Capillary Function Capillaries in major organs are usually filled to capacity Blood supply varies in many other sites

Two mechanisms regulate distribution of blood in capillary beds: Contraction of the smooth muscle layer in the wall of an arteriole constricts the vessel Precapillary sphincters control flow of blood between arterioles and venules

Fig. 42-15 Precapillary sphincters Thoroughfare channel Capillaries Arteriole Venule (a) Sphincters relaxed Figure 42.15 Blood flow in capillary beds Arteriole Venule (b) Sphincters contracted

Precapillary sphincters Thoroughfare channel Fig. 42-15a Precapillary sphincters Thoroughfare channel Figure 42.15 Blood flow in capillary beds Capillaries Arteriole Venule (a) Sphincters relaxed

(b) Sphincters contracted Fig. 42-15b Figure 42.15 Blood flow in capillary beds Arteriole Venule (b) Sphincters contracted

The critical exchange of substances between the blood and interstitial fluid takes place across the thin endothelial walls of the capillaries The difference between blood pressure and osmotic pressure drives fluids out of capillaries at the arteriole end and into capillaries at the venule end

Arterial end of capillary Venous end Fig. 42-16 Body tissue INTERSTITIAL FLUID Capillary Net fluid movement out Net fluid movement in Direction of blood flow Blood pressure Figure 42.16 Fluid exchange between capillaries and the interstitial fluid Inward flow Pressure Outward flow Osmotic pressure Arterial end of capillary Venous end

Body tissue INTERSTITIAL FLUID Capillary Net fluid movement out Fig. 42-16a Body tissue INTERSTITIAL FLUID Capillary Net fluid movement out Net fluid movement in Figure 42.16 Fluid exchange between capillaries and the interstitial fluid Direction of blood flow

Arterial end of capillary Venous end Fig. 42-16b Blood pressure Inward flow Pressure Outward flow Osmotic pressure Figure 42.16 Fluid exchange between capillaries and the interstitial fluid Arterial end of capillary Venous end

Fluid Return by the Lymphatic System The lymphatic system returns fluid that leaks out in the capillary beds This system aids in body defense Fluid, called lymph, reenters the circulation directly at the venous end of the capillary bed and indirectly through the lymphatic system The lymphatic system drains into veins in the neck

Lymph nodes are organs that filter lymph and play an important role in the body’s defense Edema is swelling caused by disruptions in the flow of lymph

Concept 42.4: Blood components function in exchange, transport, and defense In invertebrates with open circulation, blood (hemolymph) is not different from interstitial fluid Blood in the circulatory systems of vertebrates is a specialized connective tissue

Blood Composition and Function Blood consists of several kinds of cells suspended in a liquid matrix called plasma The cellular elements occupy about 45% of the volume of blood

Figure 42.17 The composition of mammalian blood Plasma 55% Constituent Major functions Cellular elements 45% Cell type Number per µL (mm3) of blood Functions Water Solvent for carrying other substances Erythrocytes (red blood cells) 5–6 million Transport oxygen and help transport carbon dioxide Ions (blood electrolytes) Sodium Potassium Calcium Magnesium Chloride Bicarbonate Osmotic balance, pH buffering, and regulation of membrane permeability Separated blood elements Leukocytes (white blood cells) 5,000–10,000 Defense and immunity Plasma proteins Albumin Osmotic balance pH buffering Lymphocyte Basophil Fibrinogen Figure 42.17 The composition of mammalian blood For the Discovery Video Blood, go to Animation and Video Files. For the Cell Biology Video Leukocyte Adhesion and Rolling, go to Animation and Video Files. Clotting Immunoglobulins (antibodies) Defense Eosinophil Neutrophil Monocyte Substances transported by blood Nutrients (such as glucose, fatty acids, vitamins) Waste products of metabolism Respiratory gases (O2 and CO2) Hormones Platelets 250,000– 400,000 Blood clotting

Plasma Blood plasma is about 90% water Among its solutes are inorganic salts in the form of dissolved ions, sometimes called electrolytes Another important class of solutes is the plasma proteins, which influence blood pH, osmotic pressure, and viscosity Various plasma proteins function in lipid transport, immunity, and blood clotting

