle. The term cardiac cycle refers to the events of one complete heartbeat, during which both atria and ventricles contract and then relax.

• Length. The average heart beats approximately 75 times per minute, so the length of the cardiac cycle is normally about 0.8 seconds.

• Mid-to-late diastole. The cycle starts with the heart in complete relaxation; the pressure in the heart is low, and blood is flowing passively into and through the atria into the ventricles from the pulmonary and systemic circulations; the semilunar valves are closed, and the AV valves are open; then the atria contract and force the blood remaining in their chambers into the ventricles.

• Ventricular systole. Shortly after, the ventricular contraction begins, and the pressure within the ventricles increases rapidly, closing the AV valves; when the intraventricular pressure is higher than the pressure in the large arteries leaving the heart, the semilunar valves are forced open, and blood rushes through them out of the ventricles; the atria are relaxed, and their chambers are again filling with blood.

• Early diastole. At the end of systole, the ventricles relax, the semilunar valves snap shut, and for a moment the ventricles are completely closed chambers; the intraventricular pressure drops and the AV valves are forced open; the ventricles again begin refilling rapidly with blood, completing the cycle.

• First heart sound. The first heart sound, “lub”, is caused by the closing of the AV valves.

• Second heart sound. The second heart sound, “dub”, occurs when the semilunar valves close at the end of systole.

cardiac output

Cardiac output is the amount of blood pumped out by each side of the heart in one minute. It is the product of the heart rate and the stroke volume.

• Stroke volume. Stroke volume is the volume of blood pumped out by a ventricle with each heartbeat.

• Regulation of stroke volume. According to Starling’s law of the heart, the critical factor controlling stroke volume is how much the cardiac muscle cells are stretched just before they contract; the more they are stretched, the stronger the contraction will be; and anything that increases the volume or speed of venous return also increases stroke volume and force of contraction.

• Factors modifying basic heart rate. The most important external influence on heart rate is the activity of the autonomic nervous system, as well as physical factors (age, gender, exercise, and body temperature).

physiology of circulation

A fairly good indication of the efficiency of a person’s circulatory system can be obtained by taking arterial blood and blood pressure measurements.

cardiovascular vital signs

Arterial pulse pressure and blood pressure measurements, along with those of respiratory rate and body temperature, are referred to collectively as vital signs in clinical settings.

• Arterial pulse. The alternating expansion and recoil of an artery that occurs with each beat of the left ventricle create a pressure wave-a pulse- that travels through the entire arterial system.

• Normal pulse rate. Normally, the pulse rate (pressure surges per minute) equals the heart rate, so the pulse averages 70 to 76 beats per minute in a normal resting person.

• Pressure points. There are several clinically important arterial pulse points, and these are the same points that are compressed to stop blood flow into distal tissues during hemorrhage, referred to as pressure points.

• Blood pressure. Blood pressure is the pressure the blood exerts against the inner walls of the blood vessels, and it is the force that keeps blood circulating continuously even between heartbeats.

• Blood pressure gradient. The pressure is highest in the large arteries and continues to drop throughout the systemic and pulmonary pathways, reaching either zero or negative pressure at the venae cavae.

• Measuring blood pressure. Because the heart alternately contracts and relaxes, the off-and-on flow of the blood into the arteries causes the blood pressure to rise and fall during each beat, thus, two arterial blood pressure measurements are usually made: systolic pressure (the pressure in the arteries at the peak of ventricular contraction)and diastolic pressure (the pressure when the ventricles are relaxing).

• Peripheral resistance. Peripheral resistance is the amount of friction the blood encounters as it flows through the blood vessels.

• Neural factors. The parasympathetic division of the autonomic nervous system has little or no effect on blood pressure, but the sympathetic division has the major action of causing vasoconstriction or narrowing of the blood vessels, which increases blood pressure.

• Renal factors. The kidneys play a major role in regulating arterial blood pressure by altering blood volume, so when blood pressure increases beyond normal, the kidneys allow more water to leave the body in the urine, then blood volume decreases which in turn decreases blood pressure.

• Temperature. In general, cold has a vasoconstricting effect, while heat has a vasodilating effect.

• Chemicals. Epinephrine increases both heart rate and blood pressure; nicotine increases blood pressure by causing vasoconstriction; alcohol and histamine cause vasodilation and decreased blood pressure.

• Diet. Although medical opinions tend to change and are at odds from time to time, it is generally believed that a diet low in salt, saturated fats, and cholesterol help to prevent hypertension, or high blood pressure.

blood circulation through the heart

The right and left sides of the heart work together in achieving a smooth-flowing blood circulation.

• Entrance to the heart. Blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart.

• Atrial contraction. As the atrium contracts, blood flows from the right atrium to the right ventricle through the open tricuspid valve.

• Closure of the tricuspid valve. When the ventricle is full, the tricuspid valve shuts to prevent blood from flowing backward into the atria while the ventricle contracts.

• Ventricle contraction. As the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery, and to the lungs where it is oxygenated.

• Oxygen-rich blood circulates. The pulmonary vein empties oxygen-rich blood from the lungs into the left atrium of the heart.

• Opening of the mitral valve. As the atrium contracts, blood flows from your left atrium into your left ventricle through the open mitral valve.

• Prevention of backflow. When the ventricle is full, the mitral valve shuts. This prevents blood from flowing backward into the atrium while the ventricle contracts.

• Blood flow to the systemic circulation. As the ventricle contracts, blood leaves the heart through the aortic valve, into the aorta, and to the body.

capillary exchange of gases and nutrients

Substances tend to move to and from the body cells according to their concentration gradients.

• Capillary network. Capillaries form an intricate network among the body’s cells such that no substance has to diffuse very far to enter or leave a cell.

• Routes. Basically, substances leaving or entering the blood may take one of four routes across the plasma membranes of the single layer of endothelial cells forming the capillary wall.

• Lipid-soluble substances. As with all cells, substances can diffuse directly through their plasma membranes if the substances are lipid-soluble.

• Lipid-insoluble substances. Certain lipid-insoluble substances may enter or leave the blood and/or pass through the plasma membranes within vesicles, that is, by endocytosis or exocytosis.

• Intercellular clefts. Limited passage of fluid and small solutes is allowed by intercellular clefts (gaps or areas of plasma membrane not joined by tight junctions), so most of our capillaries have intercellular clefts.

• Fenestrated capillaries. Very free passage of small solutes and fluid is allowed by fenestrated capillaries, and these unique capillaries are found where absorption is a priority or where filtration occurs.

age-related physiological changes in the cardiovascular system

The capacity of the heart for work decreases with age. Older peoples’ rate is slower to respond to stress and slower to return to normal after periods of physical activity. Changes in arteries occur frequently which can negatively affect blood supply.