Blood travels through peripheral tissue in a network of tiny, hair-like capillaries that connect the arterial and venous parts of the circulatory system.

• This is where nutrients and end products of metabolism are exchanged between blood and interstitial fluid.

• Water and many small solutes move freely across the capillary walls in response to differences in fluid pressure and concentration. Solutes that can cross membranes freely (passive diffusion) move from regions of high solute concentration to regions of low solute concentration.

• On the arterial ends of capillaries, blood pressure is higher than interstitial fluid pressure and solutes and water are pushed into interstitial fluid.

• On the venous ends of the capillaries, blood pressure is lower, and water and solutes from the surrounding tissues are able to re-enter the blood plasma.

• Except for protein content, blood plasma and interstitial fluid are similar in composition.

Additionally, peripheral tissue is networked with lymph capillaries. The lymphatic system collects excess interstitial fluid, debris from cellular breakdown, and proteins and lipid droplets too large to pass through capillary walls.

To maintain physiological homeostasis, the daily intake of water must roughly equal that of the daily output of water; this is approximately 2500 mL per day for an average adult.

• If output is greater than intake (as in the case of endurance athletes), body mass will be lost; 4% or greater body mass loss is considered to be dangerous. Output of water and electrolytes are very closely controlled by hormones.

• A shortage of water causes secretion of ADH. In the kidney, ADH causes a decrease in the water content of the urine, while thirst receptors in the hypothalamus, the heart, and blood vessels trigger increased water intake.

About 55% of blood is plasma, which contains the proteins and other solutes, the remaining 45% is a mixture of red blood cells (erythrocytes; RBCs), platelets, and white blood cells (leukocytes; WBCs).

• Major Components of Blood:

  • Whole blood:

    • Blood plasma—fluid part of blood containing water-soluble solutes

    • Blood cells —RBCs (carry gases)

    • WBCs (part of immune system)

    • platelets (help to initiate blood clotting)

  • Blood serum: fluid portion of plasma left after blood has clotted

Major Functions of Blood:

• Transport :The circulatory system is the body’s equivalent of an interstate highway network, transporting materials from where they enter the system to where they are used or disposed of. Oxygen and carbon dioxide are carried to and from different body parts by RBCs.

  • Nutrients are carried from the intestine to the sites of their catabolism. Waste products of metabolism are carried to the kidneys. Hormones from endocrine glands are delivered to their target tissues.

• Regulation: Blood redistributes body heat as it flows along, thereby participating in the regulation of body temperature.

  • It also picks up or delivers water and electrolytes as they are needed. In addition, blood buffers are essential to the maintenance of acid-base balance.

• Defense :Blood carries the molecules and cells needed for two major defense mechanisms:

  • the immune response, which destroys foreign invaders, and

  • blood clotting, which prevents loss of blood and begins the healing of wounds.

Antigen is a substance foreign to the body that triggers the immune response.

Inflammatory response is a nonspecific defense mechanism triggered by antigens or tissue damage.

Immune response is a defense mechanism of the immune system dependent on the recognition of specific antigens, including viruses, bacteria, toxic substances, and infected cells; either cell mediated or antibody-mediated.

Antibody (immunoglobulin) is glycoprotein molecule that identifies antigens.

Cell damage due to infection or injury initiates inflammation, a nonspecific defense mechanism that produces swelling, redness, warmth, and pain.

• Chemical messengers released at the injured site direct the inflammatory response. One such messenger is histamine, which is synthesized from the amino acid histidine and is stored in cells throughout the body.

  • Histamine release is also triggered by an allergic response.

• Histamine sets off dilation of capillaries and increases the permeability of capillary walls. The resulting increased blood flow into the damaged area reddens and warms the skin, and swelling occurs as plasma carrying blood-clotting factors and defensive proteins enters the intercellular space.

  • At the same time, WBCs cross capillary walls to attack invaders.

The cell-mediated immune response is under the control of several kinds of T lymphocytes or T cells.

• The cell-mediated immune response principally guards against abnormal cells and bacteria or viruses entering the normal cells

• it also guards against the invasion of some cancer cells and causes the rejection of transplanted organs.

The WBCs known as B lymphocytes or B cells, with the assistance of T cells, are responsible for the antibody-mediated immune response.

• Unlike T cells, which identify only antigenic cells, B cells identify antigens adrift in body fluids. A B cell is activated when it first binds to an antigen and then encounters a helper T cell that recognizes the same antigen.

• This activation can take place anywhere in the body, but it often occurs in lymph nodes, tonsils, or the spleen, which have large concentrations of lymphocytes.

Autoimmune disease is a disorder in which the immune system identifies normal body components as antigens and produces antibodies to them.

A blood clot is a multistep process that is triggered either by an intrinsic pathway that begins when blood makes contact with the protein collagen or by an extrinsic pathway that begins when damaged tissue releases a membrane glycoprotein known as tissue factor.

• The result of either pathway is a cascade of reactions in which a series of zymogens are activated, ultimately resulting in the formation of a clot composed of the insoluble fibrous protein fibrin and platelets.

Oxygen is transported attached to Fe2+ ions in haemoglobin.

• The percent saturation of haemoglobin with oxygen is governed by the partial pressure of oxygen in surrounding tissues and allosteric variations in haemoglobin structure.

• Carbon dioxide is transported in blood as a solute, attached to haemoglobin, or in solution as hydrogen carbonate ion. In peripheral tissues, carbon dioxide diffuses into RBCs, where it is converted to hydrogen carbonate ion.

• Acid-base balance is controlled as hydrogen ions generated by hydrogen carbonate formation are bound by haemoglobin. At the lungs, oxygen enters the cells, and hydrogen carbonate and hydrogen ions leave.

• A blood pH outside the normal range of 7.35–7.45 can be caused by respiratory or metabolic imbalance, resulting in the potentially serious conditions of acidosis or alkalosis.

About 25% of the blood pumped from the heart goes directly to the kidneys, where the functional units are the nephrons. Each kidney contains over a million of them.

• Blood enters a nephron at a glomerulus, a tangle of capillaries surrounded by a fluid-filled space. Filtration, the first of three essential kidney functions, occurs here.

• The pressure of blood pumped into the glomerulus directly from the heart is high enough to push plasma and all its solutes except large proteins across the capillary membrane into the surrounding fluid, the glomerular filtrate.

• The filtrate flows from the capsule into the tubule that makes up the rest of the nephron, and the blood enters the network of capillaries intertwined with the tubule.

About 125 mL of filtrate per minute enters the kidneys, and they produce 180 L of filtrate per day. This filtrate contains not only waste products but also many solutes the body cannot afford to lose, such as glucose and electrolytes.

Since we excrete only about 1.4 L of urine each day, you can see that another important function of the kidneys is reabsorption—the recapture of water and essential solutes by moving them out of the tubule.

Urine is composed of the products of filtration, minus the substances reabsorbed, plus any secreted substances. It is composed of water, nitrogen-containing wastes, and electrolytes (including H2PO4 - and NH4 +) that are excreted to help maintain acid-base balance.

The balance between water and Na+ excreted or absorbed is governed by the osmolarity of fluid in the kidney, the hormone aldosterone, and various chemical messengers.