Cellular Elements Suspended in blood plasma are two types of cells: Red blood cells (erythrocytes) transport oxygen White blood cells (leukocytes) function in defense Platelets, a third cellular element, are fragments of cells that are involved in clotting

Erythrocytes Red blood cells, or erythrocytes, are by far the most numerous blood cells They transport oxygen throughout the body They contain hemoglobin, the iron-containing protein that transports oxygen

Leukocytes There are five major types of white blood cells, or leukocytes: monocytes, neutrophils, basophils, eosinophils, and lymphocytes They function in defense by phagocytizing bacteria and debris or by producing antibodies They are found both in and outside of the circulatory system

Platelets Platelets are fragments of cells and function in blood clotting

Blood Clotting When the endothelium of a blood vessel is damaged, the clotting mechanism begins A cascade of complex reactions converts fibrinogen to fibrin, forming a clot A blood clot formed within a blood vessel is called a thrombus and can block blood flow

Fig. 42-18-1 Collagen fibers Platelet plug Platelet releases chemicals that make nearby platelets sticky Figure 42.18 Blood clotting

Platelet releases chemicals that make nearby platelets sticky Fig. 42-18-2 Collagen fibers Platelet plug Platelet releases chemicals that make nearby platelets sticky Clotting factors from: Platelets Damaged cells Plasma (factors include calcium, vitamin K) Figure 42.18 Blood clotting

Platelet releases chemicals that make nearby platelets sticky Fig. 42-18-3 Collagen fibers Platelet plug Platelet releases chemicals that make nearby platelets sticky Clotting factors from: Platelets Damaged cells Plasma (factors include calcium, vitamin K) Figure 42.18 Blood clotting Prothrombin Thrombin

Platelet releases chemicals that make nearby platelets sticky Fig. 42-18-4 Red blood cell Collagen fibers Platelet plug Fibrin clot Platelet releases chemicals that make nearby platelets sticky Clotting factors from: Platelets Damaged cells Plasma (factors include calcium, vitamin K) Figure 42.18 Blood clotting Prothrombin Thrombin Fibrinogen Fibrin 5 µm

Stem Cells and the Replacement of Cellular Elements The cellular elements of blood wear out and are replaced constantly throughout a person’s life Erythrocytes, leukocytes, and platelets all develop from a common source of stem cells in the red marrow of bones The hormone erythropoietin (EPO) stimulates erythrocyte production when oxygen delivery is low

Stem cells (in bone marrow) Lymphoid stem cells Myeloid stem cells Fig. 42-19 Stem cells (in bone marrow) Lymphoid stem cells Myeloid stem cells Lymphocytes B cells T cells Figure 42.19 Differentiation of blood cells Erythrocytes Neutrophils Platelets Eosinophils Monocytes Basophils

Cardiovascular Disease Cardiovascular diseases are disorders of the heart and the blood vessels They account for more than half the deaths in the United States

Atherosclerosis One type of cardiovascular disease, atherosclerosis, is caused by the buildup of plaque deposits within arteries

(b) Partly clogged artery Fig. 42-20 Connective tissue Smooth muscle Endothelium Plaque Figure 42.20 Atherosclerosis (a) Normal artery 50 µm (b) Partly clogged artery 250 µm

Smooth Connective muscle tissue Endothelium (a) Normal artery 50 µm Fig. 42-20a Connective tissue Smooth muscle Endothelium Figure 42.20 Atherosclerosis (a) Normal artery 50 µm

(b) Partly clogged artery 250 µm Fig. 42-20b Plaque Figure 42.20 Atherosclerosis (b) Partly clogged artery 250 µm

Heart Attacks and Stroke A heart attack is the death of cardiac muscle tissue resulting from blockage of one or more coronary arteries A stroke is the death of nervous tissue in the brain, usually resulting from rupture or blockage of arteries in the head

Treatment and Diagnosis of Cardiovascular Disease Cholesterol is a major contributor to atherosclerosis Low-density lipoproteins (LDLs) are associated with plaque formation; these are “bad cholesterol” High-density lipoproteins (HDLs) reduce the deposition of cholesterol; these are “good cholesterol” The proportion of LDL relative to HDL can be decreased by exercise, not smoking, and avoiding foods with trans fats

Hypertension, or high blood pressure, promotes atherosclerosis and increases the risk of heart attack and stroke Hypertension can be reduced by dietary changes, exercise, and/or medication