The correct answer is D. The thymus and bone marrow are the primary organs of the lymphatic system. The spleen, lymph nodes, tonsils, and appendix are secondary organs.

C is the correct answer. T-cells mature in the thymus and B-cells mature in bone marrow. After maturation, both circulate in the lymph, accumulate in secondary organs, and await recognition of antigens.

The correct answer is C. Oxygen and carbon dioxide diffuse across membranes from areas of high concentration to areas of less concentration. Active transport requires chemical energy to transport biochemicals and atomic and molecular substances across membranes. Filtration is a passive transport occurring during capillary exchange. Osmosis refers to water molecules diffusing across a selectively permeable membrane.

C is the correct answer. Neither the tonsils nor the spleen decreases in size with age. The thyroid is neither an organ nor part of the lymphatic system. It is the largest gland of the endocrine system.

The correct answer is B. The liver is a gland that produces bile. The pancreas is a gland that secretes insulin. The stomach is an organ that digests food particles, and it is a gland that secretes hormones.

B is the correct answer. A glomerulus is a compact group of interconnected capillaries in each renal corpuscle. The capillaries have pores that allow filtration of blood plasma forced through by blood pressure. Glomerulus filtration is not related to choices A, C, or D.

The correct answer is A. Seminal fluid is produced by three glands: the prostate, seminal vesicles, and bulbourethral or Cowper's. The epididymis is a tightly-coiled, narrow tube where newly created sperm cells mature. The testes produce sperm and male sex hormones. The vas deferens act only as a duct.

D is the correct answer. The antibody-mediated immune response uses antibodies developed from B-cells that target antigens. Killer T-cells are part of the cell-mediated immune response.

The correct answer is B. Respiration refers to gas and nutrient exchange at the cellular level. Expiration refers to exhalation. Ventilation refers to the inclusive act of breathing, which involves the physical movement of the diaphragm, rib cage, and lungs.

The correct answer is C. The oropharynx is the throat area that lies behind the mouth. The epiglottis is mucous membrane-covered cartilage tissue at the root of the tongue that guards the glottis entrance. The pharynx is where the digestive and respiratory tracts cross and is commonly called the throat.

The correct answer is B. The nephron is where filtration, absorption, and reabsorption occur. The renal cortex is the outermost portion of the kidney. The adrenal glands are part of the endocrine system. One sits on top of each kidney. The renal pelvis functions as a funnel for urine flow to the ureter.

The correct answer is D. The ovary produces an egg that moves to a fallopian tube, where the egg is fertilized. The egg then moves to the uterus for implantation and growth. The cervix is the top end of the vagina that joins with the lower portion of the uterus.

The correct answer is A. The total number of human chromosomes in a human body (somatic) cell is 46 (23 pairs). Each sex cell contains half the number, 23. Choice C is the number of paired human autosomes. Choice D is the number of paired sex chromosomes.

The correct answer is D. Astrocytes are star-shaped glial cells found in the brain. Ovocytes are female gametocytes. Lymphocytes are a type of white blood cell involved in the immune system.

The correct answer is B. Paracrines act locally, diffusing from the source to neighboring target cells. Exocrine refers to glands that produce substances that have an effect in the tissues where they are produced. Autocrines act upon the cell that produced them.

The correct answer is B. A sarcomere is a multi-protein complex composed of three filament systems. One is composed of the protein myosin; another is assembled by actin monomers; and the other is composed of the protein titin.

D is the correct answer. Acetylcholine is an ester and was the first neurotransmitter to be identified. Carbohydrates are the key source of energy for the body. Alkaloids have a pronounced effect on the body's nervous system. Dopamine is a neurotransmitter connected to sexual desire.

The correct answer is A. The skeletal system has three types of connective tissue. Tendons attach muscle to bone. Ligaments attach bone to bone. Cartilage is found in the rib cage, ear, nose, throat, joints, and in between intervertebral disks. Collagen is a protein that strengthens bones and connective tissue.

C is the correct answer. Blood in the pulmonary vein is coming from the lungs. Therefore, the blood is oxygen rich. It will enter the left side of the heart. The upper chamber is the left atrium. The blood will start there and then move to the left ventricle, before leaving the heart and flowing out to the body.

The correct answer is A. In some neurons, a form of supporting cell called a Schwann cell wraps around the axon. This produces a fatty, insulting covering called a myelin sheath. The myelin sheath is interrupted at intervals, leaving exposed gaps, or nodes. A nerve impulse travels down a myelinated axon and jumps from node to node. This allows the impulse to travel much faster than if it were going along the full length of a bare, unmyelinated axon.

A is the correct answer. Light will first pass through the cornea, a transparent, protective covering for the eye. After passing through the iris, light will pass through the lens. The lens completes the focusing of the image. As the light waves are focused on the retina, receptor cells will generate nerve impulses. These receptor cells are rods and cones. Cones detect color and are sensitive to edges, so they produce sharp images. Rods are sensitive to light and can detect various shades of gray, even in dim light.

The correct answer is B. The medulla oblongata, the lowest part of the brainstem, controls involuntary functions, such as breathing, heart reate, blood pressure, coughing, and swallowing. The cerebellum controls coordination and balance. The midbrain is instrumental in hearing and vision. Finally, the cerebrum is responsible for all voluntary activities of the body.

C is the correct answer. Blood has four basic functions: 1) transport nutrients, dissolved gases, enzymes, hormones, and waste products; 2) regulate body temperature, pH, and electrolytes; 3) protect the body from invaders; and 4) restrict the loss of fluid. It is the job of the lymphatic system to collect fluid that has leaked from the circulatory system.

B is the correct answer. Insulin, secreted by the pancreas, stimulates the ability of its target cells to take up and use glucose. Its target cells are those of the liver, skeletal muscles, and fat tissue. Glucagon, also secreted by the pancreas, stimulates the cells of the liver and skeletal muscles to break down glycogen and increase glucose levels in the blood. When the body is confronted with a threatening "fight or flight" situation, adrenaline is secreted by the adrenal medulla. Among its other functions, adrenaline increases the conversion of glycogen to glucose and stimualtes the rate of metabolism. PTH is secreted by the parathyroid glands. It regulates the calcium levels in the blood by increasing reabsorption of calcium in the kidneys.

The correct answer is D. After two or three hours of churning in the stomach, the food is changed into a pasty mixture called chyme. Chyme passes through the stomach into the small intestine through the pyloric valve. The sphincter is a ring of muscles at the base of the esophagus, which allows food to pass into the stomach. The mitral valve is a valve in the heart. The esophagus is the tube food passes through from the mouth to the stomach